Title:

Some problems of the interaction of laser radiation with matter

This thesis describes an investigation of the physical processes which occur when intense laser radiation is focused in a gas to produce ionization and breakdown. The ionization processes are treated numerically. The numerical analysis which is developed in the first part of the thesis is based on a solution of the rate equation of electron concentration when both multiphoton and cascade ionizations are the only active processes when ruby and Nd3+ lasers with short durations produce breakdown in argon at pressure range 8 x 102  105 torr. Solutions of this equation have been developed to provide the threshold intensities as a function of gas pressure. Reasonably good order of magnitude agreement between measured and calculated threshold intensities is obtained for ruby laser. A study is developed of laserinduced gas breakdown through a cascade process in which free electrons gain a constant amount of energy of ( .l eV) at each time step from the electromagnetic field of the laser beam, eventually causing exciting and ionizing collisions with ground state and excited state atoms. An estimate of the photoionization crosssection of the excited helium atoms, when irradiated by ruby, is obtained, where the continuity equations which represent the rate of change of the electron energy distribution function, the excited atoms, and the newlyproduced electrons with zero energy, are solved analytically. This provides an estimate for the three ruby photons photoionization absorption coefficient for helium, and breakdown threshold intensities as a function of pressure. A reasonable agreement for calculated and measured thresholds has been obtained, over the pressure range used in experiments previously carried out in this laboratory, if aberration is present in the optical system used. Improvements in the computing techniques have been introduced which are based on the assumption that electrons gain a variable amount of energy from the field, rather than the fixed amount which is exactly equal to the separation between two steps. Interpolation methods are then used to find the number of electrons at these fixed steps. A reasonable ionization growth is attained, but unexpected oscillation in the energy distribution of electrons is observed. The concept of energy diffusion of electrons in energy space is introduced into the energy gain equation of electrons. Smoothing of the energy distribution function, as well as a large amount of ionization growth, is achieved. Order of magnitude agreement between measured and calculated threshold intensities is obtained for helium irradiated by 40 n sec. ruby radiation at the pressure range 103.2 x 104 torr. An indication of the importance of a stepwise ionization process on the ionization growth and breakdown has been deduced, and the time and excited atom concentration at which this process starts to be significant, has been disclosed. Computations based on the combined effects of 3photon absorption and electron collisional ionization of excited atoms agree with experimental values of breakdown threshold intensity provided the 3photon absorption coefficient B is 1021, corresponding to a photon absorption crosssection o,r1016 cm2.
