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Soviet Journal of Quantum Electronics

Collisional and dissociative processes of the excitation of atoms and their role in lasing of alkali metal vapors

M. L. Yanson

Published Dec 31, 1982• 0 citations
Publisher

Abstract

An investigation was made of the mechanisms of quenching of excited states and of excitation of atomic levels as a result of absorption of laser radiation by alkali metal (Na, K, Cs) vapors. It was found that several processes participated in the excitation of atomic levels: collisional mechanism of excitation energy transfer from molecules to atoms, photodissociation, predissociation, and nonresonant excitation of atomic levels (radiative collisions). The partial contribution of each of these processes (dependent on the vapor density, intensity of laser radiation, and its frequency) should be allowed for in the balance of the filling and deactivation of molecular and atomic states. Qualitative characteristics were obtained for these processes and an analysis was made of their likely role in the emission of radiation from diatomic molecules in alkali metal vapors.

DOI
10.1070/qe1982v012n12abeh006311

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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)

OptSp•Aug 1, 1988•0 citations
An investigation was made of the mechanisms of quenching of excited states and of excitation of atomic levels as a result of absorption of laser radiation by alkali metal (Na, K, Cs) vapors. It was found that several processes participated in the excitation of atomic levels: collisional mechanism of excitation energy transfer from molecules to atoms, photodissociation, predissociation, and nonresonant excitation of atomic levels (radiative collisions). The partial contribution of each of these processes (dependent on the vapor density, intensity of laser radiation, and its frequency) should be allowed for in the balance of the filling and deactivation of molecular and atomic states. Qualitative characteristics were obtained for these processes and an analysis was made of their likely role in the emission of radiation from diatomic molecules in alkali metal vapors.
We have carried out a series of experiments to study collisional and radiative processes in alkali metal and alkaline-earth vapors. Our principal interest has been to understand fundamental atom-atom, atom-molecule, and atom- light interactions in order to model alkali diatomic and alkaline-earth excimer systems for possible laser applications. During this funding period, we have concentrated on collisional excitation transfer, quenching and diffusion processes involving the metastable 6s5dD sub J levels of barium. We have also investigated alkali and alkaline-earth vapors as media for observing low threshold nonlinear optical processes
Excitation transfer between the 62P126 ^{2}P_{\frac{1}{2}}62P21​​ and 62P326 ^{2}P_{\frac{3}{2}}62P23​​ resonance states in cesium, induced by Cs-N2{\mathrm{N}}_{2}N2​ and Cs-H2{\mathrm{H}}_{2}H2​ collisions, was observed in the first experiment of its kind, in which techniques of sensitized fluorescence are used to investigate collisional mixing between resonance states of alkali atoms in mixtures with molecular gases. The cesium-vapor-gas system, in which the cesium vapor pressure was low enough to avoid radiation trapping, was irradiated with a single component of the resonance doublet. The fluorescent light, which contained both components of the doublet, was monitored at right angles to the direction of excitation. Measurements of relative intensities of the fluorescent components yielded the following cross sections for excitation transfer and for quenching of resonance radiation. For Cs-N2{\mathrm{N}}_{2}N2​ collisions: ${Q}_{12}(^{2}P_{\frac{1}{2}}\ensuremath{\rightarrow}^{2}P_{\frac{3}{2}})=3.6$ A˚2{\mathrm{\AA{}}}^{2}A˚2, ${Q}_{21}(^{2}P_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=16.2$ A˚2{\mathrm{\AA{}}}^{2}A˚2, ${Q}_{10}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{1}{2}})=55.2$ A˚2{\mathrm{\AA{}}}^{2}A˚2, ${Q}_{20}(^{2}S_{\frac{1}{2}}\ensuremath{\leftarrow}^{2}P_{\frac{3}{2}})=63.5$ A˚2{\mathrm{\AA{}}}^{2}A˚2. For Cs-H2{\mathrm{H}}_{2}H2​ collisions: Q12=6.7{Q}_{12}=6.7Q12​=6.7 A˚2{\mathrm{\AA{}}}^{2}A˚2, Q21=29.6{Q}_{21}=29.6Q21​=29.6 A˚2{\mathrm{\AA{}}}^{2}A˚2, Q10=11.7{Q}_{10}=11.7Q10​=11.7 A˚2{\mathrm{\AA{}}}^{2}A˚2, Q20=6.7{Q}_{20}=6.7Q20​=6.7 A˚2{\mathrm{\AA{}}}^{2}A˚2. The large sizes of the excitation-transfer cross sections, which are comparable to the corresponding Cs-Cs cross sections, are attributed to resonances with transitions in the molecules.
We performed quasiresonant laser excitation of Li–Cd vapor mixtures at 460.3 nm (2p–4d) and 497.2 nm (2p–4s). The initial population of the Li(2p) level is caused by three processes: collisional dissociation or photodissociation of the Li 2 (B 1 Π u ) molecules or energy transfer in Li 2 (B 1 Π u ) and Li(2s) collisions. The Li(4d,4s) atoms radiatively decay towards Li(3p), from which collisional population transfer to Cd(5 3 P J ) occurs, followed by three-body reactive collisions which produce the LiCd(2 2 Π ) excimer states
Lasing on the 62P1/2→62S1/2(D1) resonance transition of atomic Cs at 894.3 nm has been demonstrated in mixtures of Ar, ethane, and Cs vapor by the photoexcitation of ground state Cs-Ar collision pairs and subsequent dissociation of diatomic, electronically-excited CsAr molecules (exciplexes or excimers). The blue satellites of the alkali D2 lines provide a pathway for optically pumping atomic alkali lasers on the principal series (resonance) transitions with broad linewidth (>2 nm) semiconductor diode lasers. In this paper we discuss different variations of this new laser system and the experiments of the initial demonstration.
Cesium molecules are pumped by a CW dye laser. Some of the excited molecules then pre-dissociate or collisionally transfer excitation to produce excited atoms at the 5D and 6P levels. The cross sections (in units of 10-14cm2) for the excitation transfer process Cs(6P)+Cs(5D) →Cs(7DJ)+Cs(6S)for J being 5/2 and 3/2 are 5. 4 ± 50% and 4. 9 ± 50%, respectively. The cross sections (in units of 10-14cm2) for the excitation transfer out of the 7D doublet [i. e. , Cs(7DJ)+Cs(6S) →states other than Cs(7D)] for J being 5/2 and 3/2 are 1. 6 ±50% and 0. 34±50%, respectively. To obtain these cross sections, the intensities of 7D and 5D fluorescence emissions and the 6P atom density are measured by using the absorption equivalent width technique. The results are discussed in eomparison with those of other experiments.
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)