J. Phys. Chem. 1988, 92, 383-388
Perturbation of Rovibronically Selected SO,(k'A,)
383
in a Supersonic Jet
Seiichiro Koda,* Department of Reaction Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo- ku, Tokyo 113, Japan
Hideyuki Yamada, and Soji Tsuchiya* Department of Pure and Applied Sciences, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan (Received: May 5, 1987; In Final Form: June 30, 1987)
The fluorescence spectrum and its intensity decay of S02(.&'A2)in single rovibronic levels of the "E" band have been observed in a supe_sonicjet to discuss the collisional transfer between rovibronic states. It was found that the undispersed fluorescence of S_02(A'A2)decayed in a nonexponential manner, while the dispersed fluorescence of the transition t t t h e (1,0,0)" state of X'A, showed almost a single-exponential decay. The fluorescence intensity of the transition to the X'AI(O,O,l)" state from certain rovibronic kvels increased after the laser irradiation and reached a peak before it decayed. These experimental facts indicate that S02(A1A2)in the initially prepared rovibronic level makes a transition to a state having a longer radiative lifetime, which is induced by low-energy collisions in a supersonicjet. The final state is probably a perturbed vibronic state of A'A,. The decay process was analyzed kinetically to obtain the relevant fluorescence lifetimes of 6-13 ,us and collisional cross sections on the order of several hundred square angstroms.
Introduction It has been noted by a number of researchers that photophysical properties of excited SO2in the A'A2 state, which is formed by light absorption in the region of 260-340 nm, are exceedingly complicated and irregular.'" Recent high-resolution spectroscopic studies in jets, however, in both the frequency domaine6 and the time domain (Zeeman quantum beat spectroscopy),' together with photophysical studies at low pr_essures,*have considerably revealed the coupling scheme o,f the A'A2 state with the other relevant electronic states. The A'A2 state borrows the transition intensity through a vibronic coupling with the 'B1state, which is the Renner-Teller pair of the ground electronic state. In the longest wavelength region of the transition, the rotationally dependent perturbations are dominantly due to the spin-orbit interaction. In the shorter wavelength region such as in the "E" band, however, the coupling mechanism is more complicated and still not understood in detail. The fluorescence decay measurements under the broad-band excitation were reviewed by Su et al.? who discussed the collisional transfer between the two groups of states having different decay lifetimes. However, the experiments were limited by the fact that many rovibronic levels were excited altogether owing to the broad line width of the excitation laser. Recently a more detailed experiment became possible by the advent of a very narrow excitation laser which enabled the preparation of a single rovibronic level. For the quenching of SO2 in various rovibronic levels of the A state, a very large cross section at room temperature was reported by Lee and co-workers.8J0 One of the most significant characteristics of the supersonic expansion jet technique is that the collision frequency of the molecules seeded in the jet is so small that these molecules can be assumed to be under an isolated condition. However, if a very (1) Hamada, H.; Merer, A. J. Can. J. Phys. 1974, 53, 2555. (2) Lee, E. K. C.; Loper, G. L. Radiationless Trunritionr;Lin, S . H., Ed.; Academic: New York, 1980; p 11. (3) Heicklen, J.; Kelly, N.; Partymiller, K. Reu. Chem. Infermed. 1980, 3, 315. (4) Fisher, A.; Kullmer, R.; DemtrGder, W. Chem. Phys. 1985,83, 415. ( 5 ) Watanabe, H.; Tsuchiya, S.; Koda, S. J. Mol. Specfrosc. 1985, 100, 136. (6) Kullmer, R.; DemtrGder, W. J . Chem. Phys. 1985, 83, 2712. (7) Watanabe, H.; Tsuchiya, S.; Koda, S. J . Chem. Phys. 1985,82, 5310. (8) Holtermann, D. L.; Lee, E. K. C.; Nanes, R.J. Phys. Chem. 1983.87, 3926. (9) Su,S.; Bottenheim, J. W.; Sidebottom, H. W.; Calvert, J. G.; Damon, E. K. Inf. J. Chem. Kinef. 1978, 10, 125. (IO) Holtermann, D. L.; Lee, E. K. C.; Nanes, R. J. Chem. Phys. 1982, 77. 5321.
0022-365418812092-0383$01.50/0
slow relative translational motion between interacting molecules results in an extraordinarily large collision cross section, it is probable that molecules in the jet are not isolated and some inelastic processes occur in the course of the expansion. Rice and co-workers reported resonance enhancement of vibrational relaxation induced by very low energy collisions of the excited 12(32,)with He",12 and proposed a scattering resonance des ~ r i p t i o n . ' ~ ,GentryI5 ~~ showed also that even nonresonance mechanisms could plausibly result in large cross sections at very low translational temperatures. As for polyatomic molecules, very efficient vibrational relaxation of glyoxal in the 'A, state was found and interpreted on the basis of the above-mentioned orbiting resonance mechanism.16 It was claimed that if near degeneracies exist in the vibrational manifold, the lowering of symmetry in the orbiting resonance can permit mode mixing that is prohibited in the isolated molecule. Besides the vibrational relaxation, intramolecular radiationless decay processes are induced or enhanced via collisions, particularly in molecules having a nonstatistical limit Theory predicts a close relationship between the collision-induced radiationless decay and the admixure of bath states in the initially excited rovibronic level. The process may be enhanced at lower tem ratures. Jouvet and S0epl9 reported a cross section of around 40 for the collision-induced intersystem crossing in the 'A, glyoxal at a translational temperature varying between 0.5and 0.1 K, which is larger by a factor of 20-30 than the room temperature value, MacDonald and Leezofound in several rovibronic levels of S02(A'A2) in the jet that the electronic quenching cross section was larger by around a factor of 10 than those at room temperature and interpreted the result by using a collision complex model. Experimental information on the collision effects on the fluorescence of excited SO2should be helpful for understanding of the coupling scheme among the relevant electronic states. Previously we reported nonexponential fluor_escencedecays from a number of single rovibronic levels of the A state of SO2in the expansion jet.21 According to the recent high-resolution laser
E
(11) Tusa, J.; Sulkes, M.; Rice, S . A. J. Chem. Phys. 1979, 70, 3136. (12) Sulkes, M.; Tusa, J.; Rice, S . A. J . Chem. Phys. 1980, 72, 5733. (13) Cerjan, C.; Rice, S . A. J . Chem. Phys. 1983, 78,4952. (14) Gray, S. K.; Rice, S. A. J . Chem. Phys. 1985,83, 2818. (15) Gentry, W. R. J. Chem. Phys. 1984, 81, 5737. (16) Jouvet, C.; Sulkes, M.; Rice, S . A. J . Chem. Phys. 1983, 78, 3935. (17) Freed, K. F. Adu. Chem. Phys. 1981,47, 291. (18) Knight, A. E. W.; Jones, J. T.; Parmenter, C. S. J. Phys. Chem. 1983, 87, 913. (19) Jouvet, C.; Soep, B. J . Chem. Phys. 1980, 73, 4127. (20) MacDonald, B. G.; Lee, E. K. C. J. Phys. Chem. 1982, 86, 323.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 2, I988
Koda et al.
studies, this phenomenon should be ascribed to some collisional origins.22 However, the detailed kinetic mechanism to explain how the fluorescence decay deviates from the single-exponential function has not been established. In the present paper, we analyze kinetically the fluorescence decay curves, taking into account the fluorescence processes as well as the transition via a low-energy collision to a state that emits fluorescence with a different lifetime. We have also observed the dispersed fluorescence spectra, and we will discuss the time evolutions of fluorescence in a certain selected wavelength region in order to elucidate the character of the collisionally populated states. Experimental Section
A pulsed nozzle having a 0.2” hole made from an automobile fuel injectot was employed for the present expansion jet of SO2 seeded up to 3% (in some cases 1%) in Ar or He into a vacuum chamber pumped by a 6-in. diffusion pump. The stagnation pressure was in the range from 150 Torr to 3 atm, and the background pressure in the chamber was 2 X IO-3 Torr at the highest. An N 2 laser pumped dye laser of a line width of 0.04 cm-’ irradiated the jet in a direction perpendicular to the jet axis, and the total fluorescence was detected by a photomultiplier (Hamamatsu R 928) which was placed in front of the nozzle at a distance of 30 cm. For observation of the fluorescence decay, the signal from the photomultiplier was digitized by an A/D converter (Iwasaki DM901) and transferred to a microcomputer (NEC PC9801F2) to accumulate the data for improvement of the S / N ratio. The time resolution of the decay measurement was about 15 ns. Geometrical considerations guarantee that the vieweing region by the photomultiplier is sufficiently large to cover the jet expansion. The dispersed fluorescence was observed by use of a monochromator (Spex 1700 11) by setting its band-path width a t 30 cm-I. The fluorescence emitted parallel to the jet axis was collected with a lens of focal length of 20 cm placed at a distance of 40 cm from the nozzle and led into the monochromator whose output was treated by a boxcar integrator (PAR 162/164) with a gate opened between 6 and 8 ps from the laser pulse. The time evolution of the dispersed fluorescence was pursued at several different wavelengths by use of a photon counting method. That is, the output photon pulse from a photomultiplier (Hamamatsu R1527) was discriminated, amplified by a broad band amplifier, and then reformed to an electronic pulse of a IO-ns pulse width, which was then digitized via the A/D converter and sent to the microcomputer. Results and Analysis A. Fluorescence Decay and the Stern-Volmer Plot. The decay
of the total fluorescence, which corresponded to the sum of emissions over the sensitive wavelength region of the photomultiplier (Hamamatsu R928; down to 930 nm), a!s observed for SO2excited rovibronically in the “E” band of the A1A2state under a variety of conditions: X/D (nozzle-laser distance X over the nozzle hole diameter D) and the stagnation pressure po. The decay shown in Figure 1 is not a single exponential. The deviation from the single-exponential function becomes larger if the X/D value is reduced or po is increased. Under the condition of a small value of po or a large one of X / D , t h e fluorescence decay is almost single
exponential though in the final stage far after the laser excitation the decay becomes slower. When the gaseous density of the jet is increased, the fluorescence decay is accelerated and becomes nonexponential, while the decay rate in the final stage is almost independent of X/D or po. This observation implies that the collisional quenching of the fluorescence occurs effectively in the initial stage since the gas density is large in the region close to the nozzle exit and is reduced significantly downstream of the jet. In order to evaluate the collisional quenching rate, the exponential (21) Watanabe, H.; Hyodo, Y.;Tsuchiya, S.;Koda, S. J. Phys. Chem. 1982, 86, 685. (22) Kullmer, R.; Demtrader, W. J. Chem. Phys. 1986, 84, 3672.
Time/ps
0
10
20
30
40
Time/ps Figure 1. (a) Total fluorescence intensity (logarithmic scale) of SO2 excited by the “E” band, ‘Q0(2)line in the jet of 3% S02/Ar mixture as a function of time and X / D . The nozzle source pressure po = 280 Torr and the nozzle diameter = 0.2 mm. (b) Total fluorescence intensity (logarithmic scale) as a function of time and po X / D = 55 and the other condition is the same as in (a). ,
decay rate of the fluorescence is defined for the initial stage and is plotted as a function of the gas density at the excitation position in the jet as shown in Figure 2. The gas density n, is calculated according to the equation n, = 0.15no(X/D)-2
(1)
which is derived by Anderson and Fenn23for the density change along the jet axis, where n, is the density in the stagnation region of the nozzle. The density can be controlled by changing either X/D or po, and in fact it is clear from Figure 2 that the data with different X/D’s and those with different p i s are represented by one common line. The concentration of SO2in the jet was changed from 1% to 3%, but no appreciable effect on the fluorescence decay was found within the experimental uncertainty. Therefore, the collision cross section of SO2 is considered to be at most 10 times (23) Anderson, J. B.; Fenn, J. B. Phys. Fluids 1965, 8, 780.
Collisional Perturbation of SO2(A1A2)
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 385
1-
0.3
0
el
j- 0.1
dl
v)
1
0
Q,
2
Number Density/ 1o15cm-3 Figure 2. Stem-Volmer plot of the initial decay rate (T-’)of SO2excited by the “E” band, ‘Q0(2) line in the jet of 3% S02/Ar mixture. 0,po = 280 Torr, various X / D 0 ,X / D = 55, various po. 1000
2000
3000
4000
I
1
I
I
1
314
315
1
316
1
317
318
Wavelengthhm
Wave num b e r / c m-’ 0
I
I
Figure 4. Dispersed fluorescence spectrum with the spectral slit width 30 cm-’ from SO2 excited by the “E” band, ‘Qo(2) line in the jet of 1% S02/He mixture. (a) X / D = 35, po = 1.4 atm; (b) X / D = 65, po = 1.0
atm.
Wavelength/nm Figure 3. Dispersed fluorescence spectrum from SO2excited by the “E” band, ‘ b ( 2 ) line in the jet of 1% S02/Ar mixture. X / D = 40; po = 310 Torr; spectral slit width = 30 cm-l fwhm. TABLE I: Collision-FreeLifetime and Quenching Rate Constant of Rovibronically Selected SO2 in the Jet lifetime/Ps
line no. rot. assignt this work K & D’ 4 7 8 11 12 14 15 17 18 20
W1) ‘Ro(2) ‘Qi(2) ‘RO(0) ‘Qo(2) ‘Qo(4) ‘PO(2) ’Ql(5) ’Qi(1)
7.5 6.6 8.6 4.9 6.7 7.2 7.8 6.0 7.9
quench. rate const/ lo-’’ cm3 s-l
6.9 8.7 6.9 8.5 7.7 17.3 8.5 8.5
7.9 6.7 7.5 4.2 6.5 5.7 6.8 8.2
Determined from Kullmer and DemtrGder.22 of that of Ar, so the observed nonexponential fluorescence decay is caused mostly by collisions with the carrier gas molecules. From the Stem-Volmer plots given in Figure 2, the collision-free lifetime 7 and the quenching rate constant k are determined and summarized in Table I. The collision-free lifetimes determined by Kullmer and DemtrijderzZ are also included in the table for comparison. B. Dispersed Fluorescence Spectra and the Effect of Pressure. The dispersed fluorescence spectrum observed by the monochromator is shown in Figure 3. Due to the low resolution of the monochromator (spectral slit width = 30 cm-’ fwhm), the fluorescence could not be rotationally resolved. The main feature of the vibrational structure was found to be independent of the excitation line within the ‘E” band. The structure is in agreement
--
5 lo Number Density /10’5cm-3 Figure 5. Intensity of the observed fluorescence to the (O,O,l)” state from SO2 excited by the ‘Ro(0) (no. 12) line (a) and by the PQl(5) (no. 18) line (b) relative to that to the (0,3,0)” state. OO
5
100
with the one observed by Holtermann et a1.* in a low-pressure gaseous SO2 Their vibrational assignments are indicated in the figure. The fluorescefce transitions to the (n,O,O)”and (O,n,O)” vibrational states of X’A2 are symmetrically allowed, but those to the (0,0,1)” state are symmetrically forbidden. Interestingly, the intensity of the fluorescence to the (O,O,l)” state relative to that to the (0,3,0)”state is found to be dependent on the molecular density as is shown in Figure 4 for the case of excitation by the ‘Q0(2) line (no. 14). It is clear that the forbidden transition occurs in the high-density condition. The extent of the dependence on the density is different for individual excitation rovibronic lines. The relative intensity increases with the increase of the density for rovibronic levels excited by the ‘&(2), ‘Ro(0), ‘Q0(2), and PQ1(l) lines (no. 8, 12, 14, and 20, respectively), while it stays almost constant for the ‘Rl( 1) line (no. 7) and decreases for the ‘Q0(4) and PQl(5) lines (no. 15 and 18, respectively). Two examples of these observations are shown in Figure 5. The values of the intercept at the zero gas density and the slope determined by assuming a linear relationship between the relative intensity and the density are summarized in Table 11. C . Time Evolution of the Dispersed Fluorescence. Because the excited molecules move downstream along the jet axis, if the slit width of the monochromator is not wide enough to allow a
386 The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
Koda et al.
TABLE II: Intercept and Slope Values for the Relationship between the Relative Fluorescence Intensity to (0,OJ)” over (0,3,0)” and the Molecular Density in the Jet intercept/ slope/ line no. assignt arb unit arb unit I ‘Ri(1) 0 0 8 ‘ROO) 0 0.4 0 0.5 12 ‘RO(0) 14 ‘Qo(2) 0 1.4 15 ‘Qo(4) 2
