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Single rovibronic level lifetimes of the 1A2 state of SO2 excited in the 3043 Å (“E”) band: rotationally resolved fluorescence emission spectrum
Volume 75, number 1
CHEMICAL PHYSICS LETTERS
1 October 1980
SINGLE ROViBRONIC LEVEL LIFETKMES OF THE 1A2 STATE OF SO2 EXCITED IN THE 3043 A (“E”) BAND: ROTATIONALLY Dennis L. HOLTERMANN, Department
of Chemirtry,
RESOLVED
FLUORESCENCE
EMISSION SPECTRUM *
Edward K.C. LEE
Umversity of Calrfomia, Irvine, Ckhfomin 92717. i&4
and Roger NANES Department
of Physzcs, colforma
State University, Fullerton, Cdforma
92634, USA
Recerved 23 June 1980
Rotationally resolved resonance fluorescence from sefected rovr%ronrclevels of the SO2 (‘AZ) molecule has been studied at Iow pressure. The coBsion-free hfetune is 13.4 r I.3 ~1s.The sum of the cross sections for the cokron-mduced electronic quenchmg and “non-specrfic” rotation-vrbra~on relaxation IS -480 A2.
The anomalous lifetime behavior of the fluorescence from SO2 excited m the 2900 A system has been extensively studred as an illustrative example of the “Douglas effect” [l-4] arismg from perturbatrons in the excited state by Iarge numbers of Ievels of another electronic state wrt.h no oscillator strength. Studies of time-resolved kinetics of the fhrorescence emission as a function of pressure at various excitation wavelengths have been performed by Brus and McDonald [S] and Su et al. [6], in order to obtain a better understanding of the specifics of the perturbations. They observed non-exponential decay of the totat ermssion intensity under all experimental conditions, and the ffuorescence decay was analyzed as brexponent&; the long-lived component of the decay was attributed to the ‘Bt state (L), perturbed by Renner-Teiler interaction with the dense manifold of ground IAt state levels, wtie the short-lived component was attnbuted to the overlapping IA2 state (S). Although the results of the above two studies agree to a large extent, some differences exist regard* Thrs research has been supported at the Umversty of Cabfornia, Irvine, in part by the Office of Naval Research Contract N-00014-7SC-0813 and the Department of Energy Contract DE-AT-O3-76-ER-70217.
mg the rmportant question of the independence of the decay processes of the emitting S and L states and the mechanism of the interconversion. Since these studies were made with relatively broadband laser excitatron (e.g., ==3 or ==I0 cm-r) which is inadequate for selectrve excitation to a single rovibronic level (SRVL) 143, and also with inadequate spectral characterization of the two emitting states, a clear understanding of this problem has been clouded by the lack of discnmination among the several (or many) rovibronic levels that contribute to the emission. The fmt strong electronic absorption system of SO, extends from 3480 to 2600 A with a banded structure belonging to the IA2 + ‘A, transition superposed on the “structureless” 1Bt + ‘A, &ansition f7] _ A partial rotational analysis of several bands with little perturbation has recently been reported [7,8] and a study of the uibmti~&@zesolved fluorescence ermssion spectra from some single vibromc levels (SVLs) has been made with an excitation bandwidth of =8 A (=80 cm-l) [9]. with a rrarrower excitation bandwidth of aO.2 cm-l from an etaIontuned, pulsed dye laser, we made an attempt to sttidy unambiguously the relaxation kinetics of the tmperturbed IA2 state by direct observation of its r&etionally-resolved fluorescence emission. We were abIe
to achieve this by the combmed use of a large volume, m~~path abso~tion/~uor~scence cell equipped with %te and Welsh optics, a 1 m monochromator, a pair of gated mtegrators, a transient digtlzer, and a srgnal averager, which greatly improves the S/N ratio. Rovlbronic levels of mterest in the Clements’ E band near 3043 A were identified using fluorescence excitation spectra. For example, the PP7(7) transItion at 32813.20 cm-l was used to selectively excite the A” = 6,J’ = 6 level. The vlbrationally-resolved fluorescence emission, recorded as a low resolution survey spectrum, was sundar to that reported by Shaw et al. [9]. The most intense vlbromc errussron band terrmnates on the (1 , 0,O) vlbratronal level in the ground electronic state, designated here as the 1: band, =l 151 cm-l (~‘i’) to the red of the laser Ime. For the present study, intensities of two groups of resonance ermsslon lines, i.e., PP7(7) and 3 unresolved r-form transitions (‘P5(7), rQ5(6), and IRS(S)) were recorded as shown m fig. 1. It should be noted that the mtensity of the r-form sub-band IS dommated by the IRS(S) tranntion. Most of the tune-resolved intensity measurements were carried out momtormg the PP7(7) emission lme of the 1: band with a 6 A emission bandpass. The fluorescence decay of the PP,(7) ermssion lme was found to be a srrtgk ex~olze~z~uZ, The SternVohner pIot of the reciprocal of +&elifetime (TV)versus the SOa pressure in the 0.12-6.5 mTorr range IS pP,ri=)
I
31711 4
51662 cm-’
2 cm-’
Fig. 1. pp,(7) and ‘Rs(5) e-non bnes of the If: band from theJ’ = 6, K’ = 6 level evclted by the pP,(7) transltion. The Weak features of ‘C&(6) and rPs(7) errussxonbnes to the red of the ‘R&5) arm&on bne arc not resolved wth the uxtrumerit resolution of ==Scm-l. 92
1 October 1980
CHCMICAL PHYSICS LETTERS
Volume 75. number 1
O,l-----l
001
00
I
I
,
IO 20 30 40 Pressure of SO2 (mtorr)
I so
Fe. 2. Stern-Volmer piot of reciprocal bfetune as a functron of pressure. pP,(73 transrtlon was excited m the “E” band, and the emtsslon mtenstty centered at the pP,(7) resonance emtsslon lmc \\asmomtored wth the instrument resolutlon of 60 cm-‘.
linear as shown 111fig. 2. It @ves a half-quenchmg pressure (P1j7) of 1 .l +-0.2 mTorr and a zero-pressure lout lifetune (r!) of 13.4 + 1.3 ps, conwderably shorter than the previously reported values of 49 ps by &us and McDonald [s], and 80 ~.lsby Su et al. [6] as the dlort-hved component ln a double ex~~i~e~l~-al decay curve. It is clear that theu: reported values of 7~ from the double exponent& decay are much longer than our value of ~8 obtamed for the lA2 state from the single exponential decay. These Iscrepanties are probably due to the lack of spectral resolution and the blended preparatibn using broadband laser excitation whch favors ‘B, aver *A, m their expenment. However, our tune-resolved lntensay measurements of the erltire unresolved emission profile gves a double exponential decay with S and L hfetune values (TV and rL) consistent with those reported earher f5,6]. Stnckler and Howell reported a calculated radiative lifetime (+ro) value of 0.6 gs usmg the integrated absorption coefficients for the entie 2900 A system [lo]. If we assign all of the banded structure to the IA2 + IA, transltion and the underlying contmuum to the lB1 + IA, transltion, the ratio of mtegrated absorption (lA, : ‘El) can be approximately patitioned 1 : (3 -i 1) *. Then, we can further estimate 7. * The values of r. were estunated after takmg mto account the ddferences m the emission frequencies for the two elec tronic transitions usmg the expression gzvenby Stnckter and Berg IllI.
Volume 75, number 1
CHEMICAL PHYSICS LIX-IXRS
for the IA, + IA, transition to be 3 +- I ys and that for the ‘B; + IA, tr~s~tlon to be 0.8 * 0.2 PS. Our observed vaIue of T$ is stall 5 times longer than the new estimate of the calculated IA, ra&ative hfetlme of =3 &ls. The m&rally prepared rovrbromc level, i.e., K’ = 6, J’ = 6 level, may undergo rotational, vibrational, and electromc relaxation through col.lis~ons v~~thground state SO, molecules. We have observed that new emissron hnes appear as srdebands of the PP7(7) resonance emission hne when SO2 pressure is increased. These are taken as evidence for “specific” rotatIonal relaxation via a AK’ = kl collisional route and detzuls shall be presented elsewhere [ 121. We designate all other rotational relaxation and vibrational rekxatron routes as “non-specific” relaxation wfuch broadly hstributes the ermssion outside the wavelength region where the PP7(7) enusslon was normally momtored for the hfetune measurements. The “electroruc quenching” reduces the overall emission intensity from the IA2 state. The kmetlc analysts of the data in fig. 2 shows that the slope represents the sum of the electromc quenching rate constant (kq) and the ‘Lnon-spec&c” ~bratlonal-rotatIonal relaxation rate constant (k,,). We obtam a value of (21 2 1) X lo-10 cm3 molecule-l s- 1 for the sum of kq + k,, correspondmg to a cross sectton of ~480 AZ. The electronic quenching rate constant
for the short-Lived component
S (ks) ex-
cxted J.SIthe “E”
band has been measured to be (I 5.8 T 0.4) X lo- lo cm3 molecule-1 s-t by
Brus and McDonald [S] and (16.9 f 1.7) X 10-I* cm3 molecule-l s- 1 by Su et al. [6]. It is mteresbng to note that these values are sun&x to our vzdue of (k,+ kns)= 21 X 10-10 cm3 molecule-l s-1, despxte the fact that our zero-pressure iifetrme value of 13.4 ~..EG IS considerably shorter than their hfetirne values of 49 or 80 ~.lswhich were used in calcuiatmg
1 October 1980
thexr quenchmg rate constants. The reason for this apparent consistency is not clear and probabIy requires a better understandmg of the relaxation mechanism. Our present result illustrates the power of the ro~tion~y-resolved ermssion me~urements in studies of the molecular dynarntcs of smal.I molecufes. We have also carned out stules of colhsion-induced rotational relaxation and electrcruc quenching using the same apparatus, x*hich will be published separately. We thank Professors J.C.D. Brand and A..#. Merer for provldmg us with high resolution SO, spectra.
References [I j A.E. Douglas, 3. Chem. Phys. 45 (1966) 1007. [ 21 J. Jortner, S.A. Rice and R.M. Hochstrasser, Advan. Photochem 7 (1969) 149. [3] P. Avours, W.hl. Gclbart and MA. El-Sayed, Chem. Rev. 77 (1977) 793. 141 G.L. Loper and E.K.C. Lee, WI*Radiationless transitions, ed. S.H. tm (Academrc Press, New York, 1977) p* 2. fS] LX. Brus and J.R. McDonald, J. Chcnt. Phyr 61(1974) 97. [6] f. Su, J.WV*Bottcnhfrm, H.W. Stdebottom, J*G* Calvert and E.R. Damon, intern. J* Chem Kmettcs 10 (1978) 125. 17’1 Y. Hamadcs and A.J. Mner,Cm. J. Phys. 51(1974j 1443,and references therein. [Sl Y. Hamada and A..!. Elerer, Can. J. Phys. 53 (1975) 2555. [9] R.J. Shaw, 3-E. Kent and h1.T. O’Dwyer, Chem Phyr; 18 (1976) 16.5, [lo] S.J.Stnckler and D-E. Howell, J. Chem Phys. 49 (1968) 1947. [ 1 I 1 S-J. Stnchler and R-A, Berg, J. Chem. Phys. 37 (196 l) 814. [ IZ] D.L. Hohermann, E.K.C. Lee and R. Nancy, lo be pubhshed.
93
CHEMICAL PHYSICS LETTERS
1 October 1980
SINGLE ROViBRONIC LEVEL LIFETKMES OF THE 1A2 STATE OF SO2 EXCITED IN THE 3043 A (“E”) BAND: ROTATIONALLY Dennis L. HOLTERMANN, Department
of Chemirtry,
RESOLVED
FLUORESCENCE
EMISSION SPECTRUM *
Edward K.C. LEE
Umversity of Calrfomia, Irvine, Ckhfomin 92717. i&4
and Roger NANES Department
of Physzcs, colforma
State University, Fullerton, Cdforma
92634, USA
Recerved 23 June 1980
Rotationally resolved resonance fluorescence from sefected rovr%ronrclevels of the SO2 (‘AZ) molecule has been studied at Iow pressure. The coBsion-free hfetune is 13.4 r I.3 ~1s.The sum of the cross sections for the cokron-mduced electronic quenchmg and “non-specrfic” rotation-vrbra~on relaxation IS -480 A2.
The anomalous lifetime behavior of the fluorescence from SO2 excited m the 2900 A system has been extensively studred as an illustrative example of the “Douglas effect” [l-4] arismg from perturbatrons in the excited state by Iarge numbers of Ievels of another electronic state wrt.h no oscillator strength. Studies of time-resolved kinetics of the fhrorescence emission as a function of pressure at various excitation wavelengths have been performed by Brus and McDonald [S] and Su et al. [6], in order to obtain a better understanding of the specifics of the perturbations. They observed non-exponential decay of the totat ermssion intensity under all experimental conditions, and the ffuorescence decay was analyzed as brexponent&; the long-lived component of the decay was attributed to the ‘Bt state (L), perturbed by Renner-Teiler interaction with the dense manifold of ground IAt state levels, wtie the short-lived component was attnbuted to the overlapping IA2 state (S). Although the results of the above two studies agree to a large extent, some differences exist regard* Thrs research has been supported at the Umversty of Cabfornia, Irvine, in part by the Office of Naval Research Contract N-00014-7SC-0813 and the Department of Energy Contract DE-AT-O3-76-ER-70217.
mg the rmportant question of the independence of the decay processes of the emitting S and L states and the mechanism of the interconversion. Since these studies were made with relatively broadband laser excitatron (e.g., ==3 or ==I0 cm-r) which is inadequate for selectrve excitation to a single rovibronic level (SRVL) 143, and also with inadequate spectral characterization of the two emitting states, a clear understanding of this problem has been clouded by the lack of discnmination among the several (or many) rovibronic levels that contribute to the emission. The fmt strong electronic absorption system of SO, extends from 3480 to 2600 A with a banded structure belonging to the IA2 + ‘A, transition superposed on the “structureless” 1Bt + ‘A, &ansition f7] _ A partial rotational analysis of several bands with little perturbation has recently been reported [7,8] and a study of the uibmti~&@zesolved fluorescence ermssion spectra from some single vibromc levels (SVLs) has been made with an excitation bandwidth of =8 A (=80 cm-l) [9]. with a rrarrower excitation bandwidth of aO.2 cm-l from an etaIontuned, pulsed dye laser, we made an attempt to sttidy unambiguously the relaxation kinetics of the tmperturbed IA2 state by direct observation of its r&etionally-resolved fluorescence emission. We were abIe
to achieve this by the combmed use of a large volume, m~~path abso~tion/~uor~scence cell equipped with %te and Welsh optics, a 1 m monochromator, a pair of gated mtegrators, a transient digtlzer, and a srgnal averager, which greatly improves the S/N ratio. Rovlbronic levels of mterest in the Clements’ E band near 3043 A were identified using fluorescence excitation spectra. For example, the PP7(7) transItion at 32813.20 cm-l was used to selectively excite the A” = 6,J’ = 6 level. The vlbrationally-resolved fluorescence emission, recorded as a low resolution survey spectrum, was sundar to that reported by Shaw et al. [9]. The most intense vlbromc errussron band terrmnates on the (1 , 0,O) vlbratronal level in the ground electronic state, designated here as the 1: band, =l 151 cm-l (~‘i’) to the red of the laser Ime. For the present study, intensities of two groups of resonance ermsslon lines, i.e., PP7(7) and 3 unresolved r-form transitions (‘P5(7), rQ5(6), and IRS(S)) were recorded as shown m fig. 1. It should be noted that the mtensity of the r-form sub-band IS dommated by the IRS(S) tranntion. Most of the tune-resolved intensity measurements were carried out momtormg the PP7(7) emission lme of the 1: band with a 6 A emission bandpass. The fluorescence decay of the PP,(7) ermssion lme was found to be a srrtgk ex~olze~z~uZ, The SternVohner pIot of the reciprocal of +&elifetime (TV)versus the SOa pressure in the 0.12-6.5 mTorr range IS pP,ri=)
I
31711 4
51662 cm-’
2 cm-’
Fig. 1. pp,(7) and ‘Rs(5) e-non bnes of the If: band from theJ’ = 6, K’ = 6 level evclted by the pP,(7) transltion. The Weak features of ‘C&(6) and rPs(7) errussxonbnes to the red of the ‘R&5) arm&on bne arc not resolved wth the uxtrumerit resolution of ==Scm-l. 92
1 October 1980
CHCMICAL PHYSICS LETTERS
Volume 75. number 1
O,l-----l
001
00
I
I
,
IO 20 30 40 Pressure of SO2 (mtorr)
I so
Fe. 2. Stern-Volmer piot of reciprocal bfetune as a functron of pressure. pP,(73 transrtlon was excited m the “E” band, and the emtsslon mtenstty centered at the pP,(7) resonance emtsslon lmc \\asmomtored wth the instrument resolutlon of 60 cm-‘.
linear as shown 111fig. 2. It @ves a half-quenchmg pressure (P1j7) of 1 .l +-0.2 mTorr and a zero-pressure lout lifetune (r!) of 13.4 + 1.3 ps, conwderably shorter than the previously reported values of 49 ps by &us and McDonald [s], and 80 ~.lsby Su et al. [6] as the dlort-hved component ln a double ex~~i~e~l~-al decay curve. It is clear that theu: reported values of 7~ from the double exponent& decay are much longer than our value of ~8 obtamed for the lA2 state from the single exponential decay. These Iscrepanties are probably due to the lack of spectral resolution and the blended preparatibn using broadband laser excitation whch favors ‘B, aver *A, m their expenment. However, our tune-resolved lntensay measurements of the erltire unresolved emission profile gves a double exponential decay with S and L hfetune values (TV and rL) consistent with those reported earher f5,6]. Stnckler and Howell reported a calculated radiative lifetime (+ro) value of 0.6 gs usmg the integrated absorption coefficients for the entie 2900 A system [lo]. If we assign all of the banded structure to the IA2 + IA, transltion and the underlying contmuum to the lB1 + IA, transltion, the ratio of mtegrated absorption (lA, : ‘El) can be approximately patitioned 1 : (3 -i 1) *. Then, we can further estimate 7. * The values of r. were estunated after takmg mto account the ddferences m the emission frequencies for the two elec tronic transitions usmg the expression gzvenby Stnckter and Berg IllI.
Volume 75, number 1
CHEMICAL PHYSICS LIX-IXRS
for the IA, + IA, transition to be 3 +- I ys and that for the ‘B; + IA, tr~s~tlon to be 0.8 * 0.2 PS. Our observed vaIue of T$ is stall 5 times longer than the new estimate of the calculated IA, ra&ative hfetlme of =3 &ls. The m&rally prepared rovrbromc level, i.e., K’ = 6, J’ = 6 level, may undergo rotational, vibrational, and electromc relaxation through col.lis~ons v~~thground state SO, molecules. We have observed that new emissron hnes appear as srdebands of the PP7(7) resonance emission hne when SO2 pressure is increased. These are taken as evidence for “specific” rotatIonal relaxation via a AK’ = kl collisional route and detzuls shall be presented elsewhere [ 121. We designate all other rotational relaxation and vibrational rekxatron routes as “non-specific” relaxation wfuch broadly hstributes the ermssion outside the wavelength region where the PP7(7) enusslon was normally momtored for the hfetune measurements. The “electroruc quenching” reduces the overall emission intensity from the IA2 state. The kmetlc analysts of the data in fig. 2 shows that the slope represents the sum of the electromc quenching rate constant (kq) and the ‘Lnon-spec&c” ~bratlonal-rotatIonal relaxation rate constant (k,,). We obtam a value of (21 2 1) X lo-10 cm3 molecule-l s- 1 for the sum of kq + k,, correspondmg to a cross sectton of ~480 AZ. The electronic quenching rate constant
for the short-Lived component
S (ks) ex-
cxted J.SIthe “E”
band has been measured to be (I 5.8 T 0.4) X lo- lo cm3 molecule-1 s-t by
Brus and McDonald [S] and (16.9 f 1.7) X 10-I* cm3 molecule-l s- 1 by Su et al. [6]. It is mteresbng to note that these values are sun&x to our vzdue of (k,+ kns)= 21 X 10-10 cm3 molecule-l s-1, despxte the fact that our zero-pressure iifetrme value of 13.4 ~..EG IS considerably shorter than their hfetirne values of 49 or 80 ~.lswhich were used in calcuiatmg
1 October 1980
thexr quenchmg rate constants. The reason for this apparent consistency is not clear and probabIy requires a better understandmg of the relaxation mechanism. Our present result illustrates the power of the ro~tion~y-resolved ermssion me~urements in studies of the molecular dynarntcs of smal.I molecufes. We have also carned out stules of colhsion-induced rotational relaxation and electrcruc quenching using the same apparatus, x*hich will be published separately. We thank Professors J.C.D. Brand and A..#. Merer for provldmg us with high resolution SO, spectra.
References [I j A.E. Douglas, 3. Chem. Phys. 45 (1966) 1007. [ 21 J. Jortner, S.A. Rice and R.M. Hochstrasser, Advan. Photochem 7 (1969) 149. [3] P. Avours, W.hl. Gclbart and MA. El-Sayed, Chem. Rev. 77 (1977) 793. 141 G.L. Loper and E.K.C. Lee, WI*Radiationless transitions, ed. S.H. tm (Academrc Press, New York, 1977) p* 2. fS] LX. Brus and J.R. McDonald, J. Chcnt. Phyr 61(1974) 97. [6] f. Su, J.WV*Bottcnhfrm, H.W. Stdebottom, J*G* Calvert and E.R. Damon, intern. J* Chem Kmettcs 10 (1978) 125. 17’1 Y. Hamadcs and A.J. Mner,Cm. J. Phys. 51(1974j 1443,and references therein. [Sl Y. Hamada and A..!. Elerer, Can. J. Phys. 53 (1975) 2555. [9] R.J. Shaw, 3-E. Kent and h1.T. O’Dwyer, Chem Phyr; 18 (1976) 16.5, [lo] S.J.Stnckler and D-E. Howell, J. Chem Phys. 49 (1968) 1947. [ 1 I 1 S-J. Stnchler and R-A, Berg, J. Chem. Phys. 37 (196 l) 814. [ IZ] D.L. Hohermann, E.K.C. Lee and R. Nancy, lo be pubhshed.
93