2564
J. Phys. Chem. 1982, 86, 2564-2566
Spectroscoplc Characterization of Repulsive Potential Energy Surfaces: Fluorescence Spectrum of Ozone Dan G. Imre, James L. Kinsey,' Robert W. Field, Depaltment of Chemistry and George Harrison Spectroscopy Laboratory. Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
and Daniel H. Katayama Air Force oeophvsics Laboratoty (LKO), Henscom Air Force Ease, Massachusetts 0 173 1 (Received: March 15, 1982; In Final Form: May 3, 1982)
It is well-known that the upper electronic state of the ozone Hartley bands is repulsive and that excitation of O3at 266 nm results in rapid dissociation into O2 + 0 with near unity quantum yield. Nevertheless, we have observed photoemission of sufficient intensity to record wejll-resolved (0.35-A fwhm) fluorescencespectra. These spectra, in addition to exhibiting vibrational levels of the XIA1 state to within 500 cm-' of the dissociation limit, also allow us to determine the geometry of the upper state potential energy surface near its saddle point. The ability here demonstrated for observing photoemission from diffuseband systems reveals hitherto unexploited possibilities for spectroscopic probing of dynamics on repulsive surfaces for polyatomic molecules.
Introduction It has often been pointed out that the separation of the fragments which result from photodissociation is like half a molecular collision. Any photons that are emitted during this process therefore provide information about the structure of such a collisional intermediate. In this Letter we report studies on photoemission from ozone following excitation at 266-nm wavelength and use the results to characterize the collisional intermediate. Although the electronic spectrum of ozone, l i e that of many polyatomic molecules, is diffuse, we show here that detailed information about electronically excited and ground-state potential surfaces may be derived from the O3photoemission spectrum. Ordinarily, one would not think of ozone as a fluorescent species because its dissociation is many orders of magnitude faster than spontaneous emission. However, some small fraction, perhaps only lo*, of absorbed photons are re-emitted before the molecule comes apart. If there are enough photons in this small fraction, they can be dispersed and detected. Whether the overall process is considered fluorescence or resonantly enhanced Raman scattering is unimportant. What is important is that the photons observed are emitted by a very small subset of the excited molecules while the two photofragments are still near each other on the upper electronic-state potential surface. The visible-UV spectrum of ozone consists of several band sy~tem~:' Chappuis (61G550 m), Huggins (374-310 nm), and Hartley (300-220 nm). All of these are known to be genuinely diffuse, owing to photodissociation. The estimated dissociation lifetimes range downward from 1 ps. In the Hartley band, which is of interest here, absorption is accompanied by photodissociation into O2 and 0 atoms within a few femtoseconds. The extremely short lifetimes of O3 excited states eliminate any possibility of studying them by conventional spectroscopictechniques. The electronically excited states correspond to unbound potential surfaces upon which dissociation takes place rapidly. Detailed studies of such (1) G. Herzberg, "Electronic Spectra and Electronic Structure of Polyatomic Molecules", Van Nostrand-Fkinhold, New York, 1966. O022-3654/02/2006-2564$O 1.2510
surfaces would enhance our understanding of transition states in chemical reactions and molecular behavior in the continuum. The frequency shifts relative to the exciting line are exclusively characteristic of the vibrational levels of the lower potential surface. A time-dependent view of the two-photon scattering p r o c e s ~ ~ reveals, -~ however, that the relative intensities of various fundamentals, overtones, and combinations are indicative of dynamics on the upper potential surface. In the time-dependent picture, the ground-state vibrational wave function is transferred at time t = 0 to the upper potential surface. This wave packet then evolves as governed by a nuclear-motion Hamiltonian whose potential is that of the upper state. The half Fourier transform (at the excitation frequency) of this moving wave packet's overlap with a given lower-state vibrational level is exactly equivalent to the conventional amplitude for resonantly enhanced scattering into that final level. Heller and co-workers have shown that it is usually unnecessary to know the complete time evolution on the upper surface, and they have derived formulas for a number of cases where the important short-time behavior is adequately given by simple approximate (classical or semiclassical) dynamics? These simple formulas make it possible to infer characteristics of the upper-state potential directly from the spectrum. Experimental Technique and Results Os, generated by a commercial ozonator, was stored on silica gel at dry ice temperature. The excess O2was removed, and the resultant mixture of about 1:l 02:03 was flowed at 0.7-1.0 torr through a fluorescence cell. The ozone was excited close to the peak of the Hartley continuum by the fourth harmonic (266 nm) of a Molectron MY34-20 Nd:YAG laser (-10 mJ/pulse at 20 Hz). The inelastically scattered light was collected with quartz lenses and imaged onto the entrance slit of a 1-m (2) E. J. Heller, J. Chem. Phys., 62, 1544 (1975). (3) E. J. Heller in 'Potential Energy Surface and Dynamics
Calculations", D. Truhlar, Ed., Plenum, New York, 1981. (4) S.-Y. Lee and E. J. Heller, J. Chem. Phys., 71,4777 (1979). (5) E. J. Heller, R. Sundberg, and D. Tennor, submitted for publication.
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2565
Letters
Flgure 1. Low-resolutlon (&A fwhm) photoemission spectrum of ozone excited at 266 nm wavelength. Inset shows high-resolution (0.35-A fwhm) scan of symmetric stretch (v,) mode. The peak marked O2 3Z,-(l,0)is the Raman fundamental for the Op X state. See text for details.
TABLE I: Ozone Energy Levels (cm-') this work f other level 10 cm-' expta 100 002 200 102 300 004 202 400 104 302 500 006 204 402 600 304 502 700 a
Reference 6.
1101 2050 2200 3077 3289 3967 4136 4357 4913 5145 5435 5749 5962 6187 6497 6897 7207 7523 Reference
calcdb
1103 2058 2201 3084 3291 4000 4142 4372 4934 5179 5444 5797 5984 6219 6506 6939 7244 7560 7.
Spex 1704A (f/8) monochromator with 8 A/mm dispersion in first order. The signal was recorded with a PAR 1621165 boxcar gated integrator. Figure 1shows a low resolution (8-A fwhm) and higher resolution (inset, 0.35-A fwhm) spectrum with tentative assignments. The spectrum consists of overtones and combination bands in v1 (symmetric stretch) and even quanta of v3 (antisymmetric stretch). No bands with u2 # 0 (bending) are evident. Table I lists vibrational energy levels derived from the observed bands (calculations based 0% constants from Barbe et al.6~'). Vibrational levels of the XIA1 state as close as 500 cm-' to the O(3P) + 02(X3Z;) dissociation limit (-88000 cm-l)' are observed here for the first time! The large number of overtones observed wjll allow us to determine vibrational constants for the X state very accu~
(6) A. Barbe, C. Secroun, and P. Jouve, J . Mol. Spectrosc., 49, 171 (1974). (7) Calculated from the spectroscopic constants of ref 6 by S. AdlerGolden, private communication.
rately. At present, we are using higher resolution spectra to refine these constants. The relative intensities of the vibrational bands contain information about the excited state. The fact that no progression in v2 (bending mode) is present indicates that the change in bond angle with excitation is very slight and that the curvature with respect to the bend angle is similar on the upper and lower potential surfaces. Quantitative information about the excited state is available from the relative intensities of the symmetric stretch fundamental (vl), ZIW, and the first overtone of the antisymmetric stretch ( v 3 ) , Zm2. The relevant equation given by Heller et aL5 can be rewritten as = w132 = u032 - 2v1w03(1002/w011100)1'2 (1) In eq 1, wOj is the frequency of the jth ground-state fundamental, VI is the first derivative of the upper-state potential with respect to the v1 normal coordinate Q1(for the ground state), V,, is the second derivative with respect to Q3, and 013 is the harmonic oscillator frequency corresponding to V33, (The first derivative, V3, is zero by symmetry.) All derivatives are evaluated at the ground state equilibrium configuration. The units of the potential derivatives are determined by use of mass-weighted normal coordinates and setting h = 1. Fermi resonance between the 002 and 200 vibrational levels, which could complicate this simple interpretation, is calculated to be very minor.' From our spectrum, a value of 0.25 is obtained for 1a02/Z100. A value for Vlz can be obtained from the Hartley band absorption spectrum: and 003 and ool are known from the spectroscopy of ground-state 03. Insertion of these data into eq 1produces a negative value for V33,Le., the upper-state potential has a maximum rather than a minimum with respect to Q3 at the ground-state equilibrium geometry. A surface schematically like that of potential surface I in Figure 2, rather than like potential surface 11, is i n d i ~ a t e d . ~The , ~ corresponding imaginary frequency is ~ 1 = 3 i(1650 f 200) cm-' (2) v33
(8) P. J. Hay, R. T. Pack, R. B. Walker, and E. J. Heller, J. Phys. Chem., 86, 862 (1982).
2566
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
Letters
I
EFXCITATION
SPFCTRUM
Figwe 3. Photoemission excltatkm spectrum of ozone in the Huggins bands. See text for explanation of assignments.
Figwe 2. Schematic diagram showing equipotential contours for two qualltatlvely different O3excited-state potential surfaces. R , and R , are the distances between the central 0 atom and each of the two other 0 atoms. The bond angle is regarded as fixed. Surface I has negathre curvature with respect to the antisymmetric stretch, whereas surface I1 has posRive curvature. The dashed lines Indicate the tendency of a trajectory on I to spread into one of the valleys and that on I1 to oscillate about the minimum in the well.
The upper state, in this case, is therefore better described as of C, symmetry rather than C2",which is the groundstate symmetry. This is in accord with ab initio calculations.s Other potential parameters are available from the data. is determined by known For example, the ratio 1200/1100 ground-state frequencies and the upper-state potential derivatives Vl and Vll. Vl, as remarked above, can be obtained from the absorption spectrum. Hence, Vll = ol? can be obtained by requiring agreement with the observed value of 1200/1100. The equation is less simple than eq 1 in this case, however.
Excitation Spectroscopy of Dissociating Molecules Since the total inelastic scattering depends on the vibrational density of states in the upper electronic state, one can obtain an excitation spectrum of scanning the laser and recording total inelastically scattered light vs. excitation wavelength. Figure 3 shows such a spectrum of O3 in the Huggins band. Vibrational assignments are made
from the work of K a t a ~ a m a . ~The spectrum resembles an absorption spectrum and shows no rotational structure (instrumental resolution 0.1 cm-'). The sharp structure is laser-induced fluorescence of the O2 B3Z,:X3Z; Schuman-Runge system of O2 and is a result of a twophoton process. The first photon is absorbed by O3 and causes rapid dissociation into 0 + 02.A second photon within the same laser pulse excites the O2 X38, u = 12 photofragment into the B32; state u = 0 level, and the undispersed B-X fluorescence is observed.
Future Work Extension of the present studies to include the Chappuis and Huggins bands is planned in the near future. It is hoped that this investigation will clarify the identification of the upper electronic state for the Huggins bands, which remains unclear at present. At least one of these band systems should correspond to an electronicstate with an equilibrium angle different from that of the X state, so that progressions in the bending mode will appear in the photoemission spectrum. Acknowledgment. This work was supported in part by a contract from the Air Force Geophysics Laobratory and in part by a grant from the National Science Foundation. We are grateful to Professor Eric Heller for useful discussions and to the George Harrison Spectroscopy Laboratory for use of the facilities of its Regional Laser Center, which is supported by the National Science Foundation. (9)
D.H.Katayama, J. Chem. Phys., 71, 815 (1979).
