The mechanisms of multiphoton ionization (MPI) and dissociation of CH3I have been studied using time-of-flight (TOF) mass spectrometer at 266 and 355 nm. MPI mass spectrum at 266 nm consists mostly of fragment ions. This is consistent with a neutral-fragment photoionization mechanism in which rapid one-photon dissociation occurs from the repulsive potential energy surface followed by MPI of neutral photofragments. The observation of parent ions at 355-nm excitation is indicative of parent-ionic ladder mechanism in which the parent ions are produced directly by two-photon excitation resonantly excited to Rydberg C state and then ionized through additional one-photon absorption to produce CH3I+. Fragment ions are produced by dissociation of CH3I+.

Theoretical studies on laser gain of a multi-stage dye laser amplifier are presented in this paper. The results show that the influences of amplified spontaneous emission (ASE) and nonlinear absorption of excitedstate on the gain are different for different input laser energies and gains. A threshold input intensity exists for a specific system. If the input intensity is higher than the threshold, the nonlinear absorption of excited-state will be the main cause for gain decrease. Otherwise, the ASE is the main cause. A new scheme is proposed to calculate the gain of the amplifier by the comparison of the input intensity with the effective saturation intensity and the choice of the calculation gain method.