Ultraviolet laser multiphoton excitation of CH2I2

Abstract

Laser multiphoton excitation of CH₂I₂ has been studied at 248 nm (KrF laser) and 193 nm (ArF laser). Intense photofragment fluorescence is observed from CH(A²Δ), with weaker fluorescence from CH(B²Σ^–) and CH(C²Σ⁺), following excitation at both wavelengths. The rotational structure in the CH(A²Δ→X²Π) system has been partially resolved and it is shown that this radical is formed with a high rotational temperature (ca. 3000 K) when CH₂I₂ is excited at 248 nm. Fluorescence from I(5p⁴6s²P_3/2) and the F¹Σ⁺_u, D¹Σ⁺_u and D′³Π_2g states of I₂ is also observed and carbon atoms in the 2¹D₂ and 2¹S₀ states are detected by laser-induced fluorescence. Mechanisms which account for the formation of these photofragments involving both multiphoton and multiple-photon excitation are discussed and the results are compared with those for single-photon excitation of CH₂I₂ in the near and vacuum ultraviolet.