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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Selective excitation and chemical reactions in alkali metal vapor

V. O. Chaltykyan, G. G. Grigoryan, Yu. P. Malakyan

Published Apr 11, 2003• 0 citations
Publisher

Abstract

We consider a gas of two-level atoms of two species (for example, natural Rb vapor consisting of two isotopes) in the field of laser radiation tuned close to resonance with the certain transition of a specific iostope. This radiation may turn to be rather close to a near lying transition of other isotope (within the linewidth of laser radiation), so it is necessary to take into account the possibility of excitation of other isotope in order to evaluate the enrichment degree if it is aimed at the isotope separation by means of selective excitation with subsequent chemcial reaction (in our example the rubidium hydride formation) of the excited isotope which is believed to take place at a rate much higher than that in the ground state of the atom.

DOI
10.1117/12.513522

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We consider a gas of two-level atoms of two species (for example, natural Rb vapor consisting of two isotopes) in the field of laser radiation tuned close to resonance with the certain transition of a specific iostope. This radiation may turn to be rather close to a near lying transition of other isotope (within the linewidth of laser radiation), so it is necessary to take into account the possibility of excitation of other isotope in order to evaluate the enrichment degree if it is aimed at the isotope separation by means of selective excitation with subsequent chemcial reaction (in our example the rubidium hydride formation) of the excited isotope which is believed to take place at a rate much higher than that in the ground state of the atom.
A theoretical analysis is made of the process of isotopically selective excitation of atoms in the field of opposite waves when the compensation of the Doppler broadening is incomplete. The density matrix formalism for a homogeneous steady-state case is used to obtain expressions for the total probability of the excitation of resonating and nonresonating isotopes and for the selectivity of the process defined as the ratio of these probabilities. In some cases analytic expressions are obtained and these make it possible to determine the nature of the dependence of the total probability of the excitation and selectivity on the temperature of a vapor, buffer gas concentration, ratio of the Rabi transition frequencies, detuning from a resonance, and isotopic shifts. Numerical estimates are obtained of the atomic parameters for transitions in potassium and rubidium atoms. It is shown that the adopted method is more promising for the separation of the isotopes of alkali metals than is one-photon excitation.
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)
Our choice of rubidium atoms as the optically active species becomes now clear. The large elasticity of collisions between the electronically excited states of the rubidium D-lines and noble gas atoms has long been proven [25]. The quasi-molecular potential has a depth of the order of kBT, allowing for a significant part of the atoms’ kinetic energy to be extracted. Finally, the cooling transitions are in the near-infrared spectral regime, making them conveniently optically accessible to commercial laser systems. The cooling power Pcool from these experiments can be estimated in a similar way as for anti-Stokes cooling [26, 27] from the mean fluorescence frequency νfl, the incident power Popt, and the absorption probability of the gas mixture at incident frequency ν, α(ν).P Pcool opt fl= -a n n nn( ) (1.1) The typical energy that can be extracted from the sample per cooling cycle is of order of the thermal energy kBT. In the experiments performed so far, we observe cooling powers Pcool in the order of up to 100 mW [17, 23, 28, 29]. Note that it is important to choose a detuning with a high-enough absorption probability a(ν). We are aware that the collisional broadening plays a crucial role here. A measurement of typical fluorescence data recorded with the dense gas mixture will be shown in Section 1.4.1. Experimentally, it turns out to be crucial that the used buffer gas is of high purity to suppress the influence of concurring heating effects, of which quenching is the most known. In the case of quenching, the excitation energy is transferred to an atom of a third species polluting the dense gas mixture when colliding with the quasi-molecule. Relaxation can then occur via nonradiative decay, and so energy is kept in the system, heating the gas. To reduce possible quenching, an effect known to occur, for example, from an admixture with nitrogen, we use a buffer gas with high purity, namely argon gas preferably with a purity of 99.9999 (argon 6.0). Residual heating can also be caused by energy pooling. In this scenario, during the collision of two excited alkali atoms one of them gets further excited. Its relaxation to the ground state implicates the emission of a fluorescence photon with higher energy. For the case of rubidium atoms, the reaction equation of that process can be written as [30]: Rb Rb Rb Rb* * **+ + + Æ + + +mv mv mv mv12 22 12 22 2 2 2 2   . (1.2) The sum of the energies of the single excited rubidium atoms before the collision is higher than the energy of the twice excited atom. The surplus of the energy is thus kept in the system as kinetic energy of the collisional partners [31]. For rubidium atoms subject to a high-pressure noble gas, fluorescence originating from the 6p Æ 5s transition then occurs, and can be detected as blue light at a wavelength λ ≈ 420 nm in the experiments. At room temperature, the alkali atoms are in the solid phase. To produce a sufficient alkali vapor number density the system is heated to temperatures of up to 680 K, while the buffer gas pressure ranges between 50 bar and 230 bar. Under these conditions, we calculate that the dense gas ensemble consists of approximately 1017 cm-3 alkali atoms and 1021 cm-3 noble gas atoms. Then, we can estimate the mean free path length λfp for a gas of colliding atoms (cross section σ) and number density n l sfp = n (1.3) Let us consider the geometric cross section πd2, where d denotes the diameter of a rare gas atom, which is twice the van der Waals radius (d = 2rvdW = 3.76 × 10−10 m for argon [32]). Then, for argon pressure of 200 bar, the mean free path length is in the order of only a few nanometers, λfp ≈ 2.3 nm. Following the Boltzmann distribution, still considering a temperature T = 680 K, one can also estimate the collisional rate to 2.4 × 1011/s. Comparing this to the natural lifetime of the first excited alkali atom level (27 ns), we calculate that 104collisions occur within this period of time. 1.3 Experimental Setup and MethodsIn the following section, we present our experimental setup in detail, including the techniques used to measure atomic fluorescence and temperature changes inside the gas mixture. We offer insights into the high-pressure cells employed (Section 1.3.1), as well as into the spectroscopic techniques (Section 1.3.2) and the thermal deflection setup (Section 1.3.3).
The effect of excited-state mixing on the optical pumping of alkali-metal vapors is considered for the case where the energy separation between the 2P12^{2}P_{\frac{1}{2}}2P21​​ and 2P32^{2}P_{\frac{3}{2}}2P23​​ first excited states is relatively large, allowing the isolation of mixing effects within a particular JJJ level. Two models for the mixing process are proposed, one a random reorientation of the total electronic angular momentum J, and the other a reorientation of J subject to the selection rule $\ensuremath{\Delta}{m}_{J}=0, \ifmmode\pm\else\textpm\fi{}2$. The probabilities for mixing transitions among JJJ-state sublevels have been calculated for an alkali atom of nuclear spin 32\frac{3}{2}23​, and can be used to calculate the optical-pumping transition probabilities for a vapor subject to any degree of excited-state mixing. Several possible experiments suggested by these calculations are discussed.