Probing the Dynamics of Atoms and Molecules by High Harmonic Generation

Abstract

In this dissertation, we have observed the phase shift of high harmonic generation (HHG) from atomic and molecular target gases by using the Gouy phase interferometric technique. The aim of this thesis is to probe the molecular structure and dynamics from the phase of emitted HHG radiation. The work mainly divided into three parts. The first part is on designing and implementing the Advanced Gouy Phase Interferometer. It includes the detail about the design and instrumentation that has been improvised and modifed from the prototype version. The interferometer consists of two thin gas jets separable in space along the laser propagation direction in single laser focus. The intensity of high harmonic generated pulses is modulated when the separation between two gas jets is increased. This modulation is observed due to the Gouy phase shift (which depends on the longitudinal position within the Gaussian beam) of the driving laser beam experienced by the two gas jets. The Gouy phase di erence imparts a delay in HHG photon emission time. The fringes visibility or temporal coherence of several consecutive harmonic orders as a function of gas jets separation is observed with atomic Ar and molecular H2 target gases. The temporal coherence properties is observed with 9 fs and 50 fs driving laser pulses. This observation of temporal coherence of the harmonics with multi-cycle driving laser pulses indicates the broader usefulness of this interferometric technique. The possible applications of this interferometer such as HHG yield enhancement, selective control of particular or range of harmonic order from the typical HHG spectrum and extracting the relative high harmonic phase shift between two gases are discussed. In the second part, we report on extracting the relative phase shift of HHG emissions by using the advanced Gouy phase interferometric technique. As a part of this, first we measured the relative phase di erence from the simplest molecular isotopes in nature i.e., hydrogen (H2) and deuterium (D2). The experimental results are presented with 9 fs and 50 fs laser pulses. The results show that the relative phase shift between these isotopes increases monotonically with the harmonic order. We have observed about 180 +/- 8 mrad phase shift for harmonic order 23 (H23) and it increases to 230 +/- 26 mrad for H35 with 9 fs pulses. The experimental results agree well with the simulation results obtained by solving the non-Born-Oppenheimer time dependent Schrodinger equation. Detailed analysis of the theoretical modelling uncover the underlying mechanisms of electrons and nuclei responsible for the measured phase di erence. The results indicate that the electron-ion Coulomb interaction during the electron acceleration in the continuum as well as the bound electron interaction with the ionized electron play the role on the observed phase shift. The phase shift measured with 50 fs pulses is 225 +/- 10 mrad for H23 and 340 +/- 38 mrad for H35. We anticipated that the contribution of high harmonic generated by the electron return to the higher excited 2p u state of H+2/D+2 molecular ion along with the ground 1s g state may lead to the larger phase shift with 50 fs pulses compared to the 9 fs pulses. The final experimental result presents the HHG phase shift measurement from methane isotopes i.e., methane (CH4) and deuterated methane (CD4). The observed phase shift as a function of harmonic order increases at a greater rate for methane isotopes compared to the hydrogen isotopes. The measured phase shift is 95 6 mrad for H23 and increases to 460 83 mrad for H31. Theoretical analysis predicted that this phase shift comes from the di erence in field free nuclear dynamics of CH+4 and CD+4 ions.