Photolysis Cross-Section of Ozone Dimer

Abstract

Clusters are important in bridging the gap between individual molecules in the gas phase and closely interacting molecules in condensed phases. The size of a cluster is a basic parameter that a researcher would like to control or measure. Whereas the size determination of ionic clusters is rather straightforward, determination of the size of a neutral cluster is much more difficult. Only a few methods are available for the size determination of a neutral cluster beam, including: 1) momentum transfer in crossed-beam scattering, 2) diffraction from a transmission grating, and 3) high-resolution spectroscopy. Ozone is not only a crucial molecule in our atmosphere but also a benchmark molecule in photochemistry and photophysics. Theoretical studies on its photodissociation were recently reviewed by Grebenshchikov et al. Although quite a few excited states and complicated non-adiabatic couplings are involved, high-level theoretical calculations can describe the relevant experimental observables of ozone such as its absorption spectrum, product distributions, etc. The detailed information about the ozone molecule may offer a good starting point for investigations of ozone clusters. Probst et al. synthesized the ozone dimer in a molecular beam and probed it with high-resolution electron-impact ionization near threshold. They also performed ab initio calculations on the ozone dimer and its possible ionic structures, mostly by means of density functional calculations with the B3LYP functional, and checked the results with a variety of other methods, such as CASSCF and QCISD or CCSD(T). For the neutral ozone dimer, their calculations indicate that two ozone molecules are only very weakly bound to each other. This result also agrees with the small MP2 dimerization energy calculated by Slanina and Adamowicz. Bahou et al. investigated the infrared spectroscopy and photochemistry at 266 nm of the ozone dimer trapped in an argon matrix. The observed frequency shift of the ozone antisymmetric stretching mode upon dimerization is small, about 1 to 3 cm , which is similar in magnitude to those induced by different trapping sites in the argon matrix, thus indicating weak interconstituent interactions in the dimer. In their photodissociation experiment, the photolysis crosssection of the matrix-isolated ozone dimer was estimated to be 1.5 10 18 cm at 266 nm, substantially smaller than the absorption cross-section of the ozone monomer in the gas phase (9.1 10 18 cm). It should be noted that the photolysis cross-section is the product of the absorption cross-section and the dissociation quantum yield; in the matrix environment the dissociation quantum yield is hard to establish. Herein, we report the synthesis of ozone clusters in a molecular beam by supersonic expansion. The photolysis crosssections of the ozone clusters were measured in a mass-resolved manner (with an electron-impact-ionization mass spectrometer) at selected excitation wavelengths (l). By tuning the temperature and backing pressure before the expansion, we were able to generate ozone clusters of different size distributions. Figure 1 shows typical electron-impact mass spectra of the ozone molecular beam under two expansion conditions. Evidence of cluster formation can be clearly seen: peaks at masses (m/z) 80 (O5 ), 96 (O6 ), 128 (O8 ), and 144 (O9 ) indicate that not only the dimer but also larger clusters could be formed. Owing to dissociative ionization in the electron-impact ionization process, the mass 80 peak is the most intense peak other than the monomer mass peaks (O3 + , masses 48 and 50). The relative intensities of [a] I.-C. Chen, A. F. Chen, W.-T. Huang, Dr. K. Takahashi, Prof. Dr. J. J. Lin Institute of Atomic and Molecular Sciences Academia Sinica PO Box 23-166, Taipei 10617 (Taiwan) Fax: (+886)2-23620200 E-mail : jimlin@gate.sinica.edu.tw [b] W.-T. Huang, Prof. Dr. J. J. Lin Department of Chemistry National Taiwan University Taipei 10617 (Taiwan) [c] Prof. Dr. J. J. Lin Department of Applied Chemistry National Chiao Tung University Hsinchu 30010 (Taiwan)