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Investigation of photoelectrons from molecules in a strong field

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Title: Investigation of photoelectrons from molecules in a strong field
Authors: Darios, Delphine
Item Type: Thesis or dissertation
Abstract: When intense and few-cycle laser pulses interact with an atomic/molecular beam highorder Above Threshold Ionisation (HATI) can take place. The resulting electrons can be re-scattered from the parent atoms/molecules and gain kinetic energy. The HATI electrons contain information about the atomic/molecular structure thus providing a method to probe atomic and molecular dynamics with sub-fs temporal and sub-angström spatial resolution. In this thesis the development of a Velocity Map Imaging apparatus capable of measuring the two-dimensional (2D) projection of the velocity distribution of electrons with energies up to 400 eV is described. This device was implemented with a molecular beam apparatus to study the electron re-scattering process that occurs when atoms/molecules are subjected to strong laser fields. Time-of-fight measurements were carried out to find the molecular beam. To perform the experiments a method to generate intense and few-cycle pulses based on hollow fibre pulse compression has been implemented. Pulses of 14 fs with energies of 500 μJ have been measured in a differentially pumped fibre set-up, with input pulses of 42 fs and 700 μJ using a home-made Frequency-Resolved Optical Gating device. The performance of the VMI apparatus was investigated by first studying the ATI rings formed by low energy electrons. Then, a study of the high energy electrons was carried out in different gases and re-scattered electrons with energies up to 100 eV were measured. The photoelectron spectra recorded with linearly polarised laser exhibit a plateau with a cut-off at 10 UP that is a characteristic of the re-scattering process. The observation of rescattered electrons was confirmed by two techniques: comparison of the data obtained with vertical polarisation (re-scattering) and circular polarisation (no re-scattering) and analysis of the structure in the angular distribution obtained in Xenon.
Issue Date: Jan-2010
Date Awarded: Feb-2010
URI: http://hdl.handle.net/10044/1/5541
DOI: https://doi.org/10.25560/5541
Supervisor: Marangos, Jonathan
Author: Darios, Delphine
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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