Ionization of lithium vapor by nanosecond resonant laser pulses tuned to 2S → 2P transition

摘要

A theoretical study is reported for the resonant excitation and ionization of dense lithium vapor induced by nanosecond laser pulses, tuned to the resonance transition 2S–2P. The lithium vapor with density (1014–1016 cm−3) is assumed to be excited and ionized by a laser beam with laser power of (105–106 W cm−2) according to the experimental conditions of Skenderovic et al. (Phys Rev A 62:052707, 2000). The time evolution of electron energy distribution function and the electron density, the population density of the excited states as well as the atomic ion, are solved numerically. The numerical calculations of the electron energy distribution function show that nonequilibrium plasmas are produced in lithium vapor by laser irradiation of the nS–nP resonance line. The electrons in these plasmas are heated by superelastic collisions with atoms in the nP state giving a distribution of electrons in energy that is characterized by a series of spikes at energy separated by the nS–nP transition energy. In addition, the competition between photoionization processes and collisional ionization processes for producing the Li + as well as energy pooling collisions process of the excited lithium atoms plays essential roles in populating the highly excited states. Moreover, the results are found to be consistent with the experimental observations.