Doctor of Philosophy / Department of Physics / Carlos A. Trallero / Ultra-fast spectroscopy has become a common tool for understanding the structure and dynamics of atoms and molecules, as evidenced by the award of the 1999 Nobel Prize in Chemistry to Ahmed H. Zewail for his pioneering work in femtochemistry. The use of shorter and more energetic laser pulses have given rise to high intensity table-top light sources in the visible and infrared which have pushed spectroscopic measurements of atomic and molecular systems into the strong-field limit. Within this limit, there are unique phenomena that are still not well understood. Many of such phenomena involve a photoionization step. For three decades, there has been a steady investigation of the single ionization of atomic systems in the strong-field regime both experimentally and theoretically. The investigation of the ionization of more complex molecular systems is of great interest presently and will help with the understanding of ultra-fast spectroscopy as a whole. In this thesis, we explore the single ionization of molecules in the presence of a strong electric field. In particular, we study molecular isomer pairs, molecules that are the same elementally, but different structurally. The main goal of this work is to compare the ionization yields of these similar molecular pairs as a function of intensity and gain some insight into what differences caused by their structure contribute to how they ionize in the strong-field limit. Through our studies we explore a wavelength dependence of the photoionization yield in order to move from the multi-photon regime of ionization to the tunneling regime with increasing wavelength. Also, in contrast to our strong-field studies, we investigate isomeric molecules in the weak-field limit through single photon absorption by measuring the total ionization yield as a function of photon energy. Our findings shed light on the complexities of photoionization in both the strong- and weak-field limits and will serve as examples for the continued understanding of single ionization both experimentally and theoretically.

A comparison of the composition and structure of carbon films deposited by ns and fs laser ablation of graphite is the subject of this thesis. In addition, the effect of irradiation on the surface of graphite has been investigated in detail. Laser-induced phase transitions from graphite to sp-bonded carbon and trans-polyacetylene chains as well as the formation of nano-diamond have been observed after irradiation with fs pulses. An optical orientation mechanism involving both electric and magnetic interactions is proposed to understand the formation of nano-stripes and other structures on irradiated graphite surfaces. These phenomena are not observed after nanosecond laser irradiation. Tetrahedral carbon (ta-C) films deposited at cryogenic temperatures using ns laser radiation consist of sub-micron graphitized grains embedded in a matrix of sp3-hybridized bonded carbon. Nano-buckling is evident in ta-C films deposited by fs ablation where the composition is found to consist of mixed sp, sp2, and sp3 – hybridized carbons species. It is found that the concentration of sp-bonded chains is negligible in ns-C films. Surface enhanced Raman spectroscopy has been used to characterize molecular species in ns and fs carbon films. Time of flight mass spectroscopy has been used to study plume species produced by laser ablation. It is also found that polyyne molecules can be formed by fs laser dissociation of small molecules in organic solvents. This process is accompanied by the deposition of hexagonal nano-diamond films on substrates placed near the laser focus during irradiation. This opens a new path in the synthesis of 1D conducting molecules and nano-diamond materials for nano-science applications. Quantum chemical calculations involving density functional theory (Gaussian '03) have been carried out in support of this work and have been used to study Raman and IR vibrational modes of several novel carbon molecules synthesized in ta-C films and in the liquid phase. These studies have been extended to assist in the identification of astronomical spectra.

A comparison of the composition and structure of carbon films deposited by ns and fs laser ablation of graphite is the subject of this thesis. In addition, the effect of irradiation on the surface of graphite has been investigated in detail. Laser-induced phase transitions from graphite to sp-bonded carbon and trans-polyacetylene chains as well as the formation of nano-diamond have been observed after irradiation with fs pulses. An optical orientation mechanism involving both electric and magnetic interactions is proposed to understand the formation of nano-stripes and other structures on irradiated graphite surfaces. These phenomena are not observed after nanosecond laser irradiation. Tetrahedral carbon (ta-C) films deposited at cryogenic temperatures using ns laser radiation consist of sub-micron graphitized grains embedded in a matrix of sp3-hybridized bonded carbon. Nano-buckling is evident in ta-C films deposited by fs ablation where the composition is found to consist of mixed sp, sp2, and sp3 – hybridized carbons species. It is found that the concentration of sp-bonded chains is negligible in ns-C films. Surface enhanced Raman spectroscopy has been used to characterize molecular species in ns and fs carbon films. Time of flight mass spectroscopy has been used to study plume species produced by laser ablation. It is also found that polyyne molecules can be formed by fs laser dissociation of small molecules in organic solvents. This process is accompanied by the deposition of hexagonal nano-diamond films on substrates placed near the laser focus during irradiation. This opens a new path in the synthesis of 1D conducting molecules and nano-diamond materials for nano-science applications. Quantum chemical calculations involving density functional theory (Gaussian '03) have been carried out in support of this work and have been used to study Raman and IR vibrational modes of several novel carbon molecules synthesized in ta-C films and in the liquid phase. These studies have been extended to assist in the identification of astronomical spectra.

The mechanochemical polycondensation between a diamine and a dialdehyde constitutes a sustainable alternative to classical solvent-based polymerization reactions. This process not only allows for a higher conversion and a shorter reaction time as compared to standard solvent-based syntheses of this conjugated polymer, but the reaction can also be adjusted by the energy introduced via the ball mill.

The mechanochemical polycondensation between a diamine and a dialdehyde constitutes a sustainable alternative to classical solvent-based polymerization reactions. This process not only allows for a higher conversion and a shorter reaction time as compared to standard solvent-based syntheses of this conjugated polymer, but the reaction can also be adjusted by the energy introduced via the ball mill.