ELSEVIER Chemical Physics 189 ( 1994) 179-204 Chemical Physics Vibrational energy transfer from CO to O2 in rare gas matrices. I. Vibrational excitation and relaxation of O2 (X, v = 4-20) Akil Salloum, Hem-i Dubost zyxwvutsrqponmlkjihgfedcbaZYXWVU Labnrutoire de Photophysique M olkculuire CNRS, Britiment 213, Universit6 de Paris-&d, 91405 Orsay Cedex, France Received 8 August 1994 Abstract Vibrationally excited oxygen Oz(u) is produced in matrix isolated CO-O2 mixtures by IR laser excitation of ‘3C’80. The quenching of the CO vibrational fluorescence by both 1602 and 1802 shows that energy transfer preferentially occurs from the high levels of CO. Laser probing of the vibrational energy contents of O2 confirms that a fraction of the energy initially deposited in CO is actually being converted into vibrational excitation of Oz. Excitation spectra of the A’ + zyxwvutsrqponmlkjihgfedcbaZYX X LIF reveal the existence of a broad O,( o) distribution extending from u = 4 to 20. Time resolvedmeasurements show an extremely slow vibrational relaxation in Oz. The 250 s lifetime of u = 4 is the longest value ever reported for a molecular vibration in solids. The u-dependence of the decay rates and their sensitivity to matrix change suggest that while vibrational relaxation of the lower levels is radiative in both Kr and Ar hosts, the upper levels are depopulated in the latter by radiationless multiphonon processes. Interstate cascading between O,( 6) and the nearby vibronic levels of the singlet manifolds is a negligible process. Quadrupolar effects are found to be important for both radiative relaxation and non-radiative energy transfer. The energy flow between the CO and O2 vibrational reservoirs is controlled by long-range dipole-quadrupole interactions. Vibrational up-pumping in 0, occurs through a sequence of CO(u) + 02( u’) transfer processes. In concentrated matrices back transfer from O,(v) to ground state CO as well as quadrupole-quadrupole mediated fusion of Oz(ti = 1) excitations compete with vibrational relaxation. 1. Introduction In recent years, highly vibrationally excited diatomic molecules have attracted considerable attention [ 1,2]. The subject is of clear chemical importance as chemical reactions often involve vibrational excitation of either the reactants or the products. In particular vibrationally excited O2 molecules play an important role in atmos- pheric chemistry. Up to now two techniques have been employed for the production of 02( u) in the gas phase. Wodtke and coworkers [ 31 have used stimulated emis- sion pumping to prepare O2 molecules in a selected high vibrational level of the ground electronic state. Park and Slanger [4] have generated a broad O,(u) distribution extending to L’ = 22 from the photodisso- ciation of ozone at 248 nm. Another technique, based on the addition of vibra- tional quanta induced by intermolecular vibration to vibration (V-V) transfer processes, is well known to produce broad distributions of highly vibrationally excited molecules. Owing to the fundamental role played by the molecular anharmonicity in the energy accumulation process, this phenomenon is often referred to as anharmonic V-V pumping. In the gas phase the subject is well documented for NO and CO [S-9]. High vibrational excitation of CO also occurs in cryogenic liquids [ 10,111 and solids [ 12-141. In solids the pumping process is called fusion of vibra- tional excitations by analogy to the triplet-triplet fusion 0301.0104/94/$07,00 0 1994 Elsevier Science B.V. All rights reserved SSDIO301-0104(94)00297-S