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Single vibronic level fluorescence of SO2
Volume 24, number 2
CHEbllCAl. PHYSICS LETTERS
SINGLE WBRONIC
15
LEVEL FLUORESCENCE
January 1974
OF SO,
J.E. KENT, h4.F. O’DWYER and R.J. SHAW Chemistry Department. Monash Urriversity. Clayton. Victoria. Australia 3I 68 Received 20 September 1973 Revised manuscript received 18 October
1973
Sin& vibronic Ievel fluorescence spectra from four levels of the first excited singlet state of sulphur dioside are presented. mey are assigned in terms of rather short progressions in the two totally symmetric vibrations for stretching b,) and bending (ui)_ The relative intensities observed present a very interesting Franck-Condon picture.
pressure (i.e., collision free) gaseous molecules creates numerous experimental problems and the resulting ‘Understanding the lower excited states of sulpbur dioxide has long plagued spectroscopists as the absorption spectrum is very complex. A recent rovibronic analysis [l] of the first weak absorption system (340 < X < 380 nm) has established this system to be due to a 3B, state as expected but not proven from previous theoretical and experimental studies. To higher energy there is a second system (250 < h < 340 nm) in which, there is evidence to suggest, lie other triplet states [2] embedded in a long progression of much more intense bands with complex and not too well resolved rotational structure. This long progression has been assumed to be the fBl state, but verification has not been possibfe to date due to the severe perturbations present. These perturbations further manifest themselves in the anomalously long lifetime measured for this state [3,4]. Recently a magnetic rotation spectrum [5] has suggested that in this so called first excited singlet state, the SO2 molecule is linear, at least in the bands examined. As a result we have been seeking additionat useable data on this system by examining the single vibronic level (SW.,) fluorescence (or at least single band ftuorescence).
thence to a chart recorder. The cell was filed on a greaseless vacuum line to a pressure of 0.1 Nm-2 (* 1 mtorr) with dried, degassed SO, supplied by BDH Chemicals.
3. Results and dhsion The SVL fluorescence spectra from four levels of the state of SO2 are shown in fig_ 2 along with our a.+ signment. The lab&@ of the four lev& follc& that .. given by Metropolis [7) with the letters D, E, c “pd C$ .: referring to band maxima at32628 cm-1 *, 32850 c;?-’, lBl
2_ Experimental
The excitation
fluorescence spectra are inherently weak and difficult
to record_. The apparatus used to obtain the spectra is shown schematically in fig, 1. A xenon 1600 W excitation source was focussed onto the entrance slit of a 0.25 m Jarrell-Ash grating monochromator with a band pass of0.8 nm. (This is approximately the width of the particular absorption bands excited.) The monochromator output is then passed through a multipIe pass fluorescence cell containing the SO,. The cell is similar to that used by Parmenter and Schuyler 161. Fluorescence radiation was dispersed by a 1 m Hilger-Engis grating monochromator operated in third order. Detection by photon counting techniques employed a cooled (-20°C) EMI6256S photomultipiier whose output was amplified (X 2400) and fed to a pulse height discriminator-ratemeter combination and
of single vibronic
levels of very low
1.5 January 1974
CHEMICAL PHYSICS LETTERS
Volume 24, number 2
Multiple-pass Emission
1600 W
O-25 m
Xe Lamp
Monochromator
Cell
Im. Spectrometer
I
I
I
I
Chart Recorder
Discriminator
Amplifier
Ratemeter
1 .
I
Cooled
H.V.Suppy
Photomultiplier
Fig. 1. Block diagram of apparatus. 33080 cm-1 and 333 13 cm-l, respectively. Thus the assignment of ly29 refers to emission from level G to one quantum of u1 and two quanta of v2 in the ground electronic state. We have tabulated the data in table I_ The spectra shown in fig. 2 have been recorded on an FSD of 150 counts set-r . For measurement of weaker lines an FSD of 20 counts se& was used. Because of the low signal level aggravated by the overlapping_ of weak bands; the tabulated frequencies are only accilrate to approximately 20 cm-l. The first noticeable feature of the spectra is that they are analysable in terms of progressions only in v1 and ~2, the symmetric stretching and bending frequenties. There is no sign of v3, the antis$mmetrik stretching vibration. :* The value for D (32607 cm”) given by Met;opolis [7] is
believed io:be in error.
‘: 22; . :-_ : .. :.~ .. _. :... ,I_ ... . ” r: ;..:.. .-- ::,..: ::_ .: . .- --.. -. I . .‘_: : 1.;L,_._’ ; ... _. .. ._ ,I.
--
The agreement between observed fluorescence lines and the previously reported [8] ground state vibrational frequencies (up to Y” = 3) is excellent and gives support to the assignment of the progressions. The second feature of the spectra is that in each of these cases, the transition 1p(N = D, E, F, G) is by far the strongest line in the spectrum. The explanation for this is by no means clear, but would seem to indicate an abnormally large Franck-Condon overlap for this partictdar transition and thus that few if any quanta of y1 are excited in the upper state. Even at the low pressures employed there is evidence for some vibrational relaxation. This would suggest that. even lower pressures and narrower bandwidths are required. However, transiticns from levels other.than that. excited occur ahdst entireIy to lo-&d 1.1. This is to be expected as these lines are the strongest in each spec:’ trrim. These hrres:c+m be easily identified by &heir pres-
_ : .,.: i.
~::
: -. .: -(I... :. ~ :_ ; .., :... -. .I : : ._ ~: _.,_; I -_,: i _- ‘- .._~~.:~l:.i-‘L~_._ ‘>.-.+ .:‘:.~l:~~.‘-_::;,,_.:~:~-i;, .:I_,.:.y;;_;$‘_:-._._I .;:-..._-1 Yzr;j _..:..y: _.r:‘c‘_.. ‘,. - i: _. :.._‘;. ___,_.:.., __, _,.., ._.._.I p .:__::._.,. ,<-‘,~-j.:;, ::.:‘-_i-.:>_ .,. ;--.:; I.-.: -...
Volume 24, number 2
CHEMICAL PHYSiCS LETTERS
;
q .’
:-
Volume 24, number 2
15 January 1974.
CHEMICAL PHYSICS LETTERS Table 1 Assignment of SVL fluorescencebands from bands G, F, E and D Band G _---Z) AC intensity
Calculatedb) A; (cm-‘)
Assignmenta)
1F
G
233(from
G)
233
3
463(from
G)
433
3
230Cfrom
F) 515
20
693
3
518 685Cfrom
G)
452(from
F)
222Cfrom
E)
936(from
G)
703(from
F)
473(from
E)
953(from
D)
915
1030
882(from
F) E)
430(from
D)
1152 1371 (from G) 908(from
E)
686(from
D)
1385(from
G) G)
1382(from
F)
6
160
1348
3
4 9
466
9
4
1589
12
1659
1837(from G)
1855
7
2P
2035
6
2088(f:om G) 18Wfrom
1037
13
912
4
1144
105
1565
10
1367
14
1687
5
1625
8
1
2065
6
F)
2105(from D)
1r2r
2177
1B
2267(&m
.2F -. 1p2p
: -:
le.
G)
2296
$
.2528 :.
... : ...-.
7
243
2
447
6
1030
13
670
3
1141
160
925
3
2031 1864
2523(fromG).
c)
7
940
3
1036
4
426
7
1152
90
704
4 3
7
1558
1687
2
1667
10
10 7
5
2026
2
12 1616
8
25
2177.
24
2191
2102 2173
2165
7
2252
5
2301
2
2300
8
2288
22
2298
16
7
._2525’
.8
2491
12
2500
2
2621
2 i838
6’..
2493
2629(from F). .1859(frotk D) .:
522
1550
1364 1869
1625(from E)
p+S
501
Ai; intensity
11
F)
1862(from E)
intensity
5
1374ifrom E) 32,140 11
A;
c)
8
1540
1667 16Wfrom
221 512
c)
3
1148
1392
1535 1615(from
intensity
690
1029 652(from
A;
Band D_~
Band E
Band F
:2528
3 :-
_.’ 2697
8
_.
‘.
Volume 24, number 2
15 January 1974
CHEMICAL PHYSICS LETTERS Table 1 (continued)
Assignrnenta)
Band F
Band E
AC
AC
Band D
Cakulatedb) A; (cm-‘)
Band G
2809 2914(from G)
4 2 5 10 5 25
2815
4
2793
3
2995
8
3009
IO
2994
7
3195
3
3194
6
3208
6
3294
11
3298
16
3320
2
3431
2801 2896 2993 3206 3301 3427
3453
3414
7
3401
2
3498
3499
3
3550
10 4
3532
6
3511
2
3607
4
3618
4
3684
3
3677
1
3731
4
3719
2
3819
7
3828
6
3944
3
410.5
4
4315
7
Ai;
3016 3178 3317
3670
intensityc)
4
3725
!10
3830
3818 3943
d)
3974
d)
intens&)
3675
2
3788
6
3937
2
4043
4443 4448
4219
5
4305
4
4439
1
4561
4559
3 25
4326
4020
3
4136
3
4330
7
6
4156 4314
intensity’)
5
63
intensi@)
2817
3
4419 4426 4554
2 20
4547
22
4460
2
4561
9
4566
lP# 14
4635
4639
2
4781(from E)
4788
4
4844
8
4803
4734
5
4843
4
4907
4921
3
4948
4940
5
5069
4817
3
4864
6 4902
4
4956
3
4945
3
4952
5
5052
4
5076 5144
3 3
5051
4
5151
4 2
5109
5147
4
5287
5292
2
5304
5341
7
5361
3
5441
5447
3
5432
5
5431
.2
5435
5
5679
5664
5
5646
6
5670
5688
9
5765
5772
1
16 2
5853
.4
5950
4.
6262
6263
2
6411-
6452
9
5365
..
5816 5929
:
5769 j947
-6452 6776
V&me 24, number 2
CHEMICAL PHYSICS LETTERS
sure dependence. That any vibrationally relaxed transitions appear at these pressures is undoubtedly a reff ection of the long excited state lifetime. As a result fluerescence from truly isolated SO, molecules would be immeasurably weak with present detection techniques. The intensities of the lines in the different progressions, as shown in table 1, are accurate to about -t 20% as in some cases the bands are not clearly resolved_ Also the relative intensity of the transitions to the 22 level is uncertain due to the fact that this line (predicted 1029 cm-l from the zero point fevel) is so close to the intense transitions to the 1 1 level (predicted 1152 cm-l). There are a number of general points that can be made from a perusal of the intensities of the various progressions. They are as follows: (i) AU the progressions are rather short indicating that the excited state may be quasi-linear as opposed to linear. (ii) There are a number of intensity dips in the progressions due to cancellations of overlap integrais of transitions originating in vibrationally excited levels of the upper state [9] _ (iii) The My1 + nv2 progression intensities are far from proportional to the intensity of the transition to mu1 indicating that the simple Franck-Condon factor picture is quite inadequate and that the al normal coordinates are quite different in the two states. One might weII expect such a difference if SO, goes from bent to quasi-linear in this transition.
15 Ianuary 1974
We are continuingwith an extensive investigation of the Fran&-Condon factors invoIved in this transition to try to verify if the states involved derive from a Renner-Teller splitting of the lL\g state of the linear structure into IAl bent and 1 B, linear or quasi-linear structures IS] _
Acknowfedgement We thank Mr. J.J. McNeiIl of C.S.I.R.O., Division of Chemical Physics, for valuable assistance. One of us (RSS) would like acknowledge the assistance of a Commonwealth Postgraduate Research Award.
References (11 J.C.D. Brand, V.7’.Jones and C. Di Laura, J. &fol. Spectry. 40 (1971) 616. [2I J.C.D. Brand, private communication. [3] K.F. Creenough and A.B.F. Duncan, J.
Am. Chem. Sac.
83 (1961) 555. [4I AX_ Doug&, J. Chem. Phys. 45 (1966) 1007. [SI I.L. Hardwick and W-H. Eberhardt, to be published. [6] C.S. Patmenter and M-W- Schuyler, J. Chem. Phys. 52 (1970) 5366. 171 N. Metropolis, Phys. Rex 60 (1941) 295. (81 R.D. Shelton, A.H. Nielsen and W.H. Fletcher, 3. Chem. fiys. 21 (1953) 2178. 191 C.S. lknenter. Advan. Chem. Phys. 12 (1972) 365.
CHEbllCAl. PHYSICS LETTERS
SINGLE WBRONIC
15
LEVEL FLUORESCENCE
January 1974
OF SO,
J.E. KENT, h4.F. O’DWYER and R.J. SHAW Chemistry Department. Monash Urriversity. Clayton. Victoria. Australia 3I 68 Received 20 September 1973 Revised manuscript received 18 October
1973
Sin& vibronic Ievel fluorescence spectra from four levels of the first excited singlet state of sulphur dioside are presented. mey are assigned in terms of rather short progressions in the two totally symmetric vibrations for stretching b,) and bending (ui)_ The relative intensities observed present a very interesting Franck-Condon picture.
pressure (i.e., collision free) gaseous molecules creates numerous experimental problems and the resulting ‘Understanding the lower excited states of sulpbur dioxide has long plagued spectroscopists as the absorption spectrum is very complex. A recent rovibronic analysis [l] of the first weak absorption system (340 < X < 380 nm) has established this system to be due to a 3B, state as expected but not proven from previous theoretical and experimental studies. To higher energy there is a second system (250 < h < 340 nm) in which, there is evidence to suggest, lie other triplet states [2] embedded in a long progression of much more intense bands with complex and not too well resolved rotational structure. This long progression has been assumed to be the fBl state, but verification has not been possibfe to date due to the severe perturbations present. These perturbations further manifest themselves in the anomalously long lifetime measured for this state [3,4]. Recently a magnetic rotation spectrum [5] has suggested that in this so called first excited singlet state, the SO2 molecule is linear, at least in the bands examined. As a result we have been seeking additionat useable data on this system by examining the single vibronic level (SW.,) fluorescence (or at least single band ftuorescence).
thence to a chart recorder. The cell was filed on a greaseless vacuum line to a pressure of 0.1 Nm-2 (* 1 mtorr) with dried, degassed SO, supplied by BDH Chemicals.
3. Results and dhsion The SVL fluorescence spectra from four levels of the state of SO2 are shown in fig_ 2 along with our a.+ signment. The lab&@ of the four lev& follc& that .. given by Metropolis [7) with the letters D, E, c “pd C$ .: referring to band maxima at32628 cm-1 *, 32850 c;?-’, lBl
2_ Experimental
The excitation
fluorescence spectra are inherently weak and difficult
to record_. The apparatus used to obtain the spectra is shown schematically in fig, 1. A xenon 1600 W excitation source was focussed onto the entrance slit of a 0.25 m Jarrell-Ash grating monochromator with a band pass of0.8 nm. (This is approximately the width of the particular absorption bands excited.) The monochromator output is then passed through a multipIe pass fluorescence cell containing the SO,. The cell is similar to that used by Parmenter and Schuyler 161. Fluorescence radiation was dispersed by a 1 m Hilger-Engis grating monochromator operated in third order. Detection by photon counting techniques employed a cooled (-20°C) EMI6256S photomultipiier whose output was amplified (X 2400) and fed to a pulse height discriminator-ratemeter combination and
of single vibronic
levels of very low
1.5 January 1974
CHEMICAL PHYSICS LETTERS
Volume 24, number 2
Multiple-pass Emission
1600 W
O-25 m
Xe Lamp
Monochromator
Cell
Im. Spectrometer
I
I
I
I
Chart Recorder
Discriminator
Amplifier
Ratemeter
1 .
I
Cooled
H.V.Suppy
Photomultiplier
Fig. 1. Block diagram of apparatus. 33080 cm-1 and 333 13 cm-l, respectively. Thus the assignment of ly29 refers to emission from level G to one quantum of u1 and two quanta of v2 in the ground electronic state. We have tabulated the data in table I_ The spectra shown in fig. 2 have been recorded on an FSD of 150 counts set-r . For measurement of weaker lines an FSD of 20 counts se& was used. Because of the low signal level aggravated by the overlapping_ of weak bands; the tabulated frequencies are only accilrate to approximately 20 cm-l. The first noticeable feature of the spectra is that they are analysable in terms of progressions only in v1 and ~2, the symmetric stretching and bending frequenties. There is no sign of v3, the antis$mmetrik stretching vibration. :* The value for D (32607 cm”) given by Met;opolis [7] is
believed io:be in error.
‘: 22; . :-_ : .. :.~ .. _. :... ,I_ ... . ” r: ;..:.. .-- ::,..: ::_ .: . .- --.. -. I . .‘_: : 1.;L,_._’ ; ... _. .. ._ ,I.
--
The agreement between observed fluorescence lines and the previously reported [8] ground state vibrational frequencies (up to Y” = 3) is excellent and gives support to the assignment of the progressions. The second feature of the spectra is that in each of these cases, the transition 1p(N = D, E, F, G) is by far the strongest line in the spectrum. The explanation for this is by no means clear, but would seem to indicate an abnormally large Franck-Condon overlap for this partictdar transition and thus that few if any quanta of y1 are excited in the upper state. Even at the low pressures employed there is evidence for some vibrational relaxation. This would suggest that. even lower pressures and narrower bandwidths are required. However, transiticns from levels other.than that. excited occur ahdst entireIy to lo-&d 1.1. This is to be expected as these lines are the strongest in each spec:’ trrim. These hrres:c+m be easily identified by &heir pres-
_ : .,.: i.
~::
: -. .: -(I... :. ~ :_ ; .., :... -. .I : : ._ ~: _.,_; I -_,: i _- ‘- .._~~.:~l:.i-‘L~_._ ‘>.-.+ .:‘:.~l:~~.‘-_::;,,_.:~:~-i;, .:I_,.:.y;;_;$‘_:-._._I .;:-..._-1 Yzr;j _..:..y: _.r:‘c‘_.. ‘,. - i: _. :.._‘;. ___,_.:.., __, _,.., ._.._.I p .:__::._.,. ,<-‘,~-j.:;, ::.:‘-_i-.:>_ .,. ;--.:; I.-.: -...
Volume 24, number 2
CHEMICAL PHYSiCS LETTERS
;
q .’
:-
Volume 24, number 2
15 January 1974.
CHEMICAL PHYSICS LETTERS Table 1 Assignment of SVL fluorescencebands from bands G, F, E and D Band G _---Z) AC intensity
Calculatedb) A; (cm-‘)
Assignmenta)
1F
G
233(from
G)
233
3
463(from
G)
433
3
230Cfrom
F) 515
20
693
3
518 685Cfrom
G)
452(from
F)
222Cfrom
E)
936(from
G)
703(from
F)
473(from
E)
953(from
D)
915
1030
882(from
F) E)
430(from
D)
1152 1371 (from G) 908(from
E)
686(from
D)
1385(from
G) G)
1382(from
F)
6
160
1348
3
4 9
466
9
4
1589
12
1659
1837(from G)
1855
7
2P
2035
6
2088(f:om G) 18Wfrom
1037
13
912
4
1144
105
1565
10
1367
14
1687
5
1625
8
1
2065
6
F)
2105(from D)
1r2r
2177
1B
2267(&m
.2F -. 1p2p
: -:
le.
G)
2296
$
.2528 :.
... : ...-.
7
243
2
447
6
1030
13
670
3
1141
160
925
3
2031 1864
2523(fromG).
c)
7
940
3
1036
4
426
7
1152
90
704
4 3
7
1558
1687
2
1667
10
10 7
5
2026
2
12 1616
8
25
2177.
24
2191
2102 2173
2165
7
2252
5
2301
2
2300
8
2288
22
2298
16
7
._2525’
.8
2491
12
2500
2
2621
2 i838
6’..
2493
2629(from F). .1859(frotk D) .:
522
1550
1364 1869
1625(from E)
p+S
501
Ai; intensity
11
F)
1862(from E)
intensity
5
1374ifrom E) 32,140 11
A;
c)
8
1540
1667 16Wfrom
221 512
c)
3
1148
1392
1535 1615(from
intensity
690
1029 652(from
A;
Band D_~
Band E
Band F
:2528
3 :-
_.’ 2697
8
_.
‘.
Volume 24, number 2
15 January 1974
CHEMICAL PHYSICS LETTERS Table 1 (continued)
Assignrnenta)
Band F
Band E
AC
AC
Band D
Cakulatedb) A; (cm-‘)
Band G
2809 2914(from G)
4 2 5 10 5 25
2815
4
2793
3
2995
8
3009
IO
2994
7
3195
3
3194
6
3208
6
3294
11
3298
16
3320
2
3431
2801 2896 2993 3206 3301 3427
3453
3414
7
3401
2
3498
3499
3
3550
10 4
3532
6
3511
2
3607
4
3618
4
3684
3
3677
1
3731
4
3719
2
3819
7
3828
6
3944
3
410.5
4
4315
7
Ai;
3016 3178 3317
3670
intensityc)
4
3725
!10
3830
3818 3943
d)
3974
d)
intens&)
3675
2
3788
6
3937
2
4043
4443 4448
4219
5
4305
4
4439
1
4561
4559
3 25
4326
4020
3
4136
3
4330
7
6
4156 4314
intensity’)
5
63
intensi@)
2817
3
4419 4426 4554
2 20
4547
22
4460
2
4561
9
4566
lP# 14
4635
4639
2
4781(from E)
4788
4
4844
8
4803
4734
5
4843
4
4907
4921
3
4948
4940
5
5069
4817
3
4864
6 4902
4
4956
3
4945
3
4952
5
5052
4
5076 5144
3 3
5051
4
5151
4 2
5109
5147
4
5287
5292
2
5304
5341
7
5361
3
5441
5447
3
5432
5
5431
.2
5435
5
5679
5664
5
5646
6
5670
5688
9
5765
5772
1
16 2
5853
.4
5950
4.
6262
6263
2
6411-
6452
9
5365
..
5816 5929
:
5769 j947
-6452 6776
V&me 24, number 2
CHEMICAL PHYSICS LETTERS
sure dependence. That any vibrationally relaxed transitions appear at these pressures is undoubtedly a reff ection of the long excited state lifetime. As a result fluerescence from truly isolated SO, molecules would be immeasurably weak with present detection techniques. The intensities of the lines in the different progressions, as shown in table 1, are accurate to about -t 20% as in some cases the bands are not clearly resolved_ Also the relative intensity of the transitions to the 22 level is uncertain due to the fact that this line (predicted 1029 cm-l from the zero point fevel) is so close to the intense transitions to the 1 1 level (predicted 1152 cm-l). There are a number of general points that can be made from a perusal of the intensities of the various progressions. They are as follows: (i) AU the progressions are rather short indicating that the excited state may be quasi-linear as opposed to linear. (ii) There are a number of intensity dips in the progressions due to cancellations of overlap integrais of transitions originating in vibrationally excited levels of the upper state [9] _ (iii) The My1 + nv2 progression intensities are far from proportional to the intensity of the transition to mu1 indicating that the simple Franck-Condon factor picture is quite inadequate and that the al normal coordinates are quite different in the two states. One might weII expect such a difference if SO, goes from bent to quasi-linear in this transition.
15 Ianuary 1974
We are continuingwith an extensive investigation of the Fran&-Condon factors invoIved in this transition to try to verify if the states involved derive from a Renner-Teller splitting of the lL\g state of the linear structure into IAl bent and 1 B, linear or quasi-linear structures IS] _
Acknowfedgement We thank Mr. J.J. McNeiIl of C.S.I.R.O., Division of Chemical Physics, for valuable assistance. One of us (RSS) would like acknowledge the assistance of a Commonwealth Postgraduate Research Award.
References (11 J.C.D. Brand, V.7’.Jones and C. Di Laura, J. &fol. Spectry. 40 (1971) 616. [2I J.C.D. Brand, private communication. [3] K.F. Creenough and A.B.F. Duncan, J.
Am. Chem. Sac.
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