Spectroscopic Studies of Rubidium with Doppler - Two-Photon Absorption.
Abstract
We have used simultaneous two-photon absorption without Doppler broadening to study the spectrum of atomic rubidium. Two series of spectral lines were observed corresponding to transitions n('2)S (<---) 5('2)S and n('2)D (<---) 5('2)S, with the principal quantum number n up to 116 and 124 respectively; the highest level, 124 ('2)D, was only 7.3 cm('-1) from the ionization limit. The Doppler-free, two-photon transitions were excited by focusing the output beam from a single mode, tunable, cw dye laser into a sample cell and retro-reflecting the emergent radiation with a concave mirror. The excited -state atom rapidly ionized, and these were detected with an electrostatically-shielded thermionic detector located inside the sample cell. The high sensitivity of the thermionic detection and the high intensities and narrow linewidths of the two-photon, Doppler-free (TPDF) transitions allowed not only the detection of highly excited Rydberg states, but also the precise determination of the transition frequencies, term values, and quantum defects for these states. Fine and hyperfine splittings, and isotope shifts were also investigated. We have also used these TPDF transitions to study the self-broadenings and self-shifts of the highly excited states in rubidium, at very low rubidium densities. Wavelengths of the TPDF transitions, n('2)S (< ---) 5('2)S (n = 9 to 116) and n('2)D (<---) 5('2)S (n = 7 to 124) were measured to an accuracy of 1 part in 10('7) with a wavemeter and, from these, the transition frequencies and term values were determined to (+OR-)100 MHz. The term values were represented by series formulae, leading to improved values for the ionization limit and for quantum defects. The presently determined value for the ionization limit of ('85)Rb (T(,(INFIN)) = 33,690.8565(15) cm('-1)) is in excellent agreement with the most accurate values published. The hyperfine splittings of n('2)S states of both ('85)Rb and ('87)Rb were determined for n = 9 to 15 and were found to be proportional to n(,eff)('-3). Also some of the fine structure splittings of n('2)D levels were remeasured to confirm earlier work in this laboratory. Self-broadenings and shifts of selected n('2)S and n('2)D Rydberg states in rubidium, up to n = 70, were measured at pressures of rubidium between 3 and 100 mTorr (densities 6 x 10('13) to 2 x 10('15) part/cc), and rates of self-broadening and of self-shift were determined. Linewidths as narrow as (TURN)15 MHz, and up to (TURN)300 MHz, were observed. All of the observed pressure shifts were towards lower frequencies, with maximum observed shift (TURN)200 MHz at the highest pressures. The rates of shift were found to have similar behaviour for both ('2)S and ('2)D series, showing a rapid initial increase and then reaching a constant rate for high levels (n > 30). The broadening rates of the ('2)S and ('2)D series also increased rapidly, peaked at n (DBLTURN) 23, and then slowly decreased for higher levels. A strong oscillatory dependence of broadening rates on principal quantum number n was observed for the ('2)S series in the region n = 20 to 30, but not for the ('2)D series. This oscillatory dependence of broadening rates on n has not been observed previously, nor are there any theoretical predictions for its occurrence in either self-broadening, or broadening due to a foreign, monoatomic gas. The observed "oscillations" have already stimulated considerable interest in the scientific community, but no clear explanation has yet been put forward.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1980
- Bibcode:
- 1980PhDT........89W
- Keywords:
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- Physics: Atomic