The intriguing near-ultraviolet photochemistry of H2Te

Article abstract of DOI:10.1016/S0009-2614(02)01102-8

The ultraviolet absorption spectrum of H₂Te has a long wavelength tail that extends to 400 nm. Photodissociation at 355 nm yields TeH(² _1/2) selectively relative to the ² _3/2 ground state; the transition moments for these channels lie in, and perpendicular to, the molecular plane, respectively. Vibrational structure in the region 380-400 nm is consistent with a shallow well in the adiabat leading to ² _1/2, akin to the one in HI leading to I(²P_1/2). These effects have no counterparts with the light Group 6A dihydrides.

Full text of DOI:10.1016/S0009-2614(02)01102-8

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26 August 2002
Chemical Physics Letters 362 (2002) 483–490
www.elsevier.com/locate/cplett
The intriguing near-ultraviolet photochemistry of H2Te
1
J. Underwood, D. Chastaing , S. Lee, P. Boothe, T.C. Flood, C. Wittig
*
Department of Chemistry, University of Southern California, Los Angeles, CA 90089-0482, USA
Received 17 May 2002; in final form 27 June 2002
Abstract
The ultraviolet absorption spectrum of H2Te has a long wavelength tail that extends to 400 nm. Photodissociation at
2
355 nm yields TeHð P1=2Þ selectively relative to the 2P3=2 ground state; the transition moments for these channels lie in,
and perpendicular to, the molecular plane, respectively. Vibrational structure in the region 380–400 nm is consistent
2
2
with a shallow well in the adiabat leading to P1=2, akin to the one in HI leading to Ið P1=2Þ. These effects have no
counterparts with the light Group 6A dihydrides. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
from molecules that contain light atoms should be
used with caution, if at all, when dealing with
molecules that contain heavy atoms.
Though the H2O molecule has been examined
extensively [2,3], prior to the recent experiments of
Plusquellic et al. [4] who measured the H/D
branching ratio in the 193 nm photodissociation of
HOD, and the corresponding theoretical study of
Effects that are of a relativistic origin, i.e.,
arising from spin-orbit interaction and/or the in-
creased mass that electrons acquire when they
approach heavy nuclei, in general become pro-
gressively more important with increasing atomic
number [1]. In the present context, this progression
occurs upon going from oxygen to tellurium in the
Group 6A hydrides: H2O; H2S; H2Se, and H2Te. A
number of these effects are, or can be, manifest in
studies of ultraviolet absorption and photodisso-
ciation, particularly at long wavelengths, where
regions of curve crossings can be accessed via one-
photon excitation processes. New vistas might
emerge from such studies, and intuitions acquired
€
Schroder et al. [5] relativistic effects in the lowest
energy absorption system were believed to be un-
important. On the basis of these studies, however,
the latter authors concluded that at long photo-
dissociation wavelengths, such as 193 nm, the
lowest triplet level, a3B1, plays a significant role.
For example, they state: ‘‘From all the experiences
gained for this system in the last decade or so it
should be clear that the surprisingly small measured
values for the H/D branching ratio and the H2O/
D2O cross-section ratio cannot be explained if ex-
clusively excitation to the first excited singlet state
is considered.’’ In their work, the a3B1 X1A1
transition dipole moment could not be calculated
* Corresponding author. Fax: +1-213-746-4945.
E-mail address: wittig@usc.edu (C. Wittig).
1
USC Women in Science and Engineering Postdoctoral
Fellow.
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)01102-8

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J. Underwood et al. / Chemical Physics Letters 362 (2002) 483–490
because spin–orbit interaction was not included in
their Hamiltonian. A value was, however, inferred
indirectly, and the transition dipole moment was
found to be 4% as large as that of the strong
A1B1 X1A1 transition. This is surprisingly large
for a singlet–triplet transition in a small, light
molecule. They also suggested that the a3B1 level
might be important in the long wavelength pho-
tolysis of H2S, accounting for the tail that extends
to ꢀ260 nm [6,7].
periment and theory, as has been done with HI
[13–16], which is isoelectronic with H2Te. Conse-
quently, Sumathi and Balasubramanian [17] have
calculated the energies of some product channels, a
number of molecular properties and parameters
for excited electronic states, and portions of their
potential energy surfaces, albeit not the reaction
paths for the different product channels. For ex-
ample, they have predicted a bond dissociation
energy of D0 ¼ 65 Æ 3 kcal/mol. No accurate ex-
perimental value was available at the time of this
prediction. It will be shown below that their pre-
diction proved to be accurate; the experimental
value reported here is 65 Æ 0:1 kcal/mol
(22,740 Æ 30 cmÀ1).
The H2Smolecule has been studied experimen-
tally [6,7] and theoretically [8–10] – not as much as
H2O, but still very thoroughly – and the lowest
energy absorption system is believed to be well
understood [3,8,10]. No serious attention, however,
has been directed toward the long wavelength tail.
The ultraviolet absorption of H2Se has also been
studied [11]. No long wavelength tail has been
noted; probably no one looked for it.
An important feature of this system is the large
TeH spin–orbit splitting of 3815 cmÀ1 [18]. Curve
crossings at large H–TeH distances can differ sig-
nificantly from those of lighter systems, as has
been seen with HI [15]. Because H2Te has addi-
tional degrees of freedom (relative to HI), and
spin-orbit interaction in TeH is smaller (by
roughly a factor of two) than in the I atom,
analogies with HI are valuable but should not be
enlisted casually, as discussed below. Note that
such curve crossings can influence reaction path-
ways much more than with light systems, even
changing qualitatively the photoexcitation and exit
channel dynamics.
There are intriguing features of H2Te that are
not evident with its lighter counterparts. For ex-
ample, in 1931 Goodeve and Stein [12] noted a
long wavelength tail that extends all the way to 400
nm. No explanation of this feature has been given,
or even attempted, in the intervening 70 years. It is
almost certainly responsible for the light sensitivity
of H2Te, because the prominent feature that peaks
ꢀ 250 nm lies well below the pyrex cutoff. Note
that the absorption of a 400 nm photon by a
ground state H2Te molecule corresponds to an
energy that is approximately 2200 cmÀ1 above the
2
TeHð P3=2Þ þ H ground state product channel and
2. Experimental methods and results
2
approximately 1600 cmÀ1 below the TeHð P1=2Þ þ
H product channel. Accessing repulsive potential
curves from the ground vibrational level via a one-
photon process at such small energies in excess of
the large-r asymptotes is unheard of with lighter
molecules. Note also that the H2 þ Te product
channel lies much lower in energy, as discussed
below.
2.1. Sample preparation
Though H2Te is the heaviest of the Group 6A
hydrides that can be synthesized by using
straightforward methods, its preparation and
handling require great care. Reports of its
sensitivities to light, impurities (especially water),
surfaces, etc. are legion [19–22]; indeed, we have
found that it can decompose at 77 K in the
dark, yielding tellurium films and enough H2 to
rupture the glass vacuum line. It is also
extremely toxic [21,22]. These obstacles probably
The long wavelength feature of H2Te tempted
us to consider the possibility that this molecule
might be a source of tunable, fairly monoenergetic
H atoms having modest translational energies. As
discussed below, this in fact turns out to be the
case.
account for the fact that H2Te
– which
The presence of pronounced relativistic effects
in H2Te invites detailed comparisons between ex-
in principle is an attractive prototype for exam-
ining relativistic effects in a theoretically and

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J. Underwood et al. / Chemical Physics Letters 362 (2002) 483–490
485
experimentally tractable molecule – has received
little attention.
lection flask. Once collection was complete, the
collection vessel was sealed and removed from the
apparatus. Yields were variable. The H2Te sam-
ples were transported to the building where the
spectrophotometer traces were obtained and the
photodissociation experiments discussed below
were carried out. Samples decomposed rapidly.
Clearly, H2Te should be used shortly after its
synthesis. It is likely that sample lifetimes can be
increased by improved purification, particularly
the removal of water.
Several methods are available for the prepara-
tion of H2Te [19,20]. The one we chose is to drop
HCl onto ZnTe powder and collect and purify the
resulting H2Te vapor [19]. All chemicals (Alfa
Aesar) were used as received and all glassware was
dried overnight at 150 °C, assembled hot, and
evacuated for approximately one hour before use.
In an N2 glove box, 12 g of ZnTe was crushed
with a mortar and pestle and added to a 100 ml
round-bottom flask equipped with a Kontes joint
sidearm. The flask was sealed and attached to a
series of two U-tubes and a collection vessel,
connected by greased ground-glass ball-and-socket
joints with Teflon stopcocks on either side of each
joint. The apparatus was connected to a vacuum
line with Tygon tubing. A pressure-equalizing
addition funnel was attached to the flask under N2
pressure and the entire setup was evacuated for
approximately 10 min. prior to beginning. Con-
centrated HCl (12.1 M) was bubbled with dry N2
for ꢀ 10 min. and then added to the addition
funnel by using 3 mm dia. Teflon tubing and N2
pressure. Steel cannulae were avoided, as low lev-
els of dissolved metal contaminants inhibited re-
action.
The reaction was performed under a static N2
atmosphere vented to a mercury bubbler. In a
darkened room, portions of HCl (4 equivalents)
were added to the ZnTe with vigorous stirring.
Reaction progress was monitored by the rate of
bubbling of the HCl/ZnTe solution, the progres-
sion of reaction color from red to gray, and the
discoloration of the Teflon stopcocks by tellurium.
The H2Te gas thus produced passed through a
trap ()15 °C dry ice/ethylene glycol cooled U-
tube) before collection ()131 °C n-pentane/LN2
slurry cooled U-tube).
2.2. Ultraviolet absorption spectra
As mentioned above, freshly prepared H2Te
samples were transported without delay to a Cary
Series 50 spectrophotometer, where ultraviolet
absorption spectra were recorded. Fig. 1a shows
the main feature, consisting of a broad back-
ground with pronounced superimposed structure
(peaked ꢀ 250 nm), as well as the long wavelength
tail extending to ꢀ 400 nm. The tail region has
been emphasized in Fig. 1b by using a higher
sample concentration. This revealed – to our sur-
prise – what appears to be a low frequency
(ꢀ 330 cmÀ1) vibrational progression at the end of
the long wavelength tail. Note that the shortest
wavelength member of this progression appears at
energies that are close to that of the TeH2P1=2
excited spin–orbit channel. This was interpreted as
a clue that the 2P1=2 level might play an important
role in the long wavelength photochemistry.
2.3. Photodissociation experiments
The photodissociation of expansion cooled
H2Te was examined by using high-n Rydberg time-
of-flight (HRTOF) spectroscopy. With cold (i.e.,
several K) parent molecules, measurements of H
atom translational energy distributions (and con-
version to center-of-mass (c.m.) translational en-
ergy distributions) yield the corresponding TeH
internal energy distributions. The experimental
arrangement is shown schematically in Fig. 2; it
has been discussed in detail previously and will not
be elaborated here [23]. The sensitivity of the ar-
rangement shown in Fig. 2 is good and signal-to-
noise ratio (S/N) has not been problematic. Fig. 3
After the reaction was complete, the reaction
flask was sealed off and the U-tubes and collection
flask were evacuated. LN2 trapping of the vacuum
line is essential. The collection vessel was cooled in
a LN2 bath and opened to the U-tubes as the
pentane/LN2 bath was removed, allowing H2Te to
diffuse into the vessel free from water and N2.
Yield was improved by back-transferring H2Te
trapped in the vacuum line LN2 trap to the col-

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486
J. Underwood et al. / Chemical Physics Letters 362 (2002) 483–490
Fig. 1. (a) Absorption spectrum of H2Te taken with a Cary Series 50 spectrophotometer. (b) A higher concentration sample em-
phasizes the long wavelength structure.
Fig. 2. Schematic of the HRTOF apparatus indicating the
(polarized) photodissociation radiations (266 and 355 nm) and
the radiations used to create high-n Rydberg H atoms (121.6
and 366 nm). The flight distance is 53.5 cm.
indicates the locations in the absorption spectra
that are accessed by using 266 and 355 nm radia-
tions. In each case, good S/N is achieved, e.g.,
because the small 355 nm absorption cross section
is overcome easily by using high 355 nm fluences.
Fig. 3. H2Te absorption spectrum and some product channel
energies. Arrows indicate the photodissociation wavelengths
The time-of-flight spectra were transformed to the
c.m. translational energy distributions shown in
Fig. 4.
used in this study. The H–TeH dissociation energy is from the
present work, the 2P1=2–2P3=2 splitting is from [18], and the
The distributions shown in Fig. 4 yield a
H—TeH bond dissociation energy of D0 ¼ 22;
H2Te energy is obtained by assuming that D0 for the TeH
diatom is 22; 740 cmÀ1
.