Heavy hydrides: H2Te ultraviolet photochemistry

Article abstract of DOI:10.1063/1.2008261

The room-temperature ultraviolet absorption spectrum of H2 Te has been recorded. Unlike other group-6 hydrides, it displays a long-wavelength tail that extends to 400 nm. Dissociation dynamics have been examined at photolysis wavelengths of 266 nm (which lies in the main absorption feature) and 355 nm (which lies in the long-wavelength tail) by using high- n Rydberg time-of-flight spectroscopy to obtain center-of-mass translational energy distributions for the channels that yield H atoms. Photodissociation at 355 nm yields TeH (Π 12 2) selectively relative to the TeH (Π 32 2) ground state. This is attributed to the role of the 3 A′ state, which has a shallow well at large RH-TeH and correlates to H+TeH (Π 12 2). Note that the Π 12 2 state is analogous to the P 12 2 spin-orbit excited state of atomic iodine, which is isoelectronic with TeH. The 3 A′ state is crossed at large R only by 2 A″, with which it does not interact. The character of 3 A′ at large R is influenced by a strong spin-orbit interaction in the TeH product. Namely, Π 12 2 has a higher degree of spherical symmetry than does Π 32 2 (recall that I (P 12 2) is spherically symmetric), and consequently Π 12 2 is not inclined to form either strongly bonding or antibonding orbitals with the H atom. The 3 A′ ←X transition dipole moment dominates in the long-wavelength region and increases with R. Structure observed in the absorption spectrum in the 380-400 nm region is attributed to vibrations on 3 A′. The main absorption feature that is peaked at ～240 nm might arise from several excited surfaces. On the basis of the high degree of laboratory system spatial anisotropy of the fragments from 266 nm photolysis, as well as high-level theoretical studies, the main contribution is believed to be due to the 4 A″ surface. The 4 A″ ←X transition dipole moment dominates in the Franck-Condon region, and its polarization is in accord with the experimental observations. An extensive secondary photolysis (i.e., of nascent TeH) is observed at 266 and 355 nm, and the corresponding spectral features are assigned. Analyses of the c.m. translational energy distributions yield bond dissociation energies D0. For H2 Te and TeH, these are 65.0±0.1 and 63.8±0.4 kcalmol, respectively, in good agreement with predictions that use high-level relativistic theory.

Full text of DOI:10.1063/1.2008261

Heavy hydrides: H 2 Te ultraviolet photochemistry
J. Underwood, D. Chastaing, S. Lee, and C. Wittig
Citation: The Journal of Chemical Physics 123, 084312 (2005); doi: 10.1063/1.2008261
View online: http://dx.doi.org/10.1063/1.2008261
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/123/8?ver=pdfcov
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THE JOURNAL OF CHEMICAL PHYSICS 123, 084312 ͑2005͒
Heavy hydrides: H2Te ultraviolet photochemistry
J. Underwood, D. Chastaing, S. Lee, and C. Wittiga͒
Department of Chemistry, University of Southern California, Los Angeles, California 90089
͑Received 19 May 2005; accepted 5 July 2005; published online 30 August 2005͒
The room-temperature ultraviolet absorption spectrum of H2Te has been recorded. Unlike other
group-6 hydrides, it displays a long-wavelength tail that extends to 400 nm. Dissociation dynamics
have been examined at photolysis wavelengths of 266 nm ͑which lies in the main absorption feature͒
and 355 nm ͑which lies in the long-wavelength tail͒ by using high-n Rydberg time-of-flight
spectroscopy to obtain center-of-mass translational energy distributions for the channels that yield
H atoms. Photodissociation at 355 nm yields TeH͑2⌸1/2͒ selectively relative to the TeH͑2⌸3/2
͒
ground state. This is attributed to the role of the 3A state, which has a shallow well at large R
Ј
H-TeH
and correlates to H+TeH͑2⌸1/2͒. Note that the 2⌸1/2 state is analogous to the 2P1/2 spin-orbit excited
state of atomic iodine, which is isoelectronic with TeH. The 3A state is crossed at large R only by
Ј
2A , with which it does not interact. The character of 3A at large R is influenced by a strong
Љ
Ј
2
spin-orbit interaction in the TeH product. Namely, ⌸1/2 has a higher degree of spherical symmetry
than does 2⌸3/2 ͑recall that I͑2P1/2͒ is spherically symmetric͒, and consequently 2⌸1/2 is not inclined
to form either strongly bonding or antibonding orbitals with the H atom. The 3A ←X transition
Ј
dipole moment dominates in the long-wavelength region and increases with R. Structure observed
in the absorption spectrum in the 380–400 nm region is attributed to vibrations on 3A . The main
Ј
absorption feature that is peaked at ϳ240 nm might arise from several excited surfaces. On the basis
of the high degree of laboratory system spatial anisotropy of the fragments from 266 nm photolysis,
as well as high-level theoretical studies, the main contribution is believed to be due to the 4A
Љ
surface. The 4A ←X transition dipole moment dominates in the Franck-Condon region, and its
Љ
polarization is in accord with the experimental observations. An extensive secondary photolysis
͑i.e., of nascent TeH͒ is observed at 266 and 355 nm, and the corresponding spectral features are
assigned. Analyses of the c.m. translational energy distributions yield bond dissociation energies D0.
For H2Te and TeH, these are 65.0 0.1 and 63.8 0.4 kcal/mol, respectively, in good agreement
with predictions that use high-level relativistic theory. © 2005 American Institute of Physics.
͓DOI: 10.1063/1.2008261͔
I. INTRODUCTION
good descriptors. Consequently, “singlet-triplet” transitions
can be quite strong and may even dominate in some regions
of the spectra.
Relativistic effects in chemistry are due mainly to the
increased effective mass of electrons when they are in the
vicinity of highly charged nuclei, and therefore have large
velocities. In systems composed of light atoms, the approxi-
mation that the speed of light c is equal to infinity holds,
whereas for systems that contain heavy atoms, this assump-
tion breaks down and relativistic effects can be manifest in
the photodissociation dynamics.
The core 1s electrons of a heavy atom travel at a signifi-
cant percentage of the speed of light, and the corresponding
increased effective mass of the electron leads to a significant
contraction of the 1s orbital. Even though valence orbitals
are affected to a lesser degree, the increased masses of the
core electrons result in ionization potentials, oxidation states,
and bond energies that would not be predicted by extrapola-
tion from lighter counterparts.1 Spin-orbit coupling also in-
creases with the mass of the atom. When this coupling is
large, spin and orbital angular momentum cease to have good
quantum numbers. Singlet-triplet selection rules are no
longer valid, and the terms “singlet” and “triplet” cease to be
The photodissociation of systems containing heavy at-
oms has received modest attention relative to lighter coun-
terparts. Many heavy atoms are toxic and their compounds
are often unstable, necessitating collaborations with chemists
who make molecules for a living. Theoretical studies are also
demanding. The large number of electrons and the need to
treat relativistic effects rigorously make such calculations
difficult.1–3
One of the few systems in which theory and photodisso-
ciation experiments have explored relativistic effects thor-
oughly and with excellent agreement is the low-energy ab-
sorption spectrum of HI, i.e., 190-315 nm.3 The absorption
of a photon in this region leads to two product channels:
HI → H + I͑2P3/2͒,
͑1͒
→H + I͑2P1/2͒.
͑2͒
Reaction ͑1͒ yields ground state I͑2P3/2͒, while reaction
͑2͒ yields spin-orbit excited state I͑2P1/2͒; these states are
separated by 7603 cm−1.4 Three excited states, A 1⌸1, a 3⌸1,
a͒
Electronic mail: wittig@usc.edu
0021-9606/2005/123͑8͒/084312/10/$22.50
123, 084312-1
© 2005 American Institute of Physics
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084312-2
Underwood et al.
J. Chem. Phys. 123, 084312 ͑2005͒
→H + TeH͑2⌸1/2,␷,J͒,
͑5͒
where ␷ and J are vibrational and rotational quantum num-
bers, respectively. Spin-orbit excited TeH͑2⌸1/2
͒ lies
3815 cm−1 above ground state TeH͑2⌸3/2͒.30 The photodis-
sociation experiments presented in this study are sensitive
only to reactions ͑4͒ and ͑5͒; the role of reaction ͑3͒ in the
gas-phase photochemistry of the low-energy absorption fea-
tures of H2Te is unknown.
There have been only a few studies of the low-energy
ultraviolet absorption features of H2Te. In 1931, Goodeve
and Stein carried out one of the earliest experiments, mea-
suring the lowest-energy absorption bands of H2O, H2S,
H2Se, and H2Te.31 Though the spectra were taken at low
resolution and there were contamination issues with the
heavier group-6 dihydride samples, the H2Te spectrum ex-
hibited a strikingly different shape compared with those of
the lighter dihydrides in the long-wavelength region. The
absorption spectra of the lighter dihydrides drop off quickly
toward longer wavelengths, and the spectra have similar
shapes throughout their first absorption bands. On the other
hand, rather than a sharp drop-off, H2Te has a long tail that
extends to ϳ400 nm. More recent studies have confirmed the
absorption spectra of H2O16 and H2S,32 and we have mea-
sured the absorption of H2Te, as shown in Fig. 1.33 At the
long-wavelength end of the absorption spectrum, though H2S
does not drop off as sharply as does H2O, it does not exhibit
the long structured tail that is seen in H2Te.
FIG. 1. H2Te absorption spectrum and possible product energies. Arrows
indicate the photolysis wavelengths used in this study, 266 and 355 nm. The
spectrum and product energies are determined in this study.
3
and ⌸0+, are involved in the ultraviolet absorption and re-
sulting dissociation that takes place throughout the HI low-
energy absorption spectrum, which is usually referred to as
the A←X band.
In a preliminary report, we proposed that the structured
long-wavelength tail in the absorption spectrum, as well as a
propensity favoring reaction ͑5͒ over reaction ͑4͒, are conse-
quences of a potential-energy surface ͑PES͒ that is bound
weakly in the H–TeH coordinate.33 This state correlates to
Recent calculations by Alekseyev et al. have increased
our understanding of photodissociation in the HI A←X band
and have guided our understanding of other heavy-atom
systems.2 They calculated the potential curves and transition
dipole moments, including relativistic effects and spin-orbit
coupling, and their results are in agreement with the numer-
ous experimental data that have been acquired with HI.5–14
the spin-orbit excited TeH͑2⌸1/2͒ product. It is similar to the
3
⌸
state in isoelectronic HI, which correlates to I͑2P1/2͒.
0+
The long-wavelength tail is intriguing, and it has been shown
that it is of a relativistic origin,3,33 namely, it is due to a
strong spin-orbit interaction in the TeH product.
1
3
Dissociation on ⌸1 and ⌸1 lead to H+I͑2P3/2͒, whereas
dissociation on 3⌸0+ leads to spin-orbit excited state I͑2P1/2͒,
Prior to the acquisition of the experimental results that
are presented below, only one theoretical study had been
carried out on the low-lying electronic states of H2Te.
Sumathi and Balasubramanian calculated the geometries,
bond energies, and ionization potentials of some of the low-
lying states of H2Te.34 They predicted a H–TeH bond disso-
ciation energy of D0=65 3 kcal/mol ͑22 730 1050 cm−1͒.
This is in good agreement with the experimental value we
determined, 65.0 0.1 kcal/mol ͑22 740 30 cm−1͒, as re-
ported earlier.33 Recently, Alekseyev et al. have calculated, at
a high level of theory, the PES’s of a number of low-lying
electronic states and the corresponding transition dipole mo-
ments connecting the excited states and the ground state
along the H–TeH coordinate.3 They predicted D0
=62 kcal/mol ͑21 640 cm−1͒, which agrees with the experi-
mental results to within the uncertainty of the calculations.
The experimental results presented below include sig-
nificant contributions from secondary photolysis of TeH that
derives from reactions ͑4͒ and ͑5͒. Namely, following pho-
tolysis of H2Te, nascent internally excited TeH can absorb a
photon and dissociate:
1
3
hereafter referred to as I*. Whereas the ⌸1 and ⌸1 states
are repulsive, 3⌸0+ is weakly bound, i.e., by ϳ600 cm−1. The
minimum at ϳ2.7 Å is shifted significantly from the ground-
3
state minimum of 1.6 Å, and calculations suggest that ⌸0+
can support two vibrational levels. At small values of R, the
1
3
absorption spectrum is dominated by the ⌸1←X and ⌸1
←X transitions. At larger R, i.e., beginning around 1.9 Å, the
3
⌸
0+←X transition dominates.
Another model system for studying relativistic effects —
and the subject of the study described herein — is H2Te. It is
isoelectronic with HI, and the lighter group-6 dihydrides
͑H2O, H2S, and H2Se͒ have been studied in varying degrees
of detail, providing bases for comparisons.15–28 Three prod-
uct channels are energetically accessible via the low-energy
absorption band, as indicated in Fig. 1.29
H2Te → H2 + Te,
͑3͒
→H + TeH͑2⌸3/2,␷,J͒,
͑4͒
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084312-3
H2Te ultraviolet photochemistry
J. Chem. Phys. 123, 084312 ͑2005͒
FIG. 2. Schematic of the high-n Rydberg time-of-flight ͑HRTOF͒ experi-
mental arrangement. Polarized photolysis radiations ͑266 and 355 nm͒ and
the radiations used to generate high-n Rydberg H atoms ͑121.6 and 365 nm͒
are indicated. The flight distance is 53.5 cm.
FIG. 3. ͑a͒ Absorption spectrum of H2Te taken with a Varian Cary Series 50
spectrophotometer, resolution Ͻ1.5 nm. ͑b͒ A higher concentration sample
emphasizes the long wavelength structure that corresponds to a vibrational
TeH͑2⌸3/2,1/2,␷,J͒ → H + Te͑3P2,1,0͒,
→H + Te͑1D2͒.
͑6͒
͑7͒
frequency of ϳ330 cm−1
.
photolysis radiation relative to the opening of the valve was
adjusted such that the photolysis radiation intercepted the
early part of the expansion, where clustering is minimal.
Nascent H atoms were probed via HRTOF spectroscopy.
Radiation at 121.6 nm excited the H atom 2p←1s transition,
followed by a radiation at ϳ365 nm that excited a transition
terminating on a high Rydberg level, i.e., n←2p, where n
was typically ϳ50. A weak dc field in the interaction region
served two purposes. It repelled positive ions and it elimi-
nated the ⌬l= 1 angular momentum photoexcitation selec-
tion rule by making space anisotropic for the weakly bound
high-n levels.39 After traveling 53.5 cm, the Rydberg atoms
were field ionized and detected with microchannel plates
͑MCP͒ arranged in a chevron configuration. The resulting
time-of-flight spectra were transformed to center-of-mass
͑c.m.͒ translational energy spectra.
Setzer et al. have calculated the low-lying electronic
states of TeH and the corresponding transition dipole mo-
ments.
͑22 380 cm−1͒,35 while Balasubramanian et al. have pre-
dicted
De=63−69 kcal/mol ͑22 030−24 130 cm−1͒.36
They
have
predicted
De=64 kcal/mol
Analyses of the secondary photolysis products in the results
presented below indicate that D0 for TeH is
63.8 0.4 kcal/mol ͑22 300 150 cm−1͒.
II. EXPERIMENT
The H2Te synthesis and the high-n Rydberg time-of-
flight ͑HRTOF͒ arrangement have been described
previously,33,37 and therefore only a brief summary is given
here. H2Te is sensitive to temperature and light, necessitating
a synthesis immediately prior to the HRTOF experiments.
Samples were prepared by dripping HCl onto a ZnTe powder
and collecting and purifying the resulting gas.38 Care was
taken to ensure that samples were not exposed to light during
the synthesis and HRTOF experiments.
The room-temperature absorption spectrum of H2Te was
obtained by filling a cell with samples and recording the
spectrum with a spectrophotometer ͑Varian Cary Series 50,
resolution Ͻ1.5 nm͒. The cell was 9.5 cm long and it was
fitted with MgF2 windows. Structure in the long-wavelength
tail was measured with a higher concentration by taking the
spectrum immediately following synthesis ͑thus avoiding
sample degradation͒ or by simply filling the cell with more
samples.
For the HRTOF experiments indicated in Fig. 2, H2Te
diluted in an argon carrier was expanded to generate the
molecular beam ͑General Valve, 0.8-mm orifice͒. The exact
percentage of H2Te in the beam is unknown, but by using
different expansion conditions it was determined that contri-
butions to the observed signals from clusters were minimal.
Possible contributions of clusters to the observed signal are
discussed below. Following expansion, the cold beam ͑sev-
eral Kelvin͒ was collimated by a 1-mm-diameter skimmer 1
cm downstream from the nozzle. At the interaction region 5
cm downstream from the skimmer, linearly polarized radia-
tion ͑355 or 266 nm͒ photolyzed the sample. The polariza-
tion was rotated with a ␭/2 plate. The time delay of the
III. RESULTS
A. Absorption spectrum
The H2Te absorption spectrum is shown in Fig. 3. Figure
3͑b͒ emphasizes the structured long-wavelength tail, and the
arrow indicates the energetic limit of the H+TeH͑2⌸1/2
͒
channel. The four peaks in the long-wavelength tail occur at
395.0, 389.0, 384.0, and 380.0 nm. The long-wavelength
structure occurs below the energetic threshold for spin-orbit
excited state TeH͑2⌸1/2͒.
B. Photodissociation
The absorption spectrum of H2Te and the relevant pri-
mary photolysis channels are shown in Fig. 1. HRTOF ex-
periments were performed at two photolysis wavelengths in
the first absorption band, 266 and 355 nm. Representative
c.m. translational energy spectra are shown in Fig. 4. All of
the data have been transformed from time-of-flight to c.m.
translational energy with the assumption that the cofrag-
ments are TeH and H. Accounting for TeH+h␯→Te+H
leads to very minor corrections because of the large Te mass.
Both primary and secondary photolysis events occur. For ex-
ample, the 355 nm spectra exhibit peaks due to the following
processes:
H2Te + ␭ ͑355 nm͒ → H + TeH͑2⌸1/2,3/2,␷,J͒,
͑8͒
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084312-4
Underwood et al.
J. Chem. Phys. 123, 084312 ͑2005͒
FIG. 4. Center-of-mass translational energy distributions obtained with photolysis radiation at ͑a͒ 266 nm and ͑b͒ 355 nm. H2Te is seeded in argon. The
distributions shown in ͑a͒ were obtained by summing 1000 TOF spectra ͑one for each laser firing͒, and then transforming from time to energy. Likewise, the
distributions shown in ͑b͒ were obtained by summing 2000 TOF spectra. Labels indicate some TeH internal states.
the case of the ultraviolet photolysis of H2Se.28 Hydrogen
TeH + ␭ ͑355 nm͒ → H + Te͑3P2,1,0͒,
TeH + ␭ ͑365 nm͒ → H + Te͑3P2,1,0͒.
͑9͒
atoms from the primary channel, TeH͑2⌸3/2͒+H, are de-
tected with maximum selectivity when the 266 nm is hori-
zontally polarized. On the other hand, vertical polarization
results in selectivity toward the detection of the spin-orbit
excited channel TeH͑2⌸1/2͒+H.
͑10͒
Reaction ͑8͒ is a primary photolysis process, while reac-
tions ͑9͒ and ͑10͒ are secondary photolysis processes. The
radiation for reaction ͑10͒ can come from the residual radia-
tion from 121.6 nm generation ͑364.8 nm͒ and/or the Ryd-
berg probe radiation ͑approximately 365 nm͒. The peaks
with a resolved rovibrational structure result from primary
photolysis events. Some of these are labeled with TeH quan-
tum states in Fig. 4. The lumpy peaks are due to secondary
photolysis processes; additional secondary photolysis peaks
underlie the primary photolysis spectra and result in unusual
peak structures. For example, a secondary photolysis peak
lies at ϳ10 500 cm−1 in Fig. 4͑a͒ and overlaps with primary
photolysis peaks.
The production of TeH͑2⌸1/2͒ results from a transition
dipole moment that lies in the plane of H2Te, whereas
TeH͑2⌸3/2͒ results from a transition dipole moment that is
perpendicular to the H2Te plane. The spin-orbit splitting
measured in this study is in agreement with previous
measurements,30 and all of the rovibrational features have
been assigned.
Photolysis with polarized 355 nm radiation yields a sig-
nificantly different result than at shorter wavelengths, as
shown in Fig. 4͑b͒. As was the case with photolysis at 266
nm, the fragments are scattered with a high degree of aniso-
tropy in the laboratory system. In contrast to the case of 266
nm photolysis, however, the excited spin-orbit state
TeH͑2⌸1/2͒ is populated preferentially. Of the H atoms that
reach the detector, ϳ90% are due to TeH͑2⌸1/2͒, corre-
Results from primary photolysis processes will be pre-
sented first. The rationale for the assignment of the remain-
ing features as secondary photolysis events, rather than dis-
sociation of clusters or other events, will follow.
C. Primary photolysis: H2Te\H+TeH
sponding to ϳ65% of the TeH product being formed in the
2
⌸
state.
1/2
Results obtained by using photolysis with polarized 266
nm radiation are shown in Fig. 4͑a͒. Unless otherwise noted,
all of the data presented here have been obtained by expand-
ing H2Te in an argon carrier. The fragments are scattered
with a high degree of anisotropy in the laboratory system.
The major channel is TeH͑2⌸3/2͒+H, which is analogous to
Assignment of the primary photolysis peaks enables D0
for H–TeH to be determined. The experimental value is
65.0 0.1 kcal/mol ͑22 740 30 cm−1͒. This is in good
agreement with theoretical predictions of 65 3 kcal/mol
34
͑22 730 1050 cm−1͒ and 62 kcal/mol ͑21 640 cm−1͒.3
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084312-5
H2Te ultraviolet photochemistry
J. Chem. Phys. 123, 084312 ͑2005͒
FIG. 5. Center-of-mass translational energy distributions for H2Te seeded in ͑a͒ argon and ͑b͒ helium. Each spectrum represents 1000 shots per point, and the
266 nm photolysis radiation is polarized. Note that there is little, if any, discernible change in the lumpy features.
D. Secondary photolysis: TeH\H+Te
H2Te in the samples. No significant change was observed in
the lumpy peaks. Thus, we conclude that clusters play a
minimal role.
Though attempts were made to minimize contributions
from so-called lumpy peaks in the spectra, it was not pos-
sible to eliminate these features, as can be seen in Fig. 4.
They contribute significantly to the observed signals, even
with low photolysis laser energies and under expansion con-
ditions that are the least favorable for cluster formation.
Similar effects have been observed in studies of H2Se
photodissociation.28 Evidence is presented below that indi-
cates that the lumpy features are due to secondary photolysis
of the type indicated in Eqs. ͑9͒ and ͑10͒. They are not, to
any significant extent, due to photodissociation of clustered
species.
It has not been possible to determine the concentration
of H2Te that is seeded in the argon carrier to form the mo-
lecular beam. H2Te is unstable and dissociates even at low
temperatures and without exposure to light. The sample de-
grades over time on the metal surfaces of the foreline and the
pulsed valve, despite the fact that the foreline has been short-
ened as much as possible. Because H2Te decomposition pro-
duces H2, the pressure in the sample reservoir cannot be used
to estimate the H2Te concentration.
Though the H2Te concentration in the expansion is not
known, a number of other factors have aided in the assign-
ment of the lumpy peaks to secondary photolysis. Specifi-
cally, the conclusion that the lumpy features are due to sec-
ondary photolysis is supported by ͑i͒ changing the expansion
conditions to alternately promote and discourage clustering,
͑ii͒ the assignment of each and every lumpy peak in the
HRTOF spectra, and ͑iii͒ the dependence of the lumpy peaks
on laser energy.
Referring to item ͑i͒, the lumpy features should depend
markedly on seeding conditions if they are due to the photo-
dissociation of clusters.40 Figure 5 shows c.m. translational
energy spectra obtained with 266 nm photolysis radiation
using argon and helium alternately as carrier gases. The
spectra were taken under otherwise identical experimental
conditions and with approximately the same percentage of
Referring to item ͑ii͒, Fig. 6 shows assignments of the
lumpy peaks that are present when using 355 nm as the pri-
mary photolysis radiation. Secondary photolysis is consid-
ered for both 355 nm and what we refer to as 365 nm. The
latter is present in the interaction region as either or both of
the separate wavelengths that are used to ͑a͒ generate
Lyman-␣ ͑i.e., 364.8 nm͒ and ͑b͒ promote the resulting ex-
cited H atoms from n=2 to a high-n Rydberg level ͑i.e.,
approximately 365 nm͒. Taken together, we refer to these
radiations as 365 nm.
The vertical grids in Fig. 6 for 355 and 365 nm are
displaced horizontally from one another by the difference in
the photon energies, i.e., 770 cm−1. Entries are included for
355 nm; it is understood that they are the same for 365 nm,
but displaced to lower energy by 770 cm−1. For example, the
peak on the far right is assigned to the 355 nm photodisso-
ciation of TeH͑2⌸1/2,␷=0,J͒, yielding H+Te͑3P2͒, and the
peak to its immediate left is assigned to the 365 nm photo-
dissociation of TeH͑2⌸1/2,␷=0,J͒, yielding H+Te͑3P2͒.
3
3
Note that the atomic tellurium P1 and P0 states are sepa-
rated by only 44 cm−1, so they are not resolved. These states
taken together are designated as P1,0. Thus, the spectrum
3
shown in Fig. 6 is rationalized to within the accuracy of the
measurements on the basis of secondary photolysis.
The above assignments yield a TeH dissociation
energy of D0=63.8 0.4 kcal/mol ͑22 300 cm−1 150 cm−1͒.
This value is in agreement with the two calculations of
De that have been carried out. Namely, Setzer et al.
have predicted De of 64 kcal/mol,35 while Balasubramanian
et al. have predicted De of 63−69 kcal/mol.36 By using the
experimental value of 2054 cm−1 for the energy of the
␷=1 level,41 our D0 value yields De=66.7 0.4 kcal/mol
͑23 330 cm−1 150 cm−1͒.
The assignment of the 355 nm secondary photolysis
peaks was relatively straightforward because a modest num-
ber of TeH states are populated via the primary photolysis
event and limited secondary photolysis channels are acces-
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084312-6
Underwood et al.
J. Chem. Phys. 123, 084312 ͑2005͒
FIG. 6. The spectrum is from Fig. 4͑b͒ ͑vertical polarization͒. The 355 nm photolysis of TeH can yield peaks at the approximate positions of the vertical lines.
2
3
The entry labeled ⌸1/2 , ␷=0→ P2 denotes the Te͑3P2͒+H channel from the 355 nm photodissociation of TeH͑2⌸1/2 ,␷=0,J͒. There are no entries listed for
365 nm. They are the same as for 355 nm, but shifted to lower energy by 770 cm−1
.
sible ͑see Fig. 7͒. On the other hand, at 266 nm, more chan-
nels are energetically accessible for both primary and sec-
ondary processes ͑see Fig. 8͒. Nevertheless, the assignment
of the secondary photolysis peaks that are observed when
as well. This D0 value is the main result of the analyses of
secondary photolysis processes. The nascent population dis-
tributions of the Te states that arise from TeH photodissocia-
tion, while interesting, are not of concern in the present
study.
Finally, referring to item ͑iii͒, we consider how the
lumpy peaks depend on laser energy. Because of sample deg-
radation, it was not possible to obtain good quality data for
peak height versus laser energy. Nevertheless, as discussed
below, the dependencies on photolysis energy shown in Fig.
9 support the assignment presented in item ͑ii͒.
photolyzing at 266 nm was accomplished in a similar manner
as for the 355 nm case above. There were no surprises, and
all of the lumpy features were reconciled by invoking sec-
ondary photolysis. Moreover, it was found that secondary
photolysis was achieved exclusively by 266 nm radiation,
rather than with contributions from 365 nm.
The D0 value of 63.8 0.4 kcal/mol ͑22 330 cm−1
150 cm−1͒ obtained from the 355 data fits the 266 nm data
Translational energy distributions for 355 nm primary
photolysis are shown in Figs. 9͑a͒ and 9͑b͒, which were re-
FIG. 7. Energy-level diagram of possible TeH secondary photolysis chan-
nels when 355 nm is the primary photolysis wavelength. Secondary photoly-
sis of the nascent TeH product is observed for radiation at both 355 and 365
nm ͑i.e., residual radiation from the generation of the 121.6 nm and/or the
Rydberg probe radiation͒. Shading indicates the internal energy range of
products observed in both. The Te energy levels are from Ref. 4. The un-
usual ordering of the 3P states is due to the strong spin-orbit coupling of 3P0
with 1S0 ͑Ref. 35͒. The 1S0 state lies 18 492 cm−1 above 3P0 ͑Ref. 4͒. Note
FIG. 8. Energy-level diagram of TeH secondary photolysis channels when
266 nm is the primary photolysis wavelength. Secondary photolysis of the
TeH product is only observed for the 266 nm radiation. Shading indicates
the internal energy range of products observed in the primary process.
that the 3P0 and 3P1 states are separated by 44 cm−1
.
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084312-7
H2Te ultraviolet photochemistry
J. Chem. Phys. 123, 084312 ͑2005͒
Regarding the ␤ peak, the signal does not increase sig-
nificantly. As discussed above, it is assigned to secondary
photolysis in which TeH͑2⌸1/2,␷=0͒ absorbs a 365 nm pho-
ton. The peak intensities of the unlabeled lumpy features
shown in Figs. 9͑a͒ and 9͑b͒ are also consistent with the
assignments shown in Fig. 6.
A similar analysis has been carried out for 266 nm pri-
mary photolysis; see Figs. 9͑c͒ and 9͑d͒, with respective en-
ergies of 26 and 49 mJ. Primary photolysis is nearly satu-
rated ͑␣d/␣c=1.1͒ because the absorption coefficient is
much larger at 266 nm than at 355 nm. Each of the ratios
␤d/␤c and ␥d/␥c is equal to approximately 1.9, which is
close to the ratio of the photolysis energies ͑49/26=1.88͒.
This is consistent with secondary photolysis in the case
where the primary photolysis process is near saturation.
There is no evidence for 365 nm secondary photolysis in
Figs. 9͑c͒ and 9͑d͒.
IV. DISCUSSION
The experimental H atom translational energy distribu-
tions for the ͑primary͒ photolysis of H2Te and the ͑second-
ary͒ photolysis of its nascent TeH product provide informa-
tion about each of these species as well as the photoinitiated
dynamical processes that they undergo. The fact that the
H2Te molecules have been expansion cooled, combined with
the good HRTOF resolution of the primary photolysis prod-
ucts, yield detailed data concerning the photodissociation dy-
namics of this system. The large range of internal energies of
the nascent TeH prohibits us from obtaining as much detailed
information as in the case of H2Te. Namely, TeH is produced
with broad ranges of internal energies: up to ϳ3900 cm−1 for
355 nm photolysis and up to ϳ23 000 cm−1 for 266 nm pho-
tolysis. Nonetheless, conclusions can also be reached about
the TeH system.
The assignment of the long-wavelength structure in the
H2Te absorption spectrum and a discussion of the photodis-
sociation dynamics at 355 nm are presented first. The photo-
chemistry in this region involves only a few low-lying elec-
tronic states. At shorter wavelengths, more electronic states
are involved. Assignment of the absorption spectrum at the
shorter wavelengths is less clear cut, but analysis of the 266
nm photodissociation results aids in determining the respec-
tive roles of the various electronic states in this region. Fi-
nally, a brief analysis of the secondary photolysis results is
presented.
FIG. 9. Dependence of spectral features on photolysis energy. At 355 nm,
c.m. translational energy distributions ͑100 shots per point͒ are given for
laser energies, ͑a͒ 37 mJ and ͑b͒ 49 mJ. See text for details. The peaks
labeled ␣ are attributed to the H2Te photolysis. Peaks labeled ␤ are due to
the secondary photolysis in which TeH absorbs a 365 nm photon. Peaks
labeled ␥ and ␦ are attributed to the 355 nm secondary photolysis. At 266
nm, c.m. translational energy distributions ͑100 shots per point͒ are given
for laser energies, ͑c͒ 26 mJ and ͑d͒ 49 mJ. Peaks labeled ␣ are attributed to
the H2Te photolysis. Peaks labeled ␤ and ␥ are attributed to the 355 nm
secondary photolysis.
corded with 37 and 49 mJ, respectively. As mentioned above,
because of sample instability, it is not possible to make a
simple comparison of peak heights from one trace to the
next. For example, the ratio of the laser energies ͑i.e.,
49/37=1.3͒ is smaller than the ratio of the ␣ peak heights for
traces 9͑b͒ and 9͑a͒ ͑i.e., ␣b/␣a=1.7͒, even though it is
known that the ␣ peaks are due to one-photon H2Te photoly-
sis. Were the samples stable, the two ratios would be the
same or close.
A. H2Te
Alekseyev et al. have calculated the low-lying PES’s in
the H–TeH coordinate ͑see Fig. 10͒, as well as the corre-
sponding transition dipole moments that connect these PES’s
to the ground state.3 One of the most striking features is the
It is possible, however, to normalize peak heights within
a given trace to facilitate comparisons between traces. Rela-
tive intensities within a trace are accurate because entire TOF
spectra are recorded with each laser firing. Let us take the
ratios of the ␥ and ␦ peaks for Figs. 9͑a͒ and 9͑b͒, i.e., ␥b/␥a
and ␦b/␦a. The ratio of the ␣ peaks, ␣b/␣a, is 1.7, and the ␣
peaks are known to be due to one-photon photolysis. Were
the ratios ␥b/␥a and ␦b/␦a due also to a one-photon process,
they would have values of approximately 1.7. Instead, they
are approximately equal to 2.8, which is close to 1.72. This is
consistent with secondary photolysis.
weakly bound 3A state. Its shape in the H–TeH coordinate is
Ј
3
remarkably similar to that of the ⌸0 curve in isoelectronic
+
HI.2 We proposed previously that there should exist a TeH
state that is analogous to HI͑3⌸ ͒ in the H–TeH
+
0
coordinate.33
The electron density of the highest energy orbital of the
2
⌸
excited spin-orbit state of TeH is smaller near the
1/2
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084312-8
Underwood et al.
J. Chem. Phys. 123, 084312 ͑2005͒
dissociation can yield a broad width. Only if dissociation is
extremely rapid, however, can this mechanism alone recon-
cile the observed widths. This region of the PES has not been
examined at a high level of theory, so this issue remains
open. Referring to Fig. 10, though 3A is crossed near its
Ј
minimum energy by 2A , vibronic coupling between these
Љ
surfaces is symmetry forbidden. Nonadiabatic coupling via
Coriolis interaction can take place, but this will be too weak
to reconcile the observed widths. Moreover, the 355 nm pho-
tolysis results indicate that coupling between 3A and 2A is
Ј
Љ
negligible for rotationally cold molecules.
Referring to item ͑iii͒, it is possible for the 2A PES to
Ј
intersect, or come close to, the 3A PES. This can be deter-
Ј
mined through an exploration of the full configuration space
using a high level of theory. If such an intersection ͑or near-
intersection͒ seam is found, it will play a prominent role in
the dynamics when it lies at energies that are below those of
the photoexcitation.
At 355 nm, ϳ65% of the H+TeH reaction products ac-
crue via the spin-orbit excited H+TeH͑2⌸1/2͒ product
channel.33 Such a propensity has been observed previously,
following the broadband photolysis of H2Te.43 It is due to
FIG. 10. H2Te curves, from Ref. 3. The H2Te bond angle and one TeH bond
length are held at their ground equilibrium values of 90.3° and 1.67 Å.
the participation of the 3A state ͑see Fig. 10͒.
Ј
In the Franck–Condon region, the 3A ←X and 2A
Ј
Ј
2
←X transition dipole moments are modest, and it has been
nucleus compared with that of the
⌸
lower spin-orbit
3/2
2
2
shown that both transitions are of primarily singlet to triplet
state, making the ⌸1/2 state more spherical than the ⌸3/2
state. Recall that the analogous atomic iodine spin-orbit state
2P1/2 is spherically symmetric.42 The significant TeH spin-
orbit splitting of 3815 cm−1 acts to preserve these states in
the large-R region of the H–TeH coordinate. Thus,
TeH͑2⌸1/2͒ is not inclined to form either strongly bonding or
antibonding orbitals with the H atom. As in HI, this results in
character.3 The 1A ←X transition dipole moment is an order
Љ
of magnitude smaller than these, and therefore it is relatively
unimportant and will not be considered further. The observed
selectivity toward photodissociation via the spin-orbit ex-
cited product channel is due to the behavior of the 3A ←X
Ј
transition dipole moment outside the Franck–Condon region.
Namely, it increases significantly with H–TeH distance be-
a shallow van der Waals well on the 3A PES that correlates
Ј
to spin-orbit excited TeH͑2⌸1/2͒. The minimum of such a
state lies at large R relative to the ground-state equilibrium
geometry, i.e., respectively, 2.4 vs 1.67 Å in H2Te, and 2.7 vs
1.6 Å in HI. Calculations along the H–TeH coordinate pre-
cause of the increasingly singlet character of 3A with in-
Ј
3
creasing RH-TeH
.
In the wavelength range of 210–300 nm, a number of
states might contribute to the absorption spectrum, as can be
dict that the 3A surface is bound by ϳ1620 cm−1.3 Prelimi-
seen in Fig. 10. The 4A ←X transition dipole moment domi-
Ј
Љ
nary calculations that include all degrees of freedom indicate
nates in the Franck–Condon region. Though transition dipole
that the binding energy is ϳ1780 cm−1.3
moments connecting the ground state to 3A , 4A , 5A , and
Ј
Ј
Ј
We attribute the long-wavelength structure in the absorp-
tion spectrum shown in Fig. 3͑b͒ to vibrations of H2Te on the
3A are all an order of magnitude smaller, they may play a
Љ
role. Analysis of the 266 nm photolysis results can help to
sort out the possible contributions to the absorption spectrum
in the main part of the absorption band.
Photodissociation with horizontally polarized 266 nm ra-
diation leads mainly to TeH͑2⌸3/2͒. This channel arises from
one or more transition dipole moments that lie perpendicular
3A PES. The separation between the peaks corresponds to a
Ј
frequency of ϳ330 cm−1, and the structured part of the spec-
trum lies at energies that are below the energetic threshold
for TeH͑2⌸1/2͒. The widths of the features can, in principle,
be due to several factors: ͑i͒ occupancy of rotational levels in
the room-temperature sample, ͑ii͒ dissociation on the 3A
PES via a pathway that leads to H2+Te, and ͑iii͒ coupling of
3A to the 2A PES.
to the plane of the molecule. We assign this to 4A ←X, as
Ј
Љ
this is the strongest of such transitions. The fact that
TeH͑2⌸1/2͒ results mainly from one or more transition dipole
moments that lie in the plane of the molecule indicates that
Ј
Ј
Referring to item ͑i͒, the observed features have widths
that are comparable to that of the band of transitions in
which each originates from a different rotational level. An
absorption or photofragment yield spectrum recorded with an
expansion-cooled sample would eliminate this source of
width.
this species does not arise from coupling of 4A to a state
Љ
that correlates to TeH͑2⌸1/2͒. The 3A , 4A , and 3A states
Ј
Ј
Љ
correlate with the spin-orbit excited channel. However, be-
cause the transition dipole moment is in the plane of the
molecule, 3A is ruled out as a major participant. In the
Љ
Referring to item ͑ii͒, the H2+Te channel is energeti-
cally accessible, and the possibility exists that dissociation
Franck–Condon region, 4A ←X has the larger transition di-
Ј
pole moment, but 3A ←X is also possible.
Ј
via this channel occurs on the 3A PES, in which case rapid
The main features of the lowest-energy absorption bands
Ј
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084312-9
H2Te ultraviolet photochemistry
J. Chem. Phys. 123, 084312 ͑2005͒
of H2O, H2S, and H2Te display structure that is superim-
posed on a broad background, with H2Te and H2S having
narrower structures than H2O.16,32 An absorption spectrum of
H2Se at a sufficient resolution to discern such structure has
not been published. Excitation in the main region of the first
absorption band of H2O involves only one excited electronic
coupling between these states is small. The long-wavelength
structure in the absorption spectrum ͑370–400 nm͒ is consis-
tent with a weakly bound well at large R on the PES leading
to spin-orbit excited state TeH͑2⌸1/2͒. The source of this
bound character is relativistic in nature: decreasing the spin-
orbit coupling decreases the bound character.
The HRTOF spectra yield bond dissociation energies.
For H2Te we have obtained D0=65.0 0.1 kcal/mol, in
agreement with theoretical predictions: 65 3 kcal/mol ͑Ref.
34͒ and 62 kcal/mol.3 For HTe, we have obtained D0
=63.8 0.4 kcal/mol. This is in agreement with theoretical
1
˜
state, A B1, and the structure has been assigned to a large-
amplitude symmetric stretching motion on the mainly repul-
18
1
˜
sive A B1 state. Two electronic states are involved in the
lowest-energy H2S absorption band. Excitation is primarily
1
to the bound B1 state, which is coupled via a nonadiabatic
1
predictions
of
De:64 kcal/mol
͑Ref.
35͒
and
interaction to the dissociative A2 state. Symmetric stretch
63–69 kcal/mol.36 Namely, by using the experimental value
of 2054 cm−1 for the energy of the ␷=1 level,41 our D0 value
yields De=66.7 0.4 kcal/mol.
motion on the bound state leads to the observed
structure.18,44
As discussed above, the 4A ←X transition dominates
Љ
An interesting issue is whether weakly bound surfaces
the main part of the low-energy H2Te absorption spectrum.
having minima at large R, such as 3A in H Te, are common
Ј
2
The 4A state is bound in the H–TeH coordinate, and struc-
Љ
in molecules that contain heavy atoms. For example, weakly
bound HI͑3⌸0+͒ can be accessed by photolyzing vibra-
tionally excited HI, and indirect evidence of HI͑3⌸0+͒ has
been observed in intracluster superelastic scattering of H
from I*.46
ture at wavelengths less than ϳ260 nm can be attributed to
3
Te–H stretch. The source of the widths is unclear. The B1
state from which 4A arises is repulsive in the H +Te coor-
Љ
2
dinate when spin-orbit coupling is not included, and calcula-
tions in this coordinate including a spin-obit coupling have
not yet been performed. Numerous other low-lying elec-
It is intriguing to consider the type of vibrational motion
associated with the long-wavelength structure. Calculations
tronic states lie in the region of 4A , and coupling to one or
Љ
on the 3A surface indicate that it can support roughly five
Ј
more of these might contribute to the widths.
vibrational levels.3
The photochemistry of molecules that contain heavy at-
oms is rich. Large numbers of low-lying electronic states and
the breakdown of singlet-triplet selection rules facilitates the
participation of a number of electronic states at modest en-
ergies. Spin-orbit coupling affects shapes of potentials, re-
sulting in unusual photochemistry, and intuition gained from
studying lighter systems may not apply to molecules that
contain heavy atoms.
B. TeH
Broadband photolysis studies of H2Te and TeH have
identified spectral regions in which their ultraviolet absorp-
tions coincide.45 The results presented above indicate that
this is also true at the relatively long wavelengths examined
here. Setzer et al. have calculated the low-lying states of
TeH, the majority of which are repulsive.35 A number of
these might participate at the energies accessed in this study,
e.g., according to the multiple dissociation channels shown
in Figs. 7 and 8. Interestingly, an ⍀=1/2 state that correlates
asymptotically with Te͑2P0͒+H is believed to be weakly
bound with a minimum at ϳ3 Å.35 The significant internal
excitation of the nascent TeH prior to its photolysis prohibits
ACKNOWLEDGMENTS
We thank Tom Flood and Paul Boothe for help with the
H2Te synthesis and Stephen Bradforth for the use of his
spectrophotometer. This research has been supported by the
U.S. Department of Energy, Office of Basic Energy Sciences,
under Grant No. DE-FG03-85ER13363.
2
any determination of the respective roles of the ⌸3/2 and
2
⌸
states in the dissociation dynamics.
1/2
V. CONCLUSIONS
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Excitation of H2Te in its lowest-energy absorption sys-
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Љ
while 3A , 4A , and 5A probably contribute, but to a lesser
Ј
Ј
Ј
4 C. E. Moore, Atomic Energy Levels ͑National Bureau of Standards,
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Ј
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Ј
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355 nm photolysis radiation and the resolved structure in the
long-wavelength tail of the absorption spectrum suggest that
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