Frequency Shifts, Line Broadenings, and Phase-Interference Effects in Rb**+Rb Collisions, Measured by Doppler-Free Two-Photon Spectroscopy

Abstract

Self-broadenings and frequency shifts of n 2 S and n 2 D Rydberg levels (up to n ~ 70) have been measured for rubidium in the pressure range of 3 to 100 mTorr, and the general dependence on principal quantum number n has been established for this low-pressure regime. A closer examination has revealed large (almost evenly spaced) oscillations in the dependence of linewidth on principal quantum number for the 2 S levels in the region of largest linewidth (n ~ 20 to 30) .The method of Doppler-free, two-photon spectroscopy, with instrumental linewidths of a few megahartz, is ideally suited to the study of small perturbations of atomic and molecular energy levels by collisions. This was demonstrated by Biraben etal. 1 in investigations of the shift and broadening of the 5 2 S and 4 2 D levels of sodium in collisions with noble gases, by Bolton, Harvey, and Stoicheff 2 for broadening of the ll 2 Dto 20 2 D levels of rubidium by argon, and recently by Weber and Niemax 3 for self-broadening of cesium levels up to 40 2 D. In this Letter, we report the results of 2 S-and 2 D -level shifts and broadenings in rubidium at pressures of 3 to 100 mTorr, for levels n = 10 to n~ 70. The present results constitute the most comprehensive study of line shift and line broadening for an atomic system to very high (n~70) Rydberg levels. Moreover, we report the first observation of pronounced oscillations in the dependence of linewidth on principal quantum number.The experimental method is based on Dopplerfree, two-photon spectroscopy with counterpropagating beams, 4 ' 5 and the use of an electrostatically shielded thermionic detector of high sensitivity. 6 A Kr + laser was used to pump a dye laser (Coherent Radiation CR599-21) producing a cw output of ~ 50 mW with a frequency jitter and linewidth of ~ 2 MHz in the wavelength region 593 to 656 nm. The dye-laser beam was chopped at 30 Hz; a small part was directed to a concave Fabry-Perot interferometer having a spectral-free range of 62.5 MHz; and the main portion was transmitted through an optical isolator (a Faraday rotator), then focused into a Rb cell, recollimated and focused into a second Rb cell, and finally, reflected back on itself with a concave mirror. Each Rb cell contained a thermionic detector. The resulting signals were amplified by lock-in amplifiers synchronized to the 30-Hz chopping frequency, and displayed along with the Fabry-Perot fringes on a three-pen chart recorder. Initially, the cells were baked and evacuated to ~ 1 X10" 8 Torr, before being charged with "lg of high-purity Rb in natural isotopic abundance (72% 85 Rb and 28% 87 Rb). Further purification was attempted in order to reduce the possible presence of Cs (occurring as an impurity at <500 ppm) by heating the cell and sidearm to ~ 220 °C for 12 h while condensing any vapors in an auxiliary cold trap. Each cell was placed in its own two-chamber oven with separate controls for the body and sidearm temperatures. In the reference cell, the sidearm was kept at 145 °C, corresponding to a Rb pressure of ~ 3 mTorr...

High-resolution two-photon spectroscopy of rubidium within a confined geometry

“…Furthermore, the Rb density was observed to decrease along the fiber's length away from the loading chamber. Nonetheless, pressure broadening and line shifts (≈ 12 kHz [8,16,17]) for the largest vapor densities (≈ 2 × 10 18 m −3 [18]) are below the resolution of this experiment.…”

Impact broadening of alkali Rydberg levels in pure and mixed alkali vapours

“…The collisional half-width of the lines measured in the mixed systems were corrected by taking into account the contributions of the selfbroadening effects. The self-broadening data of the Rb lines were taken from [7,8]. In the case of K we took the relative self-broadening rates by Stoicheff et al [6] corrected by a constant factor which was found in a separate experiment on the self-broadening of K two-photon lines [18].…”

“…Although there are, in addition to older data [-4, 5], more recent experimental data of the self-broadening and shift of potassium [6], rubidium [-7, 8] and cesium levels [9] derived also from two-photon spectra with the thermionic diode, there is still a lack of a satisfactory description of the involved collision processes [10]. In particular, strong oscillations in the optical cross sections of the levels as a function of the principal quantum number which were found by the group of Stoicheff [6,8] are still not definitively explained. In this paper we report on a completion of our earlier experiment on the self-broadening of Cs nD levels by including also measurements of the shift of the two-photon lines.…”

Probing an Electron Scattering Resonance using Rydberg Molecules within a Dense and Ultracold Gas

“…By utilizing the high densities of a Bose-Einstein condensate (BEC), the neutral atoms within the Rydberg orbit provide a probe of elastic and inelastic electron-neutral collisions for a large range of ion-neutral separations. We show in this paper that Rydberg spectroscopy in a BEC allows us to probe, with high resolution, the scattering resonance directly for the first time in this temperature regime.In a completely different temperature regime (> 400 K) than the work presented here (< 1 µK), Rydberg spectroscopy done during the 1980s in unpolarized thermal vapors investigated the line shift and line broadening of Rydberg atoms excited in a background gas of the same species atoms at similar densities [15,16]. Subsequent theory work [17][18][19][20] modeled the line shapes by taking into account the scattering resonance and an interplay of elastic and inelastic collisions.…”

“…In a completely different temperature regime (> 400 K) than the work presented here (< 1 µK), Rydberg spectroscopy done during the 1980s in unpolarized thermal vapors investigated the line shift and line broadening of Rydberg atoms excited in a background gas of the same species atoms at similar densities [15,16]. Subsequent theory work [17][18][19][20] modeled the line shapes by taking into account the scattering resonance and an interplay of elastic and inelastic collisions.…”