Abstract: The collision-free multiphoton ionization of jet cooling CH_3I was studied using a pulsed laser, tunable around 280 and 366 nm with TOF mass spectrometer and mag-netic-bottle TOF photoelectron spectrometer.
MPI of CH_3I in 280 nm region via valence à state can be explained by ladder-switch mechanism according to the following experimental results. The parent ions are almost abscent in the MPI TOF MS. I~+ MPI spectrum gives three resonances corre-sponding to two-photon excitation of I atom at 279.72, 279.25 and 278.50 nm. The I ionic state due to 2+1 REMPI of I atom can be assigned from the REMPI PES.
In 371—365 nm I~+, CH_3~+ and CH_3I~+ were observed in MPI MS. The mass-selected MPI spectra of CH_3I~+ and daughter ion I~+ have similar profiles. In the spectra the two peaks at 365.90 and 369.92 nm are assigned to be two-photon resonance with the Rydberg C state Π(0,0) and Σ(0,0) of CH_3I respectively. The formation of the CH_3I~+ ~2Π_(3/2) and ~2Π_(1/2) state created by 2+1 and 2+2 REMPI resectively were found in PES. REMPI of CH_3I around 366 nm is consistent with the ladder mechanism in which CH_3I is excited into C state upon a two photon absorption and parent ion is formed by the overall 2+1 of 2+2 MPI process. The fragment ions are produced by further dissociation of CH_3I~+ at à state.