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Photolysis Crosssection of Ozone Dimer 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- culations on the ozone dimer and its possible ionic struc- vidual molecules in the gas phase and closely interacting tures, mostly by means of density functional calculations molecules in condensed phases. The size of a cluster is a with the B3LYP functional, and checked the results with a basic parameter that a researcher would like to control or variety of other methods, such as CASSCF and QCISD or measure. Whereas the size determination of ionic clusters is CCSD(T). For the neutral ozone dimer, their calculations in- rather straightforward, determination of the size of a neutral dicate that two ozone molecules are only very weakly bound cluster is much more difficult. Only a few methods are avail- to each other. This result also agrees with the small MP2 di- able for the size determination of a neutral cluster beam, in- merization energy calculated by Slanina and Adamowicz.[13] cluding: 1) momentum transfer in crossed-beam scatter- Bahou et al.[14] investigated the infrared spectroscopy and ing,[1–4] 2) diffraction from a transmission grating,[5] and photochemistry at 266 nm of the ozone dimer trapped in an 3) high-resolution spectroscopy.[6–8] argon matrix. The observed frequency shift of the ozone an- Ozone is not only a crucial molecule in our atmosphere tisymmetric stretching mode upon dimerization is small, but also a benchmark molecule in photochemistry and pho- about 1 to 3 cmÀ1, which is similar in magnitude to those in- tophysics. Theoretical studies on its photodissociation were duced by different trapping sites in the argon matrix, thus recently reviewed by Grebenshchikov et al.[9] Although indicating weak interconstituent interactions in the dimer. quite a few excited states and complicated non-adiabatic In their photodissociation experiment, the photolysis cross- couplings are involved, high-level theoretical calculations section of the matrix-isolated ozone dimer was estimated to can describe the relevant experimental observables of ozone be 1.510À18 cm2 at 266 nm,[14] substantially smaller than the such as its absorption spectrum, product distributions, absorption cross-section of the ozone monomer in the gas etc.[9–11] The detailed information about the ozone molecule phase (9.110À18 cm2).[15] It should be noted that the photol- may offer a good starting point for investigations of ozone ysis cross-section is the product of the absorption cross-sec- clusters. tion and the dissociation quantum yield; in the matrix envi- Probst et al.[12] synthesized the ozone dimer in a molecular ronment the dissociation quantum yield is hard to establish. beam and probed it with high-resolution electron-impact Herein, we report the synthesis of ozone clusters in a mo- ionization near threshold. They also performed ab initio cal- lecular beam by supersonic expansion. The photolysis cross- sections of the ozone clusters were measured in a mass-re- [16–19] [a] I.-C. Chen, A. F. Chen, W.-T. Huang, Dr. K. Takahashi, solved manner (with an electron-impact-ionization mass Prof. Dr. J. J. Lin spectrometer) at selected excitation wavelengths (l). By Institute of Atomic and Molecular Sciences tuning the temperature and backing pressure before the ex- Academia Sinica pansion, we were able to generate ozone clusters of differ- PO Box 23-166, Taipei 10617 (Taiwan) Fax : (+886)2-23620200 ent size distributions. Figure 1 shows typical electron-impact E-mail: [email protected] mass spectra of the ozone molecular beam under two expan- [b] W.-T. Huang, Prof. Dr. J. J. Lin sion conditions. Evidence of cluster formation can be clearly + + + Department of Chemistry seen: peaks at masses (m/z)80(O5 ), 96 (O6 ), 128 (O8 ), National Taiwan University + and 144 (O9 ) indicate that not only the dimer but also Taipei 10617 (Taiwan) larger clusters could be formed. Owing to dissociative ioni- [c] Prof. Dr. J. J. Lin zation in the electron-impact ionization process, the mass 80 Department of Applied Chemistry National Chiao Tung University peak is the most intense peak other than the monomer mass + Hsinchu 30010 (Taiwan) peaks (O3 , masses 48 and 50). The relative intensities of Chem. Asian J. 2011, 6, 2925 – 2930 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2925 COMMUNICATION Figure 2. Number-density profiles of the molecular beam showing the photo-depletion signals at two detected masses, m/z=50 and 80, at differ- ent laser energies. Black and red lines are the molecular beam signals before and after laser irradiation (N2 and N1), respectively; the blue line is the difference (N2ÀN1). Under our experimental conditions where the number of photons greatly exceeds the number of molecules, we may write the related kinetic equations as: dN N À ¼ s dI ln 0 ¼ I s ð1aÞ N0 N Where N0 and N are the numbers of molecules before and after the laser irradiation, respectively; I is the laser flu- Figure 1. Mass spectrum of the ozone molecular beam seeded in Ne at ence in number of photons per unit area; s is the absorption different nozzle temperatures at a backing pressure of 1.05 bar. a) Nozzle temperature 239 K; b) Nozzle temperature 220 K. cross-section and f is the dissociation quantum yield. By re- arranging [Eq. (1a)], we then have [Eq. (1b)]. N À N these mass peaks mainly depend on two factors: 1) the pop- 0 ¼ 1 À eÀIs ð1bÞ ulation of the cluster sizes and 2) the patterns of dissociative N0 ionization of the clusters.[1,20] For example, ions of mass 80 may come from the ozone dimer (O3)2 and larger clusters We call (N0ÀN) the laser-depletion signal. [Eq. (1b)] de- and ions of mass 128 may have contributions from the scribes a saturation behavior at high laser fluences, that is, trimer and larger clusters. The formation of large clusters the laser-depletion signal (N0ÀN) will converge to N0 when was greatly suppressed at higher nozzle temperatures or at Isf@ 1. lower backing pressures. For example, by raising the nozzle After photo-excitation, the excited states of ozone that temperature from 220 to 239 K (Figure 1), most peaks at are reached are either repulsive or excited above their high masses disappeared; only the peak at mass 80 remained thresholds, which results in very short lifetimes and com- [9–11] visible among those cluster masses. plete dissociation to O+O2 [f(O3)= 1]. For ozone clus- Figure 2 shows the molecular-beam photo-depletion sig- ters which are only bound by weak van der Waals forces, nals detected at masses 50 and 80. The signal at mass 50 rep- even if the ejected oxygen atom is blocked by other O3 mol- resents an isotope of the O3 monomer that mostly consists ecules, the released energy in the photodissociation is far 18 16 of one O atom (0.2% natural abundance) and two O more than enough to boil off many O3 molecules. That is, [21] atoms. Although dissociative ionization of clusters would the dissociation quantum yield of an O3 cluster would not also contribute to ion signals at masses 48 and 50, the mono- be smaller than that of O3. Therefore, we assume f= 1in mer population was overwhelming in our experimental con- the following discussion. ditions such that the very minor cluster contributions did To avoid confusion, we call the experimental molecular not affect the photo-depletion signals at masses 48 and 50 at beam signal before laser irradiation N2 and that after laser all. irradiation N1. Owing to velocity spreads of the molecular beam and other instrument functions, N2 may be different 2926 www.chemasianj.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Asian J. 2011, 6, 2925 – 2930 Photolysis Cross-Section of Ozone Dimer from N0. For the N2 time profiles shown in Figure 2, we can see that the time profiles of the cluster (mass 80) are signifi- cantly narrower than those of the monomer (mass 50), indi- cating different velocity distributions. This is due to the fact that dimer and higher clusters only exist at lower tempera- tures, meaning that they were formed preferentially in the colder central part of the pulsed molecular beam. As a result, the velocity distribution of the cluster is inevitably narrower than that of the monomer. Similarly, the peak shape of the laser-depletion signal N2ÀN1 would also depend on the velocity distribution. In the data analysis, we obtained the numerical values for N1 and N2 by integrating the corresponding signals over an arrival time period in Figure 4. Apparent cross-section ratio at l= 248.4 nm at different cluster which the N2ÀN1 trace (e.g., see Figure 2) shows intensity concentrations (indicated by the mass intensity ratio Pm80/Pm50). above the baseline. An important issue in studying neutral clusters is size de- termination. Owing to dissociative ionization, the detected almost all clusters. The apparent cross-section ratio sm80/ m/z value does not necessarily correspond to a definite as- sm50, obtained from signals at masses 80 and 50 is shown in signment of the cluster size, but may be used to indicate the Figure 4, plotted as a function of the cluster concentration cluster formation. Figure 3 shows two saturation curves as a indicator (Pm80/Pm50). We can see that sm80/sm50 ratio is about function of laser fluence measured at masses 50 and 80, 2 when the cluster concentration is low (Pm80/Pm50 <0.07) under an expansion condition where only a very small and rises very fast to values higher than 4 at high cluster amount of mass 80 was formed.
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