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Two-photon spectra of Rydberg states of alkaline-earth atoms (strontium and barium)

G. Philip

Published Jan 1, 2008• 0 citations
Publisher

Abstract

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.

DOI
10.21954/ou.ro.0000d3bf

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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.
Odd-parity barium Rydberg states have been populated by stepwise excitation via M1-E1 and E2-E1 transitions, starting from the 6s2 1S0 ground state. Using two cw dye ring lasers and employing thermionic detection, Doppler-free spectra of 6snp 1,3P and 6snf 1,3F Rydberg states have been recorded with excellent signal/noise ratios. A systematic study of the hyperfine structure of 6snp 1P1 Rydberg states has been carried out. Between n=20 and n=23 a resonance in singlet-triplet mixing occurs caused by configuration interaction with the 5d8p 1P1 perturbing state, located close to the first ionization limit. Our results are in disagreement with an existing multichannel quantum defect analysis of this series. The singlet-triplet mixing of 6snp 1P1 and 6snd 1D2 Rydberg series is compared.
Collisional broadening and shift data for even parity states of neutral barium are presented. A two-photon absorption technique is used to populate Rydberg states of n d 1 D 2 , 3 D 2 and 1 S 0 series. High-resolution Ba excitation spectra were measured using a diode detector while scanning the frequency of an excimer pumped dye laser. Inert gases Ar, Kr and Xe were used as perturbers at pressures ranging from 10 mb to 400 mb. Shifts and broadenings of spectral lines were measured as a function of pressure and principal quantum number n . Several perturbations and deviations from expected results are discussed in this paper.
Rydberg series of msns and msnd configurations of Be (m=2) and Mg (m=3) were investigated between principal quantum numbers 10 ≤ n ≤ 70. The Rydberg states were populated using stepwise excitation or direct two-photon transitions with frequency doubled light (235 nm ≤ λ < 330 nm) from pulsed dye lasers. The excited Rydberg atoms were detected with a space charge limited thermionic diode. An analysis of the Rydberg series both in Be and Mg by means of multi-channel quantum-defect theory (MQDT) results in the determination of mp2 1D2 admixtures in msnd 1D2 states.
Cold gases of Rydberg atoms are an ideal system in which to study the novel effects of strong interatomic interactions. This thesis describes the design and construction of the world's first experiment to study Rydberg states in a cold gas of an alkaline earth metal, in this particular case strontium. We have studied a wide range of Rydberg states, and have developed a sensitive ``step-scan" spectroscopic technique that detects the spontaneous ionization of the Rydberg gas. The step-scan method is used to acquire Stark maps, and these measurements verify a single-electron model for calculating dipole matrix-elements. From the matrix-elements, interaction strengths between strontium Rydberg atoms have been calculated for the first time. The presence of two valence electrons in an alkaline earth metal, such as strontium, offers a new angle on the study of Rydberg atoms. We create doubly excited ``autoionizing" states, the first such study in a cold gas. Autoionization is used as a high yield probe of Rydberg states, and enables a study of excitation dynamics with nanosecond time-resolution. We show that autoionization can quantitatively identify and elucidate state mixing in the Rydberg gas, and probe population transfer at the very onset of ultra-cold plasma formation.