Date of Completion

3-4-2014

Embargo Period

3-2-2014

Keywords

physics, strong-field physics, molecular physics, ultrafast laser, molecular ionization, nonlinear optics, white-light supercontinuum generation, filamentation, ultrafast phenomena, lasers

Major Advisor

George Gibson

Associate Advisor

Phillip Gould

Associate Advisor

Juha Javanainen

Field of Study

Physics

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

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.

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