Of Ch <sub> 3 </ Sub> I Molecular Multiphoton Ionization Dissociation

The molecule of methyl iodide have been extensively studied in recent years by reason of the photodissociation channels of methyl iodide not only have relations with the objective of developing chemical laser but also have important significance to open out some basic theoretical questions. The methyl iodide is a representative molecule and the configuration of it belongs to C_3v Point group. The rotation period of it is more bigger than the time of its dissociation, so particularly investigate the mechanism of multiphoton ionization and dissociation of methyl iodide have evidence direction in characterizing the dynamics of congener polyatomic molecular.The mechanism of multiphoton ionization (MPI) and dissociation of methyl iodide molecule has been discussed under supersonic beam conditions by using time-of-flight mass spectrometer. Through analyzing the mass spectrums received from different wavelength of the laser beam, a conclusion is gotten that the ionization of methyl iodide at 266nm undergoes a process of neutral-fragment photoionization. Methyl iodide molecule is one-photon excited to A-band in which rapid one-photon dissociation occurs from the repulsive potential energy surface followed by (2+1) multiphoton ionization of neutral photofragments to form CH₃⁺ and I⁺ ions. The observation of parent ions at 355nm excitation is indicative of parent-ion ladder mechanism in which the parent ions are produced directly by two-photon resonantly excited to Rydberg C state and then ionized through additional one-photon absorption to form CH₃I⁺. Fragment ions are produced by dissociation of CH₃I⁺ which lies in excited state. According to the appearance potential of the parent and fragment ions, the main possible channels of the muitiphoton ionization and photodissociation of methyl have been analyzed. At 532 nm, the processes of ionization of methyl iodide molecule include two different ionization mechanisms, one is the neutral-fragment photoionization mechanism, the other is the parent-ion ladder mechanism.The mass spectrums received from different wavelength of the laser beams are different. In order to expatiate on the multiphoton ionization and dissociation mechanism of methyl iodide, we gain a series of mass spectrums for three different excitation wavelengths when we keep the laser delay time to the molecule beam unchangeable. In conjunction with the graphs of ions intensity at different laser intensities, we particularly analyzed the mechanism of multiphoton ionization and dissociation of methyl iodide molecule at different wavelength of laser beams.