International Workshop on Atomic Interactions in Laser Fields - Abstracts
G. Pichler
Institute of Physics, P.O.Box 304, HR-10000 Zagreb, Croatia
(e-mail: pichler@ifs.hr)
We have recently shown that in pure lithium glow discharge a transition from a diffuse to a constricted plasma column occurs spontaneously when lithium particle density increases [1]. The discharge in lithium vapor glows in the diffuse mode at lower metal vapor densities, and at increasing temperatures and higher lithium atom concentrations it transits into the constricted plasma mode, in which a very effective volume recombination process occurs. This narrow cylinder in which positive column glows is a very interesting plasma medium of considerable interest for various applications.
In dense lithium vapor the positive glow discharge can be laser ignited at voltages below usual breakdown voltage when using resonance, qusiresonance or two photon laser excitation, by which preionization pathway is formed between the electrodes inserted into the heat-pipe oven. When the voltage at the electrodes is below the breakdown voltage for given experimental conditions, the laser excitation at several resonance frequencies may cause multiphoton ionization along the laser beam, which enables the lowering of the breakdown voltage. If the pulsed laser is used the plasma can be ignited in this way (laser ignition), with a subsequent stable plasma glow. There are also such conditions when the electric discharge glows only during the laser excitation or it can change the electric flow from one pair of electrodes to another pair of electrodes (laser switching or laser guiding). In dense lithium vapor the resonance light at 670.8 nm (2p-2s transition) is not easily transmitted along the lengthy path between the electrodes because of large self-absorption. If a tunable laser is available then the tuning into the line wings may increase the effective pathway of this near resonance radiation, which may ionize the medium via three photon ionization process. Much better way to ionize the dense lithium vapor can be realized by using a quasiresonance transition (4d-2p) at 460.3 nm. Neutral lithium vapor can absorb this wavelength within the near resonance conditions in the Li2 B-X molecular band, which collisionally dissociates into 2p and 2s level lithium atoms, and 2p → 4d excitation at 460.3 nm and ionization at the same wavelength is then at resonance. Another very effective way of three photon ionization in dense lithium vapor is via two photon excitation from 2s into 3d level at 639.1 nm. At this frequency there is a resonance Li2 A-X molecular band excitation, which also slightly helps in the three photon ionization of Li2 molecule.
We shall describe our most recent experiments with quasiresonance (460.3 nm) and two-photon (639.1 nm) excitation and will discuss different collision processes leading to ionization. The constriction of the laser induced plasma within the narrow cylinder will also be discussed in terms of volume recombination processes among which dissociative recombination of Li2+ and Li3+ ions will be mentioned. Linear and nonlinear laser spectroscopic diagnostics of the lithium plasma will be discussed.