J . Phys. Chem. 1990,94, 1544-1549
1544
are larger than those of the trimers since closed-shellground state
are destroyed in ionizing the tetramers, while the trimer forms
a closed-shell ground state upon ionization.
1
Cu
Aq
Au
Figure 7. Periodic trends in the ionization potentials of CU,,Ag,, and
Au~.
perimental values and thus in Figure 6 every tetramer has a lower
binding energy than the true expected binding energy. As seen
from Figure 6,the total bond energy decreases considerably in
moving from Cu, to Ag, but it increases appreciably in moving
from Ag, to Au,. Once again, the anticipated trend of lower
binding energies for heavier clusters in a periodic group is violated
by Au,. This anomaly is attributed to relativistic stabilization
of the 6s orbital of the gold atom due to mass-velocity contraction
effect .
Figure 7 compares the vertical IPS of Cu,, A&, and Au,. As
noted before, although in absolute terms the IPS should be lower
than the true values by almost an electronvolt, the relative trend
with the true trend' As Seen from
very
Figure the
IP Of
is about
lower than cu4'
This trend is in accord with out anticipation as one goes down
the periodic table. However, for Au,, the IP is 1 eV larger than
for Ag, due to relativistic effects. In general, the IPS of tetramers
73
IV. Conclusions
In this investigation, we studied 14 low-lying electronic states
of Ag, and four electronic states of Cu, using CASSCF/POLCI/MRSDCI methods. The ground states of both Cu, and Ag,
were found to be of 'A, symmetry with equilibrium geometries
of a rhombus. The atomization energies of Cu, and Ag, are
obtained as 210 and 121 kcal/mol by employing the
CASSCF/MRSDCI method. The nature of the low-lying electronic states were analyzed through the coefficients of leading
configurations in the CI, Mulliken populations, the occupancies
of orbitals, and the nature of the highest occupied orbitals. For
both Cu, and A&, we found that the bonding is composed of the
s orbitals and the pz orbitals of the metal atoms. The metal atoms
located on the y axis exhibited enhanced bonding facilitated by
the pz orbitals on their centers resulting in shorter metal-metal
bonds along they axis compared to the z axis. The atoms on the
z axis move apart resulting in the acute-angled rhombus structure.
The relative trends in bond lengths, binding energies, and IPS of
the three tetramers are obtained and discussed. The gold tetramer
exhibits dramatically shorter and stable metal-metal bonds due
to relativistic mass-velocity stabilization of the outer 6s orbital
of the gold atoms.
Acknowledgment. This research was supported by the U.S.
Department of Energy under grant no. DE-FG02-86ER13558.
P.Y.F, thanks the Shanghai Institute of Metallurgyfor granting
a leave which made this joint study possible. We thank the two
reviewers for their invaluable
which improved this
significantly.
Registry No. Cud, 65357-62-2; Ag,, 64475-45-2.
Photoionization and Mass-Selected Photodlssociation of Tellurium Clusters
K. F. Willey, P. Y. Cheng, T. G. Taylor, M. B. Bishop, and M. A. Duncan*
Department of Chemistry, School of Chemical Sciences, University of Georgia, Athens, Georgia 30602
(Received: May 8, 1989; In Final Form: July 10, 1989)
Tellurium clusters in the size range of 2-20 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam
source. Cluster distributions are characterized by direct sampling of cations condensed from the vaporization plasma and
by photoionization of neutral species. Mass-selected clusters are studied with ultraviolet laser photodissociation. Fragmentation
preferentially produces the molecular species Te,, TeIt, Te5+,and TeTt.
Introduction
The chemistry of the group VIB elements is an area of growing
importance because of their utility in diverse applications such
as industrial catalysis, modeling of metalloenzymes, lubricants,
rechargeable batteries, nonlinear optics, and electronic materials.
Because of this broad interest, numerous recent studies have
focused on the synthetic chemistry of these system, with particular
attention given to the diversity of novel solid compounds
Similarities in properties have been noted for these elements.
(1) Berkowitz, J.; Liftshitz, C. J . Chem. Phys. 1968, 48, 4346.
(2) Steudel, R.; Straws, E. M.Adv. Inorg. Chem. Radiochem. 1984, 28,
135.
(3) Cooper, W. C.; Westberry, R. A. Selenium; Zingaro, R. A., Cooper,
W. C., Eds.; Van Nostrand Reinhold: New York, 1974.
(4) Bottcher, P. Angew. Chem., I d . Ed. Engl. 1988, 27, 759.
(5) Kanatzidis, M.G.;Huang, S.-P. J . Am. Chem. SOC.1989, I I I , 760.
However, there is a significant trend toward increasing metallic
character for the heavier group members. The range of structures
exhibited by these elements exceeds those of any other periodic
table group, with sulfur having the largest number of allotropes
of any element. Many of t h a e structures consist of parallel chains
or cyclic
The S8 ring present in both the solid and
the elemental vapor is perhaps the most familiar species in this
regard.' Both ring and chain structures have also been suggested
report, we
for selenium solids and v a p o r ~ . ~ *In
~ ,the
~ * present
~
describe some of the first data available for gas-phase clusters
of tellurium. Unlike sulfur and selenium, tellurium does not form
a molecular solid, but gas-phase tellurium species have not been
(6) (a) Hohl, D.; Jones, R. 0.;
Car, R.; Parrinello, M. Chem. Phys. Lett.
1987,139,540. Springborg, M.;Jones, R. 0.J . Chem. Phys. 1988,88,2652.
(7) Grimly, R. T.; Grindstaff, Q.G.; DeMercurio, T. A.; Forsman, J. A.
J . Phys. Chem. 1982,86, 976.
0022-3654/90/20941544%02.50/0 0 1990 American Chemical Society
Photodissociation of Tellurium Clusters
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1545
Reflection
6rid
Assembly
fragmentat ion
/
<
'
/
-
-
Laser
/
/
Einzei I 1
tens
Cluster Source
I
Oef l e c t i o n
Plates
j
I
I
[
Electron
Multiplier
Tube
c
I
I
1
Acceleration
Plates
Skinner
Figure 1. Reflectron time-of-flight mass spectrometer system used for
photoionization and photodissociation experiments.
studied extensively. By comparison to previous studies of sulfur
and selenium molecules, these experiments provide valuable new
insights into the fascinating properties of these group VIB systems.
Small gas-phase molecules of tellurium, primarily the dimer,
have been studied previously with photoionization and photoelectron ~ p e c t r o s c o p y . ~Laser-induced
~~
fluorescence has been
reported for some of these same species in rare gas matrices.'OJi
Neubert has analyzed molecular species in the vapor above heated
tellurium with electron impact ionization mass spectroscopy,
obtaining appearance potentials for TeN ( N = 2-7).12 In very
recent work, Bowen and co-workers have reported negative ion
mass spectra for clusters in this same size range produced in a
cold cathode Penning discharge ion source? In the present work,
we present the first data for tellurium molecular clusters produced
by laser vaporization in a pulsed-nozzle molecular beam source.
These species are detected with time-of-flight mass spectroscopy
and subjected to mass-selected photodissociation experiments. The
new data compare favorably with earlier experiments in the small
size region, while the extension to larger clusters ( N = 10-20)
provides new information on stable molecular species not recognized previously.
Experimental Section
The pulsed-nozzle cluster source and molecular beam apparatus
used for these experiments have been described in previous reports
from our research group.13 Rod-shaped samples of tellurium were
prepared by melting the solid powder in a test-tube mold under
vacuum. Laser vaporization was accomplished with either an
excimer laser (Lumonics) at 308 nm or a Nd:YAG laser (Quanta
Ray) at 532 nm. The only modification to the instrument described previously is the addition of the reflectron time-of-flight
mass spectrometer. This instrument, which is used to obtain
photoionization mass spectral distributions and mass-selected
photodissociation, is described in some detail below.
The schematic diagram of the reflectron system is shown in
Figure 1. It is located in a separate differentially pumped
(8) (a) Berkowitz, J.; Chupka, W. A. J . Chem. Phys. 1%9,50,4245. (b)
Berkowitz, J. J. Chem. Phys. 1975,62, 4074. (c) Streets, D. G.; Berkowitz,
J. J . Electron Spectrosc. Relat. Phenom. 1976, 9, 269.
(9) Snodgrass, J. T.;Coe, J. V.; McHugh, K. M.;Frcidhoff, C. 8.; Bowen,
K. H. J . Phys. Chem. 1989, 93, 1249.
(10) Ahmed, F.; Nixon, E. R. J. Mol. Spectrosc. 1981, 87, 101.
(11) Bondybey, V. E.; English, J. H. J . Chem. Phys. 1980, 72, 6479.
(12) Neubert, A. High Temp. Sci. 1978, 10, 261.
(13) LaiHing, K.; Wheeler, R. G.; Wilson, W. L.; Duncan, M. A. J. Chem.
Phys. 1987, 87, 3401.
chamber, connected to the source chamber by a 2 mm diameter
skimmer. In photoionization experiments, the collimated molecular beam is intersected with the unfocused output of an excimer
laser operating at either 193 or 157 nm (ArF or F2 mixtures,
respectively) in the ionization region. Ions produced by photoionization of neutral species in the molecular beam are accelerated into the first arm of the flight tube (1.2 m in length) with
a two-stage acceleration plate configuration employing dc voltages.
In "direct ion sampling" experiments, ions formed in the laser
vaporization plasma are expanded to the molecular beam velocity,
drifting into the spectrometer source region with the acceleration
plates grounded. Fast risetime voltages from homemade pulsed
circuits are then applied to extract the ions into the flight tube.
The reflection region of our instrument is constructed like those
described previously.'+'g The angle between the two flight tubes
is 12O, determined by the geometry of the molecular beam machine. The electron multiplier tube detector is located at the end
of the second flight tube (1.0 m long). When all parameters are
optimized, unit resolution is achieved at an e l m value of about
1000.
For photodissociation experiments, mass selection occurs at the
end of the first flight tube. Parallel plates (3 cm spacing) at this
position are biased with a positive voltage (typically 100 V), which
deflects positive ions off the flight tube axis. The risetime of this
pulse, which is about 1 ps in the present system, is the limiting
factor in mass selection capability. At the precise arrival time
of the ion packet to be selected, the deflection voltage pulses to
ground, transmitting that ion packet to the reflection region. In
the reflection region, windows are added to the vacuum system
allowing the introduction of a photodissociation laser. Specifically,
the laser position and reflection voltages are adjusted so that the
laser intersects the ion beam at the turning point of its trajectory.
At this point, the average vertical velocity component of the ion
beam is zero, making it possible to fire the dissociation laser with
the minimum timing uncertainty. Dissociation is accomplished
with a second excimer laser, a Nd:YAG laser, or a Nd:YAGpumped dye laser. Residual parent ions and daughter fragment
ions resulting from photodissociation are reaccelerated and mass
analyzed by their flight time through the second arm of the flight
tube. The lasers and pulsed nozzle for this experiment are synchronized with a digital delay generator (Stanford Research
Systems). Arrival time spectra are recorded with a transient
digitizer system (Transiac Model 2101), triggered so that time
zero is the fragment laser firing. Computer differences are typically accumulated for a sequence of 20 cycles with the fragment
laser on and off for 20 laser shots each per cycle (800 total shots).
The final spectrum presented is the resulting difference (laser
on-laser off), indicating depletion in the parent ion channel and
positive-going daughter fragments.
It should be noted that laser dissociation experiments in reflectron spectrometers have been described
However, this is the first such experiment in which dissociation
occurs in the reflection field at the turning point in the ion trajectory. We have tried experiments with dissociation at other
positions just prior to the reflection field or within the field before
or after the turning point and find that the timing is significantly
more difficult in these configurations. Although the timing is
convenient for dissociation at the turning point, it is important
to consider the possible effects of mass discrimination in this
(14) Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Zh.
Eksp. Teor. Fir. 1973, 64, 82.
(15) Boesl, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J . Phys. Chem.
1982,86, 4857.
(16) Lubman, D. M.; Bell, W. E.; Kronick, M. N. Anal. Chem. 1983,55,
..-..
1 A17
(17) Kuehlwind, H.; Neusser, H. J.; Schlag, E. W. int. J . Mass Spectrom.
ion Phys. 1983, 51, 25.
(18) Kuehlwind, H.; Neusser, H. J.; Schlag, E. W. J . Phys. Chem. 1984,
88, 6104.
(19) Kuejlwind, H.; Neusser, H. J.; Schlag, E. W. J . Phys. Chem. 1985,
89, 5600.
(20) Alexander, M. L.; Levinger, N. E.; Johnson, M. A,; Ray, D.; Lineberger, W. C. J . Chem. Phys. 1988, 88, 6200.
(21) Posey, L. A,; Johnson, M. A. J. Chem. Phys. 1988, 89, 4807.
1546 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
geometry. To do this we have used trajectory calculations on a
variety of parent ion and fragment ion masses, with dissociation
at different points along the trajectory. These calculations show
that the corresponding parent and fragment ions do not follow
exactly the same trajectory down the second flight tube and may
be laterally displaced from each other on the detector. Our present
detector has a small acceptance aperture (1 cm2), making it
difficult to achieve optimum focusing for both parent and fragment
ion beams. Therefore, whereas in an ideal experiment the integrated fragment signal would exactly add up to the parent ion
depletion, this charge conservation is not always evident in our
dissociation data. An increased area detector would eliminate
this problem. In the present experiments, we use a variety of
focusing settings to guarantee that all fragments generated are
in fact detected. Furthermore, we have calibrated the extent of
this mass discrimination by repeating previous photodissociation
experiments on bismuth clusters which were obtained in a linear
tandem time-of-flight system.25 Except for the lack of charge
conservation, fragment branching ratios obtained with the reflectron are within a few percent of those reported previously.
Another effect common to all photodissociation experiments
involves the possible release of kinetic energy into the photofragments. If there is significant kinetic energy release, fragment
ion trajectories may be displaced enough off the parent ion axis
so that they will miss the detector. Significant kinetic energy
release is not expected for unimolecular decay of large molecules.
However, the exact amount of energy release necessary to cause
a fragment ion to miss the detector depends on the mass of the
fragment, the distance to the detector, the size of the detector,
and the angular distribution of the fragment. In our instrument,
even a few tenths of an electronvolt of energy is sufficient to cause
fragment loss if the energy is directed exactly perpendicular to
the ion beam axis. Fragment ejection parallel to the beam axis
may cause broadening in the arrival time spectrum. While we
have made preliminary observations of fragment ion loss from
diatomic and triatomic metal parent ions, we have not measured
any noticeable broadening in arrival time spectra.
One of the common applications of reflectron instruments is
in the study of metastable ion d e ~ a y . ' ~ These
~ ' ~ ~experiments
'~
use the slow overall time scale for ion drift in the first flight tube,
deceleration, and reacceleration to probe microsecond metastable
lifetimes. The same experimental configuration described here
without the fragmentation laser can be used to detect unimolecularly produced, or metastable, fragment ions resulting from
the laser photoionization process.
Results and Discussion
Mass Spectral Distributions. Mass spectral distributions for
tellurium clusters produced by laser vaporization and detected
under a variety of conditions are presented in Figure 2. The upper
spectrum shows the distribution obtained by direct sampling of
the cations produced in the laser vaporization process, while the
lower two spectra represent distributions obtained with laser
photoionization of neutrals at different laser energies. It is
well-known that mass spectra such as these must be interpreted
with caution because of the uncertain roles of growth kinetics,
ionization dynamics, and fragmentation on the distributions observed.I3 However, some preliminary comments can be made
based on these data alone. Additional conclusions are possible
when these distributions are compared to the results of massselected photodissociation experiments described below.
In the upper spectrum of Figure 2 obtained by direct ion
sampling, relative intensity maxima are observed for the cluster
cations Te,+, Te5+,and Te7+. Spectrometer focusing effects in
our instrument are more severe for ion sampling because of the
reduced acceleration voltages available from our homemade
high-voltage pulsers (1500 V as opposed to 3100 V for dc acceleration in photoionization experiments). Therefore, the relative
maximum at Te2+is not as evident in Figure 2 as it is under other
focusing conditions. However, our best estimate is that the dimer
cation is present in at least the same density as the five- and
seven-atom cations. As usual, abundances in this spectrum could
Willey et al.
~
CLUSTER
+
ex
SIZE
Figure 2. Mass spectra obtained for tellurium under various conditions.
The upper spectrum is the result of direct sampling of jet-cooled cations
produced within the pulsed nozzle source by the condensation of the laser
vaporized tellurium plasma. The middle spectrum is the result of multiphoton ionization of neutral tellurium clusters at 193 nm under moderately high laser power conditions (approximately 5 mJ/cm2). The
lower spectrum represents single-photon ionization of neutral tellurium
clusters at 157 nm (0.1 mJ/cm2).
be caused by either the thermodynamic stability of the clusters
formed or the kinetics and dynamics of condensation.
The 193-nm photoionization data are accumulated at a relatively high laser power (approximately 5 mJ/cm2). As shown in
the middle spectrum of Figure 2, relative maxima are observed
for N = 2, 5,8,12, and 15. Prior experience with metal cluster
systems suggests that these conditions cause multiphoton ionization
(MPI) and consequent fragmentation.') Abundance profiles under
these conditions usually represent a complex superposition of
intensities resulting from these processes and direct ionization of
low ionization potential (IP) species. Averaged over the entire
cluster distribution, multiphoton fragmentation should enhance
the mass channels corresponding to stable cations, which are
produced preferentially in the destruction of larger species and
are themselves more resistant to further fragmentation. It is
therefore interesting to compare the abundant species produced
under MPI conditions with the cation clusters produced directly
by laser vaporization. Both spectra have abundant features at
N = 2 and 5. The occurrence of these features in both spectra
suggests that Tq+ and Te5+should be regarded as relatively stable
cations. Interestingly, electron impact ionization studies described
by Neubert also have relative maxima for these two species.12 It
is likely that electron impact also produces more intense peaks
for stable cations, subject to the exact conditions used. Further
tests of this hypothesis are provided in the photodissociation experiments described below.
Additional evidence for multiphoton absorption and fragmentation at the 193-nm wavelength is provided in Figure 3. At this
higher laser power (10 mJ/cm2), metastable fragment peaks are
observed at apparent nonintegral mass values. These peaks result
from highly excited ions produced in the initial ionization process
which dissociate after the extraction/acceleration process somewhere within the first arm of the flight tube. Deceleration and
reacceleration in the reflection region make it possible to detect
these fragments, which necessarily are produced on a microsecond
time scale.
Photodissociation of Tellurium Clusters
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1547
Te
I
:
:
:
!
:
:
:
;
:
:
!
7
'
Te
CLUSTER S I Z E
+
!
+
8
51
W
Figure 3. Mass spectrum of neutral tellurium clusters multiphoton-ionized at high laser power at 193 nm. The primary mass channels are
offscale in this figure. Intermediate mass peaks are the result of metastable ion dissociation during the transit through the time-of-flight
spectrometer.
i
Te
Te
3
+
9
T e+
4
Te
t/
t-
+
5
91
W
C
I
Te
1
6
IU
3
CLUSTER
SIZE
Figure 5. Photodissociation spectra for mass-selected TeN !N = 7-10)
at 308 nm (1 mJ/cm2). The dominant daughter fragments differ by four
cluster units from the respective parent ion in each spectrum.
wavelength ( N < 5) have ionization potentials greater than the
photon energy (7.89 eV) and that the clusters detected ( N > 5)
have ionization potential (IP) values less than this energy. This
determination is completely consistent with the known atomic IP
(9.009 eV) and the appearance potential (AP) measurements of
Berkowitz and Chupka* and those of Neubert,Iz who found AP
= 8.29, 9.3, 9.5, 7.4, and 7.2 eV for N = 2-6, respectively.
Mass-Selected Photodissociation. Mass-selected photodissociation experiments on clusters produced in pulsed molecular
beams have been described previously for metal, semiconductor,
and nonmetal (carbon) system^.^^-^^ Photodissociation experiments have been compared to the results of collision induced
dissociation.2629 These experiments have investigated the
5
SIZE
Figure 4. Photodissociation spectra for mass-selected Te, (N= 3-6) at
308 nm (1 mJ/cm2). Intensities are dependent on the spectrometer
focusing used and are believed to be accurate to 20-30% between different spectra. The dimer cation is the dominant daughter fragment in
each spectrum.
CLUSTER
I
In photoionization at 157 nm (Figure 2; lower spectrum), the
laser fluence is low enough (0.1 mJ/cm2) so that MPI processes
are expected to be relatively inefficient.13 Photoionization under
these conditions has been described previously for metal cluster
systems.13 Exact power-dependent studies at this wavelength are
difficult, but attenuation of the laser causes no significant change
in the relative peak intensities observed. It is therefore safe to
conclude that the clusters and atomic species not ionized at this
(22) Geusic, M. E.; Jarrold, M. F.; McIlrath, T. J.; Freeman, R. R.;
Brown. W. L. J. Chem. Phvs. 1987.86. 3862.
(23) Brucat, P. J.; Zheng, L. S.; Tittel, F. K.; Curl, R. F.; Smalley, R. E.
J . Chem. Phys. 1986, 84, 3078.
(24) Liu, Y.;Zhang, Q.L.; Tittel, F. K.; Curl, R. F.;Smalley, R. E. J.
Chem. Phys. 1986,85, 7434.
(25) Geusic, M. E.; Freeman, R. R.; Duncan, M. A. J. Chem. Phys. 1988,
88, 163.
(26) (a) Hanley, L.; Anderson, S . L. Chem. Phys. Left. 1986, 129, 429.
(b) Hanley, L.; Anderson, S.L. J . Chem. Phys. 1987,87, 260. (c) Ruatta,
S. A.; Hanley. L.;Anderson, S.L. Chem. Phys. Len. 1987, 137, 5.
(27) (a) Jarrold, M. F.; Bower, J. E.; Kraus, J. S . J. Chem. Phys. 1987,
86, 3876. (b) Jarrold, M. F.; Bower, J. E. J. Chem. Phys. 1987, 87, 1610.
(c) Jarrold, M. F.; Bower, J. E. J. Phys. Chem. 1988, 92, 5702.
(28) (a) Loh, S. K.; Lian, L.; Hales, D. A,; Armentrout, P. B. J. Chem.
Phys. 1988, 89,610. (b) Loh, S. K.; Lian, L.; Hales, D. A,; Armentrout, P.
B. J. Phys. Chem. 1988, 92,4009.
1548 The Journal of Physical Chemistry, Vol. 94, No. 4, 1990
,
1,
I
> I
4
To
I
Te
Te
;2
+
15
+
:E
w
1
5
10
CLUSTER S I Z E
15
Figure 6. Photodissociation spectra for larger tellurium clusters at 308
nm (1 mJ/cm*). Multichannel dissociations are observed, with Te2+,
Te5+,and Te7+occurring frequently as more abundant photofragments.
mechanism and energetics of cluster decomposition. Single atom
evaporation and molecular fission type processes have been described for different cluster systems. In general, the dissociation
data to date seems to be consistent with a mechanism of statistical
unimolecular decay. Product channels are therefore expected to
be thermodynamically, rather than dynamically, determined.
However, the exact mechanism for any new cluster system must
be determined experimentally.
Figures 4-6 present mass-selected dissociation spectra for
various sizes of tellurium cluster cations. The precursor ions for
these experiments are produced by neutral cluster photoionization
at 193 nm. This method of ion production was chosen instead
of direct ion sampling because the resulting signal levels are
significantly greater, reducing the amount of signal averaging
required. One disadvantage of this method, however, is the uncertain internal energy content of the ions prior to photoexcitation.
MPI produced ions are likely to be internally "hot", as evidenced
by the metastable decay described above, while ions produced in
the laser vaporization source are supersonically cooled as they exit
the source. However, in the experiments described, the ionization
laser power is reduced so that no metastable dissociation is detected. The difference method used for data acquisition also
ensures that the daughter ions detected are produced by the
fragmentation laser. In previously reported studies on silicon
clusters, the fragment branching ratios were independent of the
method wed for ion production.22 The photodissociation laser
used for these experiments is a XeCl excimer laser (308 nm). The
(29) Ross, M. M.; McElvaney, S. W. J . Chem. Phys. 1988, 89, 4821.
Willey et ai.
laser power was held at a low level (1 mJ/cm2) to minimize
multiphoton dissociation or reionization of neutral photofragments.
Figure 4 presents the fragmentation spectra obtained for small
tellurium cations ( N = 3-6). In each spectrum, the indicated
parent ion depletion is plotted as a negative-going peak, while the
daughter fragments are plotted in the positive direction. As usual
in these dissociation experiments, only charged fragments are
detected. Therefore, it is not possible to determine whether the
neutral species are ejected as atoms or as molecules. However,
when possible, molecular fragments would most likely provide the
lowest energy dissociation pathway. The dominant fragment in
each of the spectra shown is the dimer cation, Te2+. The common
occurrence of the dimer cation as a photofragment here, coupled
with its strong abundance in both cation and MPI mass spectra,
provides strong evidence that Te2+is indeed an especially stable
molecule.
A slightly different trend emerges as larger clusters are considered. Fragmentation spectra for intermediate-sized Te clusters
(N = 7-10) are presented in Figure 5. In each of the spectra for
N = 7,8, and 9 there is a single dominant product channel, namely
Te3+,Ted+,and Te5+,respectively. In each case, the product cation
produced has an N - 4 relationship with the respective parent ion.
This same relationship is in fact also observed for the Te6' dissociation shown in Figure 4. Dissociation of the ten-atom parent
ion produces a range of smaller products, but the most pronounced
is the six-atom daughter, also resulting from an N - 4 process.
The Occurrence of the N - 4 channel in so many dissociation events
argues strongly for a stable tetramer molecule as the common
neutral fragment.
In the larger cluster size range, another general kind of fragmentation behavior is observed. Beginning with the 10-atom
species and extending to the 18-atom species, a range of daughter
ions, rather than one dominant channel, is produced. Cations
larger than 18 atoms are not produced in great enough density
for mass-selected fragmentation. For Te12+,the two- and five-atom
channels are slightly more abundant than the others. For Teli+,
these same two channels are again prominent. For both Te16+
and Te,8+the five- and seven-atom channels are most abundant.
Throughout these data, therefore, the cations Te2+,Te5+,and Tq+
occur repeatedly as abundant daughter ions. These same species
are also prominent in cation and MPI mass spectra. Since they
are produced preferentially in both condensation and dissociation,
the conclusion is that these species are especially stable cations.
In general, then, these experiments indicate that tellurium
clusters fragment predominantly by fission, producing certain
molecular ion and neutral products. This behavior has also been
observed for other main group clusters (C, Si, Ge, Sb, Bi)22-25
but is in contrast to the single atom evaporation behavior observed
for transition metals (Fe, Nb).23
Two- and five-atom clusters were noted in previous equilibrium
mass spectroscopy studies of tellurium clusters.12 However, these
mass channels were interpreted as arising from neutral tellurium
molecules, rather than the cations suggested here. Specifically,
both second law and third law thermochemical measurements are
in agreement that Te, is more stable than Te4.I2 The seven-atom
cation and four-atom neutral molecules suggested by our study
were not observed in this previous work. It is difficult to reconcile
our photofragmentation results with these previous data. In our
work, three different measurements expected to produce stable
cation clusters favor the two-, five-, and seven-atom species. We
have no evidence for stable neutrals at these cluster sizes. There
are less data supporting Ted as a stable neutral, but it is apparently
formed as the primary fragment from 5 out of the 12 parent ions
studied. Admittedly, photofragmentation does not measure cluster
stability directly. However, barring unusual dynamical effects,
repeated observations of a common fragment molecule are compelling circumstantial evidence for its stability. In bismuth and
antimony clusters, mass-selected photofragmentation and equilibrium thermochemical measurements produce the same stable
molecules.25 It is conceivable that the two- and five-atom mass
channels assigned to neutrals in the tellurium thermochemical
experiments were in fact contaminated by fragmentation of larger
Photodissociation of Tellurium Clusters
clusters. It is also possible that the ionization cross section, which
is assumed to be constant in these experiments, is temperature
dependent. These are both well-known complications in these kinds
of experiments. Third law determinations rely on assumed
structures (in this case rings rather than chains were used) in the
calculation of partition functions. Unfortunately, the sensitivity
of the resulting thermochemical data to structural assumptions
was not reported. It is also conceivable that the N - 4 channel
observed in our experiments actually results from the sequential
loss of two neutral dimers. However, without additional information, it is impossible to determine the exact source of the
apparent discrepancies between the results of these two different
kinds of experiments.
It is worth noting that four-, six-, and eight-atom ring structures
are known or suspected for both sulfur and selenium. Previous
oven-based experiments did not produce tellurium clusters in this
size range. However, even though our photodissociation data do
extend to a large enough size, there is no evidence for the involvement of stable six- or eight-atom species. The MPI mass
spectral data in Figure 2 show a slight discontinuity a t N = 8,
and the 157-nm data show a relative maxima at N = 6, but there
is no other corroborating evidence to suggest these species as stable
molecules.
Having identified new tellurium molecules not previously expected to be stable, it is interesting to speculate on their chemical
bonding and structures. According to the conventional inorganic
chemistry reasoning, tellurium chemistry should involve primarily
the valence p electrons, with an "inert pair" of valence s e1ectromN
The relativistic contraction of the s orbital is partially responsible
for its inert ~ h a r a c t e r . ~ ~By* ~use
* of three orthogonal p orbitals
and four valence p electrons on each atom, a reasonable bonding
scheme and structure can be constructed for neutral Te,. This
scheme involves a square planar structure, with one lone pair of
p electrons on each atom in the out-of-plane orbital, and four
two-electron a bonds connecting the atoms. This arrangement
preserves the 90' angles between p orbitals and provides the least
possible steric overlap of the lone-pair electrons on adjacent atoms.
Two-coordinate Te bonding similar to this, with two lone pairs
on each atom, gives rise to the most stable modification of solid
tellurium. However, in the solid the zigzag chain structure is
(30) Cotton, F. A.; Wilkinson, G.Advanced Inorgunic Chemistry, 4th 4.;
Wiley: New York, 1980.
(31) Pitzer, K. S.Acc. Chem. Res. 1979, 12, 271.
(32) Pyykko, P.; Desclaux, J.-P.Acc. Chem. Res. 1979, ZZ, 276.
The Journal of Physical Chemistry, Vol, 94, No. 4, 1990 1549
referr red.^ Indeed, local spin density calculations on the corresponding Se, species also predict a planar zigzag chain, finding
the strained ring structure to be less favorablea6 Nevertheless,
it is difficult to explain the apparent stability of chain-Te4 relative
to other similar chains, and so the square-planar structure seems
to be more consistent with our data. While this scheme is plausible, it certainly needs to be tested against other possible structures
with rigorous quantum chemistry methods. Relativistic effective
core potential methods have been applied to tellurium dimer and
may be applicable in this regard.33 Unfortunately, no simple
bonding picture is readily evident for the two-, five-, and sevenatom cations, which are necessarily odd-electron species. Theoretical treatments would be especially valuable for both cation
and neutral tellurium species as an additional means of resolving
the experimental discrepancies described above.
Conclusion
Mass spectra and mass-selected photodissociation spectra are
presented for tellurium clusters produced by pulsed-nozzle laser
vaporization. The combined data suggests the existence of stable
two-, five-, and seven-atom cation molecules and neutral tetramer
molecules, which are produced preferentially in cluster condensation and dissociation. Some similarities to previously observed
sulfur and selenium gaseous molecules are noted, but there are
significant deviations from previously reported tellurium cluster
behavior. Theoretical studies on tellurium clusters would be
valuable for understanding the similarities and differences between
these group VIB systems. Such studies would also be useful for
understanding the unexpected stability of odd-electron molecules
such as Tez+, Te5+, and Te7+ suggested by our data.
Acknowledgment. Acknowledgment is made to the donors of
the Petroleum Research Fund, administered by the American
Chemical Society, for partial support of this research. The
Nd:YAG laser system used for this research was provided by the
Defense Department University Research Instrumentation Program.
Registry No. Te, 13494-80-9; Tezt, 12597-47-6; Te3+, 73146-28-8;
Ted, 12597-49-8;Tedt, 103218-40-2; Test, 103218-41-3; Te6', 12415313-5; Te,+, 124153-14-6;Teet, 124153-15-7; Te9+, 124153-16-8; Telot,
124153-1 7-9; TeI2', 124153-18-0; Te13', 1241 53-19- 1; Te16+,1241 5320-4; Telst, 124153-21-5.
(33) Balasubramanian,K.; Ravimohan, C. J . Mol. Spectrosc. 1987, 126,
220.