Molecular Ionization from the Ground and Excited States of I2 by Intense Laser Fields

Exposing molecules to strong laser fields produces a variety of effects, which has led to considerable work in many areas, such as enhanced ionization, high harmonic generation, nonsequential double ionization, and coherent control. However, these strong-field experiments start with neutral ground-state molecules at their equilibrium internuclear separation. In this dissertation, we experimentally investigate the internuclear-separation-dependent ionization of different molecular orbitals in neutral I2. With a pump-probe scheme, a vibrational wavepacket is launched in the B state of I2 by promoting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and then the ionization rate as a function of internuclear separation R is measured through detection of the I2+ signals as the wavepacket evolves in the B state. Moreover, since the ground to B state optical transition dipole moment is parallel to the internuclear axis, the B-state sub-population of the I2 thermal ensemble will have a high degree of alignment, allowing for angular measurements. In this way, both the angle-resolved and R-resolved measurements of the ionization rate of the LUMO are obtained, and enhanced ionization is found at 8.67 a.u.. Based on the above idea, a wavepacket is generated in the ground state by adding a dump pulse at a certain delay when the B-state wavepacket reaches the one-photon resonant crossing to return the wavepacket in the ground state. Using this returning wavepacket and Fourier-transform spectroscopy, we study the R-dependent ionization of the ground state on different molecular orbitals, HOMO, HOMO-1 and HOMO-2. We find that HOMO and HOMO-1 do not have a strong R-dependent ionization, however, HOMO-2 does. Further, HOMO-2 provides the dominant ionization pathway, which is unusual in small molecules. With the wavepacket in the B state, we discuss the enhanced ionization in HOMO-2 through detection of dissociative fragment ions, and find that enhanced ionization also happens in this inner orbital. For heavy molecules, like I2, 50-fs pulses can provide good resolution in the study based on vibrational motions, however, for light molecules, like H2, sub 10-fs pulses are needed. To obtain 10-fs pulses, we study white-light supercontinuum generation via filamentation in a gaseous medium SF6. A filament is formed at a pressure of 1 atm with an input power as low as 350 mW. The spectrum is widely broadened which is capable of producing sub 10-fs pulses