Single vibronic level fluorescence spectra of sulfur dioxide

JOURNAL

OF

MOLECULAR

Single Vibronic

SPECTROSCOPY

82, l-26 (1980)

Level Fluorescence

Spectra

of Sulfur

Dioxide

RAYMOND J. SHAW, JAY E. KENT, AND MICHAEL F. O’DWYER Department of Chemistry, Monash Universi@, Clayton, Victoria, 3168, Australia Single vibronic level fluorescence spectra of sulfur dioxide have been recorded throughout most of the region corresponding to the A -2 absorption. These spectra show progressions in the symmetric stretching mode of at least five members. Twelve origins between 30 972 and 3 1 776 cm-’ show remarkably similar Franck-Condon (FC) patterns for this progression. Seven origins between 31 840 and 32 257 cm-’ show another distinct FC pattern. This behavior is repeated for three more regions of excitation, each with a different distinct FC pattern and each containing numerous origins spread throughout a region of about 700 cm-‘. The progressions in the symmetric bending mode are essentially absent in the lowest energy excitation spectra and then slowly increase in length as the excitation energy increases. There is limited activity in both even and odd quanta of the antisymmetric stretching mode. These results are interpreted in terms of the levels of a zero-order ‘B, electronic state (with zero-order origin at around 31 240 cm-‘) that are very strongly vibronically coupled to many more ‘A, levels (with lower energy zero-order origin). The bulk of the emission is what would be expected from the zero-order ‘B, levels spread among the ‘A, levels.

1. INTRODUCTION There exists a wealth of data on the photophysics of the first excited singlet system (3400-2600 A) of sulfur dioxide, much of which has been summarized in a recent review by Heicklen et al. (1). Nevertheless there are still many controversial aspects involving the assignment, geometry, and other relevant properties of this intriguing state of sulfur dioxide. It is undoubtedly a very complex system and will require many more detailed studies to unravel all the complexities. In absorption the system begins with a series of weak bands at about 3400 A (2), slowly increasing in intensity until a fairly sharp rise occurs at about 3130 A, leading to several fairly regularly spaced bands labeled A, B, C, . . . by Clements (3) and later modified by Metropolis (4). The Franck-Condon maximum is about 5000 cm-l above the beginning of the system, suggesting a large change in geometry. Figure 1 shows part of this absorption. Below the 3400-A absorption there are further weak bands, the first few of which have been definitely identified (5) as belonging to the 3B, state, with a 3A, state probably being located at slightly higher energy (5,6). The extinction coefficient (emax = 300) of the 3400-A system indicates a spin allowed transition. The transition is traditionally labeled A-2 even though the complexity of the absorption indicates that it may be due to two or more zeroorder electronic promotions. For example, despite the fact that the observed isotope shifts (7) have been interpreted on the basis of one electronic transition

I

0022-2852/80/070001-26$02.00/O Copyright All rights

0 1980 by Academic of reproduction

Press.

in any form

Inc. reserved.

2

SHAW, KENT, AND O’DWYER

WAVELENGTH 7800 I

2900

3000

REGION

3

3100

(A) 3200

REGION

REGION

2

1

3300

FIG. 1. Part of theA-k absorption system of SO,. Very weak bands extend to 3400 A and the stronger

bands at the blue end down to below 2600 A. Three regions of the absorption spectrum that are used to classify the emission spectra are shown (see text). The arrows denote the excitations for the emission spectra in Fig. 2.

with an origin at about 28 000 cm-‘, attempts at a complete vibrational analysis (4,7,8) have not been successful. A magnetic rotation spectrum (9) and a Zeeman effect (10) have been observed. This is quite unusual for a nonlinear triatomic molecule, and again the interpretation is not as yet satisfactory. Rotational analyses of 12 bands of the PO2 and two bands of the Ss02A-x system have been presented by Merer and co-workers (8, I1 ). The rotational lines are severely perturbed and analyses could only be performed on a limited number of lines within each band of the spectrum. This analysis gave excited state geometries with a large change in bond angle (-20”) and a comparatively smaller change in S -0 bond distance. The study concluded that all the recognizable structure was due to the lA, state made allowed by Herzberg-Teller coupling with the ‘I?, state via I+(&), the antisymmetric stretching vibration. Lifetime measurements (2,22 -16) show that the lifetimes are much longer than expected from integrated absorption measurements, vary from band to band, and show a double exponential behavior. Brus and McDonald (15) interpret these results on the basis of a long and a short lifetime component attributable to the ‘B, and ‘A, states. Emission spectra have been studied previously (2,13,14,27, I8), varying from Boltzman equilibrated fluorescence to single vibronic level fluorescence. The

SULFUR

DIOXIDE

FLUORESCENCE

SPECTRA

3

latter indicate that the bulk of emission is from an excited state with the geometry predicted for the ‘B, state rather than the ‘AZ state (19,20). The present study was initiated to obtain better and more readily interpretable emission spectra and to extend the range of excitation energies further toward lower energies. 2. EXPERIMENTAL

DETAILS

A Chromatix CMX-4 dye laser was used as the excitation source operating at a pulse repetition rate of 16.7 Hz. Output from Rhodamine-6G or Rhodamine 640 was frequency doubled into the uv, giving a bandpass of about 6 cm-’ over the range of wavelengths used (3280 to 2940 A). The radiation passed directly into a Welsh-White multiple-pass fluorescence cell we have previously described (21). A three-lens system was used to transfer the emission efficiently to a l-m monochromator operated in first order with a bandwidth of 20 cm-‘. Some spectra were run with twice this resolution. The dispersed radiation was detected by an EM1 62563 photomultiplier thermo-electrically cooled to -20°C. The signal was amplified and processed by a PAR Model 212 Xl00 differential amplifier and a PAR Model 160 Boxcar integrator, used with sampling time between 10 and 35 psec after each laser pulse. The cell was filled with SOZ on a greaseless vacuum line to a pressure between 1 and 2 mTorr for all spectra. The bands positions were vacuum-corrected to cm-’ and are quoted as A\v (cm-‘), meaning shift from the exciting laser line. They give ground-state vibrational frequencies provided there is no vibrational relaxation from the initially excited levels or significant shift of the band maximum due to rotational structure. Our intensity data are unfortunately not normalized for variations in the laser pulse intensity, but for most spectra a check on the relative intensities was made by rescanning over the strongest lines after the complete fluorescence spectrum had been run. The spectra have been analyzed in terms of the ground-state vibrational energy levels calculated from the vibrational constants given by Brand et al. (22). We found however that Brand’s value ofXi3 was too low. In some of the single vibronic level spectra we observed 2~ + nz+ progressions with n values up to 5 and these data, although not as accurate as the rest of the data of Brand et al. (22), suggest that x13 should be - 14.7 cm-’ (23) rather than Brand’s value of - 12.9 cm-‘. We could also, however, fit the values we obtained for the 2v, + nv, progressions by including a yl13 term in the vibrational energy level formula. 3. RESULTS

(i) General

Description

of Spectra

We have measured over 100 fluorescence spectra and consider the best way to illustrate the conclusions in this paper to be to give some representative examples of the spectra, as shown in Fig. 2, and the relative intensities or line diagrams of the vibronic origins, as shown in Fig. 3. We have also divided the absorption spectrum into three regions, as indicated in Fig. 1, to faciiitate subsequent discussion.

4

SHAW, KENT, AND O’DWYER

4.

7

31038

L

I

I

I

I

I

I

n% &+ n?, 2% + n?, 31420

a FIG. 2. Fluorescence spectra of SO, and assignment for various excitation frequencies. The frequency scale is denoted by the shift from the exciting line in units of 1000 cm-‘. For off-scale lines the multiplicative factor underneath denotes what the line rerun on the scale should be multiplied by to compare it with the rest of the spectrum.

In region 1, extending from 3280 to 3130 A, where the absorption intensity is very low, acceptable spectra were only obtainable by varying the excitation frequency until maximum emission intensity was observed in the transition to either v1 or 2~5 in the ground state. Intermediate excitation frequencies tended to be either washed out in appearance or to give much the same spectrum only with much broader lines shifted by an amount corresponding to the shift of the exciting line from the frequency giving the maximum emission. This shift is approximate due to unresolved rotational structure of the emission. On the other hand, the rotational contours of the spectra in Fig. 2 probably correspond to exciting the densest part of the rotational structure in each band, since in general the lines are quite sharp

SULFUR DIOXIDE FLUORESCENCE

5

SPECTRA

31688 I

31776

I

I

I

I

1

I

I

I

n3, 32 + n?,

21,+ n3, 2$+n3,

b

41,+ n3, FIG. 2.-Conrinued.

with FWHM - 25 cm-‘. The sharpness of the lines is further enhanced by the fact that there is very little overlap of rotational lines from adjacent vibronic origins. Typical emission spectra from region 1 are shown in Figs. 2a and b. In region 2, extending from 3130 to 3050 A, spectra were run with excitation centered about every 20 cm-‘. Six representative spectra from this region are shown in Figs. 2c and d. This is the region where we previously (18) identified a number of new vibronic origins by observing which bands increase or decrease in intensity as the excitation is varied. This was repeated here in a more detailed way, giving improved spectra of the different origins. The spectra also show that rotational contours belonging to different vibronic origins overlap to a large extent. In spite of this there seems to be little or no background continuum in the spectra from regions 1 and 2. In region 3, again spectra were run with excitation centered about every 20 cm-’

SHAW,

KENT,

AND O’DWYER

31975

I

32120

I

I

I

I

3,+ n3,

3,+ Q2+nl), 3, +2&+n3, FIG. 2.-Continued.

and about 10 cm-’ near the most intense absorption features. In this region, where the number of vibronic levels becomes much higher, very few underlying origins could be distinguished. However, the strongest lines in the spectra showed rotational contours as the excitation was moved over the band. For example, band E emission spectra at about 20-cm-’ excitation intervals from the absorption maximum at 32 850 cm-l are shown in Fig. 2e and similar spectra for band G with an

SULFUR DIOXIDE FLUORESCENCE

I

I

I

I

I

I

SPECTRA

03+2Q2+n?, ?,+312+n?,

32463

absorption maximum at 33 310 cm-’ are shown in Fig. 2f. In region 3 the spectra appear to be composed of superpositions of emissions from a number of vibronic origins. As the excitation is moved away from the densest region ofrotational lines, where presumably the rotational shifts in emission are small, thus giving fairly sharp lines, the spectra except for the strongest emission lines become washed out

8

SHAW,

KENT, AND O’DWYER

32814

Ii

32850

I-

I

I

4?,+n3,

+23,

FIG. Z.-Continued.

and the amount of background continuum increases due simply to spectral congestion. Here presumably the rotational shifts are somewhat larger. At this juncture before discussing some of the spectra in more detail, it seems appropriate to make two general comments concerning experimental conditions and problems in assignment of the emission spectra. First, 1 to 2 mTorr pressure of SO, is a little high for collision-free conditions, particularly in view of the exceptionally long radiative lifetimes of the excited levels. However, due to the low absorption coefficient, it was necessary to use these pressures to allow recording of the spectra in reasonable times. In over 100 spectra there is no evidence of significant relaxation before emission so we presume, even at these pressures, that there is little or no observable relaxation. On the other hand, many spectra contained bands that could easily be assigned to hot excitation only; e.g., excitation at 3 1 038 cm-l in Fig. 2a, or to both cold and hot excitation, e.g., excitation at 3 1 390 cm-’ in Fig. 2d. Second, the assignments of the spectra are not straightforward as in some cases pairs of transitions are accidentally degenerate for low values of n.

SULFUR

DIOXIDE

FLUORESCENCE

SPECTRA

9 33240

*lI

-

3

2

0

1

33264

-y 7

5

5

3

2

x51

33278

*

7

6

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I

3 x5

2

x51

0

33305

Llll

I

41, + n$

’

?,+n&

+2?3

21,+nQ2

+2&

331+ 43,+

n32 + 233 nQ2 + 231

FIG. 2.-Continued.

For example, transitions to flu, + 2u, and (n + l)v, + 3~~ and similarly to n v1 + vp + 2v, and (n + l)v, + 4u, are nearly degenerate forn = 1 or 2, are separated no more than 20 cm-’ when n = 0 or 3, and become easily distinguished only at higher values of 12. In these cases when the particular progression is long the assignment is fairly certain from the higher members, but if there are only a few members the assignment is difficult. In many cases that we have examined under higher resolution both pairs of transitions seem to be present. In the spectra displayed in Figs. 2a through f only one assignment is indicated in order not to make the diagrams too complicated, although often both transitions will be present. (ii) Spectra

in Region

I (30 545-31

840 cm-‘)

In this, the region of low absorption intensity, the spectra are sharp, have very little if any background continuum, and are easily assigned as the average devia-

x40

I

o 30545

.

l

x40

x40

I

I

30850

I

l

X5

1

30972

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31138 *

31250

.

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:

::

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4

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:*,

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2

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o 31280 () r

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FIG. 3. Relative intensities of the bands in the SVL fluorescence spectra of SO2 excited at or near the various vibronic origins. The laser excitation is marked with a solid star and labeled in vacuum corrected cm-‘. The resonance band intensity is included where it has been determined from hot excitation. The intensity scale of each spectrum is 0 to 200 (arbitrary units). The factor by which some of the spectra have been expanded is indicated in the upper left-hand comer. Off-scale bands are labeled to show their relative intensities. The scale at the bottom represents the shift from the exciting line in units 10

x10

0 0 0 :

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0

of 1000 cm-‘. The symbols for the various progressions are: nv,, -; nv,, 0; v1 + nv,, 0; 2~ + ny, q; 3v, + nvz, n ; 4v, + nq, A; 5vI + nvz, A; 6v, + nvz, 0; 7v, + nvz; 0; 2~ + nv,, ---; 2v, + nzvl + nvz, same symbol as for muI + nvg with dashes in between; any bands containing q, 1; any bands containing 3u,, 1; any bands containing 4v,, + . The open stars indicate bands that occurred as doublets. 11

12

SHAW,

KENT, AND O’DWYER

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FIG. 3. -Continued.

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14

SHAW,

KENT, AND O’DWYER

FIG. 3.-Continued.

SULFUR

DIOXIDE

FLUORESCENCE

SPECTRA

15

tion of the measured frequency from that expected is only 2 to 3 cm-’ for the strong lines. There are some 17 origins in region 1 and 6 of these spectra are shown in Figs. 2a and b, with the corresponding line diagrams given in Figs. 3a, b, and c. The spectrum excited at 3 1 038 cm-’ is easily seen to originate from a hot band since the first band at 640 cm-’ from 31 038 cm-’ is 1158 cm-’ from 3 1 038 + 518(vJ = 31 556 cm-’ (cf. v1 = 1151 cm-‘). A similar analysis is valid for the rest of the nvl progression, and the hot excitation to z+ + nv, is coincident with what would be expected for a cold excitation to a avl progression. The relative intensities are close to those for the spectrum excited at 3 1 566 cm-‘, although there could be some cold excitation to nv, underneath the hot excitation to a v2 + 12~~progression. A perusal of Fig. 3 shows the main features of the emission spectra in this region. The Franck-Condon pattern for the rzvl progression is the same for all but a few of the higher-energy excitations. It shows five or six members, a maximum at n = 2, with a slow decrease to higher values of n. It also shows that there is little activity of y2 in the spectra excited between 30 545 and 31 250 cm-‘, but it becomes more active at higher-frequency excitation. While the FC patterns for nvl, v2 + nv,, and 2~ + nv, are all similar in this region, the v2 + nv, progressions are much more intense than the 2v2 + nv, progressions. The other noticeable feature is that for excitations at 30 972 and 31 776 cm-l there are nv, + 2~ progressions that have the same relative intensities as expected, relative to the nv, progressions; however, their absolute intensities are of the ratio of 1 to 3. For the case of a HerzbergTeller-induced transition this ratio should be 2 for ~4 = vi and increase as V; gets smaller than Y;. The spectrum from 3 1 776 cm-’ also contains transitions to v1 + 4~ and 2v, + 4v,. For all the other spectra in this region, the bands involving even quanta of v,, when present, are very weak. Some of the spectra also show very weak transitions to bands involving odd quanta of us with no obvious intensity relationships to the other progressions. The spectra with excitation between 31 675 and 31 720 cm-l are the exception to the generalization in region 1 that the rotational structures of different vibronic origins do not overlap. Some bands obviously occur as doublets; however, as the excitation is shifted the relative intensities become hard to interpret, making it difficult to say just how many origins are involved. (iii) Spectra

in Region

2 (31 925-32

643 cm-‘)

In this region, the rotational structure of different vibronic origins overlaps, but it is possible in most cases to classify which bands come from which origins. The FC pattern of the nvl progressions show a distinct change from region 1 and two new patterns emerge. First, at lower excitation energies around 31 925 cm-’ the p2v1 intensity has a maximum at tz = 1, decreases rapidly to a minimum at n = 4, and then increases slightly for higher values of n. Second, at higher excitation energies around 32 620 cm-’ the intensity of the nv, progression is again at a maximum for n = 1; however, the minimum now occurs at n = 3 and the n = 4 intensity is also quite strong. In most of the spectra, vl is the most intense band and shows emission envelopes, generally as doublets, as the excitation moves away from the absorption maxima. For example, the V, and 2v, bands in the 32 120 cm-’ spectrum in Figs. 2c and 3c are split into doublets and are probably due to the v1 and 2v, lines from the 32 170 cm-’ vibronic origin.

16

SHAW,

KENT, AND O’DWYER

The FC pattern for the JZQ progression also changes near 32 120 cm-l. At the beginning of region 2, it decreases monotonically from a maximum at n = 1 and then changes to a maximum at n = 2 near the 32 120 cm-’ origin. When this happens, the nv2 combination with mv, follows the relative intensities for the nv, progression in region 1. Toward the end of region 2, the nv2 progressions become more complicated again, probably due to overlapping of different origins. At 32 463 cm-’ the nv, progression and its combinations with mv, have maxima at n = 1 and n = 4. The relative intensities of the combinations are most like the m v1 intensities at 3 1 925 cm-’ for the maximum at n = 1 and like the m v1 intensities in region 1 for the maximum at n = 4. Again at 32 643 cm-’ the nv2 combinations with m v1 have a maximum at n = 4 and that for the m = 2 has the greatest intensity of the various combinations; i.e., it follows the mv, pattern in region 1. In nearly all of the spectra in region 2 there is some activity of vs. For excitation at 32 170 cm-’ there are lines for nvl + 2v2 + v3 and nv, + 3v2 + V~for n = 1, 2, 3, and 4. The fly2 maximum is at n = 2 and the combination with nv, is most intense for n = 2. In most of the other spectra the intensity pattern is not so clear as in general the progressions involving v3 are weak. Again in some of the spectra in this region lines arise involving even quanta of v3, but they are only intense enough to make an unambiguous assignment for excitation at 31 925 and 32 257 cm-‘. (iv) Spectra

in Region

3 (32 850-34

000 cm-‘)

In region 3, generally the only vibronic origins apparent are those corresponding to the absorption maxima. Here, for some of the bands such as E and G shown in Figs. 2e and f, a regular change in the rotational structure occurs as the excitation is moved across the band. For band E, in Fig. 2e, the absorption maximum is at 32 850 cm-’ and, as far as we can tell, all the bands in the emission spectrum change in the same way in that they split into doublets as the excitation wavelength is varied. The weaker lines cannot be followed as they are too numerous and when they split, they only give rise to an apparent background continuum. For band G, on the other hand, the strong lines split into triplets and the weak ones again give rise to a more or less continuous background. About 30 cm-’ below the absorption maximum of 33 310 cm-’ an nv2 progression and combinations of the progression with n v1 increase in intensity. The intensities of the combinations with n v1 follow the 31 925-cm-’ nv, pattern rather than that for band G. Furthermore, these bands split into doublets rather than triplets when the excitation is shifted to lower energies. The spectra from bands E and G suggest two important features in the interpretation of all the spectra. First, even though we know this system of SO, is very heavily perturbed, it is still a reasonable approximation to talk about vibronic origins and their associated rotational structure. Bands E and G show the same FC pattern over a range of excitation of about 100 cm-l, with splitting of the lines that, as will be seen, can be explained at least qualitatively. Second, the rotational contours observed in emission are quite dissimilar for excitation of bands E and G, suggesting the geometries may be quite different in the two bands. If the geometries are different, then these two vibronic origins cannot be described as a simple product of an electronic and a vibrational wavefunction but rather they will be linear combinations of many of these.

SULFUR

DIOXIDE

FLUORESCENCE

SPECTRA

17

Bands E and G show a relatively high amount of activity in 2v,, but not in combination with nv, alone. The main lines observed are IZV~+ 2vZ + 2u3 and nv, + 3~ + 2~~ for n = 1,2,3,4, and 5. The maxima occur for 2v, for E and 3u, for G, with the intensity dependence on tzvl like that for band A in region 2. The other bands in region 3 do not show the clear rotational structure observed in bands E and G. Generally, the emission spectra, especially at higher excitation energies, quickly degenerate into messy continua with few clearly discernible peaks as one scans the excitation across the absorption profile. 4. INTERPRETATION

The following section shows that the single vibronic level fluorescence spectra of the A-2 system of SO, are quite confusing to say the least. We see that the system has its FC maximum at least 5000 cm-’ from the onset of absorption, yet lengths of the progressions in emission are much shorter than would be expected if the system were well behaved. In particular, the nvp progressions in emission are very short until excitation is well into the middle of the absorption. The nv, progressions start off with five or six members and go to about eight near the FC maximum. These patterns suggest very little angle change in the excited state and only a moderate change in bond length. It is not easy to compare FC patterns for the intensities of progressions with calculations, because the bands giving rise to emission are very complex vibronically. However, we can make some qualitative inferences from the general features of the spectra. First, the relatively long tzv1 progression, which has the same FC pattern over a dozen or so bands between 30 545 and 3 1 776 cm-’ and showing no FC minimum, is typical of that expected for very few or no quanta of v1 in the upper state with a medium change in the origin of the normal coordinate. Second, the fact that very little 1~~ is present in emission in the lower-energy part of this region suggests that no quanta of vp are present in the excited state. Beyond 31 280 cm-‘, short mu, progressions and combinations with n v1 appear with the FC maximum at m = 1. This pattern is typical of what we expect for one quantum of r+ present in the excited state with very little change in the v2 normal coordinate origin. This pattern repeats itself over eight or more vibronic origins up to around 31 840 cm-‘. We can interpret this part of the absorption spectrum in the following manner (24,25). Imagine first, as expected from SCF calculations (19,20), that there are two zero-order electronic states of symmetries ‘A, and ‘B, in this region. The ‘A2 state carries no oscillator strength in zero order, in contrast to the ‘B, state. Let us further hypothesize that the origin of the zero-order B, state is some 3000 cm-’ higher in energy than the ‘A2 state so there are many vibronic levels of the ‘A, state near the ‘B, origin. The lowest zero-order vibronic level of the B, state is somewhere in the region where the emission activity of I+ becomes pronounced and, of course, has to be the (0.1 ,O)’ level. This level and the zero-point B, level will then interact with the many more quasi-degenerate vibronic levels of the zero-order A2 state involving odd quanta of v3 such as (nvI,mv2,v3) or (nu,,mv2,3v3), etc. Since these states are nearly degenerate, the vibronic interactions may be large, resulting in vibronic states that are mixtures of many zero-order states. The interaction matrix elements will vary, this variation depending mainly on FC overlaps. The

18

SHAW,

KENT,

AND O’DWYER

30730 31240 3178 0 a90

n=

a

7

6

5 nJ,

FIG. 4. Franck-Condon the right) of excitation.

patterns

4

3

2

1

0

PROGRESSIONS

for the nv, progressions

that occur

in various

regions

(indicated

at

final vibronic states will also be shifted by different amounts, perhaps up to 100 cm-’ from their zero-order positions, making it extremely difficult to estimate the position of the zero-order A2 levels. The emission spectra appear similar for many bands because they are dominated by theB, components. The admixture ofA, components withB, vibronic symmetry may have phases such that in emission there will be many intensity cancellations (26), hence the weakness of the bands involving 2u,. The two absorption bands, from which emission to nu, + 2~9 is relatively intense, most likely correspond to levels consisting primarily of those containing zero quanta of v2 in the zero-orderA levels such as (n,O,l)’ and (n + 1, 0, 1)‘. The zero-order position of the n vI progression in absorption of the B, state can be estimated after a perusal of Fig. 3. The FC pattern for this progression is constant over about 1000 cm-’ and then slowly varies, changing from no minima to one at n = 4, then one at n = 3, and then one at n = 2, and then finally to a pattern still with a minimum at n = 2 but with a pronounced maximum at n = 3. These patterns are summarized in Fig. 4 and show the approximate zero-order location of the (n,O,O)’ B1 levels forn = 0, 1,2, 3, and 4. Such a model places the 'B,origin at 31 240 cm-l and the 6!90-cm-’ interval is the zero-order v1 frequency of the B, excited state. A similar exercise for the ny2 progressions is not as easy because in general the intensities are much weaker. However, taking the approximate centers of the

SULFUR

DIOXIDE

FLUORESCENCE TABLE

19

SPECTRA

I

Zero-Order B, Vibronic Levels Frequency

Emission Pattern

ASSiglU?llt

length

intensity minimum,

length ;:;;zy

n value

m

value

31240

(O,O,O)

5

31700

(O,l,O)

5

31930

(l,O,O)

5

4

32160

(0,2,0)

5

4

3-4

2

32390

(1,1,0)

6

3

3-4

1 and 4

32620

(2,0,0) and (0,3,0)

6

3

32850

(1,2,0)

6

3

33080

(2,1,0) and (0,4,0)

2

33310

(3,0,0) and (1,3,0)

2 and 6

33540

(2,2,0) and (0,5,0)

33770

(3,1,0) and (1,4,0)

34000

(4,0,0) and (2,3,0) and

none

2

1

2

5

4-5

4

2 and 5

2 and 4

2

2

5

1 and 3

5-6

3

6

6

2

7

2 and 5

complex

(0,6,0).

regions where the pattern changes suggests a frequency of about 460 cm-‘. Since this value is two-thirds v~, different sets of vibronic levels of the B1 state will be coincident, or nearly so. Table I gives the location of the zero-order B, vibronic states together with a brief description of the FC patterns of the emission spectra in these regions. Two points should be noted in considering the correlation. First, near the O-O band the pattern will be simpler than that for higher-energy excitation and we have seen that this appears correct. Second, at higher energies the vibronic interaction will result in the zero-order B, states being mixed as well as spread into the zero-order A2 levels, resulting in quite complicated emission patterns at higher excitation energies. The clue to the assignment lies in considering the intensities of the combination bands nv, + mv2. Near the (O,O,O)’and (O,l,O)’ zero-order B, levels, the intensities of these bands are more or less simple products of the intensities of the n v1 and m vp progressions. This is not true at higher-energy excita-

20

SHAW.

KENT,

AND O’DWYER

tion, but in many cases, it is still possible to see what pattern of nvl progressions the mvz is built onto. For example, the relative intensities of the n z+ + mv, bands for n = 1, 2, and 3 in the 32 120-cm-’ spectrum follows more closely the 31 240cm-* n u1 pattern than the 3 1 930-cm-’ n v1 pattern, suggesting they originate from the (0,2,0)’ B, level. Note too that the IZV~progression in this spectrum gives doublets, which belong to the vibronic origin at 32 170 cm-i and has very different relative intensities. This same behavior is repeated for other bands. Particularly relevant to the assignment are the following:

Spectrum from 32 463 cm-’ 32 643 cm-’ 33 278 cm-’

nv, + mvp intensities compatible with (n ,O,O)’ B, level (1 ,O,O)’ (O,O,O)’ (1 ,O,O)’

nearest (n ,O,O)’ level (2,0,0)’ (2,0,0)’ (3,0,0)’

It is also significant that the 32 850-cm-’ spectrum is the only case where the lzvl + mv, lines show the same rotational contours as the strong nvl progression. This level has a single B, assignment and has been rated by Hamada and Merer (21) as one of the higher energy absorption bands with relatively unperturbed rotational structure. However, then V, + m v2 intensities do not follow a simple product rule as the lines for 4v, + mv, and 5v, + mvz are either missing or very weak, yet 4v, and 5v, on their own are quite strong. This is probably the result of B1 level admixture from above, e.g., (2,1,0)’ and (0,4,0)‘, and below, (2,0,0)’ and (0,3,0)‘, causing cancellations. The previous few paragraphs give a very qualitative description of the zeroorder ‘B, vibronic levels that emerge from the single vibronic level fluorescence spectra. It would also be nice to try to deduce the location of the zero-order lA, levels. Brand and Nanes (7) have suggested a bending frequency of about 300 cm-’ in the ‘AZ state. The lower-energy vibronic origins we have studied show repeated separations of 280 cm-‘, in rough agreement with Brand’s value. Combined with the tentative suggestion from the fluorescence data that 30 972 and 3 1 776 cm-’ are pure nv, + u3 levels, i.e., no v2, we give an assignment of the A, levels in Table II. Due to the huge vibronic interaction that must be present and the resulting shifts in the positions of the levels from zero order we cannot expect the progressions to be very regular. It is also not possible from the fluorescence data to determine where the AZ origin is, since we were unable to obtain fluorescence spectra anywhere near where the origin is expected to be. In Table II the numbers are rounded off to the nearest five wavenumbers from a combination of our data and those of others (4, 7). The assignments are by no means certain. In concluding this section we give in Table III a rough estimate of the quantum yield and the percentage of emission in the various progressions. This has been a guide in the assignments made and is of interest when considering the lifetime data. The quantum yields are relative and probably only accurate to a factor of about 2. They were calculated by summing the emission intensity using vertical height only and adjusting for laser intensity, photomultiplier voltage, and extinction coefficient. The largest error probably comes in from the variation of the number of rotational

SULFUR DIOXIDE FLUORESCENCE

SPECTRA

21

TABLE II Assignment of A, Vibronic Origins

,“d) 29450 29650

“U1

+

mv2

+

nvl + 3v3

u3

other

1

310

29760 29840 29940 29995

Hot to 30545

30050 Hot tcl30745

30180 30240 30280

805

Hot to 30845

30365 30465 30545 30685 30745 30845 30970 31040 31140

765

31250

Hot to 31565 (o,o,oR$

31280 31320 31420 31510

I

31565

(‘J,l,W$

31675 31720 31775 31840 31925

(l,o,o)B1

31975 32015 32120 32170 32255 32390 32460

lines in a 6-cm-’ bandwidth and the absorption intensity in a particular region of a band. Very doubtful values in Table III are indicated by a question mark. 5. SUMMARY AND DISCUSSION The previous section showed that the 3400 to 2600 A system of SO, can be considered to result from very large vibronic interaction between the zero-order 'A, and %, electronic states of SO,. The values of vi and v; in these states are approxi-

22

SHAW, KENT, AND O’DWYER TABLE III Quantum Yields and Percentage of Fluorescence Excitation frequency -1 (cm )

Intensity in Different Progressions

Relative Fluorescence quantum yield, dF

Percentage Intensity in ""1 + In"* "%

(inc.n=0)

a11 lines involving 2v3

30545

9

30745

18

30850

2

100

30972

30

31040

4

31140 31250 31280

16

31320

15

31420

30

55

35

31507

12

72

12

31566

20

36

64

31688

16

47

50

31720

5

47

53

31776

30

68

10

31840

19

21

79

31925A

75

10

15

770

18

12

68

5

27

28

72

20

68

20

10

14

85

11

4

47

30

18

63

37 6

v3

2

5

4 16

3

22

13

67

16

31975

9

24

74

2

32014

10

45

51

4

32120

24

15

3

24

64

32170B

54

30

5

11

32257

33

7.6

11

3239oc

59

38

30 7

32463

11

89

?

12

3

32620D

50

33

?

32643

35

58

7

7

32850E

48

31

14

7

9

16

33080F

39

36

33278

40

60

17

6

33310G

30

41

36

335348

11

18

70

337605

9

22

48

10

20

3401OL

30

18

56

15

11

17

12

mately 800 and 300 cm-’ (‘A,) and 690 and 460 cm-’ (‘I?,). This is based on the fact that the fluorescence spectra show well-developed, even if very complicated, FC patterns, so that there are groups of rotational lines belonging to mixed vibronic states, that could be written as

where I+!J and I#Iindicate zero-order electronic and vibrational states. +?I will be totally symmetrical, and 4:” will always contain odd quanta of v3. There are more AZ than B, states in the admixture, causing cancellations in the

SULFUR

DIOXIDE

FLUORESCENCE

SPECTRA

23

FC overlaps for emission. Overall, the resulting spectra appear to be dominated by B,-like emission. The interpretation has been, of necessity, very qualitative. At this stage we would like to comment briefly on other data concerning this system of SO,. A scan of Table III shows that the activity of vQin the spectra is small and quite variable. It also occurs even more prominently in emission from the higher-energy lBp absorption system (22) and cannot be satisfactorily accounted for there. We suggest that it is a result of Coriolis coupling between the rotational levels of the zero-order ‘A, and ‘B, states. This would involve a b, rotation, i.e., about the c axis, that is perpendicular to the molecular plane where the rotational quantum number K would not be relevant and the rotation would depend only on J. Rovibronic states, with the same J that are close together will couple, making possible transitions in absorption to levels with some A, vibronic character that can emit to B2 levels of the ground state. However, the vibronic levels of the A2 electronic state that mix will not be the same ones that mix in the vibronic interaction. The former will have total vibronic symmetry AZ, whereas the latter will be of B, symmetry. Thus, the fact that the FC patterns for emission involving va show little or no relationship to those involving i+, v2. and 2u, is understandable. Furthermore, if the presence of v:, is the result of Coriolis interaction, the rotational selection rules will be different (AK = 0, +-2) than for the normal emission (AK = ? 1). Our studies are not detailed enough to confirm this, but in at least a few cases, for example, emission from 32 170-cm-’ excitation in Fig. 2d, the lineshapes are distinctly different for the vx lines from the other lines in the spectrum. It would be interesting to see if the seemingly anomalous MRS and Zeeman spectra correlate with the activity of v,. There is a wealth of data on lifetimes and quenching for this system of SO, (I). The lifetimes have been found to be nonexponential and have been interpreted as biexponential, the slow component attributable to the B, state and the fast component to the A2 state. If our interpretation of the fluorescence spectra is correct, there are no pure B1 and A, states, except perhaps at the lowest energies, but rather large admixtures of the zero-order states. We have also found that the rotational structures of these mixed vibronic states overlap. Thus, it is hardly surprising that nonexponential lifetimes are observed because the vibronic interactions will be different and the lifetimes lengthened to different extents for different vibronic bands. A similar explanation has recently been put forward for the same effect for NO, (27). In addition, as has been pointed out previously by Caton and Gangadharan (14), one must be extremely careful in comparing lifetimes excited at different places using cut-off filters, when the emission is so highly structured and has such large variations in intensity. For example, 40% of the emission intensity for excitation at 30 972 cm-’ is at wavelengths shorter than 3550 A, yet this short wavelength cutoff filter was used in some of the more recent lifetime experiments (1.5,16). Most certainly this would invalidate some of the conclusions made concerning relative intensities of short and long wavelength components even if that description were valid. Another very striking anomaly concerns band F. We noted in all our fluorescence measurements how weak the emission is from this band compared with its neighbors and for that matter all the other bands in the spectrum. Admittedly, the yield data

24

SHAW, KENT, AND O’DWYER

0

1 3040

I 3020 WAVELENGTH

FIG.

5.

Medium resolution absorption

3000 (A)

spectrum of bands E, F, and G.

in Table III are very approximate, but the factor of 30 difference in the estimated quantum yield of emission between the maxima of bands F and G is real enough. It is then very surprising that band F shows no anomalous behavior in the lifetime experiments. We have left until last a comparison between our results and the rotational analyses (8, II) because this area is where the most surprising anomalies occur. As detailed in Section 3, bands E and G show quite different rotational structures on the strong emission lines when the excitation is moved across the band. Figure 5 shows the medium resolution absorption spectrum of bands E, F, and G and the positions of excitation for the emission spectra in Figs. 2e and fare marked. Bands E and G show the best developed gross rotational structure, presumably K subbands, of any bands in the absorption spectrum. Since SOZ is a near prolate symmetric top in its ground state (II), a ‘B, electronic transition will be polarized perpendicular to the plane and the a axis and one would expect the rotational selection rules, AK = 21, AJ = 0, tl. If B’ = C’, the K subbands of the absorption will vary, depending primarily on how much the A rotational constant changes in the excited state. There is no doubt that for SOZ, A decreases in the upper state both from theoretical considerations and from the fact there is a AK = 1, redshaded bandhead. The smaller A’ becomes, the closer this bandhead will occur to the vibronic origin. Thus, for small changes in A at around 50 cm-l to the red of the vibronic origin, there will be only AK = - 1 subbands because K” will be too high to have an appreciable population for the AK = + 1 subband. On the other hand, for larger changes in A, both AK = + 1 and - 1 subbands will occur about 50 cm-’ to the red of the vibronic origin. What this means in terms of the emission spectra is that for the first case, we would expect the rotational contours to be doublets, whereas in the latter the contours expected would be triplets. This we believe to be the origin of the different rotational structure observed in the E and G band emission spectra. Figure 6 gives the results of a very qualitative calculation of the subband structure expected with that observed for the E and G bands. Here A’ used for the E-band excitation was 1.7 cm-’ and that for the G band 1.3 cm-l. B’ -‘I C’ is taken as about 0.3 cm-‘. Of course, none of the J structure is resolved and, while this is admittedly a crude analysis, the important feature it illustrates is that the excited state geometries must be very different in

SULFUR

DIOXIDE

FLUORESCENCE

25

SPECTRA

the two states. We have also observed doublets and triplets generally on just the nv, progression for other bands in the spectrum (A, B, C, and D) but only on the strongest lines, i.e., v1 and 2~~. The rotational analyses of the absorption spectra carried out by Hamada and Merer (8, II ) give rotational constants that correspond to a medium change in bond length and a change in bond angle of about 20”. These data seem at first sight to be incompatible with our analysis of the emission spectra. To be specific, we discuss the band, the vibronic origin of which was determined to be 30 992 cm-’ from the rotational analysis (II ). We excited at this frequency and found the same emission spectrum as when we excited at 30 972 cm-l shown in Fig. 2a, but the average displacement of the strongest lines from that expected was + 19 cm-‘. The lines in the spectrum excited at 30 992 cm-’ were also much less intense and about four times broader. As can be seen in Fig. 2a, the lines are very sharp forthe 30 972-cm-l excitation. This indicates that the rotational lines at both frequencies belong to the same vibronic origin. We would expect the emission to have maximum intensity where a large number of rotational levels are populated, i.e., at 30 972 cm-‘, not necessarily at the vibronic origin. However, for the geometry obtained from the rotational analysis, we would expect medium-length progressions in vl and long progressions in v2. The former we see in the emission spectrum, but we only see L+ as one quantum and extremely weak. There could be two reasons for this apparent anomaly. In the first place, the long vp progressions expected for an angle change of 20” would have the intensity distributed between many members of the progression and the combinations with v1 would overlap, leading to a very weak, very broad emission. This would also be expected to occur in a region where the phototube is

EXCITATION

E -

60

E

-

40

E +

30

G

-

70

G -

30

OBSERVED

CALCULATED

I

A

1.1

1.3

1.5

1.7

1.9 -

FIG. 6. Comparison bands E and G.

of observed

and calculated

gross

K subband

structure

for excitations

over

SHAW,

26

KENT, AND O’DWYER

getting very much less sensitive, and we did not look very carefully in this region. The second possible reason is that the excited vibronic level is a mixture of one or a few B1 electronic levels and quite a number of AZ electronic levels and cancellations in FC overlaps in emission may result in absorption that corresponds to that expected for the A, electronic state and emission more characteristic of theB, state. This band is also the one where the lifetime data (1.5,26) have been interpreted to say that the short lifetime component suddenly increases in relative intensity. It still remains to reconcile completely all the data available on the first excited singlet system of sulfur dioxide. ACKNOWLEDGMENTS

We would like to thank the Australian Research Grants Committee for support. We would also like to thank Professor G. Wilse Robinson for his interest and willingness to discuss the SO, problem in a most helpful manner over the past few years. RECEIVED:

May

17,

1979 REFERENCES

1. J. HEICKLEN, N. KELLY, AND K. PARTYMILLER,Rev.

Chem. Intermediates,

in press.

2. S. J. STRICKLERAND D. B. HOWELL, J. Chem. Phys. 49, 1947 (1968). 3. 4. 5. 6. 7. 8. 9. 10. Il. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

J. H. CLEMENTS, Phys. Rev. 47, 224-232 (1935). N. METROPOLIS,Phys. Rev. 60, 295-301 (1941). J. C. D. BRAND, V. T. JONES, AND C. DILAURO, J. Mol. Spectrosc. 45, 404-411 (1973). J. P. VIKESLAND AND S. J. STRICKLER,J. Chem. Phys. 60,660-663 (1974). J. C. D. BRAND AND R. NANES, J. Mol. Specfrosc. 46, 194-199 (1973). Y. HAMADA AND A. J. MERER, Cunad. J. Phys. 52, 1443-1457 (1974). J. L. HARDWICK AND W. H. EBERHARDT, private communication. J. C. D. BRAND, J. L. HARDWICK, D. R. HUMPHREY, Y. HAMADA, AND A. J. MERER, Canad.

J.

Phys. 54, 186-196 (1976). Y. HAMADA AND A. J. MERER, Canad. J. Phys. 53, 2555-2576 (1975). K. F. GREENOUCHAND A. B. F. DUNCAN, .I. Amer. Chem. Sot. 83, 555-560 (1961). H. D. METTEE, J. Chem. Phys. 49, 1784-1793 (1968). R. B. CATON AND A. R. GANGADHARAN, Canad. J. Chem. 52, 2389-2398 (1974). L. E. BRUS AND J. R. MCDONALD, J. Chem. Phys. 61, 97-105 (1974). F. Su, J. W. BOTTENHEIM, H. W. SIDEBOTTOM,J. B. CALVERT, AND E. K. D~~o~,Znternat. J. Kinefics 10, 125-154 (1978). J. E. KENT, M. F. O’DWYER, AND R. J. SHAW, Chem. Phys. Letr. 24, 221-226 (1974). R. J. SHAW, J. E. KENT, AND M. F. O’DWYER,Chem. Phys. l&155-164(1976); 18,165-173 (1976). I. H. HILLIER AND V. R. SAUNDERS,Mol. Phys. 22, 193-201 (1971). D. D. LINDLEY, “Ab-Initio SCF Studies on the Ground and Excited States of Sulfur Dioxide,” masters thesis, Ohio State University, Columbus, Ohio, 1976. R. J. SHAW, Ph.D. thesis, Monash University, Australia, 1975. J. C. D. BRAND, D. R. HUMPHREY, A. E. DOUGLAS, AND I. ZANON, Canad. J. Phys. 51,530-536 (1973). R. J. SHAW, J. E. KENT, AND M. F. O’DWYER, to be published. M. BIXON AND J. JORTNER,J. Chem. Phys. 50, 3284-3290 (1969). G. WILSE ROBINSON, “Excited States,” Vol. 1, pp. l-34, Academic Press, New York, 1974. See W. H. HENNEKER, A. P. PENNER, W. SIEBRAND, AND M. Z. ZGIERSKI,J. Chem. Phys. 69, 1884-18% (1978). V. M. DONNELLY AND F. KAUFMAN, J. Chem. Phys. 69, 1456-1460 (1978).