Ionization and dissociation of $CH3I$ in intense laser field Available to Purchase

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 $⩽6.6×1014W∕cm2$⁠, $λ=798nm$⁠, and a pulse width of $180fs$⁠. 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 (⁠$<1eV$ for $CH3+$ and $<0.07eV$ 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 $I+(P23)$ and $I+(P0,13)$⁠. The dissociation $CH3I3+→CH3++I2+$ is caused by Coulomb explosion. The valid charge distance $Rc$ between $I2+$ and $CH3+$⁠, at which enhanced ionization of methyl iodide occurs, is obtained to be $3.7Å$ 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 $Rc≈3.7Å$⁠. 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.