Optical Response and Control of Molecular Systems.

Abstract

This thesis is comprised of three major parts and is concerned with the theoretical characterization of condensed phase systems within the framework of nonlinear spectroscopy experiments, using both analytical models and numerical approximation schemes.

The first part focuses on the chirped-pulse mediated coherent control of electronic population transfer, and investigates the plausibility of control in the presence of pure electronic dephasing. The molecular system is described by a same-frequency shifted harmonic oscillator model, and population transfer was computed using split-operator and direct diagonalization schemes. Dephasing effects were incorporated using a stochastic model that is able to interpolate between the homogeneous and inhomogeneous limits, and results with and without dephasing were compared as functions of the linear chirp parameter and the field intensity. The numerical findings were compared to and found to be consistent with several experimental studies performed on the laser dye LD690 in liquid methanol.

The second part is a comparative study of several approximation methods used for computing optical response functions, and is illustrated within the context of two-dimensional electronic spectroscopy. A central theme is the development of a benchmark model that can discriminate between different methods, and consists of a different-frequency shifted harmonic oscillator model. Optical response spectra were computed using four different approximation schemes, which include two distinctly different second-order cumulant approximations, a Linearized Semiclassical method, and a Forward-Backward Semiclassical method. Comparing the spectra as a function of temperature and the oscillator frequency ratio assessed the accuracy and robustness of the methods.

The final part concerned a method for computing ab initio optical response tensors in the context of two-dimensional infrared spectroscopy, and was a collaborative effort between the Geva and Kubarych groups. An excitonic Hamiltonian was used to model the photo-active modes of a vibrational system, and a direct diagonalization procedure, which utilized inputs from electronic structure calculations, was used to compute the spectra. Preliminary results for the four-mode system Mn(CO)5 are presented, and the methodology developed here was later continued and extended by other members of the collaboration.