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Journal of Physics Conference Series

Collision induced phenomena in the autoionizing spectrum of Ba and Sr

Yacoub Makdisi, Jahja O. Kokaj, K. Afrousheh, Joseph Mathew, N Thomas, Goran Pichler

Published Nov 24, 2014• 1 citation
Publisher

Abstract

Broadening and shift of spectral lines in dense Ba and Sr vapor have been studied below and above their first ionization limits. The latter are well-known autoionizing spectral lines that exhibit many different spectral features. However, at larger pressures of noble gases some previously unobserved or very weak spectral phenomena become visible and their studies might be also linked to energy conversion purposes. Originally, we used nanosecond laser which was scanned in a certain spectral region where the two-photon excitation of autoionization lines occurred. This pulsed laser was overlapped by a mode-locked femtosecond laser and a very sensitive thermionic detection was used to observe autoionizing spectral lines. We also presented our preliminary results with hot strontium vapor excited to the resonance level at 460.7 nm.

DOI
10.1088/1742-6596/548/1/012054

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Broadening and shift of spectral lines in dense Ba and Sr vapor have been studied below and above their first ionization limits. The latter are well-known autoionizing spectral lines that exhibit many different spectral features. However, at larger pressures of noble gases some previously unobserved or very weak spectral phenomena become visible and their studies might be also linked to energy conversion purposes. Originally, we used nanosecond laser which was scanned in a certain spectral region where the two-photon excitation of autoionization lines occurred. This pulsed laser was overlapped by a mode-locked femtosecond laser and a very sensitive thermionic detection was used to observe autoionizing spectral lines. We also presented our preliminary results with hot strontium vapor excited to the resonance level at 460.7 nm.
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 even-parity spectra of strontium and barium are very complex due to strong perturbation by several doubly excited configurations and this also increases discrepancies between theory and experiment in accurate level assignment and energy level and atomic structure data, particularly for the spectra of highly excited atoms and their behaviour in external fields and collisional processes which are of fundamental importance in the emerging applications in physics of cold atoms, quantum computing, precision atomic clocks and in the crossed-field problem. This thesis presents results from an experimental study to improve and extend the previously published observations in the two-photon spectra of the high-lying Rydberg states of SrI and Bal placed under a variety of test conditions such as external fields, collisions etc. Orthogonal excitation of a novel atomic jet in a purpose-built heat-pipe setup using a Xenon Chloride excimer laser pumped tunable narrow bandwidth dye laser and high sensitivity space-charge ionization in thermionic diode with complimentary optical detection was used for measurements. Presented are the two-photon spectra covering the dye laser wavelength (air) range of 460 - 425 nm for strontium and 495 - 467 nm for barium by which the J = 0,2 even-parity Rydberg series and autoionizing resonances just above the ionization limits labeled as; (4d2+5p2) 1D2 in strontium and 5d5f2p1 in barium were observed. Original new energy level values are presented with substantial extension to the data in the literature for even-parity Rydberg series: Sr I: 5sns 1S0 (12 < n < 46), 5snd 3D2 (9 < n < 47) and 5snd 1D2 (9 < n < 82) Ba I: 6sns 1S0 (13 < n < 68), 6snd 3D2 (11 < n < 46) and 6snd 1D2 (11 < n < 88) New observation of certain anomalies including a Stark shift reversal effect and zero-field thermionic diode detection with remarkable sensitivity preserving all spectral features of Rydberg states is also presented. Controlled configuration mixing by exploiting electric field and collisions has allowed new observations for the two-photon forbidden odd-parity Rydberg series of SrI: 5s2 1S0 -> 5snp 1P1o (32 < n < 46) and 5s2 1S0 -> 5snf 1F30 (29 < n < 44) Extensive study of collision involving Rydberg atoms with neutral rare gas atoms has allowed to obtain new data for shift and broadening rates for Sr I series: 5sns 1S0 and 5snd 1,3D2 in the perturbed region which clearly exhibit Ramsauer effect. Collisional evolution of a remarkably broad resonance, labeled as 4d2 1G4, also exhibiting Ramsauer effect, is presented besides new level data for strontium 4d2 3PJ states accessed by two distinct processes- direct two-photon excitation and single-photon sequential excitation. Furthermore, the thesis presents the results of a novel experimental technique involving time-resolved gated pulsed field detection technique (GPT) employed for the first time to examine the time decay of Rydberg atoms yielding their effective lifetimes.
A pressure-broadening study of three-photon and one-photon resonances was performed using counterpropagating, linearly polarized laser beams. Self-broadening, as well as the collision-induced effects due to the presence of a second noble gas, was investigated for the 6s, 6s′, 5d and 7s of xenon and the 5s′ of krypton. A multiphoton ionization scheme in a calibrated proportional counter is the basis of a new technique by which collisional processes can be investigated for high pressure regimes which have previously been inaccessible to investigation, particularly for self-broadening where the mean free path of the photons is of the order of 10−3 cm for pressures as low as 1 Torr. Using this technique, we have recorded the collision-induced widths and shifts of the resonance for pressures up to 1000 Torr. In the hundreds of torr range, asymmetric, self-broadening line shapes were observed with full widths and shifts which were greater than five times the laser bandwidth (0.002-0.005 nm).