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Dye laser excitation of the fluorosulfate radical
JOURNAL
OF MOLECULAR
SPECTROSCOPY
84,
Dye Laser Excitation
102- 112 (1980)
of the Fluorosulfate
Radical
Origin Band Excitation CHARLES H. WARREN Department
of Chemistry,
Dalhousie
University, Halifax, Nova Scotia, B3H 453, Canada
Discrete visible fluorescence is observed from excitation of the fluorosulfate radical by means of a dye laser. The frequency of the dye laser is chosen to coincide with each of the two components of the origin band observed at 19 383 and 19 354 cm-*. Ground-state fundamentals are assigned for the radical and evidence is provided to suggest that the ground state of the radical may not have CSp symmetry. INTRODUCTION
The fluorosulfate
radical, S03F, exists in equilibrium with the dimer S206F2. S&F,
3 2 S03F.
The equilibrium is temperature dependent and was first studied by Dudley and Cady (I). When the dimer, a colorless liquid, is heated to approximately 100” C, the relatively weak peroxy linkage is ruptured to yield the radical which in the liquid and gas phase has a distinctive brownish yellow color. The color of the radical occurs because the radical absorbs strongly in the visible region from MOO3800 A. The visible absorption was studied under low and high resolution by Warren and King (2-4). The origin of the system occurs around 5160 w and is observed as a doublet with a 29-cm-l separation between components. Under high resolution, the rotational structure is resolvable and from an analysis of this structure it was concluded that the transition was Y?(2)-X*A, under the C,, molecular point group. The analysis of the vibronic structure of the visible absorption system was also consistent with the assignment and further support was provided by the results of molecular orbital calculations (5). The analysis of the spectrum was carried out on the basis of a Jahn-Teller spin-orbit interaction operative in the upper *E state. Ground-state fundamentals of the radical were determined from an analysis of the hot bands to the low wavenumber side of the origin band. The six fundamentals of the radical were identified as 1055.5, 839.3, 533.5 (a, symmetry) and 1177.7, 604.1, and 369.4 cm-’ (e symmetry). The hot bands occur as doublets whose separation is the same as the doublet splitting of the origin band, 29 cm-l. The splitting was assigned as the spin-orbit splitting of the upper *E state. The majority of bands to the high wavenumber side of the origin band also occur as doublets; however, the doublet separation is not constant due to vibronic interaction in the upper 2E state. The infrared spectrum of the SOBF radical was observed at low temperatures in argon and nitrogen matrices by Suzuki et al. (6). Fundamentals were assigned 0022-2852/80/l10102-l1$02.00/O Copyright
0 1980 by Academic
All rights of reproduction
Press, Inc.
in any form reserved.
102
FLUOROSULFATE
RADICAL
FLUORESCENCE
103
which were in agreement with those determined from the hot bands of the visible absorption system. Further confirmation of the ground-state fundamentals of the radical was provided by the observation of the fluorescence spectrum of the radical in the gas phase using the following argon-ion laser lines as excitation sources: 5145, 4017, 4965, 4880, and 4765 A (7, 8). In general, the fluorescence spectra were rather complex and not all bands were assigned. Displacements arising from the fundamentals vg and Yewere often weak or overlapped by other bands and their assignments were not as definite. Similarly in the solid state matrix infrared spectrum, the peaks assigned to vg and vg were quite weak. One of the drawbacks of using the argon-ion laser as an excitation source is that the point of excitation in the upper state cannot be controlled to any degree. It was felt that by using the dye laser as an excitation source the analysis of the fluorescence spectrum would be simplified since one could selectively populate known upper-state levels. In this paper, the fluorescence spectrum of the fluorosulfate radical is reported when the radical is excited at an energy corresponding to the origin band of the visible absorption system by a dye laser. EXPERIMENTAL
DETAILS
The preparation of the dimer and evidence for the existence of the radical have been reported in a previous paper (1). Purification of the dimer was achieved by vacuum distillation and Raman spectra of the colorless dimer in the liquid state were taken and found to be identical with those reported by Aubke and coworkers (9). For the experiments in the vapor phase, the dimer was vacuum distilled into a Pyrex cell 4 cm in length with 25mm Pyrex windows fused to the ends of the cell. A small sidearm was attached to the cell and contained liquid S,O,F, under a pressure of its own vapor at room temperature. The radical was generated by heating the body of the cell to approximately 80” C by means of nichrome wire wound around the cell. The radical was excited by means of a Molectron DL-12 dye laser. The dye laser was pumped by a pulsed nitrogen laser. The nitrogen laser was constructed in this laboratory and is similar in design to the one reported by Schenck and Metcalf (IO). Its power output was approximately 150 kW. Scattered radiation at 90” to the incident laser line as it passed through the cell was collected and focused onto the entrance slit of a Spex 1401 double monochromator containing two 1200 lines/mm gratings blazed at 5000 A. Radiation emerging from the exit slit was monitored by means of a boxcar detector which was also constructed in this laboratory. All spectra were recorded with the slits of the spectrometer set at 120 p. The spectra and the frequency of the dye laser were calibrated with neon lines from a discharge lamp. RESULTS
Under low resolution, the spin-orbit doublet components of the origin band are observed at 19 383.1 and 19 353.7 cm-’ (vat.) and are designated as ‘0: and -O& respectively.
104
CHARLES H. WARREN
2500
I500 C
I
1000
0e-vw
19330 CM-’
FIG. 1. Absorption spectrum of the SO,F radical under high resolution.
In Fig. 1, a high-resolution trace of the absorption spectrum of the origin band of the radical is shown. If the dye laser was tuned a few cm-* above 19 383 cm-‘, no detectable fluorescence was observed. As the dye laser wavenumber was decreased from this point, the fluorescence very sharply increased as one entered the sharp absorption edge of the origin band. Wavelength resolved emission spectra of the radical were recorded for a large number of dye laser excitation frequencies within the band contour of the origin band. The resulting spectra in the region of 534 5 50 cm-’ below the point of excitation are shown in Fig. 2 for a few selected dye laser frequencies. The 534~cm-’ interval coincides with the fundamental y3 which is generally observed as the most intense transition in the fluorescence spectra. There was no strong correlation between the positions of the subband origins and the intensity of the resulting fluorescence indicating that the effects of pressure broadening and rotational relaxation are important. The positions and intensities of the small satellite bands attached to the main level vary as one decreases the excitation frequency. The only apparent regularity is the appearance of a band to the low wavenumber side of the main transition (indicated by an arrow in Fig. 2) and the separation between this band and the main band increases as the excitation frequency decreases. Upon excitation there is relaxation to the lowest rotational level of the high-frequency spin-orbit component of the upper electronic state (2E3,2)and since the rotational structure of the origin band is degraded to the red, transitions from this level lead to the appearance of a band to the right of the main transition. The same phenomena is observed as the excitation frequency is decreased below 19 354 cm-‘. A satellite band emerges
FLUOROSULFATE
105
RADICAL FLUORESCENCE
on the low wavenumber side which is interpreted in terms of relaxation to the lowest rotational level of the low-frequency spin-orbit component of the upper electronic state (*EIw). The results indicate that the lowest rotational levels of the
19386.2cn-’ EXCZTATION
C
19363.7 cm-
19368.2 cn-’
19373.4 cn-’ t
t
t
FIG. 2. Laser-induced fluorescence of the S03F radical obtained by exciting the radical at selected frequencies within the origin band envelope. The fluorescence is recorded in the wavenumber region corresponding to the transition 3p.
106
CHARLES H. WARREN .25ee
CPS
0
rma -%
CPS
e -leaf4
-9e
Cd FIG. 3. Laser-induced fluorescence spectrum ofthe S03F radical. Origin band excitation at 19 383 cm-‘.
two spin-orbit components of the upper electronic state occur around 19 383 and 19 354 cm-l, respectively, and that the 29-cm-’ splitting of the origin band must occur in the upper electronic state of radical; otherwise, two transitions separated by 29 cm-l and of approximately the same intensity would be observed. Wavelength resolved emission spectra of the radical were recorded for 19 383and 19 354-cm-’ dye laser excitation and the spectrum obtained from the former excitation is shown in Fig. 3. The only differences in the two spectra were with respect to the positions and intensities of the small satellite bands attached to the strong bands. Many of the bands of the 19 353-cm-l excitation have a weak band displayed 28 cm-’ to the low wavenumber side of them. This weak band is absent in the 19 383-cm-’ excitation. Since the rotational transitions associated with the
FLUOROSULFATE
RADICAL
FLUORESCENCE
107
transition ‘0: overlap those associated with the transition -O!, the higher rotational levels of the upper spin-orbit component of the ground state of the ‘E state are also populated by 19 354 cm-’ excitation. Relaxation then takes place to the lower rotational levels of the ‘0: component which subsequently emit leading to the satellite at 28 cm-‘. This process is felt to take place rather than a transfer of energy from the lower spin-orbit component to the upper spin-orbit component. If the latter process were taking place then the results summarized in Fig. 2 should indicate a substantial increase in intensity of the 28-cm-l satellite band when the -0: transition comes into resonance. Similarly, if the two upper-state levels were strongly coupled, one would expect to see transitions from the lower component when the upper component was populated by 19 383-cm-’ excitation. The results summarized in Fig. 2 do not seem to support this mechanism. Although satellite bands to the high wavenumber side of the main transition are sometimes observed, they cannot be consistently interpreted by the above mechanism. In order to determine if all of the bands observed in the spectrum displayed in Fig. 3 were associated with the radical, the excitation frequency was changed to approximately 2 cm-’ above the resonance condition at 19 383 cm-‘. All bands decreased markedly in intensity and to approximately the same degree. It is unlikely that a band due to an impurity would show the same behavior in the nonresonance condition as a band due to the radical. The fluorescence spectrum shown in Fig. 3 is characterized by a number of sharp intense bands on the Stokes side of the exciting line. Only a few relatively weak bands are observed on the anti-Stokes side. The wavenumber displacements of the strong bands can be analyzed in terms of the ground-state fundamentals of SO,F that were previously established. If the radical possesses Csa symmetry in its ground electronic state, then six fundamentals should be observed of which three have a, symmetry and three have e symmetry. The six most intense peaks in the spectrum occur at 369, 534, 605, 840, 1057. and 1179 cm-’ from the exciting line and are assigned to Q, vg, Q, u2, vl, and V+ All of the fundamentals are clearly identified in contrast to the argon-ion results in which displacements corresponding to vg and v6 generally did not appear as sharp intense bands but were often weak, missing, or overlapped. Only overtones of ug and yz are readily assigned. 2~~is observed at 1068 cm-’ as a fairly strong band and 3v3 is observed weakly at 1603 cm-‘. 2v, is seen at 1676 cm-’ as a weak band. 2v, and v2 + V~should both occur around 1208 cm-‘. Two bands are observed at displacements of 1215 and 1199 cm-‘. Results from other excitations seem to favor 1215 cm-’ as the v2 + vg assignment. Overtones of v1 and V~ may be too high to be observed with sufficient intensity; however, it seems strange that 2~ is not observed. If the mode assigned to v6 were harmonic, then 2~~ should occur around 738 cm-‘. No band is observed in this region and there seems to be no suitable candidate for 2v, within 2200 cm-’ of this except possibly a weak band at 563 cm-‘, which would appear to be too low and represent an extremely large departure from harmonicity. The majority of the remaining strong bands can be assigned in terms of combinations of the fundamentals. The most intense combinations are at displacements of 1372, 1712, and 2017 cm-’ and are assigned as ve + ug, v3 + vq, and vZ + v*. The
CHARLES
108
H. WARREN
band corresponding to y3 in the spectrum is the most intense band. Consequently, all combinations involving v3 or 2v3 with one quantum of any of the other fundamentals are observed as fairly intense bands. Similarly, the displacement assigned TABLE
I
Assignments for SO,F WAVENUMBER DISPLACEMENT
ASSlGNMENT
WAVENUMBER DISPLACEMENT
-165
-1467
w
-370
- 1506
VW
-426
-1589
m
-534
- 1603
w
-605
-1662
w
-701
-1672
m
-765
-1675
m
-840
- 1689
w
-850
-1712
ms
-861
-1733
w
-905
-1714
w
-935
-1774
w
-973
-1784
m
-1008
-1803
”
-1057
-1813
w
-1068
-1897
m
- 1099
-1906
m
-1141
- 1958
VW
-1179
-1978
w
-1199
-2003
YW
-1213
-2017
m
-1237
-2125
w
-1372
-2197
w
-1442
-2205
”
-2227
-2547
w
- 2248
-2580
w
-2267
-2728
w
-2284
-2762
”
-2309
-2047
w
-2382
-3074
w
-2426
-3089
w
ASSIGNMENT
FLUOROSULFATE
RADICAL
FLUORESCENCE
109
to vq occurs as the second most intense band and combinations of this mode and all other fundamentals except v6 are clearly observed. vg is only observed in combination with Z+and v3 whereas all the other possible combinations of ZQ,z+, p3, uq, and us taken two at a time are seen. The assignments are summarized in Table I. The stronger bands in the spectrum are readily assigned, however, many of the weaker bands are not as easily analyzed. Some of the bands seem to arise from a level above the vibrationless ground state of the upper electronic state of the radical. Weak anti-Stokes bands are observed at 195,203, and 212 cm-’ from the exciting line whose wavenumber is 19 383 cm-‘. No corresponding displacements are found on the Stokes side. In the electronic absorption system of the radical, a strong band having a band structure rather different from the origin band is observed at 19 592 cm-‘, which is 209 cm-’ above the high-frequency component of the origin band. It appears that this level is being populated by the excitation. The anti-Stokes bands must be associated with the radical and not some other species since other bands are observed in the spectrum which differ in wavenumber from the anti-Stokes bands by ground-state fundamental frequencies of the radical. The most prominent band of this latter type is the broad band having components at - 158 and - 169 cm-’ from the exciting line. This differs from the anti-Stokes bands by the interval corresponding to v6. Transitions arising from 19 592 cm-’ and terminating on levels assigned to us, v3 + z+,, 2v,, uz + v5, 2v,, and uz + E.‘~+ v5 are the most intense. These bands are also seen with the 19 353 cm-’ dye laser excitation and would require excitation of an upper-state level at approximately 19 560 cm-‘. In the absorption spectrum bands are seen in this region although a bandhead is not observed as is the case with the band at 19 592 cm-‘. The bands at - 158 and - 169 cm-’ are observed with both excitations but are approximately twice as intense with the 19 383-cm-’ excitation. An alternative assignment exists for these bands. They could be analyzed as the transition vg - v3 which corresponds to -164 cm-‘. This assignment, however, has been rejected since the anti-Stokes bands cannot be explained in this fashion. Laser excitation spectra for the radical were also observed. In Fig. 4a, the spectrometer was fixed to pass light having a wavenumber of 19 382 and the dye laser frequency scanned. Bands corresponding to the transitions +6:, +3:, ‘5: and +2? were readily assigned. Each of these bands had one strong component. The following additional transitions were observed when the dye, Molectron C-500, was replaced by the longer lasing dye, sodium fluorescein: +396:, ‘ly, +32, +3$:, +4?, +2?68, and +2?3?. The most interesting aspect of the spectrum is the cluster of bands whose wavenumbers are 190,202, and 207 cm-’ above the wavenumber of ‘Og, the wavenumber at which the spectrometer was fixed. Again bands which differ from the above bands by one quantum of vg are observed at 160 and 168 cm-1 below the wavenumber corresponding to ‘08. It appears that the level approximately 207 cm-’ above the upper spin-orbit component of the ground state of the upper electronic state must be very strongly coupled to it in order that radiation of wavenumber 19 383 can be emitted and passed by the spectrometer. No other bands of a frequency greater than 19 383 cm-’ were observed. The spectrometer was also locked at the wavenumber corresponding to the lowfrequency component of the origin band at 19 354 cm-’ and the frequency of the dye
110
CHARLES H. WARREN 2888
1 CPS
- CPS
.e I 9588
l9m3
CM-' FIG. 4. Laser excitation spectrum of the SO,F radical obtained by scanning the dye laser frequency. (a) Spectrometer locked at 19 383 cm-‘. (b) Spectrometer locked at 19 354 cm-‘.
laser scanned. For dye laser frequencies below 19 354 cm-l the spectrum was very similar to that in the previous case except that each main peak was accompanied by a small peak 28 cm-’ above it. For dye laser frequencies above 19 354 cm-‘; the spectrum shown in Fig. 4b differed substantially from the previous case in that many more bands were observed. The strongest bands were found at 19 382, 19 783, 19 879, and 19 888 cm-*. The difference between these wavenumbers and the wavenumbers at which the spectrometer was set were 28, 429, 525, and 534 cm-‘. In the absorption spectrum of the radical, bandheads were observed at 19 383, 19 784, 19 879, and 19 888 cm-‘. The 28-cm-’ satellite observed must correspond to excitation of the upper spin-orbit component of the ground state of the upper electronic state. Relaxation to the lower spin-orbit component then occurs which emits at 19 354 cm-l. The bands at 19 888 and 19 879 cm-l correspond to the transition ‘5;. This transition is observed since emission from the radical via the transition +5&3?gives a wavenumber corresponding to 19 354 cm-‘, the wavenumber at which the spectrometer was locked. Similarly the bank at 19 783 cm-l corresponds to the absorption transition previously assigned as Pi. In order for this transition to be de-
FLUOROSULFATE
RADICAL
FLUORESCENCE
111
tected, radiation must be emitted such that the radical returns to a state 429 cm-’ above the vibrationless ground state. The 429-cm-’ interval will be discussed later. A number of bands for the spectrum shown in Fig. 3 have yet to be assigned. In particular bands corresponding to displacements of 426 and 1099 cm-’ cannot be fitted into the analysis of the radical spectrum. These bands decreased in intensity in the same fashion as the other bands of the spectrum when the laser was tuned slightly off-resonance. They were also observed with the 19 354 cm-’ excitation and furthermore were observed in many of the spectra generated by argon-ion laser excitation and in the spectra generated by locking the spectrometer drive and scanning the dye laser frequency. In the matrix infrared spectrum of the radical, unassigned bands were observed at 427 and 1104 cm-‘. Similarly hot bands in the visible electronic absorption spectrum of the radical were observed at intervals of 427 and 1097 cm-’ to the low wavenumber side of the origin band. The evidence suggests that the -426 and - 1099 cm-’ displacements correspond to ground-state levels of the radical. No other bands could be found which would correspond to combinations of these levels with the other ground-state fundamentals. One possible explanation for these levels is that the molecule is distorted from Cat symmetry in the ground electronic state to C,Ysymmetry. This would lead to the appearance of nine fundamentals and the corresponding transitions would all be allowed. If the 429 and 1099 intervals correspond to fundamentals, it seems surprising that no combinations with other fundamentals are observed. A second possible explanation is that the ground electronic state of the radical has E symmetry instead of A, symmetry under the C,,. molecular point group. Vibronic coupling of one quantum of an e type fundamental with the electronic state leads to vibronic states of symmetry E + A, + A,, which would be split by the interaction. The 429- and 1099-cm-’ intervals could correspond to A, vibronic components. Because of the vibronic coupling, bands corresponding to combinations of these levels with the other fundamentals could be substantially displaced. Previous studies have assigned the electronic ground state as A,; however, the previous work is not conclusive. The A2 ground state agrees with CNDO open shell molecular orbital calculation but again this is an approximate theory. Two other bands at displacements of 935 and 1618 cm-’ are observed in the spectra with 19 383 and 19 354 cm-’ excitation but are not observed conclusively in the argon-ion laser spectra. Bands corresponding to the above intervals are observed in the visible absorption system as hot bands. The 935-cm-* intervals is observed as a fairly strong double-headed band with 29 cm-’ between components. It is also observed in the infrared matrix spectra as an unassigned band. In this paper, the 935cm-’ interval has been previously assigned as the transition 20%WV,. It should also be pointed out that the bands at 1618, 1099, and 935 cm-’ are each approximately 30 cm-’ from strong bands observed at 1589, 1068, and 905 cm-’ which have previously been assigned to transitions terminating on the levels associated with v, + v3, 2v,. and vg + v~, respectively. It seems unlikely that the 30-cm-’ separation is associated with the spin-orbit splitting of the upper state since these bands are observed with both excitations.
CHARLES
112
H. WARREN
CONCLUSIONS
The results of the present work provide strong evidence that the upper state of the radical has two components with a separation of 29 cm-‘. If one of the components of the origin band of the electronic absorption spectrum was a sequence band then the differences between the spectra with 19 383- and 19 354-cm-’ excitation should be much more pronounced. Similarly, if the lower state were split by 29 cm-l rather than the upper state, then the fluorescence spectra should consist of doublets in which each component had approximately the same intensity. The strong features of the spectra can be explained in terms of six fundamental vibrations for the radical in agreement with previous work. However, several weaker features cannot be easily explained. They do not apear to be the result of impurities and must be associated with the radical. A number of these bands can be explained in terms of excitation of a level corresponding to the absorption transition at 19 592 cm-l which was assigned as (Y,$ In the vibrational analysis of the absorption system (4), this band was difficult to assign and was tentatively assigned as 6: (j’ = 3/2), i.e., a Jahn-Teller interaction was proposed for the upper E state. The results of this work show that the v ;j = 1, j ’ = 3/2 level is strongly coupled to the upper frequency component of the ground state of the upper electronic state. The results here indicate that the ground state of the radical may not be of A2 symmetry under the C,, molecular point group. There are bands that cannot be explained in terms of this model. It appears that the ground state of the radical may be distorted but whether the distortion is dynamic or static cannot be answered. RECEIVED:
October 22, 1979 REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9. 10.
F. G. G. G. G. E.
B. DUDLEY AND G. H. CADY, J. Amer. Chem. Sot. 85, 3375-3377 (1%3). W. KING, D. P. SANTRY, AND C. H. WARREN, J. Mol. Specrrosc. 32, 108-120 (1969). W. KING AND C. H. WARREN, J. Mol. Spectrosc. 32, 121-137 (1969). W. KING AND C. H. WARREN, J. Mol. Spectrosc. 32, 138-150 (1969). W. KING, D. P. SANTRY, AND C. H. WARREN, J. Chem. Phys. 50,4565-4571 (1969). M. SUZUKI, J. W. NIBBER, K. A. OAKES, AND D. E. EGGERS, JR., J. Mol. Spectrosc. 201-215 (1975). C. H. WARREN, Chem. Phys. Lett. 68, 407-411 (1979). C. H. WARREN, J. Mol. Specrrosc. 83, 451-462 (1980). A. M. QURESHI, L. E. LEUCHUK, AND F. AUBKE, Canad. J. Chem. 49, 2544-2551 (1971). P. SCHENCK AND H. METCALF, App[. Opr. 12, 183-186 (1973).
58,
OF MOLECULAR
SPECTROSCOPY
84,
Dye Laser Excitation
102- 112 (1980)
of the Fluorosulfate
Radical
Origin Band Excitation CHARLES H. WARREN Department
of Chemistry,
Dalhousie
University, Halifax, Nova Scotia, B3H 453, Canada
Discrete visible fluorescence is observed from excitation of the fluorosulfate radical by means of a dye laser. The frequency of the dye laser is chosen to coincide with each of the two components of the origin band observed at 19 383 and 19 354 cm-*. Ground-state fundamentals are assigned for the radical and evidence is provided to suggest that the ground state of the radical may not have CSp symmetry. INTRODUCTION
The fluorosulfate
radical, S03F, exists in equilibrium with the dimer S206F2. S&F,
3 2 S03F.
The equilibrium is temperature dependent and was first studied by Dudley and Cady (I). When the dimer, a colorless liquid, is heated to approximately 100” C, the relatively weak peroxy linkage is ruptured to yield the radical which in the liquid and gas phase has a distinctive brownish yellow color. The color of the radical occurs because the radical absorbs strongly in the visible region from MOO3800 A. The visible absorption was studied under low and high resolution by Warren and King (2-4). The origin of the system occurs around 5160 w and is observed as a doublet with a 29-cm-l separation between components. Under high resolution, the rotational structure is resolvable and from an analysis of this structure it was concluded that the transition was Y?(2)-X*A, under the C,, molecular point group. The analysis of the vibronic structure of the visible absorption system was also consistent with the assignment and further support was provided by the results of molecular orbital calculations (5). The analysis of the spectrum was carried out on the basis of a Jahn-Teller spin-orbit interaction operative in the upper *E state. Ground-state fundamentals of the radical were determined from an analysis of the hot bands to the low wavenumber side of the origin band. The six fundamentals of the radical were identified as 1055.5, 839.3, 533.5 (a, symmetry) and 1177.7, 604.1, and 369.4 cm-’ (e symmetry). The hot bands occur as doublets whose separation is the same as the doublet splitting of the origin band, 29 cm-l. The splitting was assigned as the spin-orbit splitting of the upper *E state. The majority of bands to the high wavenumber side of the origin band also occur as doublets; however, the doublet separation is not constant due to vibronic interaction in the upper 2E state. The infrared spectrum of the SOBF radical was observed at low temperatures in argon and nitrogen matrices by Suzuki et al. (6). Fundamentals were assigned 0022-2852/80/l10102-l1$02.00/O Copyright
0 1980 by Academic
All rights of reproduction
Press, Inc.
in any form reserved.
102
FLUOROSULFATE
RADICAL
FLUORESCENCE
103
which were in agreement with those determined from the hot bands of the visible absorption system. Further confirmation of the ground-state fundamentals of the radical was provided by the observation of the fluorescence spectrum of the radical in the gas phase using the following argon-ion laser lines as excitation sources: 5145, 4017, 4965, 4880, and 4765 A (7, 8). In general, the fluorescence spectra were rather complex and not all bands were assigned. Displacements arising from the fundamentals vg and Yewere often weak or overlapped by other bands and their assignments were not as definite. Similarly in the solid state matrix infrared spectrum, the peaks assigned to vg and vg were quite weak. One of the drawbacks of using the argon-ion laser as an excitation source is that the point of excitation in the upper state cannot be controlled to any degree. It was felt that by using the dye laser as an excitation source the analysis of the fluorescence spectrum would be simplified since one could selectively populate known upper-state levels. In this paper, the fluorescence spectrum of the fluorosulfate radical is reported when the radical is excited at an energy corresponding to the origin band of the visible absorption system by a dye laser. EXPERIMENTAL
DETAILS
The preparation of the dimer and evidence for the existence of the radical have been reported in a previous paper (1). Purification of the dimer was achieved by vacuum distillation and Raman spectra of the colorless dimer in the liquid state were taken and found to be identical with those reported by Aubke and coworkers (9). For the experiments in the vapor phase, the dimer was vacuum distilled into a Pyrex cell 4 cm in length with 25mm Pyrex windows fused to the ends of the cell. A small sidearm was attached to the cell and contained liquid S,O,F, under a pressure of its own vapor at room temperature. The radical was generated by heating the body of the cell to approximately 80” C by means of nichrome wire wound around the cell. The radical was excited by means of a Molectron DL-12 dye laser. The dye laser was pumped by a pulsed nitrogen laser. The nitrogen laser was constructed in this laboratory and is similar in design to the one reported by Schenck and Metcalf (IO). Its power output was approximately 150 kW. Scattered radiation at 90” to the incident laser line as it passed through the cell was collected and focused onto the entrance slit of a Spex 1401 double monochromator containing two 1200 lines/mm gratings blazed at 5000 A. Radiation emerging from the exit slit was monitored by means of a boxcar detector which was also constructed in this laboratory. All spectra were recorded with the slits of the spectrometer set at 120 p. The spectra and the frequency of the dye laser were calibrated with neon lines from a discharge lamp. RESULTS
Under low resolution, the spin-orbit doublet components of the origin band are observed at 19 383.1 and 19 353.7 cm-’ (vat.) and are designated as ‘0: and -O& respectively.
104
CHARLES H. WARREN
2500
I500 C
I
1000
0e-vw
19330 CM-’
FIG. 1. Absorption spectrum of the SO,F radical under high resolution.
In Fig. 1, a high-resolution trace of the absorption spectrum of the origin band of the radical is shown. If the dye laser was tuned a few cm-* above 19 383 cm-‘, no detectable fluorescence was observed. As the dye laser wavenumber was decreased from this point, the fluorescence very sharply increased as one entered the sharp absorption edge of the origin band. Wavelength resolved emission spectra of the radical were recorded for a large number of dye laser excitation frequencies within the band contour of the origin band. The resulting spectra in the region of 534 5 50 cm-’ below the point of excitation are shown in Fig. 2 for a few selected dye laser frequencies. The 534~cm-’ interval coincides with the fundamental y3 which is generally observed as the most intense transition in the fluorescence spectra. There was no strong correlation between the positions of the subband origins and the intensity of the resulting fluorescence indicating that the effects of pressure broadening and rotational relaxation are important. The positions and intensities of the small satellite bands attached to the main level vary as one decreases the excitation frequency. The only apparent regularity is the appearance of a band to the low wavenumber side of the main transition (indicated by an arrow in Fig. 2) and the separation between this band and the main band increases as the excitation frequency decreases. Upon excitation there is relaxation to the lowest rotational level of the high-frequency spin-orbit component of the upper electronic state (2E3,2)and since the rotational structure of the origin band is degraded to the red, transitions from this level lead to the appearance of a band to the right of the main transition. The same phenomena is observed as the excitation frequency is decreased below 19 354 cm-‘. A satellite band emerges
FLUOROSULFATE
105
RADICAL FLUORESCENCE
on the low wavenumber side which is interpreted in terms of relaxation to the lowest rotational level of the low-frequency spin-orbit component of the upper electronic state (*EIw). The results indicate that the lowest rotational levels of the
19386.2cn-’ EXCZTATION
C
19363.7 cm-
19368.2 cn-’
19373.4 cn-’ t
t
t
FIG. 2. Laser-induced fluorescence of the S03F radical obtained by exciting the radical at selected frequencies within the origin band envelope. The fluorescence is recorded in the wavenumber region corresponding to the transition 3p.
106
CHARLES H. WARREN .25ee
CPS
0
rma -%
CPS
e -leaf4
-9e
Cd FIG. 3. Laser-induced fluorescence spectrum ofthe S03F radical. Origin band excitation at 19 383 cm-‘.
two spin-orbit components of the upper electronic state occur around 19 383 and 19 354 cm-l, respectively, and that the 29-cm-’ splitting of the origin band must occur in the upper electronic state of radical; otherwise, two transitions separated by 29 cm-l and of approximately the same intensity would be observed. Wavelength resolved emission spectra of the radical were recorded for 19 383and 19 354-cm-’ dye laser excitation and the spectrum obtained from the former excitation is shown in Fig. 3. The only differences in the two spectra were with respect to the positions and intensities of the small satellite bands attached to the strong bands. Many of the bands of the 19 353-cm-l excitation have a weak band displayed 28 cm-’ to the low wavenumber side of them. This weak band is absent in the 19 383-cm-’ excitation. Since the rotational transitions associated with the
FLUOROSULFATE
RADICAL
FLUORESCENCE
107
transition ‘0: overlap those associated with the transition -O!, the higher rotational levels of the upper spin-orbit component of the ground state of the ‘E state are also populated by 19 354 cm-’ excitation. Relaxation then takes place to the lower rotational levels of the ‘0: component which subsequently emit leading to the satellite at 28 cm-‘. This process is felt to take place rather than a transfer of energy from the lower spin-orbit component to the upper spin-orbit component. If the latter process were taking place then the results summarized in Fig. 2 should indicate a substantial increase in intensity of the 28-cm-l satellite band when the -0: transition comes into resonance. Similarly, if the two upper-state levels were strongly coupled, one would expect to see transitions from the lower component when the upper component was populated by 19 383-cm-’ excitation. The results summarized in Fig. 2 do not seem to support this mechanism. Although satellite bands to the high wavenumber side of the main transition are sometimes observed, they cannot be consistently interpreted by the above mechanism. In order to determine if all of the bands observed in the spectrum displayed in Fig. 3 were associated with the radical, the excitation frequency was changed to approximately 2 cm-’ above the resonance condition at 19 383 cm-‘. All bands decreased markedly in intensity and to approximately the same degree. It is unlikely that a band due to an impurity would show the same behavior in the nonresonance condition as a band due to the radical. The fluorescence spectrum shown in Fig. 3 is characterized by a number of sharp intense bands on the Stokes side of the exciting line. Only a few relatively weak bands are observed on the anti-Stokes side. The wavenumber displacements of the strong bands can be analyzed in terms of the ground-state fundamentals of SO,F that were previously established. If the radical possesses Csa symmetry in its ground electronic state, then six fundamentals should be observed of which three have a, symmetry and three have e symmetry. The six most intense peaks in the spectrum occur at 369, 534, 605, 840, 1057. and 1179 cm-’ from the exciting line and are assigned to Q, vg, Q, u2, vl, and V+ All of the fundamentals are clearly identified in contrast to the argon-ion results in which displacements corresponding to vg and v6 generally did not appear as sharp intense bands but were often weak, missing, or overlapped. Only overtones of ug and yz are readily assigned. 2~~is observed at 1068 cm-’ as a fairly strong band and 3v3 is observed weakly at 1603 cm-‘. 2v, is seen at 1676 cm-’ as a weak band. 2v, and v2 + V~should both occur around 1208 cm-‘. Two bands are observed at displacements of 1215 and 1199 cm-‘. Results from other excitations seem to favor 1215 cm-’ as the v2 + vg assignment. Overtones of v1 and V~ may be too high to be observed with sufficient intensity; however, it seems strange that 2~ is not observed. If the mode assigned to v6 were harmonic, then 2~~ should occur around 738 cm-‘. No band is observed in this region and there seems to be no suitable candidate for 2v, within 2200 cm-’ of this except possibly a weak band at 563 cm-‘, which would appear to be too low and represent an extremely large departure from harmonicity. The majority of the remaining strong bands can be assigned in terms of combinations of the fundamentals. The most intense combinations are at displacements of 1372, 1712, and 2017 cm-’ and are assigned as ve + ug, v3 + vq, and vZ + v*. The
CHARLES
108
H. WARREN
band corresponding to y3 in the spectrum is the most intense band. Consequently, all combinations involving v3 or 2v3 with one quantum of any of the other fundamentals are observed as fairly intense bands. Similarly, the displacement assigned TABLE
I
Assignments for SO,F WAVENUMBER DISPLACEMENT
ASSlGNMENT
WAVENUMBER DISPLACEMENT
-165
-1467
w
-370
- 1506
VW
-426
-1589
m
-534
- 1603
w
-605
-1662
w
-701
-1672
m
-765
-1675
m
-840
- 1689
w
-850
-1712
ms
-861
-1733
w
-905
-1714
w
-935
-1774
w
-973
-1784
m
-1008
-1803
”
-1057
-1813
w
-1068
-1897
m
- 1099
-1906
m
-1141
- 1958
VW
-1179
-1978
w
-1199
-2003
YW
-1213
-2017
m
-1237
-2125
w
-1372
-2197
w
-1442
-2205
”
-2227
-2547
w
- 2248
-2580
w
-2267
-2728
w
-2284
-2762
”
-2309
-2047
w
-2382
-3074
w
-2426
-3089
w
ASSIGNMENT
FLUOROSULFATE
RADICAL
FLUORESCENCE
109
to vq occurs as the second most intense band and combinations of this mode and all other fundamentals except v6 are clearly observed. vg is only observed in combination with Z+and v3 whereas all the other possible combinations of ZQ,z+, p3, uq, and us taken two at a time are seen. The assignments are summarized in Table I. The stronger bands in the spectrum are readily assigned, however, many of the weaker bands are not as easily analyzed. Some of the bands seem to arise from a level above the vibrationless ground state of the upper electronic state of the radical. Weak anti-Stokes bands are observed at 195,203, and 212 cm-’ from the exciting line whose wavenumber is 19 383 cm-‘. No corresponding displacements are found on the Stokes side. In the electronic absorption system of the radical, a strong band having a band structure rather different from the origin band is observed at 19 592 cm-‘, which is 209 cm-’ above the high-frequency component of the origin band. It appears that this level is being populated by the excitation. The anti-Stokes bands must be associated with the radical and not some other species since other bands are observed in the spectrum which differ in wavenumber from the anti-Stokes bands by ground-state fundamental frequencies of the radical. The most prominent band of this latter type is the broad band having components at - 158 and - 169 cm-’ from the exciting line. This differs from the anti-Stokes bands by the interval corresponding to v6. Transitions arising from 19 592 cm-’ and terminating on levels assigned to us, v3 + z+,, 2v,, uz + v5, 2v,, and uz + E.‘~+ v5 are the most intense. These bands are also seen with the 19 353 cm-’ dye laser excitation and would require excitation of an upper-state level at approximately 19 560 cm-‘. In the absorption spectrum bands are seen in this region although a bandhead is not observed as is the case with the band at 19 592 cm-‘. The bands at - 158 and - 169 cm-’ are observed with both excitations but are approximately twice as intense with the 19 383-cm-’ excitation. An alternative assignment exists for these bands. They could be analyzed as the transition vg - v3 which corresponds to -164 cm-‘. This assignment, however, has been rejected since the anti-Stokes bands cannot be explained in this fashion. Laser excitation spectra for the radical were also observed. In Fig. 4a, the spectrometer was fixed to pass light having a wavenumber of 19 382 and the dye laser frequency scanned. Bands corresponding to the transitions +6:, +3:, ‘5: and +2? were readily assigned. Each of these bands had one strong component. The following additional transitions were observed when the dye, Molectron C-500, was replaced by the longer lasing dye, sodium fluorescein: +396:, ‘ly, +32, +3$:, +4?, +2?68, and +2?3?. The most interesting aspect of the spectrum is the cluster of bands whose wavenumbers are 190,202, and 207 cm-’ above the wavenumber of ‘Og, the wavenumber at which the spectrometer was fixed. Again bands which differ from the above bands by one quantum of vg are observed at 160 and 168 cm-1 below the wavenumber corresponding to ‘08. It appears that the level approximately 207 cm-’ above the upper spin-orbit component of the ground state of the upper electronic state must be very strongly coupled to it in order that radiation of wavenumber 19 383 can be emitted and passed by the spectrometer. No other bands of a frequency greater than 19 383 cm-’ were observed. The spectrometer was also locked at the wavenumber corresponding to the lowfrequency component of the origin band at 19 354 cm-’ and the frequency of the dye
110
CHARLES H. WARREN 2888
1 CPS
- CPS
.e I 9588
l9m3
CM-' FIG. 4. Laser excitation spectrum of the SO,F radical obtained by scanning the dye laser frequency. (a) Spectrometer locked at 19 383 cm-‘. (b) Spectrometer locked at 19 354 cm-‘.
laser scanned. For dye laser frequencies below 19 354 cm-l the spectrum was very similar to that in the previous case except that each main peak was accompanied by a small peak 28 cm-’ above it. For dye laser frequencies above 19 354 cm-‘; the spectrum shown in Fig. 4b differed substantially from the previous case in that many more bands were observed. The strongest bands were found at 19 382, 19 783, 19 879, and 19 888 cm-*. The difference between these wavenumbers and the wavenumbers at which the spectrometer was set were 28, 429, 525, and 534 cm-‘. In the absorption spectrum of the radical, bandheads were observed at 19 383, 19 784, 19 879, and 19 888 cm-‘. The 28-cm-’ satellite observed must correspond to excitation of the upper spin-orbit component of the ground state of the upper electronic state. Relaxation to the lower spin-orbit component then occurs which emits at 19 354 cm-l. The bands at 19 888 and 19 879 cm-l correspond to the transition ‘5;. This transition is observed since emission from the radical via the transition +5&3?gives a wavenumber corresponding to 19 354 cm-‘, the wavenumber at which the spectrometer was locked. Similarly the bank at 19 783 cm-l corresponds to the absorption transition previously assigned as Pi. In order for this transition to be de-
FLUOROSULFATE
RADICAL
FLUORESCENCE
111
tected, radiation must be emitted such that the radical returns to a state 429 cm-’ above the vibrationless ground state. The 429-cm-’ interval will be discussed later. A number of bands for the spectrum shown in Fig. 3 have yet to be assigned. In particular bands corresponding to displacements of 426 and 1099 cm-’ cannot be fitted into the analysis of the radical spectrum. These bands decreased in intensity in the same fashion as the other bands of the spectrum when the laser was tuned slightly off-resonance. They were also observed with the 19 354 cm-’ excitation and furthermore were observed in many of the spectra generated by argon-ion laser excitation and in the spectra generated by locking the spectrometer drive and scanning the dye laser frequency. In the matrix infrared spectrum of the radical, unassigned bands were observed at 427 and 1104 cm-‘. Similarly hot bands in the visible electronic absorption spectrum of the radical were observed at intervals of 427 and 1097 cm-’ to the low wavenumber side of the origin band. The evidence suggests that the -426 and - 1099 cm-’ displacements correspond to ground-state levels of the radical. No other bands could be found which would correspond to combinations of these levels with the other ground-state fundamentals. One possible explanation for these levels is that the molecule is distorted from Cat symmetry in the ground electronic state to C,Ysymmetry. This would lead to the appearance of nine fundamentals and the corresponding transitions would all be allowed. If the 429 and 1099 intervals correspond to fundamentals, it seems surprising that no combinations with other fundamentals are observed. A second possible explanation is that the ground electronic state of the radical has E symmetry instead of A, symmetry under the C,,. molecular point group. Vibronic coupling of one quantum of an e type fundamental with the electronic state leads to vibronic states of symmetry E + A, + A,, which would be split by the interaction. The 429- and 1099-cm-’ intervals could correspond to A, vibronic components. Because of the vibronic coupling, bands corresponding to combinations of these levels with the other fundamentals could be substantially displaced. Previous studies have assigned the electronic ground state as A,; however, the previous work is not conclusive. The A2 ground state agrees with CNDO open shell molecular orbital calculation but again this is an approximate theory. Two other bands at displacements of 935 and 1618 cm-’ are observed in the spectra with 19 383 and 19 354 cm-’ excitation but are not observed conclusively in the argon-ion laser spectra. Bands corresponding to the above intervals are observed in the visible absorption system as hot bands. The 935-cm-* intervals is observed as a fairly strong double-headed band with 29 cm-’ between components. It is also observed in the infrared matrix spectra as an unassigned band. In this paper, the 935cm-’ interval has been previously assigned as the transition 20%WV,. It should also be pointed out that the bands at 1618, 1099, and 935 cm-’ are each approximately 30 cm-’ from strong bands observed at 1589, 1068, and 905 cm-’ which have previously been assigned to transitions terminating on the levels associated with v, + v3, 2v,. and vg + v~, respectively. It seems unlikely that the 30-cm-’ separation is associated with the spin-orbit splitting of the upper state since these bands are observed with both excitations.
CHARLES
112
H. WARREN
CONCLUSIONS
The results of the present work provide strong evidence that the upper state of the radical has two components with a separation of 29 cm-‘. If one of the components of the origin band of the electronic absorption spectrum was a sequence band then the differences between the spectra with 19 383- and 19 354-cm-’ excitation should be much more pronounced. Similarly, if the lower state were split by 29 cm-l rather than the upper state, then the fluorescence spectra should consist of doublets in which each component had approximately the same intensity. The strong features of the spectra can be explained in terms of six fundamental vibrations for the radical in agreement with previous work. However, several weaker features cannot be easily explained. They do not apear to be the result of impurities and must be associated with the radical. A number of these bands can be explained in terms of excitation of a level corresponding to the absorption transition at 19 592 cm-l which was assigned as (Y,$ In the vibrational analysis of the absorption system (4), this band was difficult to assign and was tentatively assigned as 6: (j’ = 3/2), i.e., a Jahn-Teller interaction was proposed for the upper E state. The results of this work show that the v ;j = 1, j ’ = 3/2 level is strongly coupled to the upper frequency component of the ground state of the upper electronic state. The results here indicate that the ground state of the radical may not be of A2 symmetry under the C,, molecular point group. There are bands that cannot be explained in terms of this model. It appears that the ground state of the radical may be distorted but whether the distortion is dynamic or static cannot be answered. RECEIVED:
October 22, 1979 REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9. 10.
F. G. G. G. G. E.
B. DUDLEY AND G. H. CADY, J. Amer. Chem. Sot. 85, 3375-3377 (1%3). W. KING, D. P. SANTRY, AND C. H. WARREN, J. Mol. Specrrosc. 32, 108-120 (1969). W. KING AND C. H. WARREN, J. Mol. Spectrosc. 32, 121-137 (1969). W. KING AND C. H. WARREN, J. Mol. Spectrosc. 32, 138-150 (1969). W. KING, D. P. SANTRY, AND C. H. WARREN, J. Chem. Phys. 50,4565-4571 (1969). M. SUZUKI, J. W. NIBBER, K. A. OAKES, AND D. E. EGGERS, JR., J. Mol. Spectrosc. 201-215 (1975). C. H. WARREN, Chem. Phys. Lett. 68, 407-411 (1979). C. H. WARREN, J. Mol. Specrrosc. 83, 451-462 (1980). A. M. QURESHI, L. E. LEUCHUK, AND F. AUBKE, Canad. J. Chem. 49, 2544-2551 (1971). P. SCHENCK AND H. METCALF, App[. Opr. 12, 183-186 (1973).
58,