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Ultraviolet excitation spectra of disulfur monoxide and sulfur dioxide
JOURNAL
OF
MOLECULAR
Ultraviolet
SPECTROSCOPY
Excitation
Spectra
(1980)
80,459-461
of Disulfur
Monoxide
and Sulfur
Dioxide
We would like to report two new findings in our laboratory: The nonexistence of the uv (23001900 & band system of S,O and the evidence of predissociation for the 2206 A band of SO,. Recently, Lakshminarayana (1) reported a new uv absorption band system which is always photographed along with the well-known near-uv (3400-2500 A) band system of S,O. The S,O molecules are formed by a rf discharge on the mixture of sulfur vapor and oxygen at low pressures. He assumes that the new uv band system belongs to S,O and gives vibrational constants for it. Also interested in the near-uv system of St0 and in being able to generate S20 molecules by an electric discharge, we have observed its absorption spectrum. We too noted the existence of such a uv band system. But we also found that the absorption intensities of these two systems vary to different degrees with the amount of oxygen added to the discharge system. This means that the uv system may not belong to the S,O molecule. As a matter of fact, when we compare our bandhead measurements of S,O to those of SO, in the same spectral region (2, 3), we find that there are many coincidences between them. Table I, which also includes the bandhead measurements for $0 by Lakshminarayana shows this point clearly. The vibrational constants from the analysis by Lakshminarayana for S,O, V; = 270 and vi = 378 cm-‘, show again the “coincidences” to those of SO, (2, 3, 4) V; = 377 and 2~; = 560 cm-‘. Thus the uv band system of Lakshminarayana for SzO may very well be the @S, + _?‘A, system of SO,.
I
/
I
I
,
2400
1900
WAVELENGTH,
ii
FIG. 1. The excitation spectrum of SO, at a pressure of 1 Torr. The decline in the increase of the fluorescence intensity is obvious at 2206 A. 459
0022-2852/80/040459-03$02.00/O Copyright
0 1980 by Academic
All rights of reproduction
Press, Inc.
in any form reserved.
NOTES TABLE I Bandhead Measurements
in Wavenumbers
for S,O and SO, in the Ultraviolet Region so2
s2O Chow'
Present
and
LaKshminarayanal Duchesne
and Rosen3
r40 cm
43 540.00
43 539
43 540
912.84
919
43 920
44 228.81
44 236
294.95
293
44 290
603.62
603
44 590
663.89
663
982.50
980
44 970
45 349.88
45 354
45 340
124.65
726
45 730
860.46
858
-1
620
995.00 46 105.79
46 106
224.52
227
350.95
348
463.36
464
572.81
570
703.67
704
824.51
829
865.23
866
47 055.36
47 052
220.45
223
402.05
391
46 070
46 430
46 840
47 200
617.54
623
47 590
997.08
48 000
48 010
48 373.86
375
48 400
776.99
781
48 780
49 129.81
49 133
49 150
541.60
49 590
900.00
49 930
I
Since most of the bands of both the “&O” and the SO, system in question are so diffuse that measurements may be in error, there is still a chance that the above conclusion is incorrect. TO clarify this, we carried out a relative quantum yield measurement experiment, where the absorption intensity and the undispersed fluorescence intensity were measured as a function of incident light wavelength. A deuterium lamp was used as a light source. An absorption-fluorescence cell, 6 in. in length, was placed behind a condenser lens which could make the light from a 0.75-m Jarrell-Ash monochromator either parallel throughout the cell for absorption measurements or focused at a point 1 in. inside the cell for total fluorescence measurements. The absorption intensity, Iab, was measured at the long end of the cell by a RCA lP28 photomultiplier. The total fluorescence intensity, I.,, was measured at right angles to the point where incident beam was focused. Absorption spectra of $0 in the uv and near-uv regions were recorded first to make sure that there were S,O molecules in the cell. The total fluorescence spectrum (excitation spectrum) was then
461
NOTES
recorded immediately after. Figure 1 shows the excitation spectrum in the uv region with a 5-A resolution. It resembles exactly the one reported by Okabe (5) for SO,. Thus, we believe that the uv system of S,O is in fact due to SOz. This conclusion is not unexpected if one uses a discharge to produce $0. By discharging oxygen over heated sulfur, the primary product should be SO and the main secondary gaseous products which exist in the afterglow are S,O and SOP, according to the foilowing reactions (6 ): 3so + so,
+ $0,
2SG + Sh”rnh + SO,, (SO), + s, + 2&O, where the last reaction occurs only at high S, concentrations. The relative quantum yield of two neighboring u’ bands, v1 and up, may be given as
where v(v’,O) is the frequency of transition from (000)” to U’of an excited electronic state. If R;; is less than one, the L+state is said to be predissociated relative to up. For the SO, uv system, the 2188-A band is obviously predissociated because of the sudden decrease of fluorescence intensity as shown in Fig. 1. Although the fluorescence intensity of the 2206-A band is greater than that of the 2224-A band, its absorption intensity is still greater. Rzzo6 2224IS calculated to be 0.7. Predissociation must have already set in at this vibrational state. Considering the fact that its fluorescence bandwidth is narrower and the quantum yield of this vibrational state (7) when excited at 2206.3 A with a bandwidth of 0.81 A is 1.O + 0.15, we believe that the predissociation occurs for some higher rotational levels but not for the whole 2206-A vibrational band. ACKNOWLEDGMENTS This work was supported by the National Science Council of the Republic of China REFERENCES I. G. LAKSHMINARAYANA. J. Mol. Spectrosc. 55, 141-150 (1975). 2. TUNG-CHING CHOW, Phys. Rev. 44, 638-643 (1933). S. J. DUCHESNE AND B. ROSEN, J. Chem. Phys. 15, 631-644 (1947). 4. J. C. D. BRAND, P. H. CHIU, A. R. HOY, AND H. D. BET. J. Mol. Spectrosc. 60,43-56 5. H. OKABE. J. Amer. Chem. Sot. 93, 7095-7096 (1971). 6. P. W. SCHENK AND R. STEUDEL, Anger. Chem. Int. Ed. Engl. 4, 402-409 (1965). 7. M. H. HUI AND S. A. RICE, Chem. Phys. Lett. 17, 474-478 (1972). P. C. Department of Chemist? National Tsing Hua University Hsinchu. Taiwan, Republic of China Received
July 25, 1979
SUNG AND C.
(1976).
L.
CHIU
OF
MOLECULAR
Ultraviolet
SPECTROSCOPY
Excitation
Spectra
(1980)
80,459-461
of Disulfur
Monoxide
and Sulfur
Dioxide
We would like to report two new findings in our laboratory: The nonexistence of the uv (23001900 & band system of S,O and the evidence of predissociation for the 2206 A band of SO,. Recently, Lakshminarayana (1) reported a new uv absorption band system which is always photographed along with the well-known near-uv (3400-2500 A) band system of S,O. The S,O molecules are formed by a rf discharge on the mixture of sulfur vapor and oxygen at low pressures. He assumes that the new uv band system belongs to S,O and gives vibrational constants for it. Also interested in the near-uv system of St0 and in being able to generate S20 molecules by an electric discharge, we have observed its absorption spectrum. We too noted the existence of such a uv band system. But we also found that the absorption intensities of these two systems vary to different degrees with the amount of oxygen added to the discharge system. This means that the uv system may not belong to the S,O molecule. As a matter of fact, when we compare our bandhead measurements of S,O to those of SO, in the same spectral region (2, 3), we find that there are many coincidences between them. Table I, which also includes the bandhead measurements for $0 by Lakshminarayana shows this point clearly. The vibrational constants from the analysis by Lakshminarayana for S,O, V; = 270 and vi = 378 cm-‘, show again the “coincidences” to those of SO, (2, 3, 4) V; = 377 and 2~; = 560 cm-‘. Thus the uv band system of Lakshminarayana for SzO may very well be the @S, + _?‘A, system of SO,.
I
/
I
I
,
2400
1900
WAVELENGTH,
ii
FIG. 1. The excitation spectrum of SO, at a pressure of 1 Torr. The decline in the increase of the fluorescence intensity is obvious at 2206 A. 459
0022-2852/80/040459-03$02.00/O Copyright
0 1980 by Academic
All rights of reproduction
Press, Inc.
in any form reserved.
NOTES TABLE I Bandhead Measurements
in Wavenumbers
for S,O and SO, in the Ultraviolet Region so2
s2O Chow'
Present
and
LaKshminarayanal Duchesne
and Rosen3
r40 cm
43 540.00
43 539
43 540
912.84
919
43 920
44 228.81
44 236
294.95
293
44 290
603.62
603
44 590
663.89
663
982.50
980
44 970
45 349.88
45 354
45 340
124.65
726
45 730
860.46
858
-1
620
995.00 46 105.79
46 106
224.52
227
350.95
348
463.36
464
572.81
570
703.67
704
824.51
829
865.23
866
47 055.36
47 052
220.45
223
402.05
391
46 070
46 430
46 840
47 200
617.54
623
47 590
997.08
48 000
48 010
48 373.86
375
48 400
776.99
781
48 780
49 129.81
49 133
49 150
541.60
49 590
900.00
49 930
I
Since most of the bands of both the “&O” and the SO, system in question are so diffuse that measurements may be in error, there is still a chance that the above conclusion is incorrect. TO clarify this, we carried out a relative quantum yield measurement experiment, where the absorption intensity and the undispersed fluorescence intensity were measured as a function of incident light wavelength. A deuterium lamp was used as a light source. An absorption-fluorescence cell, 6 in. in length, was placed behind a condenser lens which could make the light from a 0.75-m Jarrell-Ash monochromator either parallel throughout the cell for absorption measurements or focused at a point 1 in. inside the cell for total fluorescence measurements. The absorption intensity, Iab, was measured at the long end of the cell by a RCA lP28 photomultiplier. The total fluorescence intensity, I.,, was measured at right angles to the point where incident beam was focused. Absorption spectra of $0 in the uv and near-uv regions were recorded first to make sure that there were S,O molecules in the cell. The total fluorescence spectrum (excitation spectrum) was then
461
NOTES
recorded immediately after. Figure 1 shows the excitation spectrum in the uv region with a 5-A resolution. It resembles exactly the one reported by Okabe (5) for SO,. Thus, we believe that the uv system of S,O is in fact due to SOz. This conclusion is not unexpected if one uses a discharge to produce $0. By discharging oxygen over heated sulfur, the primary product should be SO and the main secondary gaseous products which exist in the afterglow are S,O and SOP, according to the foilowing reactions (6 ): 3so + so,
+ $0,
2SG + Sh”rnh + SO,, (SO), + s, + 2&O, where the last reaction occurs only at high S, concentrations. The relative quantum yield of two neighboring u’ bands, v1 and up, may be given as
where v(v’,O) is the frequency of transition from (000)” to U’of an excited electronic state. If R;; is less than one, the L+state is said to be predissociated relative to up. For the SO, uv system, the 2188-A band is obviously predissociated because of the sudden decrease of fluorescence intensity as shown in Fig. 1. Although the fluorescence intensity of the 2206-A band is greater than that of the 2224-A band, its absorption intensity is still greater. Rzzo6 2224IS calculated to be 0.7. Predissociation must have already set in at this vibrational state. Considering the fact that its fluorescence bandwidth is narrower and the quantum yield of this vibrational state (7) when excited at 2206.3 A with a bandwidth of 0.81 A is 1.O + 0.15, we believe that the predissociation occurs for some higher rotational levels but not for the whole 2206-A vibrational band. ACKNOWLEDGMENTS This work was supported by the National Science Council of the Republic of China REFERENCES I. G. LAKSHMINARAYANA. J. Mol. Spectrosc. 55, 141-150 (1975). 2. TUNG-CHING CHOW, Phys. Rev. 44, 638-643 (1933). S. J. DUCHESNE AND B. ROSEN, J. Chem. Phys. 15, 631-644 (1947). 4. J. C. D. BRAND, P. H. CHIU, A. R. HOY, AND H. D. BET. J. Mol. Spectrosc. 60,43-56 5. H. OKABE. J. Amer. Chem. Sot. 93, 7095-7096 (1971). 6. P. W. SCHENK AND R. STEUDEL, Anger. Chem. Int. Ed. Engl. 4, 402-409 (1965). 7. M. H. HUI AND S. A. RICE, Chem. Phys. Lett. 17, 474-478 (1972). P. C. Department of Chemist? National Tsing Hua University Hsinchu. Taiwan, Republic of China Received
July 25, 1979
SUNG AND C.
(1976).
L.
CHIU