Pulsed discharge jet spectroscopy of DSiF and the equilibrium molecular structure of monofluorosilylene

Article abstract of DOI:10.1063/1.473484

The jet-cooled laser induced fluorescence excitation spectrum of the ? ¹A″- X? ¹A′ band system of DSiF has been observed using the pulsed discharge jet technique. Vibrational analysis of the spectrum yielded upper state harmonic vibrational frequencies of ω¹ = 1322, ω₂ = 444, and ω₃ = 867 cm^-1. Vibronic bands involving all of the upper state fundamentals of HSiF and DSiF have now been rotationally analyzed, allowing a determination of the excited state equilibrium structure as r′_e(SiH) = 1.526 ± 0.014 ?, r′e(SiF) = 1.597 ± 0.003 ?, and θ′_e(HSiF) = 115.0 ± 0.6°. The harmonic frequencies and centrifugal distortion constants were used to obtain harmonic force fields and average (r_z) structures for the ground and excited states. The ground state average structure was used to estimate the equilibrium structure of r″_e(SiH) = 1.528 ± 0.005 ?, r″_e(SiF) = 1.603 ± 0.003 ?, and θ″_e(HSiF) = 96.9 ± 0.5°.

Full text of DOI:10.1063/1.473484

Pulsed discharge jet spectroscopy of DSiF and the equilibrium molecular structure of
monofluorosilylene
Warren W. Harper, David A. Hostutler, and Dennis J. Clouthier
Citation: The Journal of Chemical Physics 106, 4367 (1997); doi: 10.1063/1.473484
View online: http://dx.doi.org/10.1063/1.473484
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Pulsed discharge jet spectroscopy of DSiF and the equilibrium molecular
structure of monofluorosilylene
Warren W. Harper, David A. Hostutler, and Dennis J. Clouthiera)
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055
͑
Received 15 October 1996; accepted 10 December 1996͒
˜
1
˜
1
The jet-cooled laser induced fluorescence excitation spectrum of the A AЉϪX AЈ band system of
DSiF has been observed using the pulsed discharge jet technique. Vibrational analysis of the
spectrum yielded upper state harmonic vibrational frequencies of ␻ ϭ1322, ␻ ϭ444, and ␻ ϭ867
1
2
3
Ϫ1
cm . Vibronic bands involving all of the upper state fundamentals of HSiF and DSiF have now
been rotationally analyzed, allowing a determination of the excited state equilibrium structure as
rЈ(SiH) ϭ 1.526Ϯ 0.014Å,rЈ(SiF) ϭ 1.597Ϯ 0.003Å,and␪Ј(HSiF) ϭ 115.0Ϯ 0.6°. Theharmonic
e
e
e
frequencies and centrifugal distortion constants were used to obtain harmonic force fields and
average (r ) structures for the ground and excited states. The ground state average structure was
z
used to estimate the equilibrium structure of rЉ(SiH) ϭ 1.528 Ϯ 0.005 Å, rЉ(SiF) ϭ 1.603 Ϯ 0.003 Å,
e
e
and ␪Љ(HSiF) ϭ 96.9 Ϯ 0.5°. © 1997 American Institute of Physics. ͓S0021-9606͑97͒00311-5͔
e
I. INTRODUCTION
bending force constant to reproduce ␯Ј gave calculated cen-
2
trifugal distortion constants and inertial defects which were
Monofluorosilylene ͑HSiF͒ is a member of the silylene
family of reactive intermediates. Our interest in these species
stems from the role they may play in semiconductor growth
processes, as described in detail elsewhere.1 We have em-
barked on a program to study the electronic spectra of a large
in agreement with experiment for both states. Dixon and
Wright18 reported rotational constants in general agreement
17
with those of Suzuki et al. and presented a detailed simu-
lation of the anomalous branches generated by the axis-
tilting ͑or axis-switching͒19 mechanism. By fixing the bond
angles at values obtained from ab initio predictions,20 they
obtained ground and excited state molecular structures.
–5
variety of carbenes,6 silylenes,
–8
9–11
and germylenes,
12–14
in
order to determine their geometries, vibrational frequencies,
rotational constants, and modes of excited state decay. Most
of these species have ground states of singlet multiplicity,
with the lowest excited states involving promotion of an
21
In subsequent work, Suzuki and Hirota studied the ef-
0
0
fect of magnetic fields on portions of the 0 band recorded at
sub-Doppler resolution using the intermodulated fluores-
cence technique. They obtained the rotational g factors gaЉa
2
electron from a fully occupied in-plane ␴ orbital to an un-
occupied out-of-plane p␲ orbital. In most cases, only the
first excited singlet state has been accessible to our laser
induced fluorescence ͑LIF͒ techniques, although we have
been able to obtain information about the corresponding trip-
let states in favorable cases.10,12,14
and gЈ and discussed them in terms of the electronic Cori-
olis interaction between the X and A states. There have also
aa
˜ ˜
been a variety of ab initio predictions of the properties of
HSiF,2
0,22–24
including values of the vibrational frequencies
3
˜
1
20,24
and geometries for the a AЉ and A AЉ excited states.
˜
25
The study of the spectroscopy of HSiF has a short his-
tory. It was first observed by Ismail et al.15 in infrared matrix
isolation experiments in 1982, as a product of the reaction of
silicon atoms with HF and DF. The authors established the
ground state fundamental vibrational frequencies of HSiF
and DSiF and determined a force field. A year later, Lee and
In 1995, we reported the LIF spectrum of jet-cooled
HSiF, obtained by generating the radical in a chemical reac-
tion jet through the exothermic reaction of molecular fluorine
with silane, buffered by a large excess of argon. Eleven vi-
bronic bands were identified and seven of them were rota-
tionally analyzed. The upper state bending potential and
1
6
Deneufville recorded the LIF spectrum of HSiF by exciting
the products of the reaction of silane with molecular fluorine.
Analysis of the 470–390 nm vibronic band system assigned
equilibrium geometry were obtained by fitting the rovibra-
n
tional energy levels of the bending ͑2 ͒ progression to the
o
26
semirigid bender model. In these calculations, the bond
˜
1
˜
1
as A AЉϪX AЈ yielded the vibrational frequencies ␯Љ
2
angle was fixed at the ab initio value of 114.5°, as it was
found to be strongly correlated with the Si–H bond length.
Ϫ1
ϭ 860 and ␯Ј ϭ 560 cm . The fluorescence lifetime extrapo-
2
lated to zero pressure was estimated to be 185Ϯ10 ns. In
The resulting equilibrium bond lengths were r ͑SiH͒ϭ1.548
e
1
985, two research groups reported rotational analyses of
Å, r ͑SF͒ϭ1.602 Å. The rapid consumption of silane in our
e
0
Doppler-limited LIF spectra of the 0 band of HSiF. Suzuki
0
continuous flow chemical reaction jet made it prohibitively
1
7
et al. obtained ground and excited state rotational constants
and derived molecular structures by assuming values for the
Si–H bond lengths. They found that transferring the ground
state force field to the excited state and varying only the
expensive to obtain spectra of DSiF from SiD , so that nei-
4
ther the ground state ro structure nor the excited state re
structure could be obtained free of assumptions.
In the present work, we have devised an inexpensive
method of obtaining spectra of DSiF, using the pulsed
a͒Author to whom correspondence should be addressed.
discharge jet technique and DSiF as a precursor. The previ-
3
J. Chem. Phys. 106 (11), 15 March 1997
0021-9606/97/106(11)/4367/9/$10.00
© 1997 American Institute of Physics
4367
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4368
Harper, Hostutler, and Clouthier: Spectroscopy of DSiF
FIG. 1. A portion of the low-resolution discharge jet spectrum of HSiF
showing the vibrational assignments.
0
5
FIG. 2. Low-resolution spectra of the 0 and 2 bands of DSiF plotted on the
0
0
5
r
p
same wavenumber scale. The 2 band shows a greater Q Ϫ Q separation
due to a larger value of (AϪB) in the excited state.
0
¯
1
1
ously unobserved electronic spectrum of DSiF is reported,
along with new results for HSiF.
spectra and calibration signals were recorded simultaneously
on a data acquisition system developed in our laboratory.28
Trifluorosilane was synthesized by fluorinating trichlo-
II. EXPERIMENT
The stable molecules HSiF and DSiF were used as
rosilane ͑Aldrich͒ with antimony trifluoride. HSiCl ͑3–4
3
3
3
precursors for the in situ production of HSiF and DSiF by the
pulsed discharge technique. In each case, 40 Torr of the pre-
cursor was diluted with 200–400 psi of argon and the mix-
ture stored in a cylinder equipped with a gas regulator. The
gas mixture was delivered at a pressure of 40 psi to a pulsed
valve ͑General Valve, series 9͒ with a 0.8 mm diam nozzle.
The gas pulse expanded into a 6 mm diam flow channel
drilled in a 26-mm-long Delrin cylinder attached to the end
of the pulsed valve. At the appropriate time during the gas
pulse, an electrical discharge was struck between two stain-
less steel ring electrodes mounted 1 mm apart along this
channel. The discharge efficiently dissociated the precursor,
producing various fragments including the monofluorosi-
lylene, whose LIF spectrum was excited by a pulsed laser
beam 30 mm beyond the end of the Delrin cylinder. The
pulsed discharge jet was mounted inside a 20 in. vacuum
chamber pumped by a 10 in. diffusion pump equipped with a
ml͒ was condensed in vacuum onto the surface of 200 gm of
very dry, powdered SbF in a 500 ml vessel. The vessel was
3
sealed with a stopcock and the mixture allowed to warm up
to room temperature over a period of about 30 min.29 A rapid
reaction ensued with the production of white fumes and con-
siderable blackening of the solid phase. After the reaction
appeared complete, the volatile products were vaporized into
a trap cooled with liquid nitrogen, and the fraction distilling
over when the trap was warmed to Ϫ98 °C was almost pure
trifluorosilane. DSiF was synthesized in the same manner
3
using DSiCl as the reactant. The identity and purity of the
3
fluorosilanes were checked by gas phase infrared spectros-
copy. DSiCl was prepared by the heterogeneous reaction of
3
DCl30 with a mixture of 80% granular silicon and 20% CuCl2
at 380 °C.31
water-cooled baffle. The typical background pressure was
III. RESULTS
Ϫ6
2
3
ϫ10 Torr, with the chamber pressure rising to about
Ϫ5
A. Vibrational analysis
ϫ10 Torr during operation of the pulsed valve.
Low- ͑0.5 cm ͒ and medium- ͑0.1 cmϪ1͒ resolution la-
Ϫ1
Monofluorosilylene is a nonlinear molecule of C sym-
s
ser induced fluorescence spectra were collected using a
metry with 3 aЈ normal modes of vibration labeled ␯ ͑Si–H
1
Nd:YAG pumped pulsed dye laser ͑Lumonics HyperDye
or Si–D stretch͒, ␯ ͑bend͒, and ␯ ͑Si–F stretch͒. The low
2
3
3
00͒. The fluorescence was imaged onto the photocathode of
symmetry of the molecule results in unrestricted vibrational
quantum number changes for allowed electronic transitions.
The LIF spectrum of DSiF was recorded in the 455–380
nm region. The low wavenumber portion of the spectrum,
illustrated in Fig. 1, is quite simple and bands involving all
three excited state fundamentals were readily identified. At
low resolution, each individual band consists of three promi-
nent unresolved Q branches and a variety of partially re-
solved P and R branches, with a contour characteristic of a
a photomultiplier ͑EMI 9816QB͒, after having passed
through appropriate cutoff filters to reduce scattered laser
light. Etalon fringes ͑FSR 0.65 cm ͒, along with optogal-
vanic lines from various argon or neon filled hollow cathode
Ϫ1
lamps, were used for calibration. High-resolution spectra
Ϫ1
͑0.04 cm ͒ were recorded for selected bands using an exci-
mer pumped dye laser equipped with an intracavity etalon
Lambda Physik FL 3002E͒. These spectra were calibrated to
͑
Ϫ1
r
p
an accuracy of ϳ0.005 cm using the Raman shifting and I2
C-type band, as shown in Fig. 2. The Q Ϫ Q separations
1 1
LIF technique described previously.27 In all cases, the LIF
varied from band to band ͑see Fig. 2͒ and were found to give
J. Chem. Phys., Vol. 106, No. 11, 15 March 1997
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Harper, Hostutler, and Clouthier: Spectroscopy of DSiF
4369
Extensive hot bands have also been observed in similar work
on HSiCl32 and HSiBr.
11
The band origins of the assigned cold bands of DSiF
were estimated by adding the KЉ ϭ 1 Ϫ KЉ ϭ 0 interval ͑ob-
a
a
p
tained from the rotational analysis, vide infra͒ to the Q
1
¯
¯
branch maximum. For the hot bands, (AϪB)Ј and (AϪB)Љ
values were calculated from the Q branches and then the
band origins determined as before. The band origins and ap-
¯
proximate values of (AϪB)Ј are given in Table I.
In order to obtain harmonic vibrational frequencies for a
normal coordinate analysis, we first fitted the observed vi-
bronic band origins to the vibrational anharmonicity expres-
sion:
¯
3
3
3
FIG. 3. Plots of the excited state (AϪB) values vs the number of quanta in
n
1
n
0Ј
0Ј
the excited state bending mode, vЈ , for the 2 and 1 2 vibrational pro-
˜
2
0
0 0
␯ϭT ϩ
␻ ␯Јϩ
x ␯Ј␯Ј
0
0
͚
i
i
͚ ͚ ij
i
j
gressions of DSiF.
iϭ1
iϭ1 jуi
3
3
3
0
Ј
0Љ
0Љ
2
ϩ
͚ ͚ xijk
␯Ј␯Ј␯ЈϪ͓␻ ϩx ͑␯Љ͒ ͔.
͑1͒
͚
i
j
k
2
22
2
¯
near-linear plots of (AϪB)Ј vs quanta of the excited state
iϭ1 jуi kуj
bending mode, as shown in Fig. 3. These plots were charac-
¯
teristic of the particular vibrations involved, with (AϪB)Ј
As in our previous studies of HSiBr11 and HSiCl,32 we found
that the excited state modes are quite anharmonic and that
cubic terms were necessary to fit all but the lowest energy
transitions. We were particularly fortunate that in the DSiF
increasing rapidly with quanta of ␯Ј , decreasing very
slightly with quanta of ␯Ј, and decreasing strongly with the
addition of quanta of ␯Ј . These regular trends were very
useful in assigning spectra in the higher wavenumber regions
where the rotational structure of the bands overlapped, as
shown by the example in Fig. 4.
Although we did not find any hot bands in our previous
chemical reaction jet work on HSiF, a variety of weak hot
2
3
1
2
2
spectrum we were able to identify the key 1 and 3 bands,
0
0
0
Ј
so that the quadratic anharmonicity constants x of all three
ii
modes were readily determined. The hot bands were in-
cluded in the vibrational fit, but only ␻2Љ and x were de-
0
0Љ
22
bands involving 1–3 quanta of ␯Љ were identified in the
terminable. In order to keep the set of anharmonicity con-
stants manageable, we elected to omit four bands from the
least squares analysis. In the final fit, the electronic band
2
present DSiF spectra. These were assigned by matching the
¯
(
AϪB)Ј values and vibrational intervals with those of the
corresponding cold bands and from the observed ␯Љ value of
origin, T , was fixed at the value obtained from the analysis
2
00
Ϫ1
0
6
42 cm , which agreed well with the infrared matrix
of the 0 band. The quality of the fit can be judged by the
0
1
5
Ϫ1
frequency of ␯Љ ϭ 638 cm . The vibrational temperatures
observed—calculated values given in Table I. Once the as-
signed bands were fitted to the vibrational expansion, it was
then possible to calculate the upper state harmonic frequen-
cies. The resulting constants are presented in Table III.
For the sake of consistency, we have redone the vibra-
tional fit of the observed bands of HSiF,25 including more
2
in the pulsed discharge jet technique are quite high, so that
we are able to observe DSiF transitions from levels with
Ϫ1
almost 2000 cm of vibrational energy in the ground state.
constants and the more precise value of the band origin of 31
0
obtained in the present work ͑vide infra͒. The range of ob-
served bands is much less than that of DSiF, so some as-
sumptions about the anharmonicities were necessary. The
2
0Ј
HSiF 3 band was not observed, so x could not be fitted
0
33
33
directly; instead we approximated it from the relationship
xmn ␻ ␻
m
i
n
i
n
ϭ
,
͑2͒
i
x
␻ ␻
mn
m
where the superscript i denotes isotopic substitution. Any
error in this approximation is unlikely to have much effect
on the derived harmonic frequencies, as ␯Ј is not very
anharmonic in the HSiX molecules we have studied.
Of more concern was obtaining a good value for the har-
3
FIG. 4. A portion of the medium-resolution spectrum of DSiF in the high
wavenumber region identifying Q branches and showing their vibrational
assignments.
J. Chem. Phys., Vol. 106, No. 11, 15 March 1997
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4370
Harper, Hostutler, and Clouthier: Spectroscopy of DSiF
¯
TABLE I. Assignments, band origins, residuals from vibrational fitting, and approximate excited state (AϪB)
Ϫ1
values for the observed vibronic bands in the LIF spectrum of DSiF ͑in cm ͒.
Band origina
Obs.-calc.
¯
(AϪB)Ј
Assign.
Band
Obs.-calc.
¯
(AϪB)Ј
Assign.
0
2
2
0
0
23338.723
24513.046
25551.22
23763.525
24182.935
24596.49
25003.55
25403.08
25793.75
24193.120
25038.47
24908.18
25295.67
25673.66
26039.07
25907.91
24615.74
25031.75
25442.74
25846.71
25459.06
0.00
0.30
0.00
0.20
0.01
4.59
4.29
3.89
4.76
4.90
5.06
5.22
5.39
5.52
4.61
4.64
4.39
4.47
4.58
4.68
3.91
4.75
4.86
5.05
5.20
4.77
2 3
25876.50
25749.52
26130.81
22697.2
0.33
Ϫ11.05b
Ϫ13.94b
0.00
Ϫ0.10
Ϫ0.14
Ϫ0.15
0.01
4.85
4.37
4.47
•••
0
0
1
1
1
1
0
1
0
1
1 2 3
1 2 3
0
0
1
0
0
2
2
1
0
0
0
1
1
1
2
1
3
1
4
1
5
1
6
1
1
2
2
2
2
2
2
2
0
2
2
23121.7
•••
0
3
2
Ϫ0.15
Ϫ0.06
0.13
Ϫ0.02
Ϫ0.29
0.19
Ϫ0.57
Ϫ0.11
0.71
Ϫ0.34
Ϫ10.71b
0.59
23541.26
23954.97
24362.10
24761.70
25152.16
23551.36
23974.21
24390.30
24801.38
25205.24
24396.90
22058.6
4.88
5.05
5.23
5.37
5.52
•••
4.76
4.81
5.06
5.18
4.67
•••
0
4
2
0
5
2
0
6
2
0.28
0
1
3
2
Ϫ0.08
Ϫ0.52
0.59
Ϫ0.07
0.14
Ϫ0.11
0.14
0.10
0
2
0
1
0
1
0
1
0
1
0
1
0
2
0
3
2 3
0
1
1
0
1
0
1
0
1
0
2
0
1
0
2
0
3
0
4
0
1
0
1
0
2
0
3
0
4
0
1
0
1
0
1
0
1
0
1
0
2
0
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
2 3
1
2
2 3
1
3
2 3
1
4
2 3
1
0
2 3
1
0
2
2
2
2
2
1
Ϫ0.14
Ϫ0.02
Ϫ0.17
Ϫ0.66
22483.1
22902.6
23316.4
22267.2
0.01
•••
•••
•••
•••
2
2
Ϫ0.09
Ϫ0.02
0.41
2
3
2
2
b,c
2
3
a
Band origins quoted to three decimal places are from rotational analysis and are accurate to ϳ0.007 cmϪ1,
Ϫ1
those with two decimal places are estimated from medium-resolution spectra to Ϯ0.1 cm , and those with one
Ϫ1
decimal place are estimated from low-resolution spectra to Ϯ1 cm
.
b
c
Band not included in least squares fit.
Tentative assignment of a single weak band unaccompanied by other members of the hot band progression.
monic frequency of ␯Ј , since it is very anharmonic in DSiF
state. Although we only used the constants necessary to
properly fit the data, in the final analysis many of them were
determined by single bands in the spectrum, so that their
errors are likely larger than their standard errors of fit. We
estimate the uncertainties in the derived harmonic frequen-
1
0
1
Ј
ϭ Ϫ67.7 cmϪ1͒. As neither 1 nor 1 were found in the
2
3
͑x
1
0 0
spectra of HSiF, we resorted to using the product rule ex-
pression
D
D
D
1/2
MD 3/2
3
␻1
␻ ␻
2
3
ABC
m
H
Ϫ1
cies of HSiF to be of the order of 10 cm . The HSiF vi-
ϭ
ͫ
ͬ ͫ ͬ ͫ ͬ
͑3͒
D
D
D
␻1
␻ ␻
2
A B C
M
m
D
bronic band origins and observed-calculated values are pre-
sented in Table II, while the resulting anharmonicity
constants and harmonic frequencies are given in Table III.
3
to calculate the harmonic frequency. It was found that all
three calculated harmonic frequencies were strongly depen-
dent on which particular anharmonicity constants were used
in the vibrational fitting, a consequence of the limited range
of data available and the large anharmonicities in the excited
TABLE III. Vibrational constants of HSiF and DSiF ͑in cmϪ1͒.
HSiF
DSiF
HSiF
DSiF
1546.95͑9͒a
562.9͑2͒
1241.8͑7͒
426.8͑2͒
859.6͑4͒
Ϫ67.8͑4͒
Ϫ2.1͑1͒
Ϫ4.9͑2͒
Ϫ26.6͑4͒
•••
•••
•••
642.9͑3͒
Ϫ1.4͑1͒
TABLE II. Assignments, band origins, and residuals from vibrational fitting
for the observed vibronic bands in the LIF spectrum of HSiF ͑in cm ͒.
0Ј
0Љ
␻
␻
1
2
Ϫ1
0
Ј
0
Љ
␻2
x
22
0
Ј
861.77͑9͒
Band origina
Obs.-calc.
␻3
Assign.
0
1
Ј
•
••
␻Ј
1815.6͑13͒
597.1͑3͒
867.8͑5͒
1322.4͑8͒
443.5͑3͒
867.2͑5͒
x
x
x
x
x
x
x
1
1
0
0
1
2
2
2
2
23260.021
24806.970
23818.442
24366.089
24901.085
25420.527
24116.899
25298.7
0.000
0.000
0.060
Ϫ0.089
0.060
Ϫ0.015
0.000
0.000
0.000
0.000
0.000
0Ј
0
1
0
1
0
2
0
3
0
4
0
1
0
Ϫ4.1͑1͒
Ϫ4.9b
␻Ј
2
22
0
3
0
Ј
␻Ј
3
3
Ј
Ϫ66.6͑1͒
Ϫ5.6͑2͒
0.8͑3͒
•••
12
0Ј
13
0Ј
Ϫ4.1͑2͒
Ϫ2.0͑1͒
Ϫ0.15͑1͒
•••
23
3
0Ј
1
1
0
1
0
1
0
1
0
122
1
1
2
2
2
3
3
3
0
1
0
1
0
2
0
0Ј
Ϫ0.40͑2͒
Ϫ4.9͑1͒
•••
x222
25658.270
24671.2
25205.2
0
Ј
x
x
223
0Ј
1.3͑1͒
233
a
aThe values in parentheses are one standard error in units of the last signifi-
cant figure.
Constant fixed in the final fit.
Band origins quoted to three decimal places are from rotational analysis
Ϫ1
and are accurate to ϳ0.01 cm , while those with one decimal place are
Ϫ1
b
accurate to Ϯ1 cm
.
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Harper, Hostutler, and Clouthier: Spectroscopy of DSiF
4371
B. Rotational analysis
Our aim in this work was to rotationally analyze the key
bands in the spectra of HSiF and DSiF to obtain an excited
state equilibrium (r ) structure and an effective (r ) ground
e
o
state structure. We had previously obtained rotational con-
0
1
1
stants for the 0 , 2 , and 1 bands of HSiF, but were unable
0
0
0
1
to record the very weak 3 band at high resolution using the
0
chemical reaction jet technique. This problem was overcome
by the much better signal-to-noise ratios in the discharge jet
experiments, so that we have achieved rotational analyses of
1
0
0
1
0
1
0
1
0
the HSiF 3 band and the DSiF 0 , 1 , 2 , and overlapping 3
0
2
and 2 bands.
0
During the course of the high-resolution experiments, we
found that power broadening of the rotational lines in the
spectra was severe, so that the laser power had to be attenu-
ated well below 1 mJ/pulse to obtain linewidths of ϳ0.05
1
FIG. 5. The high-resolution spectrum of the 1 band of DSiF showing some
of the rotational assignments.
0
Ϫ1
cm full width at half-maximum ͑FWHM͒. A typical ex-
1
ample of the DSiF spectra is the 1 band, illustrated in Fig. 5,
0
r
r
which exhibits almost completely resolved Q and Q1
branches and R asymmetry splittings.
KЈϭ1ϪKЉϭ2 subbands. Once the assignments were com-
0
a
a
r
pleted, the rotational constants were determined by a least
squares fitting of the observed transition frequencies to Wat-
son’s A reduction of the asymmetric top rotational Hamil-
1
The rotational analyses of the spectra were similar to
that described previously for HSiF.25 The electronic transi-
tion moment is oriented out of the molecular plane, giving
r
34
1
0
tonian in the I representation. For the 1 band of DSiF, the
rise to C-type bands with the selection rules ⌬K ϭϮ1,
constants of both states were varied, with ⌬Љ fixed at the
HSiF ground state value of 4.8ϫ10 cm . The other DSiF
a
K
Ϫ4
Ϫ1
⌬
K ϭ0,Ϯ2. Although weak axis-tilting lines were found in
c
the spectra, they were not used in the least squares fitting.
Independent values of B and C were obtained from assign-
ments of the asymmetry split KЈ ϭ 2 Ϫ KЉ ϭ 1 and KЈ ϭ 1
bands were fitted with the ground state constants fixed at
1
1
those obtained from the 1 band. The 3 band of HSiF was
0
0
fitted with the ground state constants and the excited state
a
a
a
TABLE IV. Rotational line frequencies ͑cmϪ1͒ and assignments for the 1 band of DSiF.
1
0
KЈϭ1ϪKЉϭ0 subband
KЈϭ0ϪKЉϭ1subband
a a
a
a
rQ0
rP0
pR1
pQ1
pP1
JЈ
rR0
JЈ
1
2
3
4
5
6
7
8
24518.332͑Ϫ5͒a
24519.326͑4͒
24520.278͑Ϫ5͒
24521.231͑7͒
24522.137͑Ϫ9͒
24523.049͑Ϫ4͒
24523.945͑Ϫ3͒
24524.836͑Ϫ1͒
24517.252͑Ϫ2͒
24517.098͑Ϫ5͒
24516.873͑Ϫ5͒
24516.576͑Ϫ6͒
24516.223͑4͒
24515.245͑Ϫ2͒
24514.175͑1͒
24513.084͑2͒
24511.988͑11͒
24510.862͑Ϫ1͒
0
1
2
3
4
5
6
7
24508.495͑Ϫ5͒
*24509.530͑Ϫ19͒ 24507.353͑1͒
*24509.530͑13͒
24506.118͑Ϫ2͒
*24509.417͑Ϫ51͒ 24504.806͑1͒
24511.440͑Ϫ2͒
*24512.270͑15͒
24512.984͑4͒
24513.632͑13͒
24514.175͑5͒
*24509.417͑16͒
24509.320͑3͒
24509.217͑5͒
24509.084͑Ϫ3͒
24503.412͑7͒
24501.915͑Ϫ6͒
24515.787͑Ϫ5͒
KЈϭ2ϪKЉϭ1subband
KЈϭ1ϪKЉϭ2 subband
KЈϭ3ϪKЉϭ2 subband
a a
a
a
a
a
rQ1
pQ2
pP2
rR2
JЈ
rR1
JЈ
JЈ
2
24528.562͑Ϫ4͒
24526.445͑9͒
24526.640͑0͒
24526.189͑3͒
1
24501.315͑Ϫ9͒
24501.269͑Ϫ4͒
24500.243͑0͒
24500.089͑0͒
24499.135͑2͒
24498.827͑Ϫ4͒
3
4
5
6
24540.394͑Ϫ2͒
24541.227͑1͒
24541.999͑Ϫ7͒
24542.725͑Ϫ8͒
2
4528.634͑0͒
24529.382͑0͒
4529.577͑Ϫ6͒
24530.131͑13͒
4530.520͑4͒
24530.782͑8͒
4531.433͑Ϫ2͒
24531.351͑Ϫ5͒
4532.341͑2͒
24531.859͑Ϫ9͒
4533.229͑0͒
3
4
5
6
7
2
3
4
5
24503.181͑1͒
24503.340͑0͒
24502.951͑3͒
24503.274͑4͒
24502.633͑0͒
24503.181͑5͒
24502.233͑0͒
2
24526.599͑4͒
24525.851͑Ϫ3͒
*24526.599͑60͒
24525.436͑Ϫ3͒
*24526.445͑Ϫ27͒
24524.944͑1͒
2
2
2
24503.058͑Ϫ2͒
2
aThe numbers in parentheses are observed minus calculated values in units of 10Ϫ3 cmϪ1. An asterisk denotes a blended line not used in the least squares fit.
In cases where the asymmetry doubling is resolved, the transition with even parity (JϩK ϩK ) in the lower state is listed first.
a
c
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4372
Harper, Hostutler, and Clouthier: Spectroscopy of DSiF
TABLE V. Excited state rotational constants and band origins for various vibronic bands of the
˜
A
1
AЉ←X AЈ system of DSiF and the 3 band of HSiF ͑in cmϪ1͒.
˜
1
1
0
Aa,b
B
C
10 ⌬
K
3
T0
␴c
Band
0
0
5.0856 ͑35͒
0.51801 ͑51͒
0.46693 ͑87͒
0.81 ͑36͒
23338.723͑6͒
23763.525͑7͒
24182.935͑7͒
24193.120͑6͒
24513.046͑5͒
24116.899͑6͒
0.006
0.008
0.006
0.006
0.005
0.007
0
5
3
3
1
1
2
5.2517 ͑38͒
0.52105 ͑86͒
0.46518 ͑98͒
1.99 ͑42͒
0
2
1
1
7
2
2
5.3983 ͑38͒
0.52268 ͑55͒
0.46541 ͑53͒
2.74 ͑40͒
0
2
2
8
4
1
3
5.0969 ͑32͒
0.51428 ͑60͒
0.46418 ͑71͒
1.05 ͑34͒
0
8
8
6
6
1
1
4.7755 ͑23͒
0.51692 ͑34͒
0.46405 ͑44͒
1.11 ͑26͒
0
5
4
4
4
1
0
d
3
͑HSiF͒
9.2959 ͑18͒
0.54410 ͑44͒
0.51329 ͑65͒
3.876 015
8
7
0
aThe numbers in parentheses are 3␴ error limits and are right justified to the last digit on the line; sufficient
additional digits are quoted to reproduce the original data to full accuracy.
b
The ground state constants used in the analysis of the DSiF bands, obtained from fitting the upper and lower
1
states of the 1 band are Aϭ3.9972 (13), Bϭ0.54924 (40), Cϭ0.48146 (42) with ⌬ fixed at the HSiF value
0
9
2
1
K
Ϫ4
of 4.8ϫ10
.
c
Overall standard deviation of fit.
d
The upper state centrifugal distortion constants of HSiF were fixed at the following values from the analysis of
0
0
Ϫ3
Ϫ5
Ϫ7
the
0
band of Ref. 20: ⌬ ϭ3.876 015ϫ10
,
⌬JKϭ3.635 849ϫ10
,
⌬ ϭ8.005 538ϫ10
,
K
J
Ϫ5
Ϫ8
␦Kϭ3.735 918ϫ10 , ␦ ϭ5.537 164ϫ10 . The lower state rotational and quartic centrifugal distortion con-
J
stants were fixed at the values given in Ref. 20.
centrifugal distortion constants fixed at those obtained by
IV. DISCUSSION
1
7
0
Suzuki et al. for the 0 band. An example of the quality of
0
A. Harmonic force field
the fit and the range of assignments is given in Table IV. The
resulting molecular constants are summarized in Table V.
All of the bands fit very well, with no evidence of heteroge-
neous perturbations or systematic calibration errors.
In the present work, we have been able to calculate the
excited state harmonic vibrational frequencies of both HSiF
and DSiF, as given in Table III. It is unfortunate that the
TABLE VI. The harmonic force field of HSiF.
Fitted data ͑cmϪ1
͒
˜
X
˜
A
HSiF
DSiF
HSiF
DSiF
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
␻1
␻2
␻3
⌬J
⌬JK
⌬K
␦J
1913.1
859.0
833.7
1977.3
861.0
834.1
1387.4
638.4
833.4
1423.3
636.0
836.0
1815.6
597.1
867.8
1836.3
590.2
870.8
1322.4
443.5
867.2
1321.2
442.4
866.9
Ϫ7
Ϫ7
Ϫ7
Ϫ7
8.5ϫ10
2.25ϫ10
4.80ϫ10
5.7ϫ10
1.4ϫ10
9.0ϫ10
2.25ϫ10
4.81ϫ10
6.4ϫ10
1.4ϫ10
8ϫ10
3.64ϫ10
3.88ϫ10
5.5ϫ10
3.7ϫ10
8ϫ10
3.63ϫ10
3.87ϫ10
4.9ϫ10
2.3ϫ10
Ϫ5
Ϫ4
Ϫ5
Ϫ4
Ϫ5
Ϫ3
Ϫ5
Ϫ3
Ϫ8
Ϫ8
Ϫ8
Ϫ8
Ϫ5
Ϫ5
Ϫ5
Ϫ5
␦K
Internal coordinates
R ϭ⌬r͑Si–H͒
1
R ϭ⌬␪͑HSiF͒
2
R ϭ⌬r͑Si–F͒
3
Harmonic force constants
˜
X
˜
A
f11 ͑aJ ÅϪ2
͒
͒
2.24͑2͒
0.922͑9͒
4.76͑6͒
•••
0.21͑5͒
0.27͑4͒
1.92͑1͒
0.46͑2͒
5.12͑6͒
0.26͑2͒
•••
Ϫ2
f22 ͑aJ rad
f33 ͑aJ ÅϪ2͒
f12 ͑aJ Å radϪ1͒
Ϫ1
Ϫ2
f13 ͑aJ Å
͒
f23 ͑aJ Å radϪ1͒
Ϫ1
0.17͑2͒
J. Chem. Phys., Vol. 106, No. 11, 15 March 1997
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Harper, Hostutler, and Clouthier: Spectroscopy of DSiF
4373
TABLE VII. Structural parameters of HSiF.
Electronic state
˜
X
1AЈ
˜
1AЉ
A
z
e
Ab initioc
z
e
Ab initiod
ro
rz
r
ro
re
rz
r
r ͑SiF͒ Å
r ͑SiH͒ Å
1.606͑1͒a
1.548͑3͒
97.0͑6͒
1.608͑1͒a
1.542͑2͒
96.9͑4͒
1.603͑3͒b
1.528͑5͒
96.9͑5͒
1.615
1.528
96.8
1.602͑1͒a
1.557͑1͒
114.4͑3͒
1.597͑3͒a
1.526͑14͒
115.0͑6͒
1.603͑1͒a
1.555͑4͒
114.3͑2͒
1.598͑3͒b
1.536͑5͒
114.3͑5͒
1.617
1.535
114.9
␪
͑HSiF͒ °
a
Numbers in parentheses are one standard error in units of the last significant figures.
Numbers in parentheses are estimated uncertainties in units of the last significant figures.
CCSD TZ(2df,2pd) results of Ref. 24.
MRCI TZ(2df,2pd) results of Ref. 24.
b
c
d
range of HSiF vibronic bands is so restricted, as this severely
limits the accuracy of our derived frequencies. However, the
availability of a good set of centrifugal distortion constants
for the ground and excited states, from the gas phase analysis
B. Molecular structure
Several molecular structures were calculated using a va-
riety of different approaches. First, we used the ground and
excited state rotational constants to calculate effective or ro
structures. Our initial attempts at least squares fitting of the
rotational constants resulted in a very high degree of corre-
lation between the Si–H bond length and the HSiF angle
0
17
of the 0 band, makes it possible to calculate reasonable
0
harmonic force fields. The normal coordinate problem was
set up in internal coordinates and the force constants were
refined using the program of Hedberg and Mills.35 The ex-
͑
correlation coefficient greater than 0.99͒; fitting planar mo-
ments decreased the correlations substantially and gave a
more satisfactory fit. Consequently, for the r -, r -, and
cited state harmonic frequencies were used with an estimated
Ϫ1
Ϫ1
uncertainty of 10 cm for HSiF and 2 cm for DSiF, along
17
with the centrifugal distortion constants of HSiF and their
statistical uncertainties. Our re structure ͑vide infra͒ was
used in the calculation. For the ground state, the matrix in-
frared frequencies of Ref. 15 were used, with estimated er-
o
z
re-based structures, the three structural parameters were fit-
ted to the principal planar moments, P and P , calculated
A
B
from the rotational constants of HSiF and DSiF. Since the
planarity condition constrains the third planar moment to
zero, the experimental values of PC were omitted from the
Ϫ1
rors of 30 cm for the very anharmonic Si–H and Si–D
Ϫ1
stretches and 2 cm for the other frequencies. The ground
state gas phase centrifugal distortion constants of HSiF were
fit. For the r structures, a shrinkage of the Si–D bond of
o
also included in the fit with their statistical uncertainties.15
0.003 Å ͑Laurie correction͒ was included in the analysis.
The availability of rotational constants for excited state
levels involving one quantum of each of the fundamentals of
HSiF and DSiF made it possible to calculate an equilibrium
structure. Using the data in Table V and our previous HSiF
rotational constants from Ref. 25, we calculate the equilib-
The ground state structure used as input for the normal co-
z
e
ordinate analysis was the r structure given in Table VII.
The results of the force field calculations are summa-
rized in Table VI. Only five of the six force constants could
be determined in each state. In the ground state, ␻ and ␻ fit
2
3
Ϫ1
well, while ␻ was predicted to be substantially higher than
1
rium rotational constants ͑in cm ͒ to be:
observed in the matrix, consistent with the large anharmo-
nicities expected for the Si–H/Si–D stretching vibrations. In
the excited state, the DSiF harmonic frequencies fit very
well, but those of HSiF show substantial errors, consonant
with our expectations based on the lack of data for the higher
vibronic bands of HSiF. Different weighting schemes for the
vibrational frequencies did not have a substantial effect on
the resulting force constants. The centrifugal distortion con-
stants, which are the primary data determining the force
fields in both states, typically fit within the limits of experi-
HSiF
DSiF
A ϭ9.61491Ϯ0.00165
B ϭ0.55034Ϯ0.00047
C ϭ0.52094Ϯ0.00047
5.15200Ϯ0.00178
0.51890Ϯ0.00029
0.47062Ϯ0.00044
e
e
e
e
C
e
A
e
B
The inertial defect ⌬ϭI ϪI ϪI for HSiF is effectively
zero ͑⌬ϭϪ0.025Ϯ0.039 amu Å ͒ and attests to the quality
of the rotational constants. For DSiF, the inertial defect is
.061Ϯ0.038 amu Å , slightly larger than zero. This effect is
2
2
0
1
2
due to the interaction of the DSiF excited state 3 and 2
Ϫ1
levels, whose perturbed positions are only 10.2 cm apart.
Due to the low symmetry of the molecule, these levels can
interact by both Fermi and c-axis Coriolis mechanisms. We
have attempted to correct the rotational constants of the 31
level by a variety of schemes, but these did not improve the
inertial defect. Unfortunately, in the excited states of mol-
ecules of this type, the A rotational constants do not scale
linearly with the number of quanta of the bending mode, so
that the usual linear expansion of the vibrational dependence
mental error.
The HSiF ground state Si–H ͑2.24 aJ ÅϪ2͒ and Si–F
Ϫ2
͑
4.76 aJ Å ͒ stretching force constants are similar to those
Ϫ2 36
Ϫ2 37
of SiH ͑2.20 aJ Å
͒ and SiF ͑5.03 aJ Å ͒, but smaller
2
2
38
than the corresponding silane force constants which are
Ϫ2
Ϫ2
2
.84 aJ Å for SiH and 7.18 aJ Å for SiF . The force
4
4
constants show the expected trends on excitation, consistent
with the changes in the vibrational frequencies and the ge-
ometry ͑vide infra͒.
J. Chem. Phys., Vol. 106, No. 11, 15 March 1997
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4374
Harper, Hostutler, and Clouthier: Spectroscopy of DSiF
of the rotational constant is not reliable beyond vЈ ϭ 1, mak-
ing corrections for Fermi resonances difficult.
tronic excitation has on the Si–H bond length, but the ab
initio predictions and the decrease in ␻ and f suggest a
2
1
11
The r and r structures, obtained by fitting the planar
slight elongation.
o
e
moments, are given in Table VII. Comparison of the ro
structures suggests that electronic excitation produces a
slight decrease in the Si–F bond length, a slight increase in
the Si–H bond length, and a large increase in the HSiF bond
In this work, the ground and excited state molecular
structures of HSiF have been obtained free of assumptions
z
for the first time. In comparing the rЈ and r Љ structures to
e
e
the high quality ab initio predictions in Table VII, we note
that the agreement is within the uncertainties of the experi-
mental results, except for the Si–F bond lengths which are
consistently overestimated by theory. The present excited
state equilibrium bond lengths are also in general agreement
with those previously derived from our semirigid bender fit
of only HSiF rovibronic energy levels, in which the excited
state bond angle was assumed to be 114.5°.
angle. As expected, the excited state r structure has shorter
e
bond lengths than the r structure, although the error limits
o
on the Si–H equilibrium bond length are disappointingly
large.
We have also used the results of our normal coordinate
analysis to obtain average (r ) structures for the ground and
z
excited states. The harmonic contributions to the ␣ constants,
obtained from the harmonic force field, were added to the
vϭ0 effective rotational constants ͑Table V͒ and the result-
ing constants ͑transformed to planar moments͒ were fitted to
the geometric parameters. The contraction in the Si–H bond
length on deuterium substitution was calculated using the
ACKNOWLEDGMENTS
Many helpful discussions with Dr. Roger Grev are grate-
fully acknowledged and we thank him for providing us with
his ab initio predictions prior to their publication. We are
greatly indebted to Professor H. D. Rudolph for providing us
with copies of his structure programs and much advice on
their implementation. This research was supported by the
National Science Foundation. D.A.H. is an NSF summer Re-
search Experiences for Undergraduates ͑REU͒ Student.
3
9–41
equation
3
␦
rzϭ a␦
͗
u2
͘
Ϫ␦K
͑4͒
u2
2
with the zero-point mean-square amplitude of the bond,
͗
͘
,
and its perpendicular amplitude correction, K, obtained from
the force field. The Morse anharmonicity parameters a, were
calculated from the relation42
1
J. M. Jasinski, B. S. Meterson, and B. A. Scott, Ann. Rev. Phys. Chem.
38, 109 ͑1987͒.
2
3
M. J. Vasile and F. A. Stevie, J. Appl. Phys. 53, 3799 ͑1982͒.
Y. Matsumi, S. Toyoda, T. Hayashi, M. Miyamura, H. Yoshikawa, and S.
Komiya, J. App. Phys. 60, 4102 ͑1986͒.
S. Vanhaelemeersch, J. Van Hoeymissen, D. Vermeylen, and J. Peeters, J.
Appl. Phys. 70, 3892 ͑1991͒.
G. H. Kruppa, S. K. Shin, and J. L. Beauchamp, J. Phys. Chem. 94, 327
͑1990͒.
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͑1993͒.
2
2
c␮ ␣ ϩ6B
e
e
4
5
aϭ ␲␻e
ͱ
ͩ
ͪ
͑5͒
3
3
2hBe
␻e
using spectroscopic data from the SiH and SiF44 diatomic
43
6
7
molecules. Finally, estimated equilibrium bond lengths ͑we
z
have termed this the r structure͒ were obtained from the r
e
z
39–41
8
9
0
J. Karolczak and D. J. Clouthier ͑unpublished͒.
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9523 ͑1995͒.
structure using the formula
1
3
z
u2
r ϭr Ϫ
a
͗
͘
ϩK.
͑6͒
e
z
11
2
H. Harjanto, W. W. Harper, and D. J. Clouthier, J. Chem. Phys. 105,
10189 ͑1996͒.
12
J. Karolczak, Q. Zhuo, D. J. Clouthier, W. M. Davis, and J. D. Goddard,
J. Chem. Phys. 98, 60 ͑1993͒.
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Z. K. Ismail, L. Fredin, R. H. Hauge, and J. L. Margrave, J. Chem. Phys.
The bond angles were assumed to be the same in the r and
z
z
e
r structures and the results are given in Table VII.
13
z
The excited state r and r structures compare quite fa-
vorably, indicating that the r structure in the ground state is
e
e
14
z
e
probably fairly close to the equilibrium structure. Our best
estimates of the equilibrium molecular geometries in the
15
16
77, 1626 ͑1982͒.
z
e
ground and excited states are therefore the r and r struc-
H. U. Lee and J. P. Deneufville, Chem. Phys. Lett. 99, 394 ͑1983͒.
T. Suzuki, K. Hakuta, S. Saito, and E. Hirota, J. Chem. Phys. 82, 3580
e
17
tures, respectively, as given in Table VII. The ground state
bond lengths of r͑SiF͒ϭ1.603͑3͒ and r͑SiH͒ϭ1.528͑5͒ are
comparable to the equilibrium bond lengths of SiF ͑1.6011
͑
1985͒.
18
R. N. Dixon and N. G. Wright, Chem. Phys. Lett. 117, 280 ͑1985͒.
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19
44
43
20
Å͒ and SiH ͑1.5201 Å͒. The ground state bond angle of
9
6.9͑5͒° is very nearly equal to the average ͑96.43°͒ of the
21
45
equilibrium ground state bond angles of difluorosilylene
22
͑
100.77°͒ and silylene36 ͑92.08°͒. Electronic excitation de-
2
2
3
4
creases the r͑SiF͒ bond length by 0.006Ϯ0.004 Å, in accord
with the experimental and force field results which show
͑
1990͒.
K. J. Gregory and R. S. Grev ͑unpublished͒.
W. W. Harper, J. Karolczak, D. J. Clouthier, and S. C. Ross, J. Chem.
Phys. 103, 883 ͑1995͒.
␻
Ј Ͼ ␻Љ . The error bars on the excited state r͑SiH͒ bond
25
3
3
length do not allow a direct determination of the effect elec-
J. Chem. Phys., Vol. 106, No. 11, 15 March 1997
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2
2
2
6
7
8
35
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36
37
5
32 ͑1989͒.
2
3
3
3
3
9
0
1
2
3
38
H. J. Emeleus and A. G. Maddock, J. Chem. Soc., 293 ͑1944͒.
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Raman Spectra of Polyatomic Molecules ͑Van Nostrand, New York,
39
40
41
K. Kuchitsu, J. Chem. Phys. 49, 4456 ͑1968͒.
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͑1969͒.
4
4
2
3
1
945͒, p. 229.
3
4
J. K. G. Watson, in Vibrational Spectra and Structure, a Series of Ad-
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4
4
4
5
R. W. Martin and A. J. Merer, Can. J. Phys. 51, 634 ͑1973͒.
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1
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J. Chem. Phys., Vol. 106, No. 11, 15 March 1997
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
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