2-COLOR 2+2 PHOTON RESONANCE-ENHANCED IONIZATION OF BENZENE-CARBON TETRACHLORIDE BINARY CLUSTERS

Binary clusters produced from benzene-carbon tetrachloride (9:1) mixtures are ionized by two-color (UV-VIS) excitation through the resonance pumping of the benzene S2 state or the charge-transfer state of the benzene-carbon tetrachloride complex. The observed ions are C6H7+, (C6H6)(n)C6H6Cl+, C6H5Cl+, C7H6Cl+, and C7H5Cl2+ in addition to (C6H6)n+ (n = 1, 2, 3,..). Resonance two-photon excitation at 210 nm, in which the total energy (11.8 eV) is greater than the ionization potential of benzene (9.25 eV), generates neutral intermediate states with lifetimes of 250-320 ns. The visible laser ionization with a delay from the first two-photon excitation appeared to be also a two-photon resonance process, the first step of which is probably the T <-- T transition of benzene. The main reason for maintaining neutral states, in spite of the absorption of high energy, is attributed to the double excitation of C6H6 and CCl4 molecules and the subsequent dissociation of electronically excited CCl4 in the binary clusters. Condensation reactions of the component molecules in a benzene dimer ion unit and in benzene-carbon tetrachloride binary ions are found to be highly efficient for this system.