Title

Diatomic molecules in strong ultrashort pulse laser fields

Date of Completion

January 2000

Keywords

Physics, Molecular|Physics, Atomic|Physics, Radiation

Degree

Ph.D.

Abstract

The advent of ultrashort pulse laser systems has allowed the study of atoms and molecules in extreme environments. In our lab intensities up to 2·1015 W/cm2 have been achieved and pulse durations as short as 25 fs have been characterized. Characterizing these pulses and understanding their effect on diatomic molecules is the focus of this dissertation. ^ To fully characterize an ultrashort laser pulse in both electric field and phase, we have developed a compact dispersion free TG FROG (Transient-Grating Frequency-Resolved-Optical-Gate). This was done through the use of a mask that separates the input beam into three distinct beams which are focused into fused silica to create the FROG signal. ^ To understand the ionization and dissociation process in detail, a comprehensive analysis was made of diatomic nitrogen for charge states of N 2 up to N5+2 . It appears that all ionization up to N5+2 involves the charge asymmetric channel, N4+2→N1++N3+ . By determining the time between each ionization step we observe the competition between laser intensity and internuclear separation in determining the molecular ionization rate. Finally, we suggest that short pulse (<130 fs) ionization leaves fragments in electronically excited states whereas long pulse (>600 fs) ionization leaves them in electronic ground states. ^ Further investigation was made into the role of charge asymmetric dissociation with short pulses (30 fs) and we prove that charge asymmetric dissociation in diatomic molecules leaves one of the fragments in an electronically excited state. For example, using a new double pulse technique, we observed the reaction: I2+pulse1 →I2+2 **→I0++I2+ *+pulse 2→I0++I3+ demonstrating that the I2+ fragment must have been in an excited state. More generally, just as asymmetric dissociation implies that the initial molecular ion is in an excited electronic state, the observation of asymmetric channels in the post-dissociation ionization shows that the ionic fragments are themselves electronically excited. ^