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.