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Journal of Experimental and Theoretical Physics

Satellites of the Dipole-Forbidden Transitions to the Low-Lying 2S1/2 and 2D3/2,5/2 Excited States of K, Rb, and Cs Atoms in the Spectra of Gas-Phase Mixtures with CF4

Vadim A. Alekseev, T. A. Vartanyan, Anatoly S. Pazgalev, Pavel Yu. Serdobintsev

Published Sep 1, 2023• 0 citations
Publisher

Abstract

The luminescence excitation spectra of the D1 resonance line of atoms K, Rb, and Cs in gas mixtures with CF4 are found to contain satellite transitions, which correspond to the transition of an atom to the states (n – 1)d 2D3/2,5/2 and (n + 1)s 2S1/2, where n = 4, 5, and 6 for K, Rb, and Cs, respectively, with the simultaneous excitation of CF4 molecule vibrations at the IR active mode frequency ν3 with a quantum energy of 1283 cm–1. These satellite transitions are A(ns 2S1/2) + CF4(ν3 = 0) + hν → A((n – 1)d 2D3/2,5/2) + CF4(ν3 = 1) and A(ns 2S1/2) + CF4(ν3 = 0) + hν → A((n + 1)s 2S1/2) + CF4(ν3 = 1), where A = K, Rb, and Cs. The appearance of an optical coupling between the upper and lower states of these asymptotically (at $${{R}_{{{\text{A}} - {\text{C}}{{{\text{F}}}_{4}}}}}$$ → ∞) forbidden transitions is shown to be caused by the interaction of the dipole moment of the ν3 = 1 ↔ ν3 = 0 vibrational transition in the CF4 molecule with the dipole moments of the electronic transitions np 2P1/2,3/2 ↔ (n – 1)d 2D3/2,5/2 and np 2P1/2,3/2 ↔ (n + 1)s 2S1/2 in an alkali metal atom; as a result of this interaction, the upper state of the satellite transition acquires admixtures of the A(np 2P1/2,3/2)CF4(ν3 = 0) resonance states.

DOI
10.1134/s1063776123090029

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The luminescence excitation spectra of the D1 resonance line of atoms K, Rb, and Cs in gas mixtures with CF4 are found to contain satellite transitions, which correspond to the transition of an atom to the states (n – 1)d 2D3/2,5/2 and (n + 1)s 2S1/2, where n = 4, 5, and 6 for K, Rb, and Cs, respectively, with the simultaneous excitation of CF4 molecule vibrations at the IR active mode frequency ν3 with a quantum energy of 1283 cm–1. These satellite transitions are A(ns 2S1/2) + CF4(ν3 = 0) + hν → A((n – 1)d 2D3/2,5/2) + CF4(ν3 = 1) and A(ns 2S1/2) + CF4(ν3 = 0) + hν → A((n + 1)s 2S1/2) + CF4(ν3 = 1), where A = K, Rb, and Cs. The appearance of an optical coupling between the upper and lower states of these asymptotically (at RA−CF4{{R}_{{{\text{A}} - {\text{C}}{{{\text{F}}}_{4}}}}}RA−CF4​​ → ∞) forbidden transitions is shown to be caused by the interaction of the dipole moment of the ν3 = 1 ↔ ν3 = 0 vibrational transition in the CF4 molecule with the dipole moments of the electronic transitions np 2P1/2,3/2 ↔ (n – 1)d 2D3/2,5/2 and np 2P1/2,3/2 ↔ (n + 1)s 2S1/2 in an alkali metal atom; as a result of this interaction, the upper state of the satellite transition acquires admixtures of the A(np 2P1/2,3/2)CF4(ν3 = 0) resonance states.
The excitation spectrum of the resonance luminescence of Na atoms in a mixture with CF4 exhibits a satellite transition corresponding to the simultaneous optical excitation of the colliding atom and molecule, Na(2S1/2) + CF4( v3{{v}_{3}}v3​ = 0) + h vvv → Na(2P1/2, 3/2) + CF4( v3{{v}_{3}}v3​ = 1), where v3{{v}_{3}}v3​ is the infrared active mode of CF4 with a vibrational quantum energy of 1283 cm–1. It is shown that the optical coupling between the upper and lower states of this asymptotically (RNa−CF4→∞)({{R}_{{{\text{Na}} - {\text{C}}{{{\text{F}}}_{4}}}}} \to \infty )(RNa−CF4​​→∞) forbidden transition can be explained within a model that takes into account the polarization of the atom in the field of the molecule and the interaction of the dipole moment of the CF4( v3{{v}_{3}}v3​ = 1 ↔ v3{{v}_{3}}v3​ = 0) transition with the dipole moments of electronic transitions in the atom. The results of calculations based on this model are in satisfactory agreement with the experiment. It is noted that CF4 may be of interest as a component of a working medium of diode pumped alkali lasers.
6 2P1/2 ↔ 6 2P3/2 mixing and 6 2S1/2 ← 6 2P1/2,3/2 quenching in cesium, induced in collisions with ground state N2, H2, HD, and D2 molecules, have been investigated using techniques of sensitized fluorescence. Mixtures of cesium vapor and the gases, in which the cesium vapor pressure was kept very low to avoid multiple scattering, were irradiated with each component of the cesium resonance doublet in turn. The fluorescence which contained both components of the doublet was observed in a direction perpendicular to that of the exciting beam. Measurements of the relative intensities of the fluorescent components yielded the following cross sections for mixing and quenching. For Cs–N2 collisions: Q12(2P1/2 → 2P3/2) = 4.7 Å2, Q21(2P1/2 ← 2P3/2) = 25 Å2, Q10(2S1/2 ← 2P1/2) = 77 Å2, Q20(2S1/2 ← 2P3/2) = 69 Å2. For Cs–H2 collisions: Q12 = 6.7 Å2, Q21 = 44 Å2, Q10 = 7 Å2, Q20 = 5 Å2. For Cs–HD collisions: Q12 = 4.8 Å2, Q21 = 32 Å2, Q10 = 4 Å2, Q20 = 3 Å2. For Cs–D2 collisions: Q12 = 4.2 Å2, Q21 = 28 Å2, Q10 = 8 Å2, Q20 = 7 Å2. The Cs–N2 and Cs–H2 cross sections supersede the values published previously (McGillis and Krause 1967). The cross sections Q21 appear to exhibit resonances with molecular rotational transitions.
Recent absorption measurements of the self-broadening of Rb principal series lines [C. H. Greene, E. L. Hamilton, H. Crowell, C. Vadla, and K. Niemax, Phys. Rev. Lett. 97, 233002 (2006)] verified the existence of minima in highly excited long-range potentials of Rb2{\text{Rb}}_{2}Rb2​ which were predicted theoretically as bound Rydberg molecule [C. H. Greene, A. S. Dickinson, and H. R. Sadeghpour, Phys. Rev. Lett. 85, 2458 (2000)]. The present paper reports on the extension of these experimental studies to principal series lines of K broadened by K as well as Cs broadened by Cs. In addition, the broadening of some lower members of the K, Rb, and Cs principal line series due to heteroatomic interactions was investigated. The analysis of the measured satellite structures in the quasistatic line wings and their comparison to the previously published analogous data for ${\text{Rb}}^{\ensuremath{\ast}}+\text{Rb}$ system lead to the conclusion that one can expect existence of potential minima in homo- and heteroatomic alkali-metal ${A}^{\ensuremath{\ast}}+B$ systems in a wide range of principal quantum numbers, starting with K, Rb, and Cs excited to the third resonance states. The positions and strengths of the satellite structures indicate that the potential minima are due to scattering of the valence electron by ground-state perturbers.
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.
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.
The population and deactivation processes of the first excited 2S and 2D levels of alkali atoms have not been the focus of extensive attention to date, since these levels do not have dipole transitions to the 2S ground state, and the only radiative transitions to the 2p resonance level lie in the IR ( 1-3 fLm). We have demonstrated that in the case of potassium it is possible to populate the 32D and 52S levels in socalled optical collisions by laser excitation of the vapor and also by excitation energy transfer (EET) from the laser-excited Kidimers. 1 The lines of the corresponding 4D--->5P (1.529/1.475 f.lm and 6S ---> 5P (1.367/1.324 f.lm) transitions have also been identified in the laser-induced fluorescence of rubidium vapor,2 although the population mechanism of the levels is not examined in the present study. This st~dy will focus on determining the processes that result in the population of the Rb 42D and 62S levels and to determining the rate constants of these1'rocesses. The spectrum of the LGN-402 argon laser-induced fluorescence of Rb vapors was recorded in the IR by cooled Ge photo diodes selected for IR sensitivity and configured along the outlet slit of an MDR-3 monochromator. The synchronous detection method was employed. A FEU-79 photomultiplier was used in the visible range. Details on the experimental setup can be found in Ref. 3. The Rb vapor was excited by various argon laser lines (514.5-476.5 nm), although the 5l4.5-nm line was of primary interest. The C 1 ITu state of the RB2 dimer is not populated by this laser line; its predissociation efficiently populates the 52P resonance levels4 (see Fig. 1). Moreover the 5l4.5-nm laser line is closer in energy to the Rb 5S--->4D quadrupole transition (M= 75 cm-I). In the experiment at an atomic concentration [Rb] _1016 cm-3 the fluorescence lines from the 62S level (2.5 eV) were weaker by a factor of 50 compared to the lines from the 42D level (2.4 eV). A transition to the shorterwavelength laser excitation lines causes a certain increase in the population efficiency of the 4D level, although it also results in a more rapid growth of the population of the 5P resonance level due to the predissociation of Rb2 (C 1IT u ). I. In pure Rb vapors I) the line intensity of the 4D---> 5P transition has a linear relation to the laser excitation power ,(514.5 nm andp 5X 1016 cm-3 can be attributed to the onset of collisional quenching of the RB 4D state ('Trad =90 nsec). 5 So-called opticaJ.collisions lead to the population of the 4D level in these conditions (Aexc = 514.5 nm and no Na impurities)