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Journal of Physics B Atomic Molecular and Optical Physics

Two-photon excitation of Ba atoms and absolute measurement of sigma(2)

N. A. Cherepkov, A. Yu. Elizarov

Published Oct 14, 1991• 6 citations
Publisher

Abstract

Measurements of the absolute generalized cross sections for the non-resonant two-photon excitation of discrete states of barium atoms are presented. They are based on the detection of an ion current due to absorption of a third photon in two processes of both resonant and non-resonant two-photon excitation of the same discrete level. Also considered is the appearance of additional lines in the two-photon absorption spectrum due to superradiance of the dye laser, the anomalous width of some of the two-photon resonances and the use of two-photon resonances for calibration purposes in investigation of autoionization resonances.

DOI
10.1088/0953-4075/24/19/012

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Measurements of the absolute generalized cross sections for the non-resonant two-photon excitation of discrete states of barium atoms are presented. They are based on the detection of an ion current due to absorption of a third photon in two processes of both resonant and non-resonant two-photon excitation of the same discrete level. Also considered is the appearance of additional lines in the two-photon absorption spectrum due to superradiance of the dye laser, the anomalous width of some of the two-photon resonances and the use of two-photon resonances for calibration purposes in investigation of autoionization resonances.
Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned 6s6p 1P16s6p\text{ }{^{1}P}_{1}6s6p 1P1​ state of barium in the energy range between the 5d3/25{d}_{3/2}5d3/2​ and 5d5/25{d}_{5/2}5d5/2​ states of Ba+{\text{Ba}}^{+}Ba+. We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel RRR-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.
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.
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.
Two-step resonant photoionization of free Ni atoms is studied by one-color and two-color processes, using pulsed lasers with photon energies around half the ground-state ionization energy. Two-color two-step resonance ionization from the ground state a3{\mathrm{}}^{3}3F4{\mathit{F}}_{4}F4​ and the first metastable state a3{\mathrm{}}^{3}3D3{\mathit{D}}_{3}D3​ via the triplet states z3{\mathrm{}}^{3}3G3,4,50{\mathit{G}}_{3,4,5}^{0}G3,4,50​ was used to probe the autoionizing states just above the ionization limit. Several strong lines in the one-color ionization spectrum are explained as direct two-photon transitions to autoionizing states. Photoionization cross sections were deduced from saturation measurements. The large cross sections observed for specific one-color two-step transitions are due to the near double resonant excitation into autoionizing states. \textcopyright{} 1996 The American Physical Society.
Using two CW dye lasers barium Rydberg states were populated by stepwise two-photon absorption via the 6s6p 1 P 1 intermediate level. Monitoring the transmission of the first laser beam, tuned to the resonance line, and scanning the second laser across the upper atomic transition, a greatly enhanced signal was observed. Taking optical pumping into account, a simple rate equation model can quantitatively describe the increase in sensitivity. This technique was applied to measure the hyperfine structure of the 6s15d 1 D 2 Rydberg state in barium.