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Journal of Physics B Atomic Molecular and Optical Physics

Multiphoton double ionization of barium with intense picosecond pulses

Duncan Tate, Dimitrios G. Papaioannou, Thomas F. Gallagher

Published Apr 28, 1991• 13 citations
Publisher

Abstract

The authors have observed the production of Ba2+ from neutral Ba by intense picosecond laser pulses in the range 560-650 nm. Several new features have been observed, which were not observed with nanosecond pulses. Most of the strong resonances in Ba2+ production can be attributed to Ba+ transitions originating in the 6s1/2 or 5dj states, suggesting that the dominant process in double ionization in this intensity and wavelength regime is sequential removal of electrons. However, there are several resonances which cannot be attributed in this manner, evidence for possible alternative ionization mechanisms. Ba2+ production is strong throughout the region studied, at times comparable to that of Ba+. The authors have also looked for Ba2+ production in the range 280-325 nm, but no Ba2+ signal was observed in this part of the ultraviolet.

DOI
10.1088/0953-4075/24/8/013

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The authors have observed the production of Ba2+ from neutral Ba by intense picosecond laser pulses in the range 560-650 nm. Several new features have been observed, which were not observed with nanosecond pulses. Most of the strong resonances in Ba2+ production can be attributed to Ba+ transitions originating in the 6s1/2 or 5dj states, suggesting that the dominant process in double ionization in this intensity and wavelength regime is sequential removal of electrons. However, there are several resonances which cannot be attributed in this manner, evidence for possible alternative ionization mechanisms. Ba2+ production is strong throughout the region studied, at times comparable to that of Ba+. The authors have also looked for Ba2+ production in the range 280-325 nm, but no Ba2+ signal was observed in this part of the ultraviolet.
In an effort to gain more insight into the process of multiphoton multiple ionization, several experiments have recently been carried out to determine the wavelength dependence of the double ionization of Sr and Ba. 1-3 These studies were not as conclusive as one might hope, in that the observed resonances in the production of Sr++ and Ba++ did not quite match the energy levels of Sr+ and Ba+. Here we report experiments in which we have compared the spectra for the production of Ba by an intense laser with and without the population of specific states of Ba by modest secondary lasers. The results of these experiments show clearly that Ba++ is produced by first producing real intermediate states of Ba+ which are multiphoton ionized through intermediate resonances in Ba+.
Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm -2 . Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba + . The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 10 13 atoms/cm 3 ) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2 P 1/2,3/2 ), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2 P 3/2 state relative to the 6p 2 P 1/2 state.
We have investigated multiphoton ionization of Ba 6snl6snl6snl Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the 6sns6sns6sns and 6snd6snd6snd 15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of Ba+{\mathrm{Ba}}^{+}Ba+ Rydberg states and Ba2+{\mathrm{Ba}}^{2+}Ba2+ photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto Ba+{\mathrm{Ba}}^{+}Ba+ Rydberg states.
The authors have studied double ionization of Ba 6snl Rydberg states through resonant two-photon excitation of 8snl double-Rydberg states with n=13 and 14 and different l. The laser intensity dependence of Ba 2+ yield measured for l>or=8, when the electrons suffer long-range dipole interaction, shows a decrease from four-photon for l=8 to less than three for l=11. The four-photon dependence is due to autoionization decay populating the Ba + (7p) levels which gives an indirect signature to the Ba 2+ production. By increasing l this decay vanishes and the data show evidence of a direct double-ionization process via planetary-electron states for l>or=10. Additional intensity measurements performed for low l=6 states strongly mixed with 5fn'l' ones by short-range multipole interaction, reveal indirect Ba 2+ production and an individual case of partial stabilization due to destructive interference leading to direct double ionization.
Despite inherently complex multiphoton dynamics, our observations show that Ba double ionization with an infrared laser (8800--8920 cm${}^{\ensuremath{-}1}$) resembles a single-photon process; namely, its probability is proportional to the laser intensity. In this regime, single-electron ionization is due to a four-photon resonant transition through the highly perturbed state 6p21D26{p}^{2}\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}6p21D2​, whereas double ionization is realized by the two-electron mechanism. Furthermore, we argue that these conclusions are valid for other alkaline-earth-metal atoms and other parameters of laser radiation.