Nonadiabatic Molecular Alignment Measurements via Photoelectron Ionization Yields and White-Light Generation

Abstract

This dissertation reports a study of nonadiabatic (field-free) molecular alignment for linear molecules. The measurements were performed by measuring photoelectron ionization yields and white-light generation. In the first part of the dissertation, the dynamics of rotational wave packets of the molecules created by a linearly and/or circularly polarized pump pulse were measured by the electron photoionization yields that are produced by a delayed femtosecond probe beam. The photoelectron yields were measured as a function of the linearly polarized probe pulse delay for linearly and/or circularly polarized pump pulses, and revivals of the rotational wave packet were observed in N2, O2, CO2, CO, and C2H2 gases. The measured revival structures were compared to the quantum mechanically calculated time dependency of molecular alignment ((cos2θ)) parameter after the aligning (pump) pulse. The rotational constants of the molecules were also obtained by fitting the theoretically calculated alignment parameter to the measured data. The measured revival structures and rotational constants inferred from the measured data are in good agreement with the calculated results. The second part of the dissertation focuses on evolution of nonadibatic molecular alignment in nitrogen by measuring white-light generation. Again a linearly polarized pump pulse produced a molecular alignment which was measured via its nonlinear interaction effects by a variably delayed filament-producing probe pulse. The induced rotational wave packet was mapped as a function of the angular orientation difference between the polarization directions of the femtosecond pump and probe pulses. The experimental results from mapping rotational wave packets were compared with quantum mechanically calculated time dependency of molecular alignment ((cos2θ)) parameter after the pump pulse, which well reproduce all the major observed features.