Multielectron dissociative ionization of CH 3I under strong picosecond laser irradiation

Abstract

The multielectron dissociative ionization (MEDI) processes of methyl iodide (CH 3I) induced by strong ( I=10 15 W cm −2) picosecond laser light at 532 and 1064 nm are studied by means of a time-of-flight mass spectrometer. The experimental results are compared to those reported from femtosecond experiments at similar laser intensities. The influence of the laser pulse duration is clearly reflected in the kinetic energy values of the ejected fragments, which were found to be significantly lower in the picosecond experiments. Multiple charged atomic fragments (up to I 6+) have been recorded in the mass spectra. It is suggested that the higher charged atomic ions (I n+ , n≥3) are produced by field ionization processes on atomic ions which have been liberated via Coulomb explosion within unstable multiple charged parent ion in the rising time of the laser pulse. The laser intensity thresholds for the appearance of the multiple charged atomic ions have been determined and found to be reasonably close to those predicted by the barrier-suppression ionization model. Nevertheless, the observed variation of the appearance intensity thresholds with the laser wavelengths used implies that there is a contribution to the ionization by electron tunneling processes, especially at 1064 nm.