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Journal of Physics B Atomic and Molecular Physics

Continuum- and bound-state pair absorption in Cs-Cs, Rb-Rb and Cs-Rb systems

R. Hotop, Kay Niemax

Published Feb 14, 1980• 20 citations
Publisher

Abstract

The absorption spectra of pure Cs and Rb vapours and a mixture of both have been measured in the wavelength region from 2300 to 3100 AA (photoionisation continuum of the atoms) in the temperature range from 410 to 540 degrees C. Pronounced absorption bands can be detected in this wavelength region. Because the absorption was found to depend proportionally on the product of the number densities of the associated atoms (N Cs 2 , N Rb 2 and N Cs N Rb ) and the energies of many observed bands correspond roughly to the sum energies of two electronically excited species, the bands are interpreted in terms of simultaneous excitation of two atoms due to the absorption of one photon ('continuum-state absorption'). For the Cs-Cs and the Cs-Rb systems analogous bands were also observed within the discrete spectra of the atoms involved ('bound-state absorption'). Enhancement of the cross section for pair absorption near dipole-allowed transitions was found in agreement with theoretical predictions.

DOI
10.1088/0022-3700/13/3/002

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The absorption spectra of pure Cs and Rb vapours and a mixture of both have been measured in the wavelength region from 2300 to 3100 AA (photoionisation continuum of the atoms) in the temperature range from 410 to 540 degrees C. Pronounced absorption bands can be detected in this wavelength region. Because the absorption was found to depend proportionally on the product of the number densities of the associated atoms (N Cs 2 , N Rb 2 and N Cs N Rb ) and the energies of many observed bands correspond roughly to the sum energies of two electronically excited species, the bands are interpreted in terms of simultaneous excitation of two atoms due to the absorption of one photon ('continuum-state absorption'). For the Cs-Cs and the Cs-Rb systems analogous bands were also observed within the discrete spectra of the atoms involved ('bound-state absorption'). Enhancement of the cross section for pair absorption near dipole-allowed transitions was found in agreement with theoretical predictions.
This paper will review briefly a recent experiment which measured the photodetachment probability for I-ions due to simultaneous absorption of two ruby quanta. We have calculated the two-quantum photoionization rate for atomic cesium as a function of wavelength. The "transition" from the initial state to the bound intermediate state is expressed in terms of oscillator strengths. The second "transition," from these excited valence states into the coulomb continuum is expressed as a cross section. We have observed what appear to be genuine two-photon ionization signals in three cases: 1) near resonance in the10P_{3/2}state using the second-harmonic ofQ-switched ruby, 2) away from resonances, using the 5300 Å second-harmonic ofQ-switched neodymium laser, and 3) away from resonances using the first anti-Stokes line at 5750 Å from ruby-pumped D2gas. Present experiments aim at scanning the interesting range near resonance, e.g., by thermally tuning the ruby.
We experimentally study energy-pooling collision in the Rb–Cs vapour mixture at low densities in a cell. Atoms are excited to Rb(5 P 1/2 ) and Cs(6 P 3/2 ) states using two single-mode diode lasers. To isolate the heteronuclear contribution in the fluorescence spectrum, a double-modulation technique is adopted. The excited-atom density and spatial distribution are mapped by monitoring the absorption of a counterpropagating single-mode diode laser beam, tuned to Rb(5 P 1/2 →7 S 1/2 ) and Cs(6 P 3/2 →8 S 1/2 ) transitions, respectively, which could be translated parallel to the pump beams. The excited atom densities are combined with the measured fluorescence ratios to determine cross section for the energy-pooling process [i.e. Rb(5 P 1/2 ) + Cs (6 P 3/2 ) → Cs(8 S 1/2 ) + Rb (5 S 1/2 )]. The cross section is 3.79×10 −14 cm 2 ±45%.
We report on the observation of three RbCs satellite bands in the blue and green ranges of the visible spectrum. Absorption measurements are performed using all-sapphire cell filled with a mixture of Rb and Cs. We compare high resolution absorption spectrum of Rb-Cs vapor mixture with pure Rb and Cs vapor spectra from the literature. After detailed analysis, the new satellite bands of RbCs molecule at 418.3 nm, 468.3, and 527.5 nm are identified. The origin of these bands is discussed by direct comparison with difference potentials derived from quantum chemistry calculations of RbCs potential energy curves. These bands originate from the lower Rydberg states of the RbCs molecule. This study thus provides further insight into photoassociation of lower Rydberg molecular states, approximately between Cs(7s) + Rb(5s) and Cs(6s) + Rb(6p) asymptotes, in ultracold gases.
We describe a series of measurements of absorption and laser induced fluorescence on cells that contained cesium and rubidium and a rare gas: He, Ar, Kr, or Xe. These studies showed strong blue wing absorption to the short wavelength side of the alkali atom D2 lines due to collisionally formed Cs- or Rb-rare gas excimers. We also have observed an efficient two photon excitation of higher lying states in Cs and Rb that produces both intense blue emission and IR atomic emission in the 1.3 to 3.8 μm spectral region.
Rate coefficients for energy-pooling (EP) collisions Rb(5P_(J))+Cs(6P_(3/2))-->Rb(5S_(1/2))+Cs(nl_(J')) have been measured. Atoms were excited to Rb(5P_(J)) and Cs(6P_(3/2)) states using two single-mode diode lasers. To isolate the heteronuclear contribution in the fluorescence spectrum, a double-modulation technique has been adopted. The excited-atom density and spatial distribution are mapped by monitoring the absorption of a counterpropagating single-mode diode laser beam, tuned to Rb(5P_(J)-->7S_(1/2)) and Cs(6P_(3/2)-->8S_(1/2)) transitions respectively, which could be translated parallelly to the pump beams. The excited atom densities are combined with the measured fluorescence ratios to determine cross sections for the EP processes. It was found that Rb(5P_(J))+Cs(6P_(3/2)) collisions are more efficient than Rb(5P_(3/2))+Cs(6P_(3/2)) collisions for populating Cs(4F_(5/2)), while the opposite is true for populating Cs(4F_(7/2)).
The improved method for the determination of the atom number densities in pure and mixed alkali vapours has been proposed. In order to obtain the correct results one has to go beyond the simple Lorentzian shape of the quasistatic absorption profile. The method has been tested by the white-light absorption measurements in the pure K, Rb and Cs vapours. The statistical accuracy of the obtained data for the atom number density is +or-3%. The method has been applied for determining the atom number densities in the vapour over the Rb-Cs mixture. The obtained values have been used in the evaluation of the effective C 6 constants for Rb*-Cs interaction: C 6 eff =(4.9+or-0.3)* 10 -30 cm 6 s-1 for the D2 line. The theory predicts C 6 eff =4.95*10 -30 cm 6 s -1 for the D1 line and C 6 eff =2.37*10 -30 cm 6 s(-1) for the D2 line.