Multiphoton ionization of methyl iodide and selective stepwise ionization of methyl radicals

The collisionless multiphoton ionization of methyliodide in the isotopic forms CH3I and CD3I has been investigated with pulsed laser light of two wavelengths: fixed-frequency 266 nm of high intensity (∼1×107 W/cm2) and tunable around 215 nm of relatively lower intensity (∼2×106 W/cm2). The 215 nm was chosen because the methyl radicals CH3 and CD3 absorb there resonantly through the (0,0) (B̃ 2A′1←X̃ 2A′′2) band transition with the B̃ state being very heavily predissociated. Objectives have been: (1) the ionization mechanism of CD3I to form ion fragments D+, CD+, CD2+, CD3I+, and I+, obtained with 266 nm alone, and 266 nm together with tunable 215 nm, (2) the possibility of influencing the MPI by selective excitation of an intermediate (CD3, CH3), and (3) detection and spectroscopy of the methyl radical by two-photon ionization through the decaying B 2A′1 state as intermediate step. Mass spectra have been taken and measurements have been made with respect to the abundance of fragments, power dependence, sensitivity of CH3, CD3 detection and excitation–ionization spectra of the methyl radicals.