Nonresonant Multiphoton Ionization of Alkaline Earth Atoms in Intense Laser Fields.

When an atomic beam interacts with a high intensity laser beam, ionization occurs even if the photon energy is less than the threshold level for ionization of the atom. In this thesis, nonresonant multiphoton ionization of alkaline-earth atoms is examined experimentally. Since alkaline-earth atoms have doubly excited states just above and sometimes below the first ionization threshold level and are easily ionized with relatively low intensity, they are ideally suited for studying the role of electron correlations in multiphoton ionization. This motivated a careful and detailed investigation of the correlation of electrons and the multiphoton ionization mechanism. A spectroscopic study on photoelectrons arising from nonresonant multiphoton ionization of calcium and magnesium atoms up to 10$\sp{13}W$/$cm\sp2$ is described. The emphasis is placed on photoelectron spectra, and their variations with laser wavelength, intensity, and polarization. Also, the ionization process of doubly charged ions which can be produced either by a stepwise process or by the simultaneous removal of two electrons is discussed