Lead metal was vaporized and trapped in solid xenon at 12K. Electronic absorption and resonance Raman spectra were recorded of the resulting matrix, which was shown to contain Pb2, Pb3, and possibly Pb4 molecular species. The vibrational frequency for Pb2 is determined to be 108/cm for the ground state, with a dissociation energy of 82000/cm. Ad3h symmetry is indicated for the Pb3 species, with nu sub 1=117/cm and nu sub 2 = 96 /cm. The existence of Pb4 is suggested by a fundamental and overtone of 111/cm spacing

Khanna, R. K.

Stranz, D. D.

English

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Absorption and Resonance Raman Spectra of Pb 2 , Pb3 , and Pb4 in Xenon Matrices
David D. Stranz and Raj K. Khanna
Department of Chemistry
University of Maryland
^v
	
College Park, MD 20742
r
Abstract
1
Lead metal was vaporized and trapped in solid xenon at 12K. Electronic
absorption and resonance Raman spectra were recorded of the resulting
matrix, which was shown to contain Pb ? , Pbl , and possibly Pb 4
 molecular
species. The vibrational frequency for Pb 2 is determined to be 108 cm-1
for the ground state, with a dissociation energy of -18200 cm-1 . A 03h
symmetry is indicated for the Pb3
 species, with v l=117 cm-1 and v2= 96 cm-1.
The existence of Pb 4
 is suggested by a fundamental and overtone of 111 cm-1
spacing.
Introduction
The development of matrix isolation techniques has promoted an interest
in recent years in the spectroscopy of the vapors above high temperature
solids, and in particular the study of simple species which may be trapped
or grown through diffusion in low temperature inert solids. Recently, the
powerful matrix isolation method has been applied to some of the heavier
Group IV diatomic species. Teichman, et. al. [1] and Epting, et. al. [2]
have reported detailed results of absorption, fluorescence, and resonance
Raman spectra of Sn 2
 in argon matrices.
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Brewer and Chang ;;31. reported the first electronic absorption spectra
of Pb and Pb2
 in various matrices, which yielded values for the vibrational
frequency of the excited AO u state of the dimer close to the 159.2 cm-1
value first determined by Shawhan [41 in gas phase studies using a tube
furnace. Teichman and Nixon observed absorption and emission of Pb 2 in
rare gas matrices [5], From analysis of the laser-excited emission of the
A-X transition, they proposed that the ground state is actually two
near-degenerate low-lying states separated by about 110 cm -1 , each with a
vibrational frequency of approximately 220 cm -1 . The vibrational frequency
is in agreement with that reported by Weniger [6] in the analysis of the
E-X transition in gaseous Pb 2 . Finally, in the most recent work, Bondybey
and English [7] have reinvestigated the emission and excitation spectra of
Pb 2 in argon matrices. They note at least five new electronic states lower
in energy than the previous A state, and have proposed a re-labeling of the
states. The A state is now designated FOB, a notation adopted in this
paper. Analysis of the emission spectra, and reconsideration of the
Deslandres table presented by Shawhan [41 shows a single ground state for
Pb2 with a vibrational frequency of 112 cm -1 in solid argon. To date,
there have been no reported Raman spectra of molecular lead species, (as
recently noted [21), and no spectra of any type for Pb aggregates higher
than the diatomic molecule.
In our laboratory, we have initiated an experimental program utilizing
matrix isolation techniques for the study of free radical and high
temperature molecules, As a first effort to explore the problems of
obtaining Raman spectra of matrix isolated species, it was decided to
investigate the spectra of lead molecules. In particular, since the F-X
transition of Pb 2 in xenon [3] is.resonant with several of the Ar ion laser
emissions, we were interested in the possibilities of obtaining resonance
Raman spectra of this species. We report the successful results of our
efforts to record this spectrum, as well ar, data which suggests the
isolation of higher order Pb aggregates.
1
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Experimental Technique
The low temperature apparatus used in this work consisted of a CTi
Model 21 cryocooler operated at 12K, with a spectroscopic sample chamber
equipped with quartz windows. Samples were deposited on a sapphire (Al203)
window substrate mounted to the cryocooler cold station. Temperature of
the substrate was monitored t,^ a 0.07 at. % Fe-doped gold vs. chromel
thermocouple sandwiched betwecin the sapphire plate and the sample mount.
Lead.(Baker Reagent grade, granular) was vaporized from a small resistively
heated stainless steel oven with a 1 mm orifice approximately 15 cm from
the cold substrate. A stream of xenon gas (Airco) was directed against the
plate at a 45 0
 angle to the lead beam. In order to prevent fogging of the
matrix, it was necessary to maintain the temperature of the si lhstrate at
about 30K during the deposition process, which took place over a 4-6 hour
period. Spectra were recorded at the base temperature of 12-15K, except
where noted. Absorption spectra were recorded using a Cary Model 14
spectrophotometer. A custom Raman system, consisting of a Spex 1401 double
monochromator with photon counting electronics was used for the Raman work.
A Coherent Model 52 Ar ion laser, with a power estimated at 100-200 mW at
the sample on the 488 nm line was used to excite the spectra. Resolution
was typically 4 cm-1.
Results
A number of experiments were conducted to determine conditions for
obtaining optimum isolation and quality of spectra. Of these, five
experiments provided excellent results, and are presented here in the
figures and discussion. Tracings of the visible/near-ultraviolet
absorption spectra are shown in Figure 1, with data derived from these
given in Table 1. Spectra were recorded at approximately SK intervals from	
I
45K to 15K to determine frequency shifts of the absorptions with
- 3 -
II
temperature, in the hope that the maxima could be "tuned" by temperature
variation in or out of exact resonance with the laser emissions. However,
no significant shifts were noted over this range, the major change being a
strong increase in the opacity (or scattering) of the matrix as the
temperature was lowered. This limited the short wavelength accessibility
to above 350 nm. The strongest absorptions were those attributed to Pb 2 by
earlier inve t^tigators, and two weaker absorptions in the blue were also
seen. No structured absorptions which could be assigned to species other
than the dimer were recorded.
Raman spectra were recorded using the available Ar ion laser excitation
wavelengths 514.5 nm, 501.7 nm, 496.5 nm, 488.0 nm, and 476.5 nm. Of
these, the most powerful is the blue-green 488.0 nm line. The positions of
these emissions relative to the Pb 2 F-X absorption are shown in Figure I.
Strong signals were observed using all excitation frequencies. The Raman
spectra for496.5 nm, 488.0 nm and 476.5 nm excitations, shown in Figure
2, are strongly modulated by self-absorption of the scattered light. All
of the excitation lines result in spectra exhibiting one or more
progressions, the longest and most pronounced arising from excitation using
the 496.5 nm laser wavelength, which from Figure 1, can be seen to be
exactly resonant with an absorption maximum in the structured band near 500
nm. The raw Raman data for all laser excitations are presented in Table 2.
Discussion
A. Pb2
The absorption spectra presented in Figure 1 are very similar to the
spectra of lead vapor trapped in solid xenon published by Brewer and Chang
[3]. The structured band near 500 nm, in the notation proposed by Bondybey
and English [7], has been assigned to the F-X transition in diatomic lead.
Our calculated vibrational parameters for the F0+ state, from the data in
Table 1, yield we 172 cm- 1 and wexe 1.8 cm	 in  good agreement with the
IFA
- 4 -
Gvalues reported by previous investigators for both gas and matrix studies
[3-7]. The unstructured band in the red, extending from 15600-16300 cm-1,
has been assigned by Bondybey and English to the CO+-X transition in Pb2.
Spectra published by Teichman and Nixon [5] show this band with a
well-defined vibrational structure. These researchers also reported two
weak bands in the blue to ultraviolet near 23000 cm-1 and 34000 cm"19
respectively, which they tentatively attributed to Pb 2 . Although we were
unable to obtain spectra below 350 nm, our spectra show two weak and
unstructured bands near 22000 cm-1 and 24000 cm-1 . The Raman data
presented below suggest that the 22000 cm -1 band in particular may be due
to 1b 3 instead of the dimer. As mentioned above, no appreciable frequency
shifts were noted when the matrix temperature was varied.
The raw Raman data presented in Table 2 indicate 'the presence of at
least two distinct molecular species in the matrix. It can be seen that,
as the excitation wavelength is changed from the green to the blue, one
strong progression is replaced by another of different spacing. The low
fundamental frequencies of these progressions suggest that they are due to
molecular lead species. The data of Table 2 are reorganized and presented
in Table 3 to shown the progressions observed. The progression of
approximately 107 cm-1
 spacing is undoubtedly due to diatomic lead, since
the resonance of the 496.5 nm exeitation line with a maximum in the F-X
absorption spectrum induces a progression in this spacing with some 10
overtones. In addition, excitation at 476.5 nm, above the absorption band,
produces no scattering at these frequencies. A least squares fit of the
vibrational spacings gives ground state molecular parameters ``e 108 cm-
and wexe= 0.4 cm-1 . The estimated error in frequencies of 2 cm"
1
 for the
Raman data makes the anharmonicity value unreliable, however. This
confirms the calculated ground state vibrational frequency for Pb 2 in solid
argon of 112 cm -1
 deduced by Bondybey and English from their emission
spectra. From a rough calculation, the dissociation energy is estimated at
8200 cm-1 , consistent with a value of 8000 cm -1
 estimated from mass
spectral data [8].
- 5 -
^i.
}
S. Pb3
Excitation at shorter wavelengths brings another progression into
prominence with spacing of approximately 117 cm
-1 . A single feature at 117
cm-1
 first appears using 496.5 rim excitation, and becomes a progression
with five overtones with the 488.0 rim line. The much weaker 476.5 rim laser
emission still produces the same progression with two overtones; however,
no scattering at these frequencies is observed with either of the two green
laser frequencies. This implies a second species in the matrix, which most
probably is the triatomic Pb 3 . If a 0 3h geometry is assumed for the
trimer, two vibrational modes of Ai and E" symmetry, both Raman active, are
allowed. When the dimer stretching force constant, f r 0.71 mdyne/9, is
assumed for the trimer, with an interaction constant f rr= -0.1 fr , the
frequencies of these two modes are calculated as 118 cm
-1 and 98 cm-1,
respectively. Therefore, it is reasonable to assign the 117 cm-1
progression to the totally symmetric stretching of the 0 3h trimer. A least
squares fit of these frequencies to a line gives We= 117 cm-1 and wexe= 0.2
cm-1 . Closer inspection of the Raman spectra reveals a feature at 191 cm-1
with 488.0 rim and 476.5 rim excitation of an intensity proportional to the
117 crr-,-1
 feature in both. This may be assigned to the first overtone of
the degenerate stretch, giving the fundamental frequency for this; mode as
approximately 96 cm-1 . Recalculation of the vibrational parameters using
these frequencies gives f r= 0.68 and frr= -0.06, in units of mdyne/R•
An argument may be advanced for the choice of D U symmetry over the two
other choices, C 2v and linear. For the bent model, three frequencies are
expected, corresponding to a symmetric, and asymmetric stretch and a
symmetric bend. Only the symmetric modes are expected to show resonance
effects, and the bending mode should be much lower in frequency than the
stretches. The observation on two resonance enhanced modes is inconsistent
with this interpretation. A similar argument holds for the linear model.
In it, only the symmetric stretch is Raman active. For the D3h model, both
modes are close in frequency as the calculation indicates, and the overtone
of the degenerate mode contains totally symmetric character, which permits
resonance enhancement.
- 6 -
The increase in intensity and number of overtones of the Pb3
progression with increasing excitation frequency implies that the weak
absorption band near 22000 cm-1 is attributable to this species rather than
the dimes. An attempt was made to promote diffusion and reaction in the
matrix by raising the temperature; however, no changes were observed, the
same result noted by earlier researchers [31. This indicates that the
heavy lead species insufficiently mobile in the nearly as heavy Xe matrix
to diffuse and give detectable rooucts in a reasonable time. Therefore,
the 22000 cm"
1
 feature can only be tentatively assigned to Pb3.
C
The Raman spectrum excited by the 488 nm laser line gives two weak
features at 111 cm-1 and 222 cm-1 , whose intensities relative to the dimer
and trimer peaks as well as to each other suggest their assignment to the
fundamental and first overtone of the totally symmetric stretch of a Pb4
molecule. Two symmetric structures for a tetramer are possible. For a
square planar (DO ) model, the distribution of vibrational modes is:
A 1 + A 2 + B 1 + 82g + E 
	 (°4h);
whereas for a tetrahedral structure, the distribution takes the form:
Al
 + E + F 2	(Td).
In either case, only one totally symmetric mode is expected with
significant intensity in the Raman spectrum even though three modes are
Raman active. A clear distinction between the two structures is,
therefore, not possible. However, the cubic close-packed structure of bulk
lead [91 suggests that a tetrahedral model is preferable. This choice is
consistent with a mechanism for the growth of the tetramer from a 03h
trimer by simple atom addition and new bond formation, a process which, for
a resulting tetrahedral Pb4 , requires no bond breaking. Such bona breaking
is required for the square planar structure.
- 7 -
The Ill cm-1
 feature is therefore attributed to the A l (Td)
fundamental. For the totally symmetric species, the G and F matrix
elements are evaluated to be 4u Pb and (fr + 4frr ), respectively. A value
for fr
 = 0.62 mdyne /R is estimated if the value of the interaction term frr
is carried over from the trimer. The lower force constant value for the
tetramer is possibly due to delocalization of the valence electrons in
forming more bonds (compared to dimer and trimer) with neighboring atoms.
Conclusions
Analysis of the visible electronic absorption and resonance Raman
spectra of lead vapors isolated in solid xenon matrices has provided strong
evidence for the existence of several molecular species, some of which have
not previously been reported. The presence of Pb 2
 is confirmed by the
visible absorption, and Raman spectra yield a vibrational frequency for the
ground state of 108 cm
-1 , and a dissociation energy of 8200 cm -1 . A second
resonance Raman progression indicates a Pb 3 species of 03h symmetry.
Finally, two additional Raman features at approximately 111 cm
-1 spacing
are evidence for the existence of a third species, tentatively identified
as Pb4 . The assumption of tetrahedral symmetry for this species is
supported by estimated stretching force constant decrease consistent with
the trend shown from dimer to trimer.
Acknowledgements
This work was supported by a grant from NASA (NSG5172) through the
Astrochemistry branch at Goddard Space Flight Center, Greenbelt, MD. The
authors would like to acknowledge Dr. B. Donn for his interest in this
work, and Mr. John Trembly for his assistance in the design and fabrication
of the oven enclosure.
- 8 -
ITable 1: Electronic absorption maxima of Pb 2 (FO+ -- XOu) in xenon
%,	 nm v, cm-1 ev, cm
-1
Assignment
521.30 19183 0 -- 0
169
516.75 19352 1 -- 0
164
512.40 19516 2 -- 0
163
508.15 19679 3 -- 0
156
504.15 19835 4 -- 0
153
x500.3 19988 5 __ n
152
A96.5 20140 6 -- 0
Note: Estimated frequency accuracy is 2 cm-1.
Table 2: Observed Raman frequencies of molecular Pb species in xenon matrices
Excitation wavelength (in nm):
514.5 501.7	 496.5 488.0 476.5 Assignment
107 107
	
107 107 Pb2	 (v1)
111 Pb 4	 (v1)
117 117 116 Pb3	 (v1)
191 191 Pb3	 (2v2)
212 213	 213 213 Pb 	 (2x11
222 Pb4	 (2v1)
233 233 Pb3	 (2v1)
319 319	 319 319 Pb2	 (3v1)
350 349 Pb3	 (3v1)
425 424 425 Pb 	 (4v 1 )	 ?
466 Pb3	 (49 1 )	 1
530 528 Pb2	 (5v1)
581 Pb3	 (5v1)
632 Pb2	 (6v1)
696 Pb3	 (6v1)
735 Pb2	 (7v1)
835 Pb2	 (8v1)
937 Pb2	 (9v1)
1038 Pb2	 (1Ov1)
1138 Pb2	 (11x1)
Note: All frequencies are in cm" 1 units. Frequency accuracy is estimated at
2 cm-1.
Table 3: Observed Raman frequencies of molecular Pb species in xenon
Species;
Pb2 Pb 	 A'	 Pb	 ^, Pb  4
107 117	 --- 111
213 233	 191 222
319 349
424 466
528 581
632 696
735
835
937
1038
1138
Note: All frequencies in cm-1 , with estimated accuracy of 2 cm-1.
Y1
References
	 {
[1] R. A. Teichman III, M. Epiing, and E. R. Nixon, J. Chem. Phys. 68, 336
(1978)
[2] M. A. Epting, M. T. McKenzie, Jr., and E. R. Nixon, J. Chem. Phys. 73,
134 (1980)
[3] L. Brewer and C.-A. Chang, J. Chem. Phys. 56, 1728 (1972)
[4] E. N. Shawhan, Phys. Rev. 48, 343 (1935)
[5] R. A. Teichman III and E. R. Nixon, J. Molec. Spectrosc. 59, 299 (1976)
[6] S. Weniger, J. Phys. (Paris) 68, 596 (1967)
[7] V. E. Bondybey and J. H. English, J. Chem. Phys. 67, 3405 (1977)
[8] J. Drowart and R. E. Honig, J. Phys. Chem. 61, 980 (1957)
[9] L. Pauling, The Nature of the Chemical Bond, 3 rd^ ed. (Cornell Univ.
Press, Ithaca) 1960
Figure 1: UV-visible absorption spectrum of Pb species in a solid xenon
matrix at 12K. Arrows indicate the positions of the argon ion laser
wavelengths used to excite the Raman spectra,
^m ,
T ..
................ .,......	 ,..... ... ........... .
Q
^f
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NN
I
id	 t7	 d	 p
I^^
Figure 2; Resonance Raman spectra of molecular Pb species in a solid xenon
matrix, recorded at 12 K. Vertical bars denote the count rates, not
normalized to laser power or instrumental response. (a) Excitation using the
476.5 nm laser wavelength. (b) Excitation at 488.0 nm. (c) Excitation at
496.5 nm.
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Absorption and resonance Raman spectra of Pb2, Pb3 and Pb4 in xenon matrices

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