Understanding and controlling laser-matter interactions: From solvated dye molecules to polyatomic molecules in gas phase.

摘要

The goal of my research is to obtain a better understanding of the various processes that occur during and following laser-matter interactions from both the physical and chemical point of view. In particular I focused my research on understanding two very important aspects of laser-matter interaction; 1) Intense laser-matter interactions for polyatomic molecules in the gas phase in order to determine to what extent processes like excitation, ionization and fragmentation can be controlled by modifying the phase and amplitude of the laser field according to the timescales for electronic, vibrational and rotational energy transfer. 2) Developing pulse shaping based single beam methods aimed at studying solvated molecules in order to elucidate processes like inhomogeneous broadening, solvatochromic shift and to determine the electronic coherence lifetimes of solvated molecules.;The effect of the chirped femtosecond pulses on fluorescence and stimulated emission from solvated dye molecules was studied and it was observed that the overall effect depends quadratically on pulse energy, even where excitation probabilities range from 0.02 to 5%, in the so-called "linear excitation regime". The shape of the chirp dependence is found to be independent of the energy of the pulse. It was found that the chirp dependence reveals dynamics related to solvent rearrangement following excitation and also depends on electronic relaxation of the chromophore. Furthermore, the chirped pulses were found to be extremely sensitive to solvent environment and that the complementary phases having the opposite sign provide information about the electronic coherence lifetimes. Similar to chirped pulses, the effects of a phase step on the excitation spectrum and the corresponding changes to the stimulated emission spectrum were also studied and it was found that the coherent feature on the spectrum is sensitive to the dephasing time of the system. Therefore a single phase scanning method can provide the fundamental information regarding solvated species which could be eventually used to interrogate single molecules under a microscope.;I also investigated the dynamics and control of large aromatic molecules following ionization with an intense laser pulse in order to understand the mechanism of ionization and further excitation to cationic excited states. The strong-field photofragmentation of a large family of substituted aromatic ketone molecules was explored. The results are consistent with single electron tunnel-ionization leaving the cation with little internal energy in the remaining laser field. In the presence of electronic resonance with the excitation field, subsequent fragmentation takes place. Advanced ab initio electronic calculations confirm our observations that similarly consider the initial point to involve a molecule in its ground state configuration that suddenly loses an electron. This study serves to provide a model for the behavior of polyatomic molecules under strong fields that is consistent with a 'sacrificial electron' that takes most of the energy of the field as it promptly leaves the molecule. I also investigated the dynamic behavior of a symmetric organic molecule known to undergo reverse Diels Alder reaction following strong field ionization and found that the molecular ion has signatures of vibrational coherence that corresponds to a C-C Raman stretching mode in the neutral connecting the two rings. These kinds of studies could help us to understand the electron delocalization in a complex gas phase molecule following excitation and devise novel control schemes for studying important reactions under strong fields.