1822
J. Phys. Chem. 1983, 87, 1822-1829
Higher Electronic Excited States of Carbon Disulfide. An Electron Impact Investigation J. P. Doerlng Department of Chemistry, The Johns Hopkins University, &/timore, Maryland 2 12 I8
and Ruth McDlarmid' Laboratory of Chemical Physics, National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20205 (Received: June 14, 1982;I n Final Form: December 16, 1982)
The electron impact spectrum of CS2 has been remeasured and reanalyzed. Transitions are observed to the ns(n,),npa(II,), and one other orbital Rydberg series to the fist ionization potential. Singlet-triplet designations are shown to be valid only for n 5 5. Valence transitions are observed to the (2?rp)-'(7ag)', ngand (2rU)-'(7og)l, II, states.
Introduction CS2 exhibits a richly structured near-ultraviolet absorption spectrum and a more diffuse vacuum-ultraviolet absorption spectrum. The present investigation is concerned mainly with the latter region. The literature regarding the energy levels of CS2is extensive. However, it can be summarized as follows. Price1 first reported the absorption frequencies of most of the optically observable vacuum-ultraviolet transitions of CS2 and constructed one distinct Rydberg series leading to each of the lower two ionization potentials of the molecule.2 The observed series were not given either orbital or symmetry assignments. Later Price's series were confirmed and additional bands approximately 400 cm-' to the blue of each of the system origins were r e p ~ r t e d . Next, ~ CS2 was considered as part of an investigation of many 16electron molecule^.^ Occupied-orbital energies were calculated for C02, and the calculated orbital ordering was assumed to be valid for all 16 electron molecules despite its disagreement with the experimental orbital ordering of C 0 2 (and CS2). Many valence transitions were assigned on the basis of this calculated occupied-and vacant-orbital ordering. Rydberg transitions were given vacant atomic orbital assignments (s, p, d). The bases for these assignments were not given. One of the ns Rydberg series had a smaller quantum defect than a np Rydberg series leading to the same core. The optical electronic spectra of CS2and C 0 2 were next studied in an investigation in which the Rydberg-term values were calculated by the pseudopotential method. The derived promotion energies were compared with the experimental values to assign state and orbital symmetries to the Rydberg transitions originally identified by Price as well as some, but not all, of those subsequently rep ~ r t e d . In ~ this investigation it was explicitly noted for the first time that at low primary quantum number, where U,2) coupling applies, the corresponding members of series leading to the two lowest ionization potentials are transitions to singlet and triplet components of the same orbital state whereas at higher primary quantum number, where (Q,w) coupling appIies, the former triplet series (1) W. C. Price and D. M. Simpson, Proc. R. SOC. London, Ser. A, 165, 272 (1938). (2) The symmetry y o u p of the lowest energy molecular ion is split by ( Q , w ) coupling to give and 2E3,2states. For the ground state of CS2+,the splitting is 0.054 eV. (3) Y. Tanaka, A. S. Jursa, and J. J. LaBlanc, J. Chem. Phys., 32,1205 (1960). (4) J. W. Rabalais, J. M. McDonald, V. Scherr, and S. P. McGlynn, Chem. Reu., 71, 73 (1971). ( 5 ) F. R. Greening and G. W. King, J . Mol. Spectrosc., 59,312 (1976).
should approach the intensity of the former singlet series.6 In fact, the previously unambiguously assigned spectral region is where (Q,o) coupling applies. The members of' the original Rydberg series were in part identified by the presence of a secondary band 440 cm-' to the red of each primary Rydberg transition. Another significant contribution to an understanding of the electronic spectrum of CS2 has come from electron impact spectroscopy. The earliest investigationsconfiied Price's Rydberg series and suggested two others, whose assignments were based on theory.' More recently, a higher resolution variable incident energy and angle electron impact energy loss investigation was conducted.* For the first time it was potentially possible to differentiate experimentally between symmetry- and/or spin-forbidden and symmetry- and/or spin-allowed transition^.^ In addition to confirming the long, strong Rydberg series identified by all previous investigators, a previously assigned allowed np Rydberg series was unambiguously assigned as a symmetry-forbidden ns Rydberg series and several other transitions were reassigned on a firmer experimental basis. Several transitions, however, were left unassigned. In addition to spectroscopic investigations of gaseous CS2, the spectrum of CS2 has been obtained in argon, krypton, xenor., and neon matricedo and in liquid krypton, argon, and xenon." Two structured systems, whose transition energies are host invariant, and two single, very strong transitions, whose transition energies are host dependent, were observed around 6.2,8.2, 7.0, and 8.50 eV, respectively.lOJ1The photoelectron spectrum of CS2 has also been extensively investigated.'* It has been shown that the unambiguous occupied orbital energy ordering is ...(5qJ2(2~,)4(21r,)4. This orbital ordering is in agreement with that deduced from high-resolution optical spectroscopic investigations of the CS2+molecular ion.13 The I _
(6) For a discussion of the types of coupling of electronic motions, see G. Henberg, "Spectra of Diatomic Molecules", Van Nostrand, New York, 1950, pp 333-8. (7) V. Y. Foo, C. E. Brion, and J. B. Hasted, h o c . R. SOC. London, Ser. A, 322, 535 (1971). (8) D. G. Wilden and J. Comer, Chem. Phvs., 53, 77 (1980). (9) For a discussion of the application of electron impact spectroscopy to polyatomic molecules, see J. P. Doering and R. McDiarmid, J. Chem. Phys., 73, 3617 (1980). (10) J.-Y. Roncin, N. Damany, and B. Vodar, Chem. Phys. Lett., 3,197 (1969). (11) G. Ya. Zelikina, V. V. Bertaev, and 'T. G. Meister, Opt. Spektrosk., 42,997 (1977); Opt. Spectrosc. (USSR), 42,575 (1977); G. Ya. Zelikina and T. G. Meister, Opt. Spektrosk., 43, 85 (1977); Opt. Spectrosc. (USSR), 43, 46 (1977). (12) M. J. Hubin-Franskin, J. Delwiche, P. Natalis, and G. Caprace, J. Electron Spectrosc. Relat. Phenom., 18, 295 (1980), and referenre contained therein.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983 1823
Higher Electronic Excited States of CS2
I
1
eV. IO"
eV, 0"
U J
I
7 70
I
I
t
1
780 790 ENERGY LOSS, eV
Flgure 2. 5.5-1 1.5-eV energy-loss survey spectrum of CS, taken at the following incident energies and scattering angles: 100 eV, Oo; 100 eV, 10'; and 28 eV, . ' 0 The zero of the vertical axes has been displaced upward for the various scans as indicated.
Flgure 1. Detailed scan of the 7.65-7.9O-eV region of the CS2 energy-loss spectrum taken at 1 0 0 4 incident energy and 2' scatterlng angle. This spectrum illustrates the avallabie energy resolution. The small "hitches" on the low-energy side of the main peak correspond to lines observed in the optical spectrum.
lowest energy valence transition must, then, involve transitions out of these orbitals. The lowest vacant orbital is predicted to be rU,a prediction well confirmed by the high-resolution analysis of the lower energy transitions of CS2.14 The next lowest vacant molecular orbital is uncertain. Since none of the optical investigations were able to distinguish forbidden from allowed transitions, and the previous electron impact investigations were incomplete, we initiated a detailed electron impact investigation of the electronic spectrum of CS2,with the goals of identifying the previously unidentified transitions and of clearly delineating the energy region where the transition from (AJ) to (Q,w) coupling occurs. From our analysis, we conclude that several of the previous assignments are incorrect or are correct for invalid reasons. These assignments are discussed in detail and our results are compared with those obtained from previous work.
Experimental Section The apparatus and procedures have been described previously?J5 Reagent-grade CS2was used without further purification except for repeated freeze-thaw degassing cycles under vacuum. An energy resolution better than 15 meV (fwhm) was used for the detailed studies. The resolution is demonstrated in Figure 1. The two prominent bands are separated by 82 meV (666 cm-'), which can be compared with the optical value' of 694 cm-'. The small "peaks" on the side of the intense 7.75-eV peak are incompletely resolved bands observed optically' -128 and -53 cm-' from the most intense peak. Somewhat lower resolution was used for the survey spectra. Results Three survey spectra of CS2 taken at 100-eV incident energy and scattering angles of 0" and loo, as well as a 28-eV, Oo spectrum, are presented in Figure 2. Those members of Price's series that occur in this energy region are marked 0,those of the ns Rydberg series are marked +, and another series, to be discussed, are marked 0. The (13) W.J. Balfour, Can. J.Phys., 64, 1969 (1976). (14) Ch.Jungen,D.N.Malm, and A. J. Merer, Can.J.Phys., 51,1471 (1973).
I
.
570
1
I
590
1
1
1
I
6.10 6.3 ENERGY LOSS, eV
I
I
650
I
670
Figure 3. Detailed 5.70-6.704' energy-loss spectrum showing the superposition of two scans taken at 100 eV and scattering angles of 2' and 6'. The spectra were normalized at the strongest peak. The lack of significant change in the relative intensttiis of the vibronic bands with scattering angle shows that withln this allowed transition the vibronic band Intensities are determined solely by the Franck-Condon principal.
100-eV spectrum resembles both the optical spectrum4v5 and the previous electron impact spectrum? To investigate the effect of varying the experimental parameters on the energy-loss spectrum, the various regions of the spectrum are expanded and discussed individually, as follows. Only results which are significantly different from those presented by WC8 will be presented here. Expanded spectra of CS2at 5.7-6.7-eV energy loss, taken at an incident energy of 100-eV and scattering angles of 2O and 6O, are superimposed in Figure 3. There is no significant change in the spectrum: The close correspondence of the two high-resolution spectra illustrates the stability and reproducibility of the energy-loss spectra. The expected lack of significant change in the relative vibrational band intensities is a consequence of this being a fully optically allowed transition. Expanded spectra of CS2 from 6.50 to 8.00 eV are presented in Figure 4. This spectral region contains five discrete electronic transitions. Increasing the scattering angle from 2 O to 5 O at 100-eVincident energy increases the intensity of the 6.7-eV shoulder of the 6.8-eV peak. Increasing the scattering angle also appears to shift the
1824
The Journal of Physical Chemistry, Vol. 87,No. 10, 1983
I
Doering and McDiarmid
t
1
I
v
-
, , , ,
i " " 1
700 750 ENERGY LOSS, e V
6.50
~-
__ 8.00
1
8.20 ENERGY LOSS, eV
8.00
'
1
840
Flgure 6. 8.00-8.5O-eV energy-loss spectra taken at 1 0 0 4 incident energy and scattering angles of 2 O and 5'. The vertical scale has been displaced upward for the 5' spectrum as indicated. 0
n
Flgwe 4. 6.50-8.OO-eV energy-loss swvey scan. Conditions: (A) 100 eV, ; ' 2 (B) 100 eV, 5'. A' and B' are X5 amplifications of the A and 6 scans. The zero levels have been displaced upward for the various scans as indicated. The -tsymbols indicate the positions of Rydberg origins discussed in the text.
0 IO"
5"
I
,
'
I
'
cs2
I
'
i
'
850 870 8 90 ENERGY LOSS, e V Flgure 7. 8.45-8.95-eV energy-loss spectra taken at 1004J incident 0 and 5'. The vertical scale has been energy and scattering angles of ' and 0 displaced upward for the 5' spectrum as indicated. The symbols indicated the positions of Rydberg origins discussed in the text.
+
01
6.50
,
I
L0" I
I
t
6.X
6.90 ENERGY LOSS, eV
Flgure 5. Detailed 6.50-7.OC-eV energy-loss spectra taken at 1 0 b V incident energies and scattering angles from ' 0 to 13' as Indicated. The vertical scale has been displaced upward as indicated. The figure illustrates the violent changes in these features of the spectrum which are observed as the scattering angle is varied. The symbols indicate positions of Rydberg origins discussed in the text.
+
Franck-Condon maximum of the 7.0-7.5-eV system to lower vibrational quantum numbers, thus changing the relative vibrational intensities. However, the total intensity of the system relative to that of the 6.8- and 7.75-eV bands does not appear to change. Our work and ref 8 show that decreasing the incident energy at 0' scattering angle diminishes the intensities of the 6.8- and 7.75-eV bands relative to that of the 6.25-eV band but incremes the 6.8-eV band intensity relative to that of the 7.75-eV band. Figure 5 shows the dramatic changes in the 6.8-eV bands as the scattering angle is changed from 2O to 1 3 O in detail. The weak, lower energy component is seen to increase in intensity relative to the 6.8-eV band up to loo and then
decrease slightly with increasing scattering angle. Figure 6 shows the diffuse 8.2-eV system at 100-eV incident energy and scattering angles of 2 O and 5O. The 8.2-eV system is seen to be composed of several subbands, as was observed in the optical ~ p e c t r u m . ' Increasing ~~~~ the scattering angle from 2' to 5' appears not to alter the intensity of the 8.2-eV system, but rather to increase its broad band component with the result that the diffuse structure disappears. Similarly, decreasing the incident energy at Oo scattering angle appears to have no effect on the relative intensity of this system (Figure 2). As shown in Figure 7, at the resolution of the electron impact investigation, an incident energy of 100 eV, and a scattering angle of Oo, the 8.5-9.0-eV region of the spectrum appears to be composed of three distinct peaks and several shoulders. Increasing the scattering angle from Oo to 5O, at 100-eV incident energy, appears to increase the intensities of the 8.52- and 8.6-eV peaks relative to those observed at 8.65 and 8.77 eV. All of these peaks appear to increase in intensity relative to the 6.25-eV peak as the scattering angle is increased (Figure 2), but, since the
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983 1825
Higher Electronic Excited States of CS2
5"
0'
,
30
I
,
I
9.10 ENERGY LOSS, e V 9.00
Flgure 8. 8.90-9.15-eV energy-loss spectra taken at 1OO-eV incident
energy and scattering angles of 2' and 5'. The vertical scab has been displaced upward for the '5 spectrum as indicated. The 0 symbols indicate the positions of Rydberg origins discussed in the text.
general background scattering at energy losses greater than 8.0 eV increases with increases in the scattering angle at 100-eV incident energy, it is difficult to assign individual intensity changes to the discrete peaks. At an incident energy of 100 eV and a scattering angle of 2O, the 9.0-eV energy-loss region of the spectrum, presented in Figure 8, is quite complex. There are several quite distinct peaks (at 8.96,9.01, and 9.04 eV) and suggestions of shoulders in the vicinity of 8.98 eV. Increasing the scattering angle to 5" appears to increase slightly both the intensity of the 8.96-eV peak and the resolution of the shoulders in the 8.98-eV region. The discrete peaks appear to retain their intensities relative to each other and relative to the 6.25-eV reference peak after the scattering intensity in Figure 2 is corrected for the diffuse background. The last energy-loss region covered in this investigation, from 9.10 to 9.85 eV, is presented in Figure 9. This and the 9.0-eV energy-loss region are the only ones studied here that overlap the Rydberg bands studied by Price.' The many distinct peaks seen in the higher resolution optical spectrum merge at the lower resolution of the lOO-eV, 2 O electron impact spectrum to form a quasi-continuum on which are superimposed the stronger peaks. Increasing the scattering angle from 0" to 5' at 100-eV incident energy increases the scattered intensity from some of these peaks relative to that from others (see Figure 9). In addition, our data show that the intensity of the whole region increases relative to the 6.25-eV reference band, and, unlike the 9.0-eV region, this increase does not appear to be due solely to an increase in the background scattering. The positions of all of the bands discussed above and the effect of changing the experimental conditions are summarized in Table I. Assignments of these bands and their quantum defects, calculated by assuming that the stronger, higher energy peak of the 0.05-eV pairs converges on the 10.13-eV IP,2912,13are also presented in Table I. Discussion In this section the experimental electron impact spectrum of CSz will be analyzed. Since this is a low-resolution (15)J. P.Doering and R. McDiarmid, J. Chem. Phys., 75,87(1981). (16)A. D.Walsh, J. Phys. Radium, 15,501 (1954).
1
0
'
1
'
1
'
1
'
1
'
1
'
1
'
9.30 9.50 9.70 ENERGY LOSS, eV
Flgure 9. 9.10-9.85-eV energy-loss spectra taken at 1OO-eV incident energy and scattering angles of 0' and 5'. The vertical scale has been displaced upward for the 5" spectrum as indicated. the 0,and 0 symbols indicated the positions of Rydberg origins discussed in the text.
+,
technique, ancillary techniques must be used in the interpretation of the data. It is useful to first summarize what is known about the electronic states of CSz, what conclusions can be deduced from the available data, and what can be induced from comparable systems. Several semiempirical theoretical calculations have been carried out on COz and the results assumed to be transferable to CS2. Unfortunately, the theoretical electron orbital orderine5J7does not predict the ordering deduced from photoelectron spectroscopy or from high-resolution optical spectroscopy of the molecular Both of these techniques concur in ordering the outer three occupied molecular orbitals of CS2 (and COz) as (a,)2(7r,)4(7r,)4. Assignments based on the theoretical orbital ordering must, therefore, be fortuitous or incorrent. In the discussion that follows, the experimental occupied-orbital ordering will be assumed. Independent of these calculations, considerations of the nodal and bonding properties of the experimental molecular orbitals18enable the likely geometric structures of the assorted excited states to be predicted. The (27rJ orbital is predicted to be strongly bonding. Thus, the removal of ~ ) is predicted to lead to an increase in the a ( 2 ~electron C-0 (C-S) bond length. This prediction is confirmed by the observed structure of the photoelectronlZand optical13 spectra of the CSz molecular ion. The (37ru)vacant orbital is predicted to strongly favor a bent molecular geometry. Thus, the promotion of an electron to this orbital is predicted to cause excitation of bending vibrations which should be reflected in the Franck-Condon envelope of the relevant transition(s). On this basis, the probable geometries and vibrational structures of the lower energy valence transitions from each of the three highest filled molecular orbitals are presented in Table 11. The relative energy ordering assumes that the orbital energy differences (17)N.W.Winter, C. F. Bender, and W. A. Goddard, 111, Chem. Phys. Lett., 20, 489 (1973). (18)A. D. Walsh, J. Chem. SOC.,2266 (1953);also discussed in G. Herzberg, "Electronic Spectra of Polyatomic Molecules",Van Nostrand, Princeton, NJ, 1966,pp 312-21. The molecular orbital numbering in these references assumes first long row elements. For sulfur, these numbers must be incremented to take the n = 2 shell into account.
1826
The Journal of Physical Chemistry, Vol. 87,No. 10, 1983
Doering and McDiarmid
TABLE I: Electron Energy-Loss Spectrum of CS, 100 eV, 0"
scatt angle, eV 6.7 2 6.82 7.00 7.10 7.21 7.31 7.42 7.51 7.69 7.75 p 7.85 7.97 8.18 sh 8.188 8.22 8.258 8.325 8.535 8.61 8.655 8.72 8.78 p 8.83 sh 8.96 8.985 sh 8.998 sh 9.010 p 9.043 9.17 9.22 9.31 9.35 9.42 9.48 p 9.54 p 9.625 9.68 10.074' 1O.12gc
-~ IP = 10.074c
100 eV, 5" scatt angle
greatly enhanced enhanced about constant most intense peak shifts t o lower vibrational overtones
1.98
about constant constant
1.61
ha
assignment b
IP = 1O.12gc
orbital
A(s-
state
T),
eV
0.06
enhanced structure becomes more diffuse greatly enhanced enhanced
2.02
? diminished constant enhanced
1.83
about constant
1.46
0.06 2.00 0.04 1.82 0.058
1.46 2.12 enhanced enhanced
0.050 2.13
1.78
0.040 1.82
constant constant enhanced constant
1.43
0.060 1.42 2.19
1.49
0.055 1.49 0.055
The s values are chosen so the n* values calculated from the term values yield reasonable values of n. See ref 16. Orbital assignments designate changes t o the ground-state configuration. Singlet and triplet designations are only valid below n = 6, where ( A , X ) coupling applies. Reference 12. Another Rydberg transition occurs in this energy region as does another valence transition. See text. e Assignment tentative. a
TABLE 11: Valence Excited States of CS,' configurationb
state(s)
. . .(5~,)'(2n~)'(2~~)'(3n,)'
bent linear bent C-S lengthened C-S lengthened bent linear
1 q 1 ( ) 3 u + , Xu-, A , )
. . .(5uU)'(2nu)'(2ng)'(7ug)'
1.3nu
. . .(5uu)'(2nu)'(2ng)'(3n,)' . . .(5uU)'(2nu)'(2ng)'(7ug)' . ..(50,)'(2n,)'(2~~)'(3~~)' . . .(50,)' ( 2ru)'(2ng)'( 7 0 ~ ) '
active vibration(s)
geometry
' + 3 ( Z g cX, g - , A g ) 1,3n
1,'n
wcg+
v2
? VI? v2
v2 V2
?
a In order of increasing energy. State energy differences assume that orbital energy differences exceed correlation differences. (27rg) is essentially nonbonding. ( 2 ~ is~a strongly ) bonding orbital.
exceed the correlation changes. This may not be correct. Assuming, for the present, that it is, we see, from Table 11, that the lowest energy transition is the well-studied (27r,) (37ru)promotion. The next highest energy valence transition could arise from either the (27rg) (u,)lgor the (2au) ( 3 ~transition. ~ ) Both transitions are forbidden. The latter transition is expected to give rise to a bent
-
-
-
(19) There is some question as to whether an antibonding u orbital can be distinguished from the lowest ns Rydberg orbital (M. B. Robin, 'Higher Excited States of Polyatomic Molecules",Vol. 1, Academic Prees, New York, 1974, Chapter 1). We shall assume that they are distinguishable and look for experimental data to confirm or refute this as-
sumption.
molecule, to lead to a great increase in the C-S bond length, and to be split into three components, as in the (27rJ (3au)transitions. These components should be characterized by complex Franck-Condon envelopes involving progressions in both the C-S stretching and the S-C-S bending vibrations. The (27rJ (a,) transition should not possess much vibrational structure. The next higher energy transitions, (27ru) (ug) or (5uJ (37ru), are respectively allowed and forbidden. They have considerably different predicted Franck-Condon structures, as indicated in Table 11. In the discussion to follow, these predicted differences in Franck-Condon structure will be used as aids in assigning the valence transitions.
-
-
-+
+
Higher Electronic Excited States of CS2
In addition to the valence transitions discussed above, assorted Rydberg transitions are also expected for CS2at energy losses below 10 eV. The lowest ionic state of CS2 is split by (Q,w) coupling into 2E1/2and 'E312 states 0.06 eV apart.'s3J3 Rydberg series to both these states are possible and have been observed. The second set of ionization potentials are at 12.69 and 12.71 eV,I2 approximately 2.6 eV above the first. As the first Rydberg transition leading to an ionization limit is 3.5 eV or less below its limit,20Rydberg transitions to the second ion core are predicted only in the highest energy region of our spectrum, if at all. We will, therefore, assume that all observed Rydberg transitions are to the lowest ion states but be alert to a possible failure of this assumption in the higher energy-loss region of the spectrum. Rydberg transitions can be to s, p, d, f, ... atomic-like orbitals. In nonspherically symmetric molecules, n (and 1) may fail to be good quantum numbers.21 We shall assume that they are. The vibrational structures of the Rydberg transitions should resemble that of the photoelectron spectrum of the molecular ion. For CSz, the origin band carries most of the intensity of the transition to the ground state of the ion; l2 thus, the origin of each Rydberg series member to this core should also carry most of the intensity of that transition. Since the transitions originate from a rgorbital, only transitions to the p and f levels are permitted by optical selection rules.22 Transitions to s and d orbitals are permitted by vibronic coupling. The hydrogenic orbitals are further split by interactions with the molecular core.z1.23,24 In general, within a hydrogenic orbital, u,r,8 sublevels have decreasing quantum defects.% It has been further noticed that in planar molecules the sharpest, strongest Rydberg transitions are usually to the out-of-plane component of the given (primary quantum number) hydrogenic orbital.%n For CSz, the out-of-plane p orbital is the pa orbital. We thus expect this to be the stronger, or only, observed p Rydberg series. We will return to this point below. Portions of the spectrum of CS2have also been obtained in liquid" and solidL0argon, neon, krypton, and xenon. Theoretical and experimental analyses of the spectra of impurities in rare gas hostsz8have demonstrated that valence transitions are essentially unchanged in these hosts while Rydberg (extravalency) transitions are greatly altered. The first Rydberg transition is greatly blue shifted, the magnitude of the shift increasing with host in the order Xe, Kr, Ar, Ne. Higher primary quantum number Rydberg transitions lose their molecular identities and become transitions of the host-impurity system. The fates of the different 1 components of the first Rydberg transition are currently unknown. Theoretically they are predicted to retain their molecular identities but experimentally they have not been observed in solid rare gas matrices.% In CS2, two different types of bands are observed in the condensed phases. The 6.82- and 7.75-eV bands are observed as single (20) Rydbergterm values are predicted by the evaluation of the ) ~ , R is the Rydberg constant Rydberg equation, IP - Y, = R / ( ~ I * where (13.60 eV) and n* the effective quantum number. As the minimum n* = 2 (hydrogen), the maximum term value = 3.5 eV. (21) Ch. Jungen, J . Chem. Phys., 53,4168 (1970). (22) correlates with a d atomic orbital; thus (1 and u,g) allowed transitions are to p and f atomic orbitals. (23) A. D. Liehr, 2.Naturforsch. A, 11, 752 (1956). (24) E.Lindholm, Ark. Fys., 40, 97 (1969). (25) V. G. Hammond, W. C. Price, J. P. Teegan, and A. D. Walsh, Discuss. Faraday SOC., 9, 53 (1950). (26) R. McDiarmid, J . Phys. Chem., 84,64 (1980);R. McDiarmid, J. Chem. Phys., 69, 2043 (1978). (27) J. P.Doering and R. McDiarmid, J. Chem. Phys., 76,1838 (1982). (28) A. Gedanken, B. Raz, and J. Jortner, J. Chem. Phys., 58, 1178 (1973), and references contained therein.
The Journal of Physical Chemistv, Vol. 87, No. 10, 1983
1827
TABLE 111:
Comparison of the Vapor- and Condensed-Phase Spectra (in e V ) of CS, this work
6.82 b 7.75 8.155 8.188 8.22 8.258 8.33
condensed phasesa
Ne
Ar
Kr
Xe
8.71
7.75 7.26-7.07 8.50
7.08-6.91 8.16
6.98-6.85 7.94
8.182
8.172
8.251
8.239
8.309
8.311
8.373
8.373
8.439
8.441
etc.
etc.
wcc
8.283
8.30
8.348
8.34
8.387
8.38
8.416 8.459 etc. Condensed phases are rare earth matrices, ref 10, e x c e p t where indicated. These condensed-phase spectra are liquefied rare gases, ref 31, reported over t h e following temperature ranges: Ar, 92-125 K ; Kr, 124-194 K ; Xe, 178-250 K. Wilden and Comer, ref 8.
intense peaks in liquid and/or solid rare gases, whose positions depend on the host (see Table 111). The relative energies of these transitions in the different hosts Xe, Kr, Ar, and Ne, and the magnitudes of the spectral shifts between different hosts parallel those observed for several other molecules in the same hosts.2s It seems reasonable to conclude that these are Rydberg transitions. Parenthetically, we note that, in CS2, transitions to two 1 components of the first Rydberg shell are clearly observed. In addition to these Rydberg transitions, two other transitions, centered around 6.2 and 8.2 eV, are observed in condensed phases which are weaker, show extensive vibrational structure, and are essentially unshifted in transition energy either by solvation or by changing the host. The vibrational structures of these transitions correspond closely to those observed in the vapor phase (Table 111). It seems reasonable to assign these as valence transitions. The assignments of these transitions will be discussed below. The last spectroscopic techniques that has been applied to CS2is electron impact spectroscopy. I t has long been known that at low incident energy and large scattering angle spin-forbidden transitions are enhanced relative to fully allowed transitions, although most of the previous investigations have been conducted on first-row elementsm It has more recently been shown that higher incident energy, large scattering angle variables enhance symmetryforbidden transitions relative to symmetry-allowed trans i t i o n ~ .There ~ is also evidence that valence transitions peak in intensity at lower incident electron energy than Rydberg transition^,^ although this result, again, was demonstrated only for first-row elements. The best established Rydberg series is Price's original series.' This series is composed of strong, sharp transitions, each of which is accompanied by a weaker, sharp band 400 cm-' to its red. Little vibrational structure has been reported for the members of this series, an observation with which we concur. At higher resolution than attained here, the lower component of the 9.04-eV band (n = 5) appears to split into several components, the lowest energy com(29) S. Trajmar, J. K. Rice, and A. Kuppermann, Adu. Chem. Phys., 18,15 (1957);A. Kuppermann, W. M. Flicker, and 0. A. Mosher, Chem. Reu., 79, 77 (1979).
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The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
Doering and McDiarmid
ponent of which was assigned as a member of the Rydberg bands b and k), which, since they are also forbidden, series converging to 10.08 eV. This transition, barely themselves increase in intensity relative to the members distinguishable as a shoulder in the high incident energy, of the allowed np series as the optical selection rules are small angle electron scattering spectrum, becomes relarelaxed. We assign this second series of transitions as the tively more intense at very low incident energy and large (3n,)ns(nd?) Rydberg series in parallel with the assignscattering angle.8 It is also relatively more intense for ment of the (311,)np Rydberg series. As for the ('v3nU) pa higher n. This observation indicates that for CS2,for n Rydberg series, the transitions corresponding to singlet= 5 the dominant spin-orbit coupling mechanism is ( h , Z ) ; triplet transitions are predicted to grow in intensity relative hence, the lower energy spin component of a molecular to the same orbital singletsinglet transition as n increases. orbital pair is described as a triplet state. Optical selection Both series are also expect to become weaker at higher rules forbid singlet-triplet transitions. The violation of Unfortunately, the spectrum becomes so congested at higher energy that this prediction cannot be verified. In optical selection rules at low incident energies and large summary, we concur with WC that the transitions observed scattering angles in electron spectroscopy is expected to at 6.82,8.61,9.22, and 9.54 eV are singletsinglet members enhance such transitions, as observed. It is interesting to note that the triplet is observed even under optical conof an orbital ns(nd?) Rydberg series and propose that the ditions, a consequence of the large spin-orbit coupling of weak transitions observed 400 cm-' to their red at 6.72, this second-row-atom-containing molecule. The analogus 8.54, and 9.17 eV are their corresponding singlet-triplet triplets in C 0 2 are barely ~ i s i b l e .The ~ n = 6 member of transitions at low n where (A,X) coupling is applicable. this series, the 9.42-eV band, shows no such enhancement. The only other sharp, vibrationless band observed in the optical and electron impact spectra of CS2 is at an energy It thus appears that in CS2the shift from predominantly loss of 8.78 eV. This transition was previously assigned ( L Z ) to predominantly (Q,w) coupling of the strong Rydas the 5pa Rydberg t r a n ~ i t i o n .The ~ 8.78-eV band is acberg series occurs at n = 5. Are there n = 4 members of companied by a weaker band approximately 0.06 eV to its these series? On the basis of its intensity and approximate position, the very intense 7.75-eV transition has been asred, as is observed for each of the low-n members of the signed as the n = 4 member of the singlet series, notns and npa Rydberg series. Under our experimental withstanding its significantly different quantum d e f e ~ t . ~ , ~conditions ~~ the intensity of this shoulder did not increase, The matrix investigations proved that the 7.75-eV tranbut we did not go to the low incident energies and large sition is definitely a Rydberg transition. The transition angles necessary to relax the optical selection rules prohibiting singlet-triplet transitions of orbitally allowed has a weaker transition approximately 400 cm-I to its red which is enhanced by the relaxation of the optical selection transitions. On the basis of the existence of the low-energy band and its vibrationless contour, it seems reasonable to rules, as was observed for the 9.0-eV series member. It, thus, seems reasonable to assign this transition as a memassign the 8.78-eV transition as a Rydberg transition. The ber of Price's series, despite its different quantum defect. quantum defect of this transition is reasonable for a p This Rydberg series has been assigned as a p Rydberg Rydberg transition and, if correctly assigned, is, by elims e r i e ~ .The ~ , ~ sulfur atom np Rydberg series has a quantum ination, to the pa orbital. However, if this were a p defect, 1.57, that approximates the 1.50 observed here,30 Rydberg transition, it would be to the 5p state. There is and thus supports this assignment. There are two possible absolutely no indication of an experimental state anywhere p Rydberg orbitals (states): pa(II,) and pa(&,). On the around 7.3 eV, the calculated location of the 4p band. The basis of semiempirical calculations, previous investigations other possible assignment is to a 5f Rydberg orbital. This have assigned Price's series as p ~ ( n , ) . Although ~ this assignment has the advantage that no lower energy memassignment appears to be inconsistent with the body of ber of the series is anticipated. On the other hand, the observed quantum defect is grossly inconsistent with this available information concerning molecular Rydberg transition^,^^-^^ it is supported by a high-resolution rotaassignment unless the state is interacting with the pa series, thus perturbing both quantum defects. The n = tional analysis of the 7.75-eV tran~ition.~'We therefore 6 members of this (orbital) series are weakly observed concur that the observed p Rydberg series in CS2is to the npa Rydberg orbitals (nu).The -400-cm-' series is obaround 9.3 eV. This transition is not observed in conviously the corresponding (low n) 311u Rydberg series. CS2 densed media. At the present time, this transition cannot thus manifests the n = 4, 5 , 6 , 7 , ... members of the 't3pabe convincingly assigned on experimental grounds. ('Jn,) Rydberg series. Several valence transitions are observed in CS2. The The next reasonably firmly established Rydberg series lowest energy valence transitions, 3.25,3.65, 3.82, and 6.2 is that proposed by Foo et al.; and WCe8 The members eV, have been assigned as 3Au(3A2), 'AJ'AJ, 'AU('Bz),and of this series are observed to increase in intensity as the 'Z,+('B,), re~pectively.'~,~~ The next experimental valence optical selection rules are relaxed (see Figure 2); thus, a transition is observed between 7.0 and 7.5 eV by both symmetry-forbidden ns orbital assignment was proposed. optical'^^^^ and electron impact spectro~copy.'~~ Increasing Obviously a symmetry-forbiddennd assignment would also the scattering angle at high incident energy does not alter fit the experimental data and might better agree with the the intensity of the transition but shifts its Franck-Condon S atom quantum defects. That at least the first member envelope to lower vibrational overtones as shown in Figure of this series, the 6.80-eV transition, is, in fact, a Rydberg 4. Reducing the incident energy at high scattering angles transition is demonstrated by the rare gas solution indisplaces the Franck-Condon progression approximately vestigations of CS2." Each member of this series has a 0.05 eV to the red.a Although this interval is reminiscent weaker band, approximately 400 cm-' to its red, as was of the spin-orbital splitting observed in the assorted observed for the members of Price's series. In the electron Rydberg transitions, the relative appearances of the two impact investigations, the intensity of the lower energy sets of transitions differ from those observed for the band increases relative to that of the members of WC's Rydberg transitions. We propose that the 7.0-7.5-eV series, both here (Figure 2) and in WC8 (their unassigned transition arises from an orbitally forbidden transition (30) C. E. Moore, *Atomic Energy Levels", Vol. I, National Bureau of Standards, Washington, DC. 1971. ( 3 1 ) A. d. Merer, personal communication.
(32) T. Dunn in 'Studies on Chemical Structure and Reactivity", J. H. Rudd, Ed., Methuen, London, 1966, p 103. (33) A. E. Douglas and D. Zanon, Can. J. Phys., 42, 627 (1964).
Higher Electronic Excited States of CS2
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
neon and argon are substituted for each other, but shifted somewhat in krypton (Table 111). We propose that the 8-eV energy loss region of the spectrum of CS2 contains both a Rydberg transition at 8.18 eV and at least one valence system. The Rydberg transition probably arises from a forbidden 4d Rydberg transition. The valence (3a,), E,+, transition (Table 11) could be either (2aJ E;, and Ag, or (2aJ (7ag),II,. While the former may be predicted to be lower in energy in a zeroth-order approximation, a II, assignment would both have the required intensity and provide a basis for the displacement of the 4pa(II,) Rydberg transition from its calculated transition energy, -7.95 eV, to its observed transition energy, 7.75 eV. We thus tentatively propose that the 8-eV energy region of the spectrum of CS2contains at least one forbidden Rydberg and an allowed II, valence transition. All of the Rydberg and valence assignments discussed above are summarized in Figure 10.
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I
I
70 80 ENERGY LOSS, eV
60
90
I
IO 0
Figure 10. Assignments in the spectrum of CS,. Rydberg series are identified both by Rydberg orbital and by state. Valence transitions are idmthd by state only. The correspondingorbitals are as follows: E,+, (2aJ (3aJ; (2XJ (7aJ (2*J (7aJ
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n,,
nu;
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whose true origin (and accompanying vibrational structure) becomes active as the optical selection rules are relaxed, as was observed for a~etone.~'This transition is optically enabled by one quantum of v2, a a, vibration which has a fundamental frequency of 397 cm-' (0.049 eV) in the ground state2 and -400 cm-l (0.05 eV) in the 7.0-eV excited state. The intensity-donating electronic transition is most probably the strong, adjacent 6.35-eV valence system. The 7.0-eV system is, consequently, deduced to be II,. The spacing of the vibrational progression built on both the true and false origins, 0.10 eV (800 cm-l), arises from successive quanta of 2v2. Since the vibrations involve 2v2,the molecule must retain its u,g symmetry, hence be linear. We thus deduce from Table I1 that the 7.C~7.5-eV transition is a (ag) (7ug),'II transition where the (5aJ orbital is distinct from the 4s Rydberg orbital. (The (2aJ (4s), lIIS transition occurs at 6.8 eV.) Since the (2n,J3(7aJ1, lJIg upper state is linear, the vibrational progression in 2v2must be induced by coupling with the bent 'B,+('B1) state and the shift in the Franck-Condon maximum with experimental conditions reflects a dependence of coupling on vibrational quantum number. The one remaining unanalyzed energy-loss region is around 8.0 eV. WC's results indicate that this region contains both a peak, at 8.18 eV, and a system of bands (their peak i and bands j) that become much more intense at low incident energy.* The optical spectrum of this region is essentially uninterpretable. The spectrum of CS2 in rare gas matrices displays a corresponding structured spectrum around 8.2 eV that is essentially unchanged when
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(34) G. Herzberg, "ElectronicSpectra of Polyatomic Molecules",Van Nostrand, Princeton, NJ, 1966,p 601.
1829
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Conclusion A high-resolution, variable incident energy and scattering angle electron energy loss investigation of CS2has been carried out and the results correlated with all known relevant data concerning the electronic excited states of CS2 below the first ionization potential. Transitions are assigned to orbital ns,npa and probably 5 and 6pa Rydberg states. For n 5 5 the (A,B) coupling mechanism is shown to predominate and the orbital Rydberg states are assigned as singlets or triplets. For n 2 5, the (Q,w) coupling mechanism predominates and the designation "singlet" or "triplet" no longer applies. Transitions to both the 4s and 4pa Rydberg states are demonstrated to have been observed in rare gas solutions or matrices. Valence transitions are observed to at least two states above the (2aJ (3a,), Zu+, E ; , A, states. These are assigned as the (2aJ (7ag), IIg and the (2aJ (7cg), II, states. The (2x ) (7aJ transition is distinct from the (27r.J (4s) Ryaberg transition: It is enabled by one quantum of v2 and it manifests a vibrational progression in 2v2. The II, valence state is proposed to cause the observed displacement of the 4pa(II,) Rydberg state.
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Note Added in Proof. A new variable incident energy, variable scattering angle electron energy loss investigation of CS25 has appeared since this manuscript was submitted for publication. The assignment deduced therein are not in agreement with those presented here. Acknowledgment. J.P.D. acknowledges support from the National Science Foundation under grant CHE 7926051. Registry No. CSz, 75-15-0. (35) M.-J. Hubin-Franskin,J. Delwiche, A. Poulin, B. Leclerc, P. Roy, and D. Roy, J. Chem. Phys., 78,1200 (1983).
