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AIP conference proceedings

Energy-pooling ionization produced by the collisions of In atoms in presence of a resonant laser field

Paola Bicchi, C. Marinelli, E. Mariotti, M. Meucci, L. Moi

Published Jan 1, 1997• 0 citations
Publisher

Abstract

We report the production of electrons and ions in the collisions of In(62S1/2) atoms resonantly excited by laser radiation. The cross section for the ions production is reported.
DOI
10.1063/1.51842

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The quantitative investigation of the energy pooling collision (EPC) process between resonantly laser excited In atoms is reported. The process is studied in a laser induced fluorescence (LIF) experiment where the nP populations, with n=11, 10 and 9, are monitored. The population mechanism for these levels is verified via a temporal analysis of the fluorescence signals. This is possible in In due to the strongly reduced self trapping of the resonant radiation. The cross section for energy pooling collision to these nP levels has been derived by considering the Rydberg character of these levels.
Merging two subfields, collisions and spectroscopy, this book presents the latest research proposals, innovative experiments and fresh ideas. Topics considered include radiative collision induced electron continuum-continuum scattering; models of laser-induced chemical reactions; experimental studies of laser-induced ionizing collisions; laser photodissociation of Na/sub 2/ + ions produced by laser resonant ionization of Na vapor; steady-state electron-atom collision in a laser field; conditions for the observation of the predicted splitting of the laser assisted electron impact of the differential cross section of atoms; transitions between resonances; some proposals for experiments; above-threshold multiphoton ionization; observability of optical collisions in the frequency and the time domain; laser switched collisions - a diagrammatical approach; polarization of atomic fluorescence excited by molecular photodissociation; multiphoton selective excitation; on the role of chaotic dynamics with many basins of attraction; predissociation in a strong electromagnetic field; theory of double resonance; and coherent excitation of polyatomic molecules up to highly excited vibrational states by laser light.
The production of Rydberg atoms via collisions in a resonantly laser excited dense In vapour is reported. When a dense In vapour confined in a quartz cell, whose temperature is raised above 820 °C, is excited by 100 kW cm−2 laser pulses at 410.3 nm, resonant with the fundamental transition 5 2P1/2 → 6 2S1/2, the fluorescence spectrum shows radiative emissions from Rydberg states, namely those corresponding to the n 2P → 6 2S1/2 series with 9 ≤ n ≤ 23. Fluorescences corresponding to the 5 2D → 5 2P1/2,3/2 transitions are also observed. Analysis of these signals' intensity as a function of several different experimental parameters, such as the atomic density and the laser power density, indicates that the population in the Rydberg levels is produced mainly by energy pooling collisions (EPC). The EPC cross section for the population of the Rydberg levels is also given. The influence on these effects of the presence of a fine structure in the In ground state (5 2P1/2,3/2) is also discussed, as well as the presence of a buffer gas inside the quartz cell.
We observe the production of Na2+ and Na+ arising from single collisions between crossed beams of sodium atoms when a laser field is tuned near the Na(3p 2P3/2) and Na(3p 2P1/2) transitions. Measurements of ion intensity vs laser intensity show that at moderately high power true laser-induced processes dominate over purely collisional effects. Relative intensity of mass-selected ions produced at either member of the Na resonance doublet shows conclusively that Na+ does not arise simply from photodissociation of Na2+ but must result from a direct, laser-induced collisional ionization.
We report on the electronic detection of the energy-pooling ionization (EPI) process in resonantly laser-excited gallium vapour. The formation of electrons and ions in the collision of two Ga atoms excited to the 5 2 S 1/2 level is analysed as a function of several different parameters such as the laser power density, the atomic density and the temperature. The influence of the strong chemical reactivity of Ga at high temperatures on the process is also evidenced. Finally the cross section for the EPI process is derived from the electronic signals detected. It turns out to be (1.1±0.5)×10 -15 cm 2 , a large value that states EPI as a very efficient process in Ga vapour.
The recently observed efficient ionization of sodium vapor by a laser tuned to the sodium resonance line can be explained in terms of the large population of resonance-state atoms created by laser-induced radiative equilibrium. This large population of excited atoms represents (a) a reservoir of energy that can rapidly be transferred to the free electrons via superelastic collisions and (b) a source of pseudo-ground- state atoms possessing a much reduced ionization energy. Three-photon ionization is proposed as the mechanism for creation of the primary pool of free electrons.