The photodissociation dynamics of ICl is studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin−orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of ICl at this wavelength. Two sharp velocity distributions are observed for the I channel, suggesting the existence of two dissociation pathways that correlate with ground state iodine formation. Based on the expected translational energy release and the energy separation between those peaks, the two distributions are assigned to dissociation of ICl to I(2P3/2) + Cl(2P3/2) and to I(2P3/2) + Cl*(2P1/2); the former channel appears at higher translational energy. The distribution of I* also shows two strong peaks, indicating that there are two dominant channels for the formation of I* atoms at this wavelength which we assign to the dissociation of ICl forming I*(2P1/2) + Cl(2P3/2) and I*(2P1/2) + Cl*(2P1/2), respectively. The quantum yield of I*(2P1/2) is determined to be 0.30, indicating that the formation of ground state iodine is clearly the favored dissociation channel at 304 nm. The observed anisotropy in the angular distribution of dissociation products (β) indicates that the I* + Cl and I* + Cl* channels are formed predominantly from the parallel transition (β = 1.7 for both channels) while the I + Cl and I + Cl* products are formed mainly from perpendicular transitions (β = −0.5). The decrease in the anisotropy parameter of the I formation channels from their limiting value of −1 is attributed to the presence of more than one path for the formation of I + Cl/Cl* photoproducts with opposite polarization for their absorbing transitions. The possible excited state dynamics, which give the observed results, are discussed in terms of the previously proposed energy correlation diagram for ICl.

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Proton-transfer reactions that proceed within methanol cluster ions were studied using an electron impact time-offlight mass spectrometer. When CH,OH seeded in helium is expanded and ionized by electron impact, the protonated species, (CH,OH),H+, are the predominant cluster ions in the low-mass region. In CH,OD clusters, both (CH,OD),H+ and (CH,OD),D+ ions are observed. The ion abundance ratios, (CH30D),H+/(CH30D),D+, show a tendency to decrease as the methanol concentration increases, which is apparently related to the cluster structure and reaction energetics. The results suggest that the effective formation of (CH,OD),H+ ions at low concentration of CH,OD in the expansion is the result of the relatively facile rotation of methanol molecules within the smaller clusters that tend to form at low CH,OD concentration. Ab initio molecular orbital calculations were carried out to investigate the rearrangement and dissociative pathways of ionized methanol dimer. Ion-neutral complexes, [CH,OH,+-.O(H)CH,] and [CH,OH,+-OCH,I, are found to play an important role in the lowenergy pathways for production of CH30H2+ + CH,OH (and OCH,) from ionized methanol dimer.

KCT0001502.

The intracluster ion-molecule reactions occurring within the titanium/methanol heterocluster ions are studied using a simple cluster source based on the mixing of the laser-ablated plasma with the supersonic expansion of a methanol vapor seeded in Ar carrier gas. The product ions are detected using a reflectron time-of-flight mass spectrometer. The mass spectra exhibit a major sequence of cluster ions with the formula Ti + (CH 3 O) 2 (CH 3 -OH) n , which are attributed to the hydrogen elimination reactions (-2H or -H 2 ) of Ti + ion within the heteroclusters. In addition, heterocluster ions of the types Ti + (CH 3 O)(CH 3 OH) n and Ti + (CH 3 O) 3 (CH 3 OH) n also emerge throughout the observed mass spectra, indicating that consecutive H-eliminations by the Ti + ion occur for up to three methanol molecules. The results from isotope-labeling experiments suggest that H-elimination is the reaction path for O-H bond breaking in Ti + -methanol heteroclusters. The observed TiOH + and TiO + ions are interpreted as arising from the insertion of Ti + ions into the C-O bond in CH 3 OH, followed by CH 3 -and CH 4 -elimination, respectively, from the [HO-Ti + -CH 3 ] intermediate. The experiments also show that the chemical reactivity of heterocluster ions is greatly influenced by cluster size and argon stagnation pressure. The reaction energetics and formation mechanisms of the observed heterocluster ions are also discussed.

The photodissociation dynamics of IBr has been studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin–orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of IBr at this wavelength. Two sharp velocity distributions observed for the I channel suggest the two dissociation pathways that correlate with ground-state iodine formation. Based on the expected translational energy release and the energy separation between the peaks, the two distributions have been assigned to dissociation of IBr to form I(2P3/2)+Br(2P3/2) and I(2P3/2)+Br*(2P1/2) with the former channel appearing at higher translational energy. The I* distribution shows one strong peak indicating that there is one dominant channel for formation of I* atoms at this wavelength which has been assigned to dissociation of IBr to form I*(2P1/2)+Br(2P3/2) with a quantum yield of 0.1. The I* signal formed from the I*(2P1/2)+Br*(2P1/2) dissociation channel is observed very weakly. The observed anisotropy parameter indicates that the I+Br* product (β=−0.7) is formed mainly from the perpendicular 1Π1(2341)←X transition while the I*+Br channel (β=1.8) is formed predominantly from the parallel 3Π0+(2341)←X transition followed by curve crossing to the 3Σ0+−(2422) state. The recoil energy dependence of the anisotropy parameter in the I atom produced in the I+Br channel shows a positive β value above maximum of the peak recoil energy and a negative value below the peak maximum of the recoil energy distribution. These results are interpreted in terms of the presence of more than one path for the formation of I+Br photoproduct with opposite polarization for their absorbing transitions, most likely the 3Π0+(2341)←X and the 3Π1(2341)←X transitions. The possible excited state dynamics which give the observed results are discussed in terms of the previously proposed potential energy diagrams for IBr and ICl.

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Photodissociation of n-alkyl iodides and CF3I has been studied using state-selective ionization and pulsed-field time-of-flight (TOF) mass spectrometry. The (2+1) resonance-enhanced multiphoton ionization cross-section ratio of I(2P1/2) and I(2P3/2) is measured from I2 photodissociation in the 304 nm region. Using this ratio, the relative populations of I(2P1/2) and I(2P3/2) by a pure parallel excitation to the 3Q0 state of n-alkyl iodides and CF3I are obtained. This method can exclude the effects of clusters and spectrum overlap in different dissociation paths from branching ratio calculations. The product quantum yield of I(2P3/2) obtained from branching ratio reveals the 3Q0–1Q1 curve crossing probability—which increases from 0.70 for CH3I to 0.86 for n-C4H9I (with recoil velocity decrease of fragments at the curve crossing point). The potential energies at the crossing point and the Landau–Zener parameters ζ, calculated from our modified model, are 385 kJ/mol and 1100 m/s for n-alkyl iodides, and 391 kJ/mol and 177 m/s for CF3I, respectively.