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Journal of Physics B Atomic Molecular and Optical Physics

Ionization of calcium atoms through resonant laser pumping of3P metastable states

Muhammad Arshad Khan, E.E. Khawaja, H.A. Al-Juwair

Published Sep 28, 1990• 18 citations
Publisher

Abstract

Ionization of calcium atoms through resonant laser pumping of the 4s4p 3 P 1 metastable state is investigated. The ionization yield is measured as a function of laser power, oven temperature and vapour density. The small bias field between the collector electrodes is observed to have a definite role in initial 'seeding' and 'priming' of the medium prior to the arrival of the laser pulse in that it accelerates some of the thermionically emitted electrons through the vapour.

DOI
10.1088/0953-4075/23/18/004

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Ionization of laser-excited strontium vapor: The dynamics of superelastic collisions with metastable populations

Catherine Bréchignac, Ph. Cahuzac, Anne Débarre

We report on the observation and absolute measurements of the population dynamics for several Sr i and Sr ii levels when the vapor is illuminated by a short laser pulse. When the laser beam excites the 5s5p ${}^{1}$${P}_{1}$ resonance level, we show that a pool of metastable population is created, allowing an efficient heating of the medium by superelastic collisions with seed electrons, and a strong final ionization is achieved. When the laser source excites the 5s10s 7thinsp${;}^{1}$${S}_{0}$ Rydberg level, superelastic collisions do not occur and a drastic change in the population behavior is observed. In both cases the dynamics of the energy transfer towards the plasma is detailed.

Physical Review A•May 1, 1985•11 citations
Ionization of calcium atoms through resonant laser pumping of the 4s4p 3 P 1 metastable state is investigated. The ionization yield is measured as a function of laser power, oven temperature and vapour density. The small bias field between the collector electrodes is observed to have a definite role in initial 'seeding' and 'priming' of the medium prior to the arrival of the laser pulse in that it accelerates some of the thermionically emitted electrons through the vapour.
The ionization of calcium atoms resonantly excited to the 4s4p 3 P 1 metastable state is investigated. In particular, the influence of various experimental parameters on the total ionization yield and the rate of its growth are reported. Maximum ionization yield was obtained with the buffer gas (Ar) pressure in the range 100-650 mbar. Furthermore, the time history of depopulation of the metastable state 4s4p 3 P 1 monitored in a 'pump' and 'probe' experiment using a second dye laser tuned to the 4s4p 3 P 1 -4s5s 3 S 1 transition is presented. The effective lifetime of the 4s4p 3 P 1 state is shortened to 31+or-2 mu s at 850 degrees C and 800 mbar Ar pressure from the known value of about 350 mu s.
Experimental results on the ionization of calcium atoms following resonant pumping of the 4s 2 1 S 0 -4s4p 3 P 1 transition with a CW ring dye laser are reported. Absorption profiles of the 4s 2 1 S 0 -4s4p 3 P 1 transition around 6572.78 AA are measured under different conditions of temperature (T) and buffer gas pressure (p) where ionization was observed. Voigt profile fits are seen to adequately describe the observed absorption profiles. The results on collisional widths are in agreement with some previously published measurements. At low buffer gas pressures, collisional widths are found to scale as T 0.5 .
Experimental results are reported where the rates of collisional energy transfer in optically prepared atomic metastable states are considerable compared with the rate of normal radiative decay. A remarkable consequence is a large increase in the effective lifetime of some excited states lying above the optically prepared state. The specific case of calcium is presented here as an example where over a 1000-fold increase in the lifetime of 4s3d 3 D states has been experimentally measured following pulsed-laser pumping of 4s4p 3 P 1 metastable state. This can have important applications in the development of quasi-CW lasers. The possibility of populating 4s4p 1 P 1 and 4s5s 3 S 1 states through energy pooling collisions is also investigated. The rate coefficient for pooling to the 4s5s 3 S 1 state is estimated to be 10 -8 -10 -9 cm 3 s -1 .
An experiment is described in which large numbers ( approximately 10 16 ) of calcium atoms are placed in the 4s4p 3 P metastable level by optical pumping with a tunable dye laser and then studied spectroscopically. Equations governing the degree of excitation by the laser are derived. Predictions of the theory are shown to be in good agreement with experimental results. In particular, at low optical depths produced by 10 15 ground state atoms cm -2 the dye laser raises approximately 50% of the calcium atoms into the metastable state. At higher optical depths associated with 10 17 ground state atoms cm -2 the number of excited states is limited only by the laser energy and line width. For the dye laser used the largest number of metastables produced was approximately 10 16 . The absorption spectra of the 4s4p 2 P level is seen out to 4s4p 3 P to 4s22d 3 D. The relaxation rate between the excited state fine structure is studied directly with an optically delayed dye laser continuum. Other applications of this technique of producing large numbers of excited states are briefly discussed.
Using a pulsed tunable dye laser pumped by a nitrogen laser, the 4s4p 1 P 1 0 level in Ca I was selectively populated from the 4s 2 1 S 0 ground state. Transitions from the 4s4p 1 P 1 0 level to 4sns 1 S 0 and 4snd 1 D 2 and, for n approximately=20, also to 4snp 1 P 1 0 and 4snf 1 F 3 0 were observed. The latter transitions arise because of Stark mixing of Rydberg states in the field of laser-produced ions and electrons. By applying a voltage to electrodes in the calcium vapour, Stark shifts were measured for levels belonging to the 4snf 1 F 3 0 and 4s(n+1)d 1 D 2 series in a region where the series cross. Diagonalisation of the energy matrix for interacting states with hydrogenic dipole matrix elements gave theoretical Stark shifts in reasonable agreement with those measured. The 4snf 1 F 3 0 series has been extended from n=12 to n=28 by absorption from 4s3d 1 D 2 . The latter level was found to be populated by stimulated emission from 4s4p 1 P 1 0 .
We report on the observation and absolute measurements of the population dynamics for several Sr i and Sr ii levels when the vapor is illuminated by a short laser pulse. When the laser beam excites the 5s5p 1{}^{1}1P1{P}_{1}P1​ resonance level, we show that a pool of metastable population is created, allowing an efficient heating of the medium by superelastic collisions with seed electrons, and a strong final ionization is achieved. When the laser source excites the 5s10s 7thinsp;1{;}^{1};1S0{S}_{0}S0​ Rydberg level, superelastic collisions do not occur and a drastic change in the population behavior is observed. In both cases the dynamics of the energy transfer towards the plasma is detailed.