Two-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 C₆H⁺₇, (C₆H₆)_n C₆H₆Cl⁺, C₇H₅H₇Cl⁺, and C₇H₅Cl⁺₂ 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.