Nonlinear photoionization of supersonically expanded molecular pulses of iodomethane (CH₃I)

In this paper, the ultraviolet output of frequency-doubled OPO laser at 266 nm was employed to obtain highly-resolved (2+1) resonance-enhanced multiphoton ionization (REMPI) and mass spectra of supersonically-expanded and jet-cooled molecular pulses of iodomethane. The molecular sample consisted of a 3% neat iodomethane gas dissolved in helium. The REMPI and mass spectra of iodomethane were obtained using a two meter time-of-flight (TOF) mass spectrometer coupled with a microchannel plate detector that was kept at 4K volts. The laser power density effect on the (2+1) REMPI process was also investigated by focusing and defocusing the laser pulses using 30 cm and 75 cm biconvex lenses, respectively. By monitoring I+ and CH3+ fragments, it was evident that 3 photons were involved in the REMPI process.