The structure, spectroscopy, and excited state predissociation dynamics of GeH2

Article abstract of DOI:10.1063/1.470520

The spectroscopy and excited state dynamics of <*> ¹B₁ germylene (GeH2) have been investigated experimentally and theoretically.Jet-cooled laser-induced fluorescence spectra of GeH2 were obtained by subjecting germane (GeH4) to an electric discharge at the exit of a pulsed nozzle.The band origins of ten vibronic transitions were determined, giving values for the upper state fundamentals of ν₁=783.0 cm^-1 and ν₂=1798.4 cm^-1.Sufficient numbers of 0⁰₀ band rovibronic transitions were observed to give the ground and excited state structures as r =1.591 (7) Angstroem, θ =91.2(8) deg and r'=1.553(12) Angstroem, θ'=123.4(19) deg.Fluorescence lifetime measurements show that the O_0,0 rotational levels decay radiatively; higher J rotational states in the 0⁰ vibronic level decay much faster, due to a heterogeneous predissociation in the excited state.High quality ab initio studies are consistent with a model in which the lower vibronic levels of the <*> state predissociate through the <*> ³B₁ state to produce Ge(3P)+H2(¹Σ⁺_g).The transition state for this process has been located and the barrier to dissociation is 15.2 kcal/mol above the <*> ¹B₁ state, so that tunneling through the barrier must occur.Above 4000 cm^-1 of vibrational energy in the <*> state, a breaking off of fluorescence is observed as a second predissociation channel involving GeH2(<*> ¹B₁)->Ge(1D)+H2(¹Σ⁺_g) becomes accessible.This process is also found to have a barrier, in contrast to previous theoretical studies of SiH2, where the analogous dissociation was predicted to be barrierless.

Full text of DOI:10.1063/1.470520

The structure, spectroscopy, and excited state predissociation dynamics of GeH2
J. Karolczak, Warren W. Harper, Roger S. Grev, and Dennis J. Clouthier
Citation: The Journal of Chemical Physics 103, 2839 (1995); doi: 10.1063/1.470520
View online: http://dx.doi.org/10.1063/1.470520
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The structure, spectroscopy, and excited state predissociation dynamics
of GeH2
J. Karolczak,a) Warren W. Harper, Roger S. Grev, and Dennis J. Clouthierb)
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
͑Received 1 May 1995; accepted 17 May 1995͒
1
˜
The spectroscopy and excited state dynamics of A B1 germylene ͑GeH2͒ have been investigated
experimentally and theoretically. Jet-cooled laser-induced fluorescence spectra of GeH2 were
obtained by subjecting germane ͑GeH4͒ to an electric discharge at the exit of a pulsed nozzle. The
band origins of ten vibronic transitions were determined, giving values for the upper state
fundamentals of ␯1ϭ783.0 cmϪ1 and ␯2ϭ1798.4 cmϪ1. Sufficient numbers of 000 band rovibronic
transitions were observed to give the ground and excited state structures as rЉϭ1.591͑7͒ Å,
␪Љϭ91.2͑8͒° and rЈϭ1.553͑12͒ Å, ␪Јϭ123.4͑19͒°. Fluorescence lifetime measurements show that
the 00,0 rotational levels decay radiatively; higher J rotational states in the 00 vibronic level decay
much faster, due to a heterogeneous predissociation in the excited state. High quality ab initio
˜
studies are consistent with a model in which the lower vibronic levels of the A state predissociate
through the a B1 state to produce Ge͑3P͒ϩH2͑1⌺ϩg ͒. The transition state for this process has
3
˜
1
˜
been located and the barrier to dissociation is 15.2 kcal/mol above the A B1 state, so that
tunneling through the barrier must occur. Above 4000 cmϪ1 of vibrational energy in the A state, a
˜
breaking off of fluorescence is observed as a second predissociation channel involving
1
1
1 ϩ
˜
GeH2(A B1)→Ge͑ D͒ϩH2͑ ⌺g ͒ becomes accessible. This process is also found to have a barrier,
in contrast to previous theoretical studies of SiH2, where the analogous dissociation was predicted
to be barrierless. © 1995 American Institute of Physics.
I. INTRODUCTION
mane in the gas phase show that the initial process is the
production of GeH2 and H2.11 GeH2 has been postulated as
the primary precursor in the germane laser-assisted chemical
vapor deposition process and a model has been developed for
the subsequent film growth.12 GeH2 and GeH3 have been
detected by infrared absorption spectroscopy of the matrix-
isolated products of a low-pressure dc discharge through
germane.13 In studies of the interaction of digermane with
germanium surfaces, infrared spectroscopy revealed the pres-
ence of GeH3, GeH2, and GeH in varying quantities at dif-
ferent temperatures.14 The interaction of diethylgermane with
Si surfaces, as characterized by photoelectron spectroscopy,
also revealed GeH2 as a major surface adsorption product.1
One of the major impediments to ascertaining the impor-
tance of gas phase GeH2 in semiconductor growth processes
is the lack of a sensitive spectroscopic method for detecting
and quantifying it. Indeed, it is astonishing how little is
known about germylene, considering the importance of ger-
manium hydrides in semiconductor processing and the exten-
sive research activity on the corresponding carbene and si-
lylene species. Until recently, the only available
spectroscopic data were the ground state vibrational frequen-
cies, obtained in 1972 from matrix isolation studies15 of the
photolysis of GeH4. The gas phase microwave, infrared and
photoelectron spectra are unknown. In 1993, Saito and Obi
reported the laser-induced fluorescence ͑LIF͒ spectra of
GeH2 and GeD2, produced by the photolysis of phenylger-
mane in a supersonic free jet expansion.16,17 They observed
five vibronic bands for each species ͑2n0, nϭ0–4͒, in the
612–514 nm region. Each band consisted primarily of a tran-
sition to the lowest rotational level in the upper state, sug-
The production of semiconductors involving Si/Ge al-
loys or heterostructures has received considerable attention
in recent years. These systems have great potential for the
fabrication of electronic devices, because they offer the pos-
sibility for band gap engineering through modifications of
the stoichiometry or growth patterns.1,2 The primary method
for growing such structures is chemical vapor deposition
͑CVD͒, in which feed gases ͑SiH4, GeH4, etc.͒ flow over the
heated substrate surface in vacuum, depositing thin layers of
silicon and/or germanium of the desired composition. The
low temperature growth of germanium films by laser photo-
dissociation of GeH4 on or near the substrate surface has also
been investigated as a way of producing more carefully con-
trolled deposition.2–4 In addition, laser processes have the
potential for direct writing of circuit elements onto surfaces.
Other techniques for growing germanium films include mo-
lecular beam epitaxy5,6 and various electric discharge
methods.7–9
Silanes, germanes, and their halogen-substituted analogs
are the most common precursors used in the chemical vapor
deposition of silicon and germanium. As pointed out by Lu
and Crowell10 in 1993, ‘‘although the chemical vapor depo-
sition of Si or Ge from these gases is well-established tech-
nology, the mechanistic details are far from well under-
stood.’’ In the case of germanium, germylene ͑GeH2͒ has
often been cited as a reactive intermediate in these processes.
For example, studies of the thermal decomposition of ger-
a͒
Permanent address: Quantum Electronics Laboratory, Institute of Physics,
A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland.
b͒
Author to whom correspondence should be addressed.
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2840
Karolczak et al.: Spectroscopy predissociation of GeH2
II. EXPERIMENT
gesting an excited state predissociation process similar to
that in SiH2. The authors obtained the harmonic bending
frequencies and x022 anharmonicity constants for the ground
and excited states. While the present work was in progress,
Saito and Obi published a further study18 in which they mea-
sured fluorescence lifetimes of the excited state levels, ob-
served the 250 and 206 bands of GeD2 and postulated a mecha-
nism for the predissociation process. By analogy with
SiH2,19 they suggested that the S1 excited state is Coriolis
coupled to the ground state, which then interacts with the 3B1
state by spin–orbit coupling. The known thermochemistry is
consistent with dissociation to Ge͑3P͒ϩH2 occurring on the
triplet state surface.
Germylene ͑GeH2͒ was made using an electric discharge
jet similar to many of the designs in the literature.40–45 In our
apparatus, a mixture of 5% GeH4 in argon ͑40–60 psi,
Matheson͒ was expanded through the 0.8 mm orifice of a
pulsed valve ͑General Valve, Series 9͒ into a 7 mm diameter
flow channel drilled into a 14 mm long Teflon cylinder at-
tached to the end of the valve. Halfway down the channel,
two tungsten electrodes with pointed ends were mounted per-
pendicular to the gas flow with a 2 mm gap. At the appro-
priate time after triggering the valve, a 500 ns duration high
voltage ͑2–6 kV͒ pulse was applied to one electrode, with
the other electrode grounded. The resulting discharge effi-
ciently dissociated germane, producing readily detectable
quantities of GeH2.
For measurements of the LIF spectrum of GeH2, the
pulsed free jet expansion was crossed with a tunable dye
laser beam ͑Lambda-Physik FL 3002͒ 24 mm downstream
from the end of the Teflon discharge block. The light emitted
at right angles to the laser beam was imaged through appro-
priate cutoff filters onto the photocathode of a photomulti-
plier tube ͑EMI 9816QB͒ and the pulsed signals were pro-
cessed with gated integrators. Low resolution spectra ͑0.1
cmϪ1͒ were calibrated with optogalvanic lines of various
neon- and argon-filled hollow cathode lamps. High resolu-
tion spectra ͑0.04 cmϪ1͒, taken with an angle-tuned etalon in
the laser cavity, were calibrated with laser induced fluores-
cence lines of iodine vapor. The GeH2 laser induced fluores-
cence and calibration spectra were digitized and recorded
simultaneously on a data acquisition system of our own
design.46
In contrast to the paucity of experimental data, there
have been a substantial number of theoretical studies of
GeH2.20–32 Only the most relevant of these will be discussed.
The ground state geometric parameters of GeH2 have been
predicted20,26,30 in the range r͑Ge–H͒ϭ1.587–1.607 Å and
␪͑HGeH͒ϭ90.4–91.5°. By fitting 37 calculated points on the
ground state potential energy surface, Bunker et al.30 ob-
tained the vibrational frequencies ␯1ϭ1857 cmϪ1, ␯2ϭ923
cmϪ1, and ␯3ϭ1866 cmϪ1, in good agreement with the ma-
trix data.15 The energy separation of the excited triplet and
ground singlet states is predicted20,26,29 to be in the range of
22.8–23.6 kcal/mol ͑7974–8254 cmϪ1͒, with an increase in
the bond angle of 28.4–29.8° and a corresponding decrease
in the bond length of 0.046–0.056 Å on excitation. The trip-
let state vibrational frequencies obtained from the calculated
potential energy surface29 were ␯1ϭ1991 cmϪ1, ␯2ϭ763
cmϪ1, and ␯3ϭ2012 cmϪ1. The first excited singlet ͑1B1͒
state was calculated by Balasubramanian26 to be 47.3 kcal/
mol ͑16 543 cmϪ1͒ above the ground state with r͑Ge–H͒
ϭ1.553 and ␪͑HGeH͒ϭ122.1°, using relativistic CI methods.
Barthelat et al.31 obtained ⌬E(S1ϪS0)ϭ46.7 ͑16 334 cmϪ1
͒
The pulsed discharge jet was mounted near the center of
a large cylindrical ͑20 in. o.d.͒ vacuum chamber pumped by
a 10 in. diffusion pump. The background chamber pressure
was typically 2ϫ10Ϫ6 Torr, rising to about 1ϫ10Ϫ5 Torr
when the pulsed valve ͑200–220 ␮s gas pulses͒ was in op-
eration. For the measurement of fluorescence lifetimes, con-
siderable care was taken to guarantee that the data were col-
lected sufficiently far downstream of the jet to ensure
collision-free conditions. In a series of experiments, the dis-
tance of the excitation laser beam from the jet was aug-
mented until the longest measured lifetimes ͑2.3 ␮s͒ were
invariant to further increases. All of the final measurements
were done with the excitation laser beam 61 mm from the
end of the Teflon discharge block, with optics that ensured
that fluorescent molecules would not leave the detector view-
ing zone over at least two lifetimes. Fluorescence decays
were recorded by digitizing the PMT output on a digital stor-
age oscilloscope ͑LeCroy 9450A͒ with a temporal resolution
of 2.5 ns per data point. The scattered laser light background
was subtracted from the decay curve by slightly detuning the
laser wavelength from the feature of interest. The decay
curves were averaged over 500–1000 laser shots, back-
ground subtracted and transferred to a computer ͑IBM-PC͒
for analysis.
with r͑Ge–H͒ϭ1.566 and ␪͑HGeH͒ϭ123.2° and vibrational
frequencies of ␯1ϭ1864, ␯2ϭ860, and ␯3ϭ2011 cmϪ1 for the
S1 state, using CI methods with core potentials.
In recent years, we have embarked on a program to
study the jet-cooled electronic spectra of divalent group IVA
reactive intermediates. We have succeeded in producing LIF
spectra of CCl2,33 CClF,34 CF2,35 SiCl2,36 SiF2,37 GeCl2,38
39
and GeF2 with sufficient resolution to resolve the vibra-
tional and, in favorable cases, the rotational structure in spec-
tra that are often impossibly congested at room temperature.
In the case of GeCl2, GeF2, and SiF2, we have also been able
to study the first excited triplet states through direct laser
induced phosphorescence excitation spectroscopy. In the
present work, we report the detection of jet-cooled GeH2 by
subjecting GeH4 to an electric discharge at the exit of a
pulsed valve. Several new bands have been found, the geom-
etry of GeH2 has been obtained from the rotational structure
of the 000 band, fluorescence lifetimes have been measured
for several excited state levels, and the germanium isotope
effects have been measured. We have investigated the ener-
getics of the excited state predissociation process with the
aid of high quality ab initio calculations of the transition
states and potential curves. Our more extensive results are in
general agreement with previous work,16–18 although there
are important differences.
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Karolczak et al.: Spectroscopy predissociation of GeH2
2841
TABLE I. Vibrational assignments and estimated band origins of the ob-
served vibronic bands of 74GeH2 .
Assignment
Band origina
Obs.–Calc.b
000
16 325.544
17 108.516
17 889.550
18 668.381
19 445.015
20 219.80
18 123.977
18 888.566
19 651.671
20 413.20
Ϫ0.08
Ϫ0.04
0.12
201
202
203
0.11
204
Ϫ0.07
Ϫ0.06
0.26
Ϫ0.27
Ϫ0.24
0.25
205
101
110210
110220
110230
aBand origin of the 000 band from Table IV; other bands from PP1͑1͒ line
ϩ13.538 cmϪ1 ͑11,0–00,0 interval in ground state͒. Origins quoted to three
decimal places are Ϯ0.01 cmϪ1; others Ϯ0.05 cmϪ1
.
bCalculated using Eq. ͑1͒ and the parameters T0ϭ16 325.63͑22͒, ␻01Ј
ϭ1798.09(28),
␻02Јϭ783.94(20), x20Ј2ϭϪ1.018(38), x102Ј
ϭϪ17.8(8); values in parentheses are standard errors of 1␴.
smaller quantities of the germylene. Each band was studied
at high resolution but only the lowest energy vibronic band at
613 nm shows rotational structure; the other bands consist of
a single line for each of the five naturally abundant isotopes
of germanium, as illustrated in Fig. 1. These single lines are
p
assigned as the P1͑1͒ transition terminating on the lowest
rotational level ͑00,0 JKa,Kc͒ in each upper vibronic state, by
analogy with SiH2.16,19 The absence of any evidence of
r-form branches originating in KЉa ϭ 0 is a strong indication
of an inhomogeneous predissociation in the excited state.
Using the previous assignments of the bending
FIG. 1. Examples of high-resolution spectra of various vibronic bands of the
1
B1 –X 1A1 system of GeH2 . In each case, the five features are due to the
˜
˜
A
pP1͑1͒ transitions of the naturally abundant germanium isotopomers, as il-
lustrated in the bottom spectrum.
progression16 and our ab initio estimate of ␯Ј ϭ 1809 cmϪ1
1
͑vide infra͒, vibrational assignments were readily made for
all the observed bands. The results are summarized in Table
I. All of the bands are assigned as cold bands originating in
the lowest vibrational level in the ground state, as expected
III. EXPERIMENTAL RESULTS
A. Spectra
The lowest spin-allowed electronic transition of GeH2
Љ
for a molecule whose lowest vibrational frequency (␯2) is
917 cmϪ1 16 The band origin for the 74Ge isotopomer has
been estimated in each case by adding the ground state
1,0Ϫ00,0 interval ͑13.538 cmϪ1͒ to the observed pP1͑1͒ tran-
1
1
˜
˜
has been predicted to be the A B1 –X A1 band system with
onset near 610 nm,26,31 in excellent accord with the spectrum
reported by Saito and Obi.16 Ab initio calculations26,31 sug-
gest a bond angle increase of 30° and a bond length contrac-
tion of 0.034–0.041 Å on excitation, so that pronounced ac-
tivity in both ␯Ј and ␯Ј would be expected in the spectrum,
.
1
sition frequency. To obtain the vibrational constants, the vi-
bronic band origins were fitted to the anharmonic expansion
␯ϭT00ϩ␻0Ј Јϩ␻0Ј Јϩx0Ј Ј2ϩx0Ј Ј Ј .
͑1͒
1
2
¯
v1
v2
2
22v2
12v1v2
1
while the vibrational selection rules preclude activity in
␯3Ј . GeH2 is predicted to be a prolate asymmetric top in both
The results are reported in Table I, along with the observed–
calculated values.
˜
˜
the ground and excited states and the A–X transition mo-
ment should be oriented out of the molecular plane, generat-
ing C-type vibronic bands obeying the selection rules ⌬Jϭ0,
Ϯ1; ⌬KaϭϮ1 and ⌬Kcϭ0, Ϯ2. Germanium has five natu-
rally occurring isotopes of significant abundance
͑70Geϭ20.5%, 72Geϭ27.4%, 73Geϭ7.8%, 74Geϭ36.5%, and
76Geϭ7.8%͒ so that each vibronic band is expected to show
a multitude of isotopic splittings.
The discharge jet technique produced moderately strong
LIF signals from GeH2 in the 615–485 nm region, allowing
us to positively identify ten vibronic bands, some examples
of which are shown in Fig. 1. Saito and Obi16,18 were only
able to observe the five 2n0 ͑nϭ0–4͒ bands, presumably be-
cause the laser photolysis of phenylgermane produces
The lines of the different germanium isotopes were re-
solvable at high resolution for many of the higher vibronic
bands. The wave numbers of the individual isotopic lines are
given in Table II. As shown in Fig. 2, the germanium isotope
shifts are linear with increasing quanta of the bending mode
and extrapolate back to very small values for the 000 band.
In contrast to Saito and Obi,16 who only observed three
rotational lines in the 000 band, we were able to observe a
total of 18 lines, providing important information about the
ground and excited states. The rotational structure of the
band is shown in Fig. 3 and the measured transition frequen-
cies and rotational assignments are summarized in Table III.
Each rotational line in the band consists of contributions
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2842
Karolczak et al.: Spectroscopy predissociation of GeH2
TABLE II. Measured PP1͑1͒ isotopic line frequencies ͑cmϪ1͒ for various
vibronic bands of GeH2 .
76GeH2
74GeH2
73GeH2
72GeH2
70GeH2
201
202
203
204
205
101
17 094.693 17 094.978 17 095.122 17 095.280 17 095.589
17 875.515 17 876.012 17 876.274 17 876.539 17 877.081
18 654.131 18 654.843 18 655.205 18 655.601 18 656.398
19 430.556 19 431.477 19 431.966 19 432.447 19 433.467
•••
18 110.295 18 110.439
20 206.26a
•••
•••
20 207.44
18 110.612 18 110.801
20 208.68
110210 18 874.641 18 875.028 18 875.238 18 875.437 18 875.883
110220 19 637.521 19 638.133 19 638.460 19 638.781 19 639.464
aThe lines of the 205 band were outside the I2 LIF range and were calibrated
with optogalvanic lines to Ϯ0.05 cmϪ1
.
from each of the five isotopomers of GeH2, as the isotope
shifts are unresolved at our laser resolution. It is noteworthy
that we were able to observe a few very weak lines in the rR0
r
and Q0 branches, terminating on upper state levels involv-
ing KaЈ ϭ 1. Two weak lines at 16 344.760 and 16 267.934
cmϪ1 remain unassigned, although they may involve p-form
transitions from KЉa ϭ 2. The ground and excited state rota-
tional constants were determined by a least-squares fitting of
Watson’s A reduction of the asymmetric top rotational
Hamiltonian in the Ir representation to the observed transi-
tion frequencies. In the fitting process, the ground and ex-
cited state rotational constants A, B, and C and the band
origin, T0, were varied, yielding an overall standard devia-
tion of fit of 0.008 cmϪ1 for the 16 fitted transitions. The
70
x
FIG. 2. Germanium isotope effects ͓␯ ͑ GeH2͒–␯͑ GeH2͔͒ for the pP1͑1͒
lines of various vibronic bands of GeH2 . The isotope effects in ͑a͒ are for
the 2n0 progression; those in ͑b͒ are for the 10120n progression. The symbol
designations are: closed circleϭ76GeH2 , open triangleϭ74GeH2 , closed
triangleϭ73GeH2 , and open squareϭ72GeH2 .
¯
¯
FIG. 3. The 00 band of the A B1 –X 1A1 system of GeH2 . Rotational assignments of various lines are shown in the low-resolution spectrum at the bottom.
The pP1͑4͒ assignment refers to the shoulder on the right-hand side of the lowest wavenumber feature in the spectrum. The insets show high-resolution spectra
of portions of the band with the associated rotational assignments.
0
1
˜
˜
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Karolczak et al.: Spectroscopy predissociation of GeH2
2843
TABLE III. Rotational line frequencies ͑cmϪ1͒ and assignments for the 000
band of GeH2 .a
Upper state
Lower state
Observed
frequency
J
Ka
Kc
J
Ka
Kc
Obs.–Calc.
4
2
1
4
3
2
2
3
4
1
3
2
0
1
2
3
1
1
1
0
0
0
1
1
0
0
0
0
0
0
0
0
3
1
0
4
3
2
2
3
4
1
3
2
0
1
2
3
3
1
0
3
2
1
2
3
4
1
3
2
1
2
3
4
0
0
0
1
1
1
0
0
1
1
1
1
1
1
1
1
3
1
0
2
1
0
2
3
4
1
3
2
0
1
2
3
16 372.604
16 353.644
16 346.461
16 337.119
16 336.622
16 335.895
16 333.504
16 332.381
16 324.000
16 323.189
16 322.836
16 322.559
16 312.006
16 297.064
16 282.200
16 268.815
Ϫ0.003
0.016
Ϫ0.016
Ϫ0.004
Ϫ0.003
0.012
Ϫ0.003
0.007
Ϫ0.007
Ϫ0.003
0.008
0.004
Ϫ0.009
Ϫ0.003
0.004
FIG. 4. Typical fluorescence decay data obtained on excitation of the 000
band pP1͑1͒ transition. ͑A͒ Plot of the natural logarithm of the fluorescence
intensity vs time. The least squares fitted line gave a lifetime of 2.28 ␮s over
2.2 lifetimes. ͑B͒ The same data with the ordinate plotted on a linear scale.
0.000
aTwo additional unassigned lines were observed at 16 344.760 and
16 267.934 cmϪ1
.
bands were too weak for reliable fluorescence lifetime mea-
surements. In all cases, good single exponential decays were
obtained over 2–3 lifetimes, with no evidence of geometric
or collisional effects influencing the measurements. A typical
fitted line frequencies and observed–calculated values are
given in Table III and the resulting constants in Table IV.
The rotational constants obtained for the ground and ex-
cited states allow a determination of the effective ͑r0͒ mo-
lecular structure in both states. Since only two moments are
necessary to obtain a structure, three distinct calculations are
possible, which should give the same structure if there is no
inertial defect or experimental error in the constants. In gen-
eral, one obtains slightly different structures even with very
precise constants, due to the finite inertial defect. In the
present case, we have calculated all the possible structures
and report average values for the ground and excited states in
Table IV.
decay curve following excitation of the P1͑1͒ line of the 000
p
band is illustrated in Fig. 4. The measured fluorescence life-
times are reported in Table V. For the higher vibronic bands,
no significant variations in fluorescence lifetimes could be
found on excitation of the different GeH2 isotopomers.
While our fluorescence lifetime measurements were in
progress, Saito and Obi18 reported similar results for the
bending levels of GeH2 and GeD2. Table V shows that the
present results are in excellent accord with their data for the
rotational levels of the lowest vibronic level but show sub-
stantial differences for higher vibronic levels, with our life-
times being longer in all cases. The source of the discrepancy
is difficult to ascertain. We have repeated our measurements
under a variety of experimental conditions with consistent
results. We also measured fluorescence lifetimes of SiH2,
prepared in the discharge jet by substituting SiH4 for GeH4.
For the 22 ͑00,0͒ and 26 ͑00,0͒ levels we obtained lifetimes of
B. Fluorescence lifetimes
We have measured fluorescence lifetimes of three rota-
tional states in the 00 vibronic level and for the 00,0 rotational
state for six higher vibronic states; the 250, 110202, and 101230
TABLE IV. Comparison of experimental and ab initio molecular constants and geometric parameters of
74GeH2 .
Experiment
Theory
1A1
A
1B1
16.415 ͑21͒
X
1A1
a
3B1
A
1B1
˜
˜
˜
˜
˜
X
a
a
A
B
C
T0
6.9978͑145͒
6.5314͑84͒
3.3318͑32͒
0
6.9582b
6.4504
3.3474
0
14.3103c
4.6810
3.5272
15.2037d
4.5288
4.5182͑30͒
3.4601͑36͒
16 325.544͑18͒
3.4894
7975
16 543
r͑Ge–H͒͑Å͒
␪͑H–Ge–H͒͑°͒
1.591͑7͒
91.2͑8͒
1.553͑12͒
123.4͑19͒
1.591
91.4
1.545
119.8
1.553
122.1
aValues in parentheses are 3␴ values. The geometric parameters are averages over the three possible structures
͑see text͒ and the values in parentheses are the maximum deviation from the average.
bReference 30.
cReference 29.
dReference 26.
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2844
Karolczak et al.: Spectroscopy predissociation of GeH2
TABLE V. Fluorescence lifetimes ͑in ␮s͒ of single rovibronic levels of
GeH2 .
Vibronic
level
Rotational
level ͑JKaKc
This
work
͒
Ref. 18
00
000
101
202
000
000
000
000
000
000
2.28 Ϯ0.02
0.967Ϯ0.010
0.136Ϯ0.005
1.79 Ϯ0.02
1.45 Ϯ0.02
1.26 Ϯ0.02
1.06 Ϯ0.02
2.32 Ϯ0.02
1.94 Ϯ0.02
2.29Ϯ0.07
0.97Ϯ0.01
0.13Ϯ0.03
1.30Ϯ0.26
0.81Ϯ0.01
0.72Ϯ0.04
0.64Ϯ0.05
•••
21
22
23
24
11
1121
•••
1.07Ϯ0.01 and 0.79Ϯ0.01 ␮s, respectively, comparable to
literature values19 of 1.0Ϯ0.27 and 0.60Ϯ0.03 ␮s. An exami-
nation of the published decay curve for the GeH2 22 level18
shows noticeable deviations from single exponential behav-
ior at longer times ͑beyond 2 ␮s͒, indicative of geometric
effects due to excited state molecules leaving the detector
viewing zone. The experimental parameters18 ͑800 ␮m ori-
fice, 1 atm argon, excitation 19 nozzle diameters downstream
of the jet and 12.5 diam after photolysis, chamber pressure
10Ϫ4 Torr͒ also appear insufficient to ensure collision-free
conditions. In our work, we found that the fluorescence life-
times were quite sensitive to the experimental conditions,
particularly the distance downstream of the jet, as expected if
collisional quenching is important. If all of our lifetimes,
including those of the 00 level, were longer than those of
Saito and Obi, the discrepancy could probably be satisfacto-
rily explained as due to collisional or geometric effects. The
fact that the longest lifetimes are in agreement is more diffi-
cult to rationalize, although the effect may be due to more
efficient collisional quenching of the higher vibrational lev-
els.
FIG. 5. Plots of the inverse lifetime of the 00,0 rotational levels of the 2n
vibronic levels of the A states of SiH2 and GeH2 vs the third power of the
transition energy of the vibronic band. In ͑a͒, the results for SiH2 ͑open
triangles, Ref. 19͒ and GeH2 ͑closed circles, present work͒ are compared. In
͑b͒, previous data for GeH2 ͑open squares, Ref. 18͒ are presented.
˜
zero intercept, suggesting that the current results are more
reliable.
IV. THEORETICAL STUDIES
A. Methods
The present lifetime measurements are in much better
agreement with expectations from theory than were previous
results. In published work on SiH2, it was shown19 that plots
of the 2n ͑nϭ0–6; 00,0 rotational state͒ level fluorescence
decay rates vs the cube of the transition energy times the sum
of the Franck–Condon factors yielded a straight line with
near-zero intercept. These results imply that the lowest rota-
tional levels decay purely radiatively, as expected for a pre-
dissociation process that involves Coriolis coupling to the
ground state. Similar results would be expected for GeH2,
but Saito and Obi18 found that their plot gave a straight line
with a substantially negative intercept. In Fig. 5, we have
replotted the SiH2 and GeH2 data including the present re-
sults. We have neglected the Franck–Condon factor summa-
tion, as the sum is found to be nearly constant for the present
range of upper state vibronic levels18,19 and the Franck–
Condon factors can only be approximated without more
complete information on the ground and excited state vibra-
tional force fields.47 We find that our lifetime data gives a
good linear plot with an intercept closer to zero than previ-
ous SiH2 data, whereas the literature lifetimes for GeH2 have
considerably more scatter and deviate much further from a
The goal of these theoretical studies was to characterize
those features of the excited state potential energy surfaces
responsible for the excited state dissociation processes. Thus,
we used the full-valence complete-active-space self-
consistent-field method ͑CASSCF͒, because it can describe
regions of the potential energy surface where bond breaking
and spin recoupling are important.48 The equilibrium geom-
1
3
1
˜
˜
˜
etries of the X A1 , a B1 , and A B1 electronic states were
precisely determined at the CASSCF level of theory using
analytical gradient techniques, as were the transition states
corresponding to the reactions
ϩ
1
1
GeH A B ͒→Ge D͒ϩH 1⌺g ͒,
͑2͒
͑3͒
˜
͑
͑
͑
2
2
1
ϩ
GeH a B ͒→Ge P͒ϩH 1⌺g ͒.
3
3
˜
͑
͑
͑
2
2
1
All stationary points were characterized as minima or transi-
tion states by determining the nuclear Hessian, and associ-
ated harmonic vibrational frequencies, via central finite dif-
ference methods.
Polarized triple-zeta quality basis sets were used for the
geometry optimization. Specifically, for hydrogen we em-
ployed Dunning’s (5s/3s) contraction of Huzinaga’s primi-
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Karolczak et al.: Spectroscopy predissociation of GeH2
2845
9s7p5d1f ) and H(5s2p/3s2p)—contains 80 functions.
We will refer to this basis set as TZ(2df,2p).
Our final energy predictions used larger atomic natural
orbital ͑ANO͒ basis sets51 and second-order configuration
interaction ͑SOCI͒ energies at the CASSCF TZ(2df,2p) op-
timized geometries. The hydrogen ANO set is the density
matrix averaged (8s4p3d)/[3s2p1d] generalized contrac-
tion of Widmark et al.,52 while the germanium ANO set is of
the form (20s15p11d3f1g)/[7s6p4d2f1g]. The latter
was constructed from the valence CISD natural orbitals ob-
tained with Partridge’s 20s15p9d primitive gaussian basis
set,53 and the following appended polarization functions:
␣dϭ0.1431, 0.05724; ␣fϭ0.85, 0.34, 0.136; ␣gϭ0.48. The
SOCI is a single and double excitation multireference CI
method in which all configurations of the CASSCF act as
references; it is known to yield energies in excellent agree-
ment with full CI for chemical systems where the full CI can
be obtained.54
B. Results
The CASSCF/TZ(2df,2p) optimized geometries of the
three electronic states of GeH2 and the transition states and
products for reactions ͑2͒ and ͑3͒ are shown in Fig. 6. Be-
cause the CASSCF method mainly includes near-degeneracy
electron correlation effects, but lacks significant dynamical
correlation, bond distances are usually too long at this level
of theory.48 GeH2 appears to be normal in this respect: the
bond distances are longer than experimental values for the
ground and excited singlet state by 0.02–0.03 Å. For similar
reasons, CASSCF harmonic vibrational frequencies are often
in fortuitously good agreement with experimentally observed
fundamental frequencies, as is found in the present case ͑see
Table VI͒.
FIG. 6. Stationary points on the GeH2 potential energy surfaces at the
complete-active-space self-consistent-field level of theory using a polarized
triple zeta basis set, TZ(2df,2p). The structures on the left are the reactants
and their corresponding transition states ͑or products͒ are shown on the
right.
tive set,49 with two sets of p functions added ͓␣p(H)ϭ1.5
and 0.375͔. The germanium basis set was derived from Dun-
ning’s 14s11p5d primitive set,50 initially contracted to
8s6p3d in a ͑61121111/611111/311͒ fashion. Subsequently,
the two valence s and p functions were replaced by a set of
three functions with exponents ␣s͑Ge͒ϭ0.376, 0.151, 0.06,
and ␣p͑Ge͒ϭ0.382, 0.153, 0.061 corresponding to 2.5␮, ␮,
and ␮/2.5 where ␮ is the corresponding geometric mean of
the original two s and p functions. Finally, additional sets of
d-functions ͑␣dϭ0.25, 0.08͒ and a single f function
͑␣fϭ0.34͒ were appended to the germanium basis set. Using
pure spherical harmonic d and f gaussians, the resulting ba-
The transition state geometries for dissociation of the
3
1
˜
˜
a B1 and A B1 states ͑reactions 2 and 3͒ have Cs symme-
3
1
try, resulting in AЉ and AЉ states ͑see Fig. 6͒. The lowered
symmetry results from an avoided crossing with electronic
states of A2 symmetry, as discussed previously for SiH2.55,56
These transition state geometries are intermediate between
those of the reactants and products; for example, one of the
Ge–H distances is 0.4–0.5 Å longer than in GeH2, but the
H–H distance is still far from the equilibrium distance in H2.
Transition state geometries midway between reactants and
products are expected for an approximately thermoneutral
sis
set—technically
designated
Ge(15s12p7d1f/
TABLE VI. Experimentally determined and theoretically predicted vibrational frequencies ͑cmϪ1͒ for GeH2 .
Experimenta
Theory
␯
␯
␯
Reference
␯
␯
␯
Reference
1
2
3
1
2
3
1A1
3B1
1A1
1887
•••
•••
•••
•••
920
•••
•••
•••
ϳ780
783
1864
•••
•••
•••
•••
15
•••
•••
•••
18
1857
1840
1991
1998
1864
1809
923
913
763
801
860
783
1866
30b
this work
29b
this work
31
˜
X
1840
2012
2054
2011
1909
˜
a
˜
A
1798
•••
this work
this work
aFundamental vibrational frequencies.
bFundamental vibrational frequencies from a fitted potential surface.
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2846
Karolczak et al.: Spectroscopy predissociation of GeH2
TZ(2df,2p) results. In terms of quantities that can be di-
rectly compared to experiment, the largest effect of these
theoretical enhancements is for the Ge͑3P͒–Ge͑1D͒ energy
difference, which changes from 7808 cmϪ1 ͑22.3 kcal/mol͒
at the CASSCF/TZ(2df,2p) level of theory to 6251 cmϪ1
͑17.9 kcal/mol͒ at the SOCI/ANO level of theory. To directly
compare these values to spectroscopically determined atomic
state energy differences, we need to apply spin–orbit correc-
tions, because the quantities we obtain from our theoretical
studies correspond to term averages; i.e., averages over all
the J states for a given term. Thus, the term average energy,
Eav , obtained theoretically should be corrected by
TABLE VII. Relative energies ͑kcal/mol͒ of the various optimized geom-
etries of GeH2 at the complete-active-space self-consistent-field ͑CASSCF͒
and second-order configuration interaction ͑SOCI͒ level of theory using ba-
sis sets of polarized triple zeta ͑TZ(2df,2p)͒ and [7s6p4d2f1g/3s2p1d]
atomic natural orbital ͑ANO͒ form.
CASSCF
SOCI
Electronic state
TZ(2df,2p) ANO TZ(2df,2p) ANO
GeH2(A 1B1)
46.3
72.0
50.3
21.4
66.7
28.0
0.0
46.2
72.0
49.8
21.7
66.9
28.0
0.0
46.1
69.0
48.9
22.7
64.2
29.9
0.0
45.7
69.4
49.5
23.0
64.9
31.6
0.0
˜
GeH ͑1A transition state͒
Љ
2
Ge͑1D͒ϩH2͑1⌺ϩg ͒
GeH2(a 3B1)
˜
GeH ͑3A transition state͒
Љ
2
Ge͑3P͒ϩH2͑1⌺ϩg ͒
Eav 3P͒ϪE 3P0͒
͑
͑
GeH2(X 1A1)a
˜
ϭ E 3P ͒ϩ3E 3P ͒ϩ5E 3P ͒ /9ϪE 3P0͒
͑4͒
a
The absolute energies of the ground X 1A1 electronic state are: CASSCF/
TZ(2df,2p) –2076.410 223; CASSCF/ANO –2076.568 711; SOCI/
TZ(2df,2p) –2076.487 485; SOCI/ANO –2076.651 699.
͓ ͑
͑
͑
͔
2
͑
˜
0
1
or 969 cmϪ1 from spectroscopy.57 The final result is a spin–
orbit corrected prediction for E(1D2)–E(3P0) of 7220 cmϪ1
͑20.6 kcal/mol͒ at the SOCI/ANO level of theory, in excel-
lent agreement with the experimental value of 7125 cmϪ1
͑20.4 kcal/mol͒.57
reaction, which is the case here. The three harmonic vibra-
1
tional frequencies associated with the AЉ state are 1540,
677, and 785i cmϪ1, while those for AЉ are 1633, 78, and
3
To compare most energy differences with experimental
quantities we must correct for zero-point vibrational energy
differences. Here we have approximated58 the zero-point en-
ergies as one-half the sum of the predicted harmonic vibra-
tional frequencies, except for H2, where the exact value of
6.21 kcal/mol is used.58 After making these corrections, we
obtain the schematic potential energy surface in Fig. 7. Note
that the triplet germanium atom enthalpy includes the spin–
orbit correction discussed above, but that no correction has
been made to the other triplet state energies. For GeH2͑3B1͒,
this is justified from previous relativistic CI calculations that
showed the three spin–orbit components of this state differed
by only 0.12 kcal/mol.26 For the transition state, it is still
probably reasonable to assume that spin–orbit effects are
small.
1628i cmϪ1; the single imaginary frequency in each case
verifies that these are transition states. The larger magnitude
imaginary frequency associated with the triplet state reaction
correlates with the larger barrier height found for that case
͑vide infra͒.
The relative energies are shown in Table VII. Note that
increasing both the quality of the basis set and the correlation
method leads to only small ͑Ͻ4 kcal/mol͒ changes in the
relative energies compared to our base CASSCF/
In those cases where we can compare our theoretically
predicted enthalpies to experiment, the agreement is excel-
1
1
˜
˜
lent. For example, the A B1 –X A1 enthalpy difference is
predicted to be 45.6 kcal/mol, compared to the experimental
result
of
46.7
kcal/mol.
Also,
from
the
1
3
1 ϩ
˜
GeH2(X A1)→Ge͑ P0͒ϩH2͑ ⌺g ͒ enthalpy difference, and
the experimental value for the heat of formation of the ger-
manium atom ͑0 K͒ of 88.2Ϯ0.7 kcal/mol recommended by
Berkowitz,59 we predict ⌬H0f ͑GeH2͒ϭ59.7 kcal/mol, com-
pared to experimental values of 60.4Ϯ3.8 ͑Ref. 60͒ and
Ͼ59.3 ͑most probably 61.8͒ kcal/mol.59 In conclusion, the
theoretical predictions for the atomic germanium states, the
GeH2 singlet states and the heat of formation of GeH2, which
connects the atomic and molecular states, all agree with ex-
periment to within about 1 kcal/mol. This gives us confi-
dence in those theoretical predictions which cannot be di-
rectly compared with experiment.
V. DISCUSSION
FIG. 7. A schematic potential energy profile for the three electronic states of
GeH2 and their asymptotes, relative to the standard state of solid germa-
nium, Ge(s), and molecular hydrogen. Relative energies ͑kcal/mol͒ are de-
termined at the second-order configuration interaction ͑SOCI͒ level of
theory using atomic natural orbital ͑ANO͒ basis sets, and have been cor-
rected for zero-point energies and atomic spin-orbit splitting.
In this work, we have been able to determine the ground
and excited state molecular structures of GeH2 for the first
time. Previous ab initio predictions of the equilibrium geo-
metric parameters ͑Table IV͒ are in good agreement with the
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Karolczak et al.: Spectroscopy predissociation of GeH2
2847
present r0 values. It is fortunate that the discharge jet tech-
nique provided strong enough LIF signals and sufficiently
high rotational temperatures to allow us to record a more
extensive range of transitions than in previous work.16
The present ab initio predictions of the excited state har-
monic vibrational frequencies are in excellent accord with
the experimentally observed fundamentals ͑Table VI͒. For
the ground state, our theoretical predictions are in agreement
with previous more detailed calculations,30 which involved
fitting points on the ground state potential energy surface.
There is still some uncertainty as to whether ␯ or ␯ is
ing the observation that fluorescence from this level persists
even at higher excited state vibrational energies. The differ-
ences in the jet spectra of SiH2 and SiD2 have been
interpreted19 as due to a potential barrier in the dissociation
3
of a B1 to Si͑3P͒ϩH2, in qualitative agreement with ab
˜
initio studies56 of the energetics and transition states in-
volved in the dissociation of SiH2.
At high vibrational energies in the excited states of SiH2
and SiD2, the abrupt onset of the Si͑1D͒ϩH2͑1⌺gϩ͒ dissocia-
tion channel appears, as indicated by Si͑1D͒ photofragment66
and fluorescence lifetime studies.56,19 Ab initio studies56 sug-
gest that there is no reverse barrier to this process, so that
1
3
larger, as theory and experiment give conflicting results.
However, the matrix spectrum15 was assigned with ␯1Ͼ␯
dissociation occurs once the A state energy equals that of the
˜
3
solely on the basis of an observed splitting in one of the
fundamentals of GeD2, attributed to Fermi resonance be-
products. However, lifetime studies19 of SiH2 and SiD2 show
a distinct isotope effect for this process, suggesting the ex-
istence of a potential barrier and a tunneling predissociation
mechanism.
tween ␯ and 2␯2, and there is some question about the va-
1
lidity of this conclusion.
As a check on the vibrational analysis of the electronic
All of the available experimental data support the view
that GeH2 undergoes predissociation processes from the
spectrum, we have calculated the expected isotope shifts for
0
1
˜
˜
˜
the 00 band of the A–X transition, from the ground and
excited state ab initio harmonic frequencies. For
74GeH2/74GeD2, we predict a 14.3 cmϪ1 shift on deuteration,
in good agreement with the 11Ϯ2 cmϪ1 shift measured by
A B1 state, similar to those in SiH2, as originally proposed
by Saito and Obi.16 The Ј ϭ 0 level of GeH and the
v2
2
Ј ϭ 0, 1, and 2 levels of GeD only show fluorescence
v2
2
from low angular momentum rotational states, indicative of a
Saito and Obi.16 Similarly, we predict small ͑Ͻ0.2 cmϪ1
͒
J-dependent predissociation process. The measured fluores-
cence lifetimes of various rotational states of the GeH2 00
level ͑Table V͒ decrease by more than an order of magnitude
from Jϭ0 to Jϭ2, clearly showing the J dependence. For
higher vibrational levels, only transitions to the 00,0 rota-
tional level are observed by LIF; the excited state rotational
levels with nonzero rotational angular momentum do not
give rise to detectable fluorescence. The smooth variation of
the measured fluorescence lifetimes for the 00,0 rotational
levels with excited state vibrational energy ͑Fig. 5͒ indicates
that the decay is purely radiative, without a substantial non-
radiative component. Thus, without the enabling effect of
shifts for the various germanium isotopomers, in agreement
with those observed ͑see Fig. 2͒. If the vibrational numbering
of the 2n0 progression were shifted by one unit in either di-
rection, the discrepancy between the observed and calculated
isotope shifts would be unacceptably large. Similarly, the
inertial defect for the 00 level of GeH2 ͑0.1140 amu Å2͒ is
comparable to that of SiD2 ͑Ref. 61͒ ͑0.138 amu Å2͒; if our
000 band were actually 210 the upper state inertial defect would
be expected to be substantially larger ͑SiD2 21ϭ0.4067 amu
Å2͒. The isotope shift and inertial defect data leave little
doubt that the vibrational analysis is correct.
Much of the spectroscopy and excited state decay dy-
namics of GeH2 can be understood in the context of our more
extensive knowledge of SiH2, which we briefly summarize
here. Silylene absorbs in the 650–480 nm region,62 much of
the rotational structure is perturbed and the fluorescence life-
times of single rovibronic levels vary widely.63 At low ex-
cited state vibrational energies, SiH2 predissociates to
Si͑3P͒ϩH2͑1⌺ϩg ͒, as shown by photofragment excitation
spectroscopic detection of Si͑3P͒.64 Jet spectroscopy of SiH2
reveals that only the few lowest rotational levels in the lower
vibronic states of SiH2 fluoresce and that the fluorescence
lifetimes decrease with increasing rotational angular
momentum.19 More extensive rotational structure is found in
the LIF spectra of jet-cooled SiD2, but most of the structure
disappears at higher vibrational energies.61 Fukushima
et al.19 concluded that all these observations were consistent
with a rotationally dependent predissociation process involv-
1
˜
molecular rotation, the lower vibrational levels of A B1
GeH2 decay radiatively, whereas levels with rotational angu-
lar momentum decay nonradiatively.
Saito and Obi18 found a breaking off in the LIF spectra
at Ј ϭ 4 ͑3120 cmϪ1 of vibrational energy͒ for GeH2 and
v2
Ј ϭ 6 ͑3337 cmϪ1 of vibrational energy͒ for GeD2. With
v2
the increased sensitivity of our experiment, we were able to
observe the Ј ϭ 5 level and ␯Ј ϩ ␯Ј combination levels
v2
1
2
up to 4088 cmϪ1 of vibrational energy in GeH2. Calculated
Franck–Condon factors18 show that the absorption spectra
extend well beyond the breaking off of the LIF spectra, im-
plying that a second nonradiative channel involving even the
00,0 levels appears at high vibrational energies.
Referring to Fig. 7, the present experimental results on
GeH2 and previous work16–18 on GeD2 can be rationalized by
a model similar to that used for SiH2. Employing the best
available thermodynamic and spectroscopic data, ͑experi-
mental data in Fig. 7͒, we calculate that the Ge͑1D͒ϩH2
products are 0.5–2.5 kcal/mol above the S1 vϭ0 level, so
that predissociation of this level yields Ge͑3P͒ϩH2. The
lower vibrational levels of GeH2 and GeD2 show
J-dependent and isotope-dependent nonradiative decay pro-
cesses which can be ascribed to second-order coupling from
S1 through S0 to T1. Although the energy of the Ge͑3P͒ϩH2
ing
second-order
coupling
of
˜
the
type
1
1
3
˜
˜
A B1 –͑Coriolis͒→X A1 –͑spin–orbit͒→a B1 state. Ener-
getic and spin conservation rules dictate that the predissocia-
1
˜
tion should occur from the triplet manifold and the A B1
3
˜
state cannot interact directly with the a B1 state through
spin–orbit coupling.65 Coriolis coupling between the A and
˜
˜
X states would not occur for the 00,0 rotational level, explain-
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129.97.125.158 On: Wed, 10 Dec 2014 21:56:23

2848
Karolczak et al.: Spectroscopy predissociation of GeH2
products is below the S1 state, our ab initio results show a
barrier to the dissociation, suggesting that tunneling through
the barrier must occur, consistent with the observed differ-
ences in dynamics between GeH2 and GeD2.
process in the excited state also implies that it is inaccurate
to use the breaking off of fluorescence18 or the onset of the
second reaction channel19 as the dissociation limit in calcu-
lating the heats of formation of SiH2 or GeH2.
For higher vibrational levels of GeH2 and GeD2, there
are two possible processes: coupling through S0 to T1 and
tunneling through the barrier to yield Ge͑3P͒, or direct tun-
neling through the potential barrier from S1 to form
Ge͑1D͒ϩH2. In the former case, the 00,0 levels, which cannot
Coriolis couple to S0, would be expected to decay radia-
tively, as observed. In the latter case, the fluorescence life-
times of the 00,0 levels would be expected to decrease sub-
stantially at vibrational energies above the enthalpy of the
ACKNOWLEDGMENTS
The authors thank Chris Chan of the University of Brit-
ish Columbia for providing us with his design for a pulsed
discharge circuit and Yukio Yamaguchi for the CASSCF pro-
gram used in this work. We acknowledge the University of
Kentucky Center for Computational Sciences and the Com-
puter Center of the University of Kentucky for aid in the
theoretical calculations. This research was supported by the
National Science Foundation.
Ge͑1D͒ϩH products, which should occur between Ј ϭ 0
v2
2
and 2 for both GeH2 and GeD2. The fact that there are no
anomalies in the rotational structure or fluorescence lifetimes
of the 210 or 220 bands of either isotopomer leads us to con-
clude that dissociation to form Ge͑1D͒ is not the predomi-
nant decay process for any of the levels observed in the LIF
experiments.
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The breaking off of fluorescence from higher vibronic
levels of germylene suggests that the second dissociation
process to form Ge͑1D͒ becomes important at about 4000
cmϪ1 or 11.4 kcal/mol of vibrational energy in the S1 state.
As the barrier to this dissociation is calculated to be 20.5
kcal/mol above the S1 state, tunneling through the barrier
must occur. If sufficiently sensitive LIF experiments could be
done, we would anticipate that GeD2 levels above those of
GeH2 would be detected and that the 00,0 level lifetimes of
both isotopomers would decrease substantially in the region
where the second dissociation process becomes significant.
To clarify the role of the two dissociation processes, we are
currently designing an experiment to observe the Ge͑3P͒ and
Ge͑1D͒ products spectroscopically following state-selected
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