Ionization and dissociation of CH3I in intense laser field

The ionization-dissociation of methyl iodide in intense laser field has been studied using a reflection time-of-flight mass spectrometry (RTOF-MS), at a laser intensity of <= 6.6x10(14) W/cm(2), lambda=798 nm, and a pulse width of 180 fs. With the high resolution of RTOF-MS, the fragment ions with the same M/z but from different dissociation channels are resolved in the mass spectra, and the kinetic energy releases (KERs) of the fragment ions such as Iq+ (q=1-6), CHm+ (m=0-3), C2+, and C3+ are measured. It is found that the KERs of the fragment ions are independent of the laser intensity. The fragments CH3+ and I+ with very low KERs (< 1 eV for CH3+ and < 0.07 eV for I+) are assigned to be produced by the multiphoton dissociation of CH3I+. For the fragments CH3+ and I+ from CH3I2+, they are produced by the Coulomb explosion of CH3I2+ with the interaction from the covalent force of the remaining valence electrons. The split of the KER of the fragments produced from CH3I2+ dissociation is observed experimentally and explained with the energy split of I+(P-3(2)) and I+(P-3(0,1)). The dissociation CH3I3+-> CH3++I2+ is caused by Coulomb explosion. The valid charge distance R-c between I2+ and CH3+, at which enhanced ionization of methyl iodide occurs, is obtained to be 3.7 A by the measurements of the KERs of the fragments CH3+ and I2+. For the CH3In+ (n >= 3), the KERs of the fragment ions CH3p+ and Iq+ are attributed to the Coulomb repulsion between CH3p+ and Iq+ from R-c approximate to 3.7 A. The dissociation of the fragment CH3+ is also discussed. By the enhanced ionization mechanism and using the measured KER of Iq+, all the possible Coulomb explosion channels are identified. By comparing the abundance of fragment ions in mass spectrum, it is found that the asymmetric dissociation channels with more charges on iodine, q > p, are the dominant channels. (c) 2007 American Institute of Physics.