6460
-
i22+
J. Phys. Chem. 1986, 90, 6460-6464
g211i Emission Spectra of Fluoro- and Chlorothioboron Cations, FBS' and
GIBS'
Michael A. King, Robert Kuhn, and John P. Maier* Institut f u r Physikalische Chemie der Uniuersitat Basel, CH-4056 Basel. Switzerland (Received: July 3, 1986)
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The A2Z+ R2niemission spectra of GIBS' and FBS' in the gas phase have been obtained. The cations were produced by electron impact on an effusive beam of the products of the high-temperature reaction between solid boron and the corresponding halogensulfide. A vibration$ analysispf the band systems has been made, and the frequencies of the stretching fundamentals of these cations in both the A2Z' and XzIIi states have been inferred. The rotational profiles of the emission bands indicate that B' > B" for both cations.
1. Introduction Emission spectroscopy in conjunction with electron-impact ionization is an attractive gas-phase technique for the investigation of polyatomic open-shell cations which undergo radiative relaxation from their excited states.' The resulting emission spectra are valuable sources of information on the electronic, vibrational, and, in some cases, rotational structure of the lowest lying doublet states of such cations. The main interest to date has been concentrated on ions derived from substituted acetylene, polyacetylene, and halobenzene precursors.* These spectra also provided the vital initial data for subsequent high-resolution measurements by means of laser excitation of the fluore~cence.~The most recent application of the electron-impact approach has been the observation of the emission spectra of several phosphorous-substituted alkyne ions, HCP+,4 FCPf,5 and CH3CP+.6 The current studies are concerned with the isoelectronic sulfidoboron cations, XBS', with X = H, F, CI. The spectrum of the simplest species, HBS', has been discussed in detail,' and in this article we present the results on CIBS' and FBS'. The first known species exhibiting a formal boronsulfur double bond was HBS. It was detected by mass spectrometry as a product in the high-temperature reaction of hydrogen sulfide and solid boron.* The syntheses of CIBS and FBS follow in an entirely analogous way, except that H,S is replaced by the corresponding halogen-sulfide (X-SS-X).9 Although little is known about the mechanism of the reactions leading to these rather unusual species, a considerable amount of structural data on these sulfidoborons has been obtained by microwave,I0 infrared," and photoelectron spectroscopy. *,I The results of the photoelectron investigations are of particular relevance to this study, because they yield the ionization energies and lead to an assignment of the observed emission systems as ( 1 ) Maier, J. P. In Kinetics oflon-Molecule Reactions; Ausloos, P., Ed.; Plenum: New York, 1979; p 437. (2) For a recent review, see: Maier, J. P. J . Electron Specrrosc. Relat. Phenom. 1986, 40, 203 and references therein. (3) Maier, J. P. Acc. Chem. Res. 1982, 15, 18. Klapstein, D.; Maier, J. P.; Misev, L. In Molecular Ions: Spectroscopy, Structure and Chemistry; Miller, T. A., Bondybey, V. E., Eds.: North-Holland: New York, 1983; p 175. (4) King, M. A.; Klapstein, D.; Kroto, H. W.; Maier, J. P.; Marthaler, 0.; Nixon, J. F. Chem. Phys. Lett. 1981, 82, 543. King, M . A,; Klapstein, D.; Kroto, H. W.; Maier, J. P.; Nixon, J. F. J . Mol. Spectrosc. 1982, 80, 23. (5) King. M. A.; Klapstein, D.; Kroto, H. W.; Kuhn, R.; Maier, J. P.; Nixon, J. J . Chem. Phys. 1984, 80, 2332. ( 6 ) Lecoultre, J.; King, M. A.; Kuhn. R.; Maier, J. P. Chem. Phys. Lett. 1985, 120, 524. ( 7 ) King, M. A.; Klapstein, D.; Kuhn, R.; Maier, J. P.; Kroto, H. W. Mol. Phys. 1985, 56, 871. (8) Kirk, R.; Timms, P. L. J . Chem. Sot., Chem. Commun. 1967, 18. (9) See: Kroto. H . W. Chem. Sot. Reo. 1982, 11, 435 and references therein. (10) Kirby, C.; Kroto, H. W. J . Mol. Specrrosc. 1980, 83, 130. ( 1 1 ) August, J. A.; Aziz, S.; Kroto, H. W.: Suffolk, R. J., private com-
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TABLE I: Wavenumbers and Proposed Assignments of the Observed Bands in the A2Z+ S'n, and B211,12 9zI13,z (B) Emission Spectrum of GIBS
tlcm-' 3sCIlOB32S+ 25 477.3 25 094.4 25 004.4 24961.5 24929.1 24 67 1.5 24 620.9 24 578.3 24 545.5 24 453.3 24068.8 23 949.6 23613.6 23 568.6 23 329.2 23 230.6 23 112.6 22 728.3 21 999.1 20 676.6 19371.7
'7CI1lB32S+
24960.5 24933.7 24 576.9 24 549.9 24 458.8 24074.6
35CllOB32S+
24959.6
24 575.9 24 449.9 24065.1
23617.1
23557.5
23 333.4 23 233.9
23 173.3
22006.7
R
assignt
001-000 00 1-000 100-100 OOO-OOO 010-010 000-100 100-1 00 OOO-OOO 0 10-0 10 000-001 000-001 000-002 000-100 000-002 000-200 000-100 000-101 000- 101 000-300 000-400 000-500
3/2
I/* 3/2
3/* '/2
'/I
'/I
3/2 '/2
3/2 3/2 '/2
(B)
'/2
'/2 '/2 '/2
(B) (B) (B)
"he values (f0.4 cm-') refer to Q-branch and P-branch maxima for the two band systems, respectively.
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the A22+ R2n,transitions of FBS' and CIBS'. As expected, the spectra are qualitatively similar to those of HBS" and HCP'! Whereas the emission spectrum of CIBS' consists of short progressions in the stretching vibrations, in the case of FBS' strong excitation of the ground-state bending mode is also observed. In addition, in the s p e c t r y of CIBS'-a weak band system is tentatively assigned to the B211,j2 X2113,, transition.
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2. Experimental Section The crossed sample-electron beam apparatus used has been described p r e v i ~ u s l y . ~However, the inlet system had to be modified for the production of the chemically very reactive FBS and CIBS. A 40-cm-long quartz tubing (id. 10 mm) was coupled directly to the ionization chamber through water-cooled O-ring seals and was terminated by an electrically grounded stainless steel nozzle (aperture 2-3 mm). The tube was filled with boron chips along 10 cm of its length. An electric furnace heated the tube to about 1050 OC. FBS and CIBS were produced by flowing F2S2 and C12S2, respectively, through the packed tube. FzSz was synthesized by the reaction of AgF with sulfur at 130 'C;I3 C12S2was a commercial sample. The precursors were admitted to the reactor tube from Pyrex vessels held at -95 O C with F,S2 and at room temperature with CI,S,. The products of the pyrolysis were passed directly into the ionization chamber. The resulting effusive jet was crossed with a magnetically collimated 200-eV electron beam (- 3 mA) 3-4 mm above the nozile. The pressure,
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0022-3654/86/2090-6460$01 S O / O
(8)
0 1986 American Chemical Society
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A22+
R211i Emission Spectra of C l B S and FBS'
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 6461
000 - 000 r----7 x2
I
I
3
4
5
I
I
22000 000 - 001
r-----
r
B2n-X2n
000-voo I
000-voo
21000
20000
vIcm-1
-
000 100
m B2n-F2n 2
1
i 25000 24500 24000 23500 23000 v/cm-1 Figure 1. Emission spectrum obtained by -200-eV electron impact on the pyrolysis products of CI2Szand solid boron (fwhm = 0.026 nm). The main band system is the A2Z* %IIj transition of CIBSt.
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measured downstream from the interaction region, was 6 Torr. The resulting emission was dispersed by a 1.25" fl9.5 monochromator. The detection system consisted of a cooled GaAs photomultiplier tube coupled to single-photon-counting electronics and a dedicated microcomputer (LSI 11/23) that also controlled the stepper motor of the monochromator grating. Resolutions of 20.007-mm fwhm were employed. The spectra were calibrated by simultaneously recording the spectrum of a Ne-filled U-shaped hollow cathode lamp as well as with the numerous concomitantly excited halogen and sulfur emission lines. The accuracy of the reported band maxima positions is considered to be f 1.O to f0.4 cm-l depending on the wavelength region.
3. Results and Discussion 3.1, CIBS'. Emission bands were observed in the 25 60018 000-cm-' region following electron impact on the reaction products of C12S2and crystalline boron. The main portion of the spectrum is shown in Figure 1, and the positions of the band maxima are collected in Table I together with the proposed assignments. The spectrum consists primarily of four doublets separated by 383 cm-I. Their origins are located a t 24 961.5 and 24 578.3 cm-l. The following co>siderations lead to the assignment of these bands to the '2' X211 transition of the CIBS' ion. The photoelectron spectral study of the C_lBS, backed up by calculations, showed that the ground-state X of the cation has 211 symmetry, whereas the excited 2X' and 211 states are almost degenerate.I2 The adiabatic ionization energy leading to the 22' state is clearly determined because the origin band is dominant in this transition. The difference between that value and-the ionization energy leading to the ion in the lowest level of the X211 state is measured to be 25 000 f 500 cm-', which is close to the mean of the origin bands in the emission spectrum, 24 770 cm-' (Table I). The intensity distribution of the bands in the obsecved X211 emission system (Figure 1 ) is consistent with the 2Z' transition, as the origin bands are the most intense. In contrast, the photoelectron band corresponding to the excited 211 state of ClBS' shows a broad Franck-Condon distribution, indicative of a considerable geometry change. Consequently, the location of the adiabatic ionization energy to this state is not certain, except Lhat it probably lies close to that of the 2Z' state.I3 The 211 -, X211 emission system is thus also expected to show a broad vibrational intensity distribution. Furthermore, the 383-cm-' separation of the origin bands in the emission spectrum is in accord with the spin-orbit splitting in the cationic ground state estimated from the photoelectron spectrum (-370 f 30 cm-l).l2 The final evidence that the e_mission band system is the 2 X + X211 rather than the 211 X211 transition of CIBS' comes
-
-
-
-
-
a)
24980
24970
24960
--
V/cm-l
Figure 2. (a) Higher resolution scan (fwhm = G.0065 nm) of the 7 i= 3 / 1 component of the origin band in the A22+ X211iemission spectrum of CIBS'. (b) Computer simulation of a z2+ 2113j2transition using E' = Bo(35CI'1B3ZS+) = 0.0933 cm-' l o and E" = 0.0925 cm-'. A triangular instrument function was used with the experimentally determined fwhm of 0.4 cm-l ( T = 300 K).
from the rotational contours. In Figure 2a a higher resolution scan of the band at 24961.5 cm-' is shown together with a computer simulation of a 2Z+ 2113,2 transition (Figure 2b). The B'value was set equal to Bo(3sC1"B32S) = 0.0933 cm-l.Io The B"va1ue was then varied until the observed profile was reproduced, Le., with B" = 0.0925 cm-I. The spin-orbit constant for the 211 state was set to -383 cm-I, which is the separation of the doublets in the emission spectrum. Due to the similarity of the rotational constants in the two states, only one maximum (in the Q branch) is obtained (Figure 2). In contrast, a satisfactory fit to the observed contour could not be obtained for a 211-211transition using any reasonable set of rotational constants. A second weak emission system is also apparent. It consists of a progression with five members, the first of which is located at 24671.5 cm-' (cf. Figure 1). The successive intervals decrease fairly irregularly, 1342, 1330, 1322, and 1305 cm-I. By comparison with the vibrational analysis of the main system (vide infra), these bands are attributed to a progres_sionof the v I (BS stretch)" mode in the cationic ground-state X2113/2. However, some of the bands show a complicated structure which we have not been able to account for. Comparing the first interval, 1342 cm-', with the vibrationalfundamental of 1347.8 cm-' (Table I) derived from the 2Z+ X2113/, system, the first member_of this progression is tentatively assigned to the 000 2113/2-100X2113,2 Lransition of CIBS'. By extrapolation, the origin of the 'II3,? X2II3,? transition is located at 26019.3 cm-' which determines
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6462
King et al.
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986
TABLE 11: Fundamental Frequencies (hO.8 cm-') of the Detected Isotopic Species of GIBS' together with the Molecular Ground-State Values" state
uI
R*n,
1347.8 1390.6 1407.8
A2Zi
XIZ+
V3
508.9 516.0 520-530
VI
V3
1'1
VI
1343.2
502.0
1402.4
510.3
1464
-
the order of the excited states, Le., A2Z' and B211,-and thus the transitions ar_e labeled in the tables and figures as A2Z+ R211 and B211 X2n,The absence of a transition involving excited levels within the B211 manifold of GIBS' could be due to vibrationa_lly induced int_ernal conversion process of such levels within the B state to the A state. The vibrational structure of the emission spectrum of CIBS' is complicated by the prese_nce of eight naturally occurring isotopic species. In the '2' X'n, system, bands of the three most abundant species (Le., 3sC1'1B32Sf(57.6%),37C1"B32S+(18.6%), and 35C1'oB32S+(14.2%)) could be identified, and in the weak B2n3 R2113,2 system only bands due to 3sC110B32S+and 37C11dB32S+ could be-assigned, In the case of the A'Z' X2n,emission system, most of the bands can be attributed to the single excitation of the totally symmetric stretching modes, v l or u 3 , in both electronic states. The values inferred for vl" (B=S stretch) and u j / / (CI-B stretch) of 1347.8and 508.9 cm-' (for 35C11'B32S')may be compared with the molecular ground-state values of 1407.8 and 530 cm-'.I' On the basis of the value v,' 1400 f. 30 cm-' obtained from the photoelectron work,'* the weak sequence bands to the blue of the
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-
-
-
-
origins are assigned to the 100-100 transitions, and accordingly a more precise value of vl' = 1390.6 cm-I (35C1'oBj2S') is deduced. Then the stronger bands -32 cm-' to the red of the origins have to be assigned to the 010-010 sequence transitions. The derived fundamental frequencies are summarized in Table 11, together with the molecular ground-state values. The radiative lifetime of the lowest vibrational level of the A2B+ state of CIBS' was also measured by using pulsed electron-impact excitation to be 240-A 13 ns. This value is considerably less than the lifetime of the A2Z' state of HBS', where a lower limit of 2300 f 200 ns was determined. The lifetime shortening is suggestive of intfamolecular interactions due to the energetically overlapping A'Z' and B211 states of CIBS'. 3.2. FBS'. The emission spectrum obtained with the products of the high-temperature reaction of F2S2and boron is shown in Figure 3. The measured positions of the assigned or prominent bands are collected in Table 111. The spectrum consists of a series of composite doublets separated by -370 cm-'. The lower energy components of these doublets again show a multiplet structure. These bands can be grouped into two progressions with intervals of -635 and 1720 cm?. The combination bands involving pairs of the three modes are quite strong. The origins of the band system are located s t 26 873.1 and 26 502.7 cm-', respectively. In some of the transitions satellites with about 25% intensity of the main bands are discernible. These must be due to a second isotopic species. The intensity ratio (-4:l) suggests that the emitting molecule contains one boron atom. Furthermore, the average subband separation of -370 cm-' is of the order expected for the spin-orbit splitting in a 211 state of a sulfur-containing species (cf. section 3.1). These observations together with the photoelectron study on FBS13 indicate that the emission is due
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TABLE 111: Wavenumberso and Proposed Assignments of the Observed Band Maxima in the i * C + t/cm-l transition tlcnii'
19F11B32S+i9FioB32S+ A2Z+ state 100 2Z+ 28 590.7 28 655.1 28 583.0 28 25 1.4 28 242.9 28 220.6 27 955.0 27 848.6 27 564.0 27 556.0 27 228.7 27 193.7 26 896.2 26 889.0 26 887.5 26873.1 26 870.0 26 866.4 26551.5 26 533.7 26 527.6 26 502.7 26496.6 26 494.5 26 299.6 26 288.1 26 273.8 26 236.6 26 229.7 26 167.5 26 142.0 26 134.2 26 129.7 26 122.4 25 923.3 25 904.0 25 897.8 25 874.1 25 870.1 25 863.8
100 2z+ I00 2z+
26 867.7 26 53 1.4 26 525.2
26 233.3 26 226.2
010 p 2 s
001 2z+ 001 001 *E+
000 *ni,2 000 *n,2 010 p 2 z 000 *n, 2
000 TI3,* 100 TI,, 2 000 2ni 2
000 *z+ 000 2s+ 000 *z+ 000 2z+
010 p 010 p
000
*z+
000
*z+
000 *z+ 25 899.9 25 893.3
000 *z+
25 860.3 25 854.7
000 *z+ 000 *z+
000
2z+
QI
000 *nl2
000 2z+ 100 2z+ 000 *z+
000 2z+ 000 *z+
branchb QP21 +
010 p 22
IO0 *z+ IO0 2\.+
*z+ 001 zz+ 26 874.6
g2n,state 000 TI,,1 000 2n3 2
2 s
ooo *n,z
000 TI,,?
001 TI3,* 001 TI,,* 010 h
*z
020 020 011 011
p 2r13,2 p* 2
n,
k fi
28 22
001 001 TIl
*
Q P 1 2+ Q I PI QP12+ Q ! P, Pz + 'QI2 QP,,
All values 1 0 . 8 cm-I. * A n asterisk denotes a tentative assignment.
- R'n,
Emission Spectrum of FBS+ transition
19F"B322s+ 19F'oB32S+h2Z+state 25 790.9 25 783.8 25 749.3 25 743.5 25 608.6 25601.6 25 539.7 25 533.2 25 510.6 25 502.8 25 283.5 25 278.0 25 240. I 25 233.8 25 151.7 25 145.8 25 134.4 24987.8 24980.3 24814.6 24 808.9 24 801.9 24 796.6 24 784.7 24778.9 24 6 15.9 24609.8 24 541.6 24 535.3 24 520.5 24514.5 24 191.1 24 154.4 24 149.0 23 897.7 23 891.6 23 530.0 23 524.8
R2n,state
25 598.6 25 59 1.8 Oll
25 227.8 25 092.8 25 086.5
k-25
012 p 22 012 p ' 2 002 2ni12 002 *nip 100 TI3,> 100 *n3;* 003 211,12 003 211ji, 110 p 2z IIOp2C
24 726.9 24721.3
IO0 2n1,2 100 * r I , 2
003 *111,> 003 2n1,2 101 2n3,2 101 2n3,2 111 p 2 z
101
2ni,2
101 *11112
102 102 2n3;* 102 *nII2 102 2 r 1 1 , 2
branch*
A2Z+
-
A211i Emission Spectra of CIBS' and FBS' 000 - 000
m ,
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986 6463 100 - 000
m
I
I
I
I
28500
28000
27500
G1cm-l
000-voo
i
2
1
000 - 100
m
I
-
27000
001 - 000
m
26500
2 5 500
25000 Tlcm-1 Figure 3. Emission spectrum of the A2Z* x2n,transition of FBS', obtained by -200-eV electron impact on the pyrolysis products of F2S2and boron (fwhm 0.026 nm). Bands denoted with a dot are atomic (F, S ) emission lines. The double markers in the vibrational assignment on top of the low-energy components refer to the two possible assignments (cf. section 3.2).
-
26000
-
to the A2Z' A211itransition of FBS'. As is the case with HBS', the symmetry of the cationic ground state is 211iand that of the first excited state is 2Z'. By use of the adiabatic ionization energies of 10.91 f 0.05 and 14.20 f 0.05 eV leading to these states,I3 the transition frequency for the origin of the A2Z' X2nitransition of FBS' is predicted at 26 500 f 600 cm-I. This value is in agreement with the mean of the origin bands, 26 688 f 1 cm-l, in the emission spectrum. -The secfnd excited state of FBS' lies at 17.2 eV,I3 and thus the B211 X211 transition would lie in the vacuum-UV, inaccessible t_o our instrumentation. Regardless, emission is not likely, as the B211state lies well above the dissociation limit. Higher resolution scans (0.0065-nm fwhm) of the origin bands are shown in Figure 4. The rotationa_lcontou? of the bands are X2nitransition of quite different from the ones in the A2Z' CIBS'. As in the corresponding transition in HBS',' the bands show two distinct heads separated by -6.5 cm-l. Assuming B' (FBS+:2Z+) = B,(FBS) = 0.165 ~ m - ' the , ~ lower state rotational constant B"is estimated to be 0.157 cm-' according to the relation 1/B" - 1/B' = 2/6, where 6 is the separation of the Q- and P-branch heads. The ratio AB/B'- 4.8% may be compared with the corresponding ratios in HBS' (6.5%)' and CIBS' (0.9%) (cf. section 3.1). Whereas the assignment of the higher energy component (Figure 4a) is straightforward, two different assignments are feasible in the case of the low-energy bands (Figure 4b). This is because the frequency of the ground-state bending- mode of FBS' is close to the effective spin-orbit splitting in the X211i state of FBS'. This accidental near-degeneracy of the 000 2111j2and 010 p 2Z states suggests that Coriolis plus vibronic coupling mechanism mixes these two states (and hence may explain the similar intensity of the two main bands in Figure 4b). A similar mechanism for the 000 *II3/2 and 010 K 2Z states is much less likely since these are separated by -700 cm-I, and in fact the 000-010 K 22 transition is only detected as a weak band. A similar pattern is also observe? in the transitions from the vibrationally excited levels of the A2Z' state. However, in the absence of a rotational analysis for these transitions, the order of the 010 p * 2and 000 2111,2levels cannot be decided on unambiguously. The reasons which led us to prefer the assignment shown in Figure 4 and Table I1 are summarized b_elow. The effective spin-orbit splitting constant in the X211istate of the isoelectronic cation FCP' is -190.2 ~ m - l and , ~ that for HCP' is -146.97 One expects a similar change in the constant on passing from HBS' to FBS'. If the band a t 26 533.7 cm-I is assigned to the 000 *Z+ 000 2111,2transition, an effective
ooo %+- ooo 2n1,, F"0S' F'005*
4 7 h
-
,I
loo 21'- too Zn,,? I
-
b)
-
-
I
I
I
26570
26560
26550
265L0
1
I
,
26530
26520
26510
-
. "
,
c
26500
_
G/cm-'
Figure 4. (a) Higher resolution scan (0.0065-nm fwhm) of the il = 3 / 2 component of the origin bands in the A28+ R2n,transition of F B S . (b) Higher resolution scan (0.0065-nm fwhm) of the low-energy components of the origin bands in the A22+ R211, transition of FBS. For a discussion of the assignment, see section 3.2.
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spin-orbit splitting of -339 f 2 cm-l-is deduced. This value is only slightly larger than the one in the X2njstate of HBS', -322.6 f 0.4 cm-'.' On the other hand, the assignment of the transition to the band at 26 502.7 cm-I yields A"(X 211j) = -370 f 2 cm. Furthermore, as is apparent from Figure 3, the lower energy components in the transitions involving the excitation of the stretching vibrations, u1 and u3, in the electronic ground state, are also split into two main bands. Whereas the rotational profiles of these bands to lower energy are similar to the profiles of the corresponding Q = 3/2 bands, the intensity ratio of the two maxima in the high-energy components is quite different. Also, the intensity of these low-energy components varies roughly proportionally to the intensity of the corresponding fl = 3 / 2 bands, whereas the components to higher energy vary strongly with increasing vibrational excitation. This effect is most pronounced in the 000-OOu, u3 = 1, 2 transitions (cf. Figure 3). Thus, these intensity considerations favor the assignment of the high-energy component to the 000 22'-010 p 'Z and the low-energy component to the 000 22'-000 2r11,2transition. The pattern of the isotopic satellites in the origin bands also supports this interpretation. The S I = 3 / 2 origin transition of 19FloB32S+ is shifted 1.5 cm-' to the blue relative to the '9F'1B32S'
J . Phys. Chem. 1986, 90, 6464-6470
6464
to be 0.2 i 1 cm-' to the red of the origin band and is thus obscured by it. The proposed assignmen: then leads to an interpretation of most of the strong bands in the A2Z+ X2nitransition of F B S . The spectrum consists mainly of short progressions in u3" (B-F stretch) and ul" (B=S stretch)." The inferred vibrational intervals are collected in Table IV. It should be noted that there exists a small but significant variation of the separation of the spin-orbit doublets (370.4-366.5 cm-l) in the different vibrationally excited levels of the g211iground state (cf. Table 111). The reason for this may be Fermi resonance, but the present data are of insufficient quality for a quantitative analysis. In addition, transitions involving the excitation of the ground-state bending mode in a single quantum are apparent. These occur as combination bands with the stretching vibrations. In the case of the 000 2Z+-01c3 c3 = 0, 1 transitions, the K 2Z components are also weakly observed. However, there still remain a few weaker bands which have not been assigned.
TABLE IV: Vibrational Fundamentals of FBS' (*2 cm-') Determined from 0-Branch Maxima
m,,:
"I
u2
9n,
637
64 1
VI
1776 642 1781
U?
69 I
Ul
u3 A22+
1782 343
1718 633 1718
"3 12
1721 339
-
origin (Table 111). In the region of the S I = origin bands, a transition due to F'OBS' is discernible to the red of the high-energy band (cf. Figure 4b). The isotopic splitting is -2.3 cm-I, and kence the frequency difference of the bending fundamental in the X2JI3,*state in F"BS+ and F'OBS' is calculated as -3.8 cm-I. Alternatively, if the band a t 26 533.7 cm-' is due to the 000 ,2+ 000 %,,, transition of FBS', the isotopic splitting between the two spin-orbit components would be -3.8 cm-I. This value seems too large compared with the values measured in the emission spectrum of 35CI'1B32S+/3SClioB32S+0.5 cm-' (cf. Table I) and Hi'B32S+/H10B32S+ 0.3 cm-'.' Assigning the band a t 26502.7 cm-' to the R = origin of both F'IBS' and F'OBS' yields 1.5 cm-I for the isotopic separation in the two spin-orbit origins. The preferred assignment also explains the apparent absence of the 100 100 2111;z sequence band. Its location is calculated
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-
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Acknowledgment. This work is part of project no. 2.429-0.84 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung. Ciba-Geigy SA, Sandoz SA, and F. Hoffmann-La Roche & Cie SA, Basel, are thanked for financial support and SERC (U.K.) for the award of an overseas Fellowship to M.A.K. Prof. Harry W. Kroto, University of Sussex, England, is thanked for his friendly advice concerning the syntheses of FBS and ClBS and the fruitful collaboration over the past years.
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An IR-UV Double Resonance Study of CBr,F,. States'
Evidence for V-V' Vibrational Steady
Nancy C. Rothman, David F. Dever, Dana Garcia, and Ernest Grunwald* Department of Chemistry, Brandeis University, Waltham, Massachusetts 02254 (Received: August 21, 1985; In Final Form: May 19, I9861
CBr2F2gas a t pressures above 10 Torr was vibrationally excited with an IR laser pulse at 1088 cm-I, and relaxation of the vibrational distribution was probed by UV absorption in double resonance. The UV probe band ,,X(, = 227 nm) is due to an orbitally forbidden vibronic transition; the vibronic coupling mode is identified as the CBr, asymmetric stretch at 829 cm-'. Equilibrium temperature profiles of UV absorptivity were measured at 229 and 257.6 nm. The latter is "hot band" absorptivity and is found to indicate the temperature of the 829-cm-I mode. At 229 nm the time evolutions of IR and UV resonance are similar, but at 257.6 nm the UV resonance lags significantly behind the IR resonance. Time lags at 257.6 nm agree approximately with V-T/R relaxation times under all conditions. The dynamical properties of CBr2F2satisfy sufficient conditions for the existence of V-V' vibrational steady states, in which each vibrational mode has a characteristic temperature depending on the instantaneous T / R temperature according to eq 1-3. Computer simulations of the time evolution of the 829-cm-' mode temperature in V-V' vibrational steady state reproduce the UV resonances at 257.6 nm. An appendix reports a detailed study of infrared absorptivity of CBrzFzas a function of IR fluence, IR pulse length, and CBr2F2pressure.
W e report an IR-UV double resonance study of dibromodifluoromethane, CBr,F2, in the gas phase a t pressures above 10 Torr. The infrared was absorbed near the center of the sym-CF, stretching band' from a 1088-cm-' laser beam at MW/cm* power levels. The IR absorption produced very significant vibrational excitation. The simultaneous absorption of low-intensity ultraviolet radiation was measured at selected wavelengths in the first UV band of CBr2F2 (A,,, = 227 nm).3,4 This absorption probes the relaxation of the vibrational excitation energy introduced by I R absorption. The facts a t hand suggest that relaxation proceeds ( I ) It is a pleasure to acknowledge financial support by the National Science Foundation and by the Edith C. Blum Foundation. (2) Plyler, E. K.; Acquista, N. J . Res. Natl. Bur. Stand. 1952, 48, 92. (3) Davidson, N. J . Am. Chem. SOC.1951, 73, 467. (4) Doucet, J.: Gilbert, R.; Sauvageau, P.; Sandorfy, C. J . Chem. Phys. 1975. 62. 366
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by a mechanism of time-dependent V-V' vibrational steady states .s-lo Vibrational steady states can exist when the vibrational energy is either too great or too small to be consistent with the T / R temperature TTIR.6i11 For absorption from high-power infrared lasers, such steady states form when both intermode (V-V') and resonant (V-V) vibrational exchange are fast relative to V-T/R ( 5 ) Treanor, C. E.; Rich, J. W.; Rehm, R.G. J . Chem. Phys. 1968, 48, 1798, 1806. (6) Teare, J . D.; Taylor, R. L.; von Rosenberg, C. W . Nature (London) 1970, 225, 240.
( 7 ) Shamah, I.; Flynn, G. J . Chem. Phys. 1978, 69, 2474. (8) Steel, C., cited in ref 9, footnote 49. (9) Grunwald, E.; Liu, S.-H.; Lonzetta, C. M. J . Am. Chem. SOC.1982, 104, 3014.
(10) (a) McNair, R. E.; Fulghum, S . F.; Flynn, G. W.; Feld, M . S.; Feldman, B. J. Chem. Phys. Lett. 1977. 48, 241. (b) Shamah, 1.: Flynn, G . J . A m . Chem. Soc. 1977, 99, 3191. (11) Shamah, 1.; Flynn. G . J . Chem. P h ~ , s1979, . 70, 4928.
CZ 1986 American Chemical Society
