DOI: 10.1002/asia.201100526 Photolysis Cross-Section of Ozone Dimer I-Cheng Chen, [a] Andrew F Chen, [a] Wen-Tsung Huang, [a, b] Kaito Takahashi, [a] and Jim J. Lin* [a, b, c] Dedicated to Professor Yuan T. Lee on the occasion of his 75th birthday Clusters are important in bridging the gap between indi- vidual 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 avail- able for the size determination of a neutral cluster beam, in- cluding: 1) momentum transfer in crossed-beam scatter- ing, [1–4] 2) diffraction from a transmission grating, [5] and 3) high-resolution spectroscopy. [6–8] Ozone is not only a crucial molecule in our atmosphere but also a benchmark molecule in photochemistry and pho- tophysics. Theoretical studies on its photodissociation were recently reviewed by Grebenshchikov et al. [9] 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. [9–11] The detailed information about the ozone molecule may offer a good starting point for investigations of ozone clusters. Probst et al. [12] synthesized the ozone dimer in a molecular beam and probed it with high-resolution electron-impact ionization near threshold. They also performed ab initio cal- culations on the ozone dimer and its possible ionic struc- tures, 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 in- dicate that two ozone molecules are only very weakly bound to each other. This result also agrees with the small MP2 di- merization energy calculated by Slanina and Adamowicz. [13] Bahou et al. [14] 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 an- tisymmetric stretching mode upon dimerization is small, about 1 to 3 cm 1 , which is similar in magnitude to those in- duced by different trapping sites in the argon matrix, thus indicating weak interconstituent interactions in the dimer. In their photodissociation experiment, the photolysis cross- section of the matrix-isolated ozone dimer was estimated to be 1.5  10 18 cm 2 at 266 nm, [14] substantially smaller than the absorption cross-section of the ozone monomer in the gas phase (9.1  10 18 cm 2 ). [15] It should be noted that the photol- ysis cross-section is the product of the absorption cross-sec- tion and the dissociation quantum yield; in the matrix envi- ronment the dissociation quantum yield is hard to establish. Herein, we report the synthesis of ozone clusters in a mo- lecular beam by supersonic expansion. The photolysis cross- sections of the ozone clusters were measured in a mass-re- solved manner [16–19] (with an electron-impact-ionization mass spectrometer) at selected excitation wavelengths (l). By tuning the temperature and backing pressure before the ex- pansion, we were able to generate ozone clusters of differ- ent size distributions. Figure 1 shows typical electron-impact mass spectra of the ozone molecular beam under two expan- sion conditions. Evidence of cluster formation can be clearly seen: peaks at masses (m/z) 80 (O 5 + ), 96 (O 6 + ), 128 (O 8 + ), and 144 (O 9 + ) indicate that not only the dimer but also larger clusters could be formed. Owing to dissociative ioni- zation in the electron-impact ionization process, the mass 80 peak is the most intense peak other than the monomer mass peaks (O 3 + , 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) Chem. Asian J. 2011, 6, 2925 – 2930  2011 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 2925