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Journal of Physics B Atomic Molecular and Optical Physics

Atomic population redistribution in a dense Ga vapour proceeding via energy pooling ionization induced by resonant laser-assisted collisions

S Barsanti, Paola Bicchi

Published Oct 28, 2002• 1 citation
Publisher

Abstract

In this paper we report on the atomic population redistribution originating from the ionization that takes place in a dense Ga vapour kept in quartz cells and resonantly excited by laser radiation, in the collisions between two excited atoms. This ionization process is known as energy-pooling ionization (EPI). The electron/ion recombination that takes place in the low density plasma produced gives rise to population in the atomic Rydberg levels and from the latter via cascade transitions to lower lying ones. We have monitored the fluorescences relative to the radiative emissions from such levels, namely those corresponding to the n P → 5S 1/2 series, with 9 ≤ n ≤ 26, and the 4D → 4P 1/2,3/2 transitions. Their characteristics testify to their origin as being due to the EPI process. Further confirmation is obtained by performing a time-resolved analysis of such fluorescences, whose appearance and time evolution is strongly influenced by the dynamics of the process. The effect of the introduction of a few Torr of buffer gas inside the quartz cell, resulting in the quenching of all the fluorescences for n ≥ 12, is also discussed.

DOI
10.1088/0953-4075/35/21/316

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Physical Review A•Oct 21, 2004•105 citations
In this paper we report on the atomic population redistribution originating from the ionization that takes place in a dense Ga vapour kept in quartz cells and resonantly excited by laser radiation, in the collisions between two excited atoms. This ionization process is known as energy-pooling ionization (EPI). The electron/ion recombination that takes place in the low density plasma produced gives rise to population in the atomic Rydberg levels and from the latter via cascade transitions to lower lying ones. We have monitored the fluorescences relative to the radiative emissions from such levels, namely those corresponding to the n P → 5S 1/2 series, with 9 ≤ n ≤ 26, and the 4D → 4P 1/2,3/2 transitions. Their characteristics testify to their origin as being due to the EPI process. Further confirmation is obtained by performing a time-resolved analysis of such fluorescences, whose appearance and time evolution is strongly influenced by the dynamics of the process. The effect of the introduction of a few Torr of buffer gas inside the quartz cell, resulting in the quenching of all the fluorescences for n ≥ 12, is also discussed.
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 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.
This work illustrates the temporal analysis we have done on the fluorescence signals originating from levels with pecularly different characteristics populated either via Energy Pooling Collision (EPC) or via Energy Pooling Ionization (EPI). The fluorescence from the first excited level (S ia) gives informations about the limited radiation trapping due the fine structure of the ground state l/2 3/2 the ones from the 1 1P and 5D levels allow to discriminate between the different population mechanism, EPC and EPI respectively. The cross sections for the two different processes may also be obtained.
Energy-pooling collisions between low-lying excited states of Ba have been studied. We have used a CW dye laser to excite the level in Ba. Radiative decay and collisions with Ba or buffer-gas atoms (Ar, He) populate also the , and metastable levels. The spatial density distributions of these excited levels were measured by the absorption of lines from a Ba hollow-cathode lamp as a function of laser frequency. When the laser was on the exact atomic resonance we observed fluorescence from the , , , , , , and states populated by energy-pooling collisions of Ba( , ). By changing the buffer gas pressure the fluorescence intensity ratio between - and - transitions changes dramatically. This is found to be due to redistribution of population among metastable levels, both in magnitude and space. In addition, we have found that the and metastable levels are also populated when the laser is tuned to the blue from the 553.5 nm line under which conditions a pronounced red-shifted light cone (cone emission) is observed.
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.
Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as $100\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high nnn states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.