Decay of fluorescence from single vibronic states of SO2

Volume 1 i, number 4

DECAY

CHEMCAL

OF FLUORESCENCE

15 December

PHYSICS LETTERS

FROM SINGLE

VIBRONIC

STATES

1972

OF SO2

Man-Him HUI * and Stuart A. RICE Depamnenr

of Chetnisrry and The James Franck instirute, Chicago. Illinois60637, USA ,

Received 20 September

ZlJniversitJ? of Chicago,

1972

The fluoresccncc from ten vibronic states in the 2300 to 2100 A absorption region of SO2 has been studied. A sharp drop in fluorescence quantum yield and fluorescence lifetime for wavelengths shorter than 2206 A is a:t:ibuted to the dissociation of SO,. ‘Ihe quantum yield of fluorescence in the spectral region where there is no dissociation is unity at zero pressure, within our experimental error. We observed neither anomalously long lifetimes of

lIuoresccnce, nor non-exponential studied and other states.

decay of fluorescence,

indicating

that there is weak coupling between

the state(s)

Comparison of our experimental data with calculations based on RRXM theory suggests that energy randomization is not complete before dissociation, and that coupling between the symmetric bending mode and tic asymmetric stretching

mode is wry weak.

The #otochemical

and photophysical

properties

of SO, have been studied for many years, with most attention focussed on the effects generated after excitation of the 3B1 and ‘B1 states [ 11. Much less is known about the fate of molecules excited to the ? and g states [2,3], lying above 40000 cm-l. Yet, it is just this region of excitation that leads to photo fragmentation [4,5] of SO,. The work reported in this letter was undertaken for two purposes: (1) A study of the competition between photon emission *andphotofragmentation can, under favorable circumstances, give information about the distribution of excitation energy in the molecule. Theoretical studies suggest that in a triatomic molecule energy ranlllomization may The fluorescence

not be faster than chemical reaction. of SO2 can be used to examine this

possibility. (2) The theory of radiationless processes explicitly distinguishes between the behavior to be expected of an excited “large molecuie” and an excited “small molecule” [6]. Although there have been several reports of studies of fluorescence decay of prepared single vibronic levels of larse molecules [7,8], few corresponding data are available for small molecules. l

Harper Fellow i971-1972.

434 ;

Testing and refinement of the theory of radiationless processes require such data. We have studied, by the method of single photon counting, the lifetimes and quantum yields of fluorescence from ten vibronic levels of SO, in the ? and 5 states. The details of our equipment have been described fully in other publications [7,9]. For the present we need make only three remarks. First, to eliminate the effects of low order diffracted light (the grating in the monochromator was used in the fourth order) an interference filter (band pass 2.50 .@ was placed in front of the monochromator entrance slit. For the study of three states, from which fluorescence was very weak, the filter could not be used. Second, so as to prepare single vibronic states we iised exciting radiation with a bandpass of only C.81 i%.Third, all samples were Merck 99.98% moisture free SO2. As described in earlier papers 171, the fluorescence cell was loaded using a greaseless, Hg-free vacuum line. &I absorption spectrum of SO, in the energy range of interest to us is displayed in fig. I. We have found that the extinction coefficient is pressure dependent even in the range 1 to 0.1 torr, the effect being that the extinction coefficient increases as the pressure decreases. The most plausible interpretation of this observation attributes the effect to collisional

Volume

2lOO

2150

2200

2250

WAVEtENGTH Fig. 1.

2300

(%,

broadening of the rotational envelope. In ali our measurements the extinction coefficient was determined down to the same pressure as was the fluorescence decay. The lifetimes and quantum yields of fluorescence determined in these experiments are recorded in table 1. These data were obtained at 3 pressure of about 0.1 torr. At this pressure we estimate, using a hzrd sphere collision model, that 90% of the excited molecules emit a photon before undergoing collision with another molecule. There is not now available an absoIute quantum for any substance

1972

2200 A. For this reason we have been forced to refer our relative quantum yield measure~ler~ts to the quantum yield of fluorescence fro-m the benzene level lRZu (6:) [IO], which is centered at 2590 A. Insofar as is possible, changes in apparatus constants between 2600 and 3200 a were accounted for by using ratios of relative intensity measurements and including changes in photomultiplier response, but there remains a residual uncertainty related to the change of reflectivity of the mirror used in the cell when the scattered light intensity is determined. We beiieve that it is because of this uncertainty that the raw values of the quantum yields relative to that of benzene exceed unity for four of the prepared states. We have, therefore, set the quantum yield of fluorescence from the state CY~(1,2) equal to unity and scaled all others correspondin&. As can be estimated from the data tabulated, the overall uncertainty in the quantum yields is of the orcier of 15%. The mpsi striking features of the data obtained are that the fluorescence lifetime ar,d yield drop suddenly for excitation energy in excess of 45 400 cm-l [i.e., above the state ~~~(0.3) **], and that the ftuorescence is too weak and the iifetime too short for us

i\bsorption spectrum of SO2.

yield determination

IS December

CHEMiCAL PHYSICS LETTERS

17, numbcr 4

to measure when the excitation energy exceeds 46 500 cm- 1 [i.e., above the state CQ(0,6)] (see

fig. 2). (on the other hand, Douglas and Zanon have ob-

served resonance

fluorescence

from suiphur dioxide

* Assignment and nomenclature of states arc according to ref. ( 111. For example, UZ,(0,3) means a transition to the 012electronic state with 0 quantum in .v~, 3 quantum in ~2 and no quantum in y3 from vibrationless ground electronic state.

at or near

Table I Fluorescence

data of individual vibronic states of sulphur dioxide -Normalized

a)

State

h(K)

r(nsec)

Qlf

@f

knrcsec-l)

1.

Ql(I,1)

2. 3.

crl(1,2? Ctz@,Bl q (l-3) rt, (0, 1) ff2 (0,3J (0.2) ol, or2 @,4) az(O,5) qJ (0,6)

2297.5 2277. I 2260.9 2258 2243 22336.3 2224.1

31.82 32.77 35.23 33.39 38.70 45.41 41.27

1.028 1.134

I.042 0.753 0.845

0.907 1.0 0.98 b, 0.956 0.919 0.664 0.745

2.94 0 0.567 I.31 2.10 7.41 6.18

2187.8 2169.3 2152.4

28.11 9.63 7.81

0.172 0.081 0.0277

0.151 0.0714 0.0244

4. :: 7. 8. 9.

1.084

a) Data for states I to 6 are obtained with the use of interference b) Quant~~m yield estimated by extrapolation.

kr(seCC1) 106 106 106 106 106 106

2.85 X IO’ 3.05 x 107 2.iS.x 10’ 2.86 x IO7 2.37 x 10’ 1.46 1.80 x!X IO’ lo7

30.19 x 106 96.43 x lo6 124.91 X 106

0.54 3 10’ 0.74.X 10’ .0.31 X 107

filter while those of stites

X x x x x x

7 to 9 are obtained

without

use of filter. 47.5

Volume 17, ntimber 4

CHEMICALPHYSICSLETIERS

15 December 1972

[ 121 when excited with the strong Zn lines at 2139 and 2 100 A This, however, does not contradict our

FLUORESENCE LIFE-TIME OF INDIVIDUAL VIBRONIC STATES OF SO,

data because their sensitive photographic detection technique is not affected by the short life time of fluorescence.) Our observations strongly support the view that the threshold for an important radiationless process lies just above q,(O, 3). Clearly, they are in agreement with Okabe’s suggestion [5] that SO, fragments when excited to an energy greater than that corresponding to the state LYZ (0,3). Our raw data and our normalized data show sys-

X,

WAVELENGTH 61

ia) I

cl

-I w

,

I

I

FLUORESCENCE QUANTUM YIELD OF INDIVID’JAL Vl6RONlC

A, WAVELENGTH

(ii,

(b)

\

X,WAVELENGTH

(Cl 476

(1)

,

tematic decreases of the fluorescence quantum yield as the molecule is excited from cr2(0,0) to cy2(G,3). Since the density of states at the energy of the prepared state in any manifold of SO2 is very small, 1-2 per cm-l, we expect the quantum yield of fluorescence to be unity unless some chemicaI process or collisions between molecules seNe as routes for radiationless degradation of the energy. But we have argued that a2 (0,3) lies below the threshold for dissociation, so we must rule out chemical reaction for the case under examination. There is lefi, then, only the possibility +hat collisional quenching is the source of the observed trend. To verify this deduction we have studied the pressure dependence of the fluorescence lifetimes of SO2 prepared in the states a2(0,3), q(O,2) and a2 (0,l). Stem-Volmer plots of the inverse lifetime versus pressure (fig. 3) yield the self-quenching rates 45.90 t 0.8,40.36 5 0.42 and 37.62 + 0.56 X 106 set-1 torr- 1 for the states 0z2(0,3), 0~~(0,2) and CY~ (0,l j, respectively. After extrapolation to zero pressure, the quantum yields of fluorescence from these three states become equal within experimental error, as expected. We thus believe that the quantum yields are unity for all the first six states studied when the molecules are truly isolated, It is interesting that the observed self-quenching rate for the state a2(0,3) is about three times larger than the rate of quenching of SO2 fluorescence by Ar [S]. The observed rate of self-quenching corresponds to about one deactivation in ten hard core collisions. The data described above can now be used to partially answer the questions which underly the stated

Fig. 2. (a) Fluorescence life times, (b) fluorescence

quantum yields and (c) nonradiative decay rates of individual vibronic states of S02.

VoIume 17, isumber 4

7,-

i

s

I

CHEMICAL PHYSICS LETTERS

I

SELF -QUENCHING FLUCRESCEKE OF

-I ‘i

r” p \ c J g

z =:

,w

I

I

I

t

OF SO,

40

30

7r

PRESSURE

1 TORR)

Fii. 3, S&f-quenching of fluorescence of SOz: Stern-Volmer plot of inverse fluorexencc Life time versus pressure.

of this investigation. First, we note that even with the uncertainties in the values of the quantum yields of fluorescence, the de&iced nonradiative lifetimes (table 1) smoothly decrease as the energy of excitation increases. The lifetime of the excited moiecule calculated according to the RRKM [13J theory is about lo-l1 to lo-l2 sec. When compared with the experimental data, this suggests that in SO2, r~dom~za~ion of the excitation ener,gy is not complete before decomposition occurs. In this sense, SO, is like chloro and bromoacetylene [ 143. The long lifetime after dissociation limit also is consistent with the suggestion [4, S] that SO2 decomposes into SO + 0. Asymmetric stretching is expected to be responsible for this decompositiori and since the asymmetric mode of vibration is not excited in the experiment, it would take a considerable period of time for energy to trzmsfer from..the symmetric stretehing mode to the asymmetric mode. Of course, the net effect of slow vibrational relaxation is a lowering of the rate of decomposition. Furthermore, our interpretation suggests that the coupling between ZQr&d v3 is very weak, in the order of 0.01 cm-l. Second, within our experimental error, the lifetimes of the prepared states do not appear to be anomalously long fl5], nor are there observable deviations from simple exponential behavio_r. ThEe observations suggest that the levels of the C and D states which we have excited are not strongly coupled to a small number of states in one or more other electronic m~anifofds. purposes

1.5 December

1972

In this sense the ? and 3 states are different from the lBl state [ 11. Finally, within our experimental error the decay of fluorescence from IX~(0,3), cy2(0,2) and “2 (0, I) was of the simple exponential form at all pressures studied. At low pressure there are no collisions and the decay should be exponential. At comparatively high pressures where there are collisions, the fact that non-exponential behavior was not observed is easily shown to be in agreement with the predictions of the model proposed by Freed and Heller f 161 since the rate of variation of the 7N and rr with Fergy? rather small for the vibronic levels of the C and D states. In contrast, the rates of decay of fluorescence from the vibronic levels of the lB1 state are nonexponential [I] in form just because TV and rr vary rapidly with energy. This, in turn, is probably related to the nature of the coupling of the levels of the lBt state and the ground state, which coupling leads to a marked increase in the radiative lifetimes of said levels. The point is that the relative!y we& coupling of 3 given vibronic level to a large number of levels of another manifold leads to an energy distribution which is susceptible to perturbation by collision; #his perturbation then leads to a redistribution of energy which alters the apparent lifetime if the lifetimes of the component states differ enough. This research has been supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research. We have also benefitted from the use of faciiities provided by the Natiomd Science Foundation - Material Research Laboratory program at the University of Chicago. We wish to thank Professor K.F. Freed for some helpful exchanges of information.

References H.D. Mettee. J. Chem. Phys. 49 (1968) 1784; H.W. Sidebottom, K. Otsuka, A. Horowitz, J-G. Calvert. B.R. Rabe and EX. Damon, Chem. Phys. Letters 13 (1972) 337, and references therein. G. Nerzberg, Electronic spectra of electronic structure of polyatomic molecules (Van Nostrand, Princeton, 1966) p. 5 13-605, and references therein. J.C.D. Brand and K. Srikameswaran, Chefi, Phys. Letters 1.5 (1972) 130. P. warneck, F.F. Marmo and J-0. Sullivan, J. Chem. Phys. 4C (1963) 1132.

477

Voiume

17, number 4

CHEhfICAL

1972

(121 G. Hex&erg, Electronic

spectra of electronic structure of polyatomic molecules (Van Nostrand, Princeton, 1966) p. 512. 1131 R.A. hinrcus and O.K. Rice, J. Phys. Coiloid Chem. 55 (1951),894. [ 141 K, Evans and %A. Rive, Chem. Phys, Letters 14 (1972)

[5] H. Okabe, 3. Am. Chem. Sot. 93 (1371) 709.5. f6] J; Jortner and S.A. Rice, Advan. Photochem. 7 (1969) 149; K.F. Fieed, Bu?l. Am. Phys. Sot. 16’(1971) 1339. $71 K. Spears and S.A. Rice, J. Chem. Phys. 55 (1971) Z&l. [S] A& Abramson, KG. Spears and S.A. Rice, J, Chem. Phys. 56 11972) 2291. [9] K. Spears, Ph.D. Thesis, University of Chicago. [IO] C.S. Parmenter and M.W, Schuyler, Chem. Phys, Letters 6 (1970) 339. fll] J. Duchesne and 3. Rosen. J. Chem. Phys. 15 (1947) 631.

.’

15 December

PHYSICS LETTERS

[15] kE. Douglas, Y. Chem. Phys. 45 (1966) 1007. [ 161 K. Freed and D.F. He&x, to be published.

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