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Acta Physica Sinica

COLLISIONAL ENERGY POOLING BETWEEN EXCITED SODIUM ATOMS

SHEN YI-FAN, LI WAN-XING, 新疆大学物理系

Published Jan 1, 1996• 0 citations
Publisher

Abstract

The collisional energy pooling process in a sodium vapor [ Na(3PJ)+Na(3PJ′)→Na(nLJ″)+Na(3S1/2)] is experimentally investigated. At sufficiently low sodium densities,such that only the optically excited 3PJ state is significantly populated, the ratios of nLJ″ fluorescence intensities and the relative density of Na(3P1/2) versus the density of Na(3P3/2) are measured. The ratios of rate coefficients k(nLJ′)(J, J′) relative to k5S(3/2,3/2) are given for nLJ″= 4D3/2, 4D5/2, 4F5/2, 4F7/2, and 5S1/2, each for J = J′ =1/2 and 3/2. At high sodium densities, the 3P states are populated in nearly statistical ratios, the ratios of the k(nLJ″)(1/2,3/2) relative to k5S(3/2,3/2) are obtained.

DOI
10.7498/aps.45.29

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The collisional energy pooling process in a sodium vapor [ Na(3PJ)+Na(3PJ′)→Na(nLJ″)+Na(3S1/2)] is experimentally investigated. At sufficiently low sodium densities,such that only the optically excited 3PJ state is significantly populated, the ratios of nLJ″ fluorescence intensities and the relative density of Na(3P1/2) versus the density of Na(3P3/2) are measured. The ratios of rate coefficients k(nLJ′)(J, J′) relative to k5S(3/2,3/2) are given for nLJ″= 4D3/2, 4D5/2, 4F5/2, 4F7/2, and 5S1/2, each for J = J′ =1/2 and 3/2. At high sodium densities, the 3P states are populated in nearly statistical ratios, the ratios of the k(nLJ″)(1/2,3/2) relative to k5S(3/2,3/2) are obtained.
We report the measurements of the temperature dependence of the cross section 4D for the energy pooling process Na(3P)+Na(3P) Na(4D)+Na(3S). The latest two, as yet undisputed, results for 4D obtained by different authors at T = 597 K and T = 483 K suggest that this cross section decreases with increasing T , which contradicts the theory and other experiments on similar processes. To resolve this controversy and to examine the temperature trend of the cross section, we have measured the 4D in the temperature range 567-705 K, covering the high-temperature region that has not yet been investigated experimentally. To determine 4D we have excited sodium atoms in the quasistatic wing of the D1 line using a cw dye laser and measured the fluorescence intensity for the 4D 3P 3/2 transition, relative to the intensity of the optically thin quasistatic wing of the D2 line. The spatial distribution of the number density of the sodium atoms in the 3P 3/2 state and the sodium ground-state number density were measured too. The method used for the determination of the cross section is advantageous since it entirely circumvents the need to account for the radiation trapping of 3P level radiation, which was substantial under experimental conditions of the ground-state densities being 10 14 -10 16 cm -3 . The measurements of the cross section 4D in the investigated temperature range have shown that it increases as ~exp(- E / kT ). From the experiment we obtained E = (608±95) cm -1 , which is in excellent agreement with the energy defect (613 cm -1 ) for the considered process, and in fair agreement with the values which follow from recent theoretical calculations.
The observation of a heteronuclear reverse energy pooling process [Rb(7D)+Na(3S)→Rb(5P)+Na(3P)] is reported. The dye-laser beam populated the Rb(7D) state by a stepwise excitation process. A double-modulation technique has been used to select the fluorescence contribution at the Na(3P_J)exit channels due only to the above reactions. The ground-state sodium atom density is obtained by measuring the wing absorption coefficient. The measured density and fluorescence intensities have been used to obtain reverse energy-pooling rate coefficients. The contribution to the rate coefficients from other processes is discussed.
The observation of a heteronuclear reverse energy pooling process is reported. The cross sections for the collision processes Rb(7D)+Na(3S)→Rb(5P)+Na(3P) have been measured. In the experiment the rubidium atoms were excited in two steps to the 7D(subscript 3/2) state. A double-modulation technique has been used to select the fluorescence contributions at the Na(3P(subscript j)) exit channels due only to the above reactions. The ground state sodium atom density was measured by the absorption of line from a Na hollow-cathode lamp. The measured density and fluorescence intensities have been used to obtain reverse energy pooling rate coefficients. The contribution to the rate coefficients from other processes is discussed.
We measured the cross section for the collision transfer process Na(4D)+Na (3S)Na(4F)+Na(3S). The 4D state was excited by two photon absorption using a tunable dye laser. Since the direct 4F→3D fluorescence can not be detected with our apparatus, we monitored instead the cascade 3D→3P and 4D→3P fluorescence. Relative intensity of the fluorescence, combined with the measurement of sodium density, yielded cross section σ_(4D)→(4F)=1.3×10~(-14)±28％(cm~2). Our result is compared to other experiments.
The formation of Na (nl\mathrm{nl}nl) and Na2+\mathrm{Na}_{2}^{}{}_{}{}^{+}Na2​​+ in sodium vapor (\ensuremath{\sim}1013{10}^{13}1013 ${\mathrm{cm}}^{\ensuremath{-}3}$) irradiated by a single-frequency (20-MHz bandwidth) cw dye laser tuned to one of the DDD lines has been studied. The dependence of product signal on laser power density suggests that Na 3p3p3p-Na 3p3p3p energy pooling collisions are responsible for these species. Rate constants for specific reaction channels and temperature dependences of rate constants are also reported.