Theoretical Studies of Laser-Molecule Interactions.

In this work, two projects under the broad umbrella of laser-molecule interaction studies are presented. The first project is a classical analysis followed by a semiclassical quantization of a diatomic molecule irradiated by a continuous wave (cw) infrared (IR) laser. The second project is a theoretical study of a diatomic irradiated by a picosecond IR pulse train. Both projects help elucidate the dynamics of the energy acquisition process. The diatomic coupled to a cw laser study uses a Birkhoff-Gustavson normal form (BGNF) analysis to semiclassically quantize the conservative Hamiltonian for the full system. In transforming the Hamiltonian into second order normal form in order to apply the BGNF algorithm, a novel approach for preserving the nonlinearity of the molecular oscillator is presented. The power series expansion is truncated at fourth order. The classical Chirikov resonance behavior of the system is lost in the transformation. Quantum Rabi oscillations in the molecular action are seen as result of the BGNF quantization which agree with the calculated Rabi period based on a quantum mechanical two state analysis. In the diatomic irradiated by a picosecond IR pulse train study the field is modulated in a novel approach for overcoming molecular anharmonicities and possibly intramolecular vibrational energy relaxation (IVR). By appropriately chosing the pulse spacing in the train, the field appears as a series of sidebands that correspond to a series of vibrational transitions of the diatomic molecule. The availability of many molecular transition frequencies allows the molecule to achieve ultrahigh vibration population inversion. For example, with the proper choice of initial conditions the probability of being in the seventh excited vibrational state of the molecule is greater than 0.7 in less than 20 picoseconds. Classical, resonance, and time -dependent quantum analyses are presented to explain and interpret the dynamics. Also, a quantum dressed state analysis that is equivalent to a Floquet Hamiltonian approach is presented. The pulse train parameters are optimized, and an explanation of the combination of multiphoton versus sequential one photon events that occur is included. The two most significant results of this work are (1) the extreme vibrational excitation achieved in the diatomic-picosecond IR pulse train study, and (2) the recovery of the correct Rabi oscillations from a classical analysis of the HF-cw IR laser system. The extreme vibrational excitation with an IR laser has never been achieved before, either experimentally or theoretically. This is also the first study to recover the correct quantum Rabi oscillations from a classical analysis of the system.