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Applying the cw laser absorption and fluorescence method the cross sections for the fine-structure mixing and quenching of the caesium 5DJ states have been measured in pure caesium vapour. Caesium atoms were optically excited to the 5D5/2 state via the quadrupole-allowed 6S1/2→5D5/2 transition. The ground-state caesium density N0, obtained from the absorption at the pumped quadrupole transition, and the fluorescence intensity I689 of the sensitized 5D3/2→6S1/2 emission were measured simultaneously in the range 3×1014 cm-3≤N0≤1×1016 cm-3 at constant temperature T = 585 K. It was found that the quantity N02/I689 exhibited a parabolic dependence on N0, confirming that the quenching of the Cs 5DJ states is due to collisions with Cs ground-state atoms, not molecules. The coefficients of the second-order polynomial fitted through the measured data yielded the cross sections σ5/2→3/2 = (57±19)×10-16 cm2 and σD = (35±10)×10-16 cm2 for the Cs 5DJ fine-structure mixing and quenching, respectively, due to collisions with caesium ground-state atoms. Using recently calculated Cs*+Cs potentials we performed an analysis which shows good agreement between the measured values and the theoretical predictions.

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 4D3P3/2 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 3P3/2 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 1014-1016 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.

Applying cw laser fluorescence method the cross sections for the fine-structure mixing and quenching of the Cs 6P state, induced by collisions with the ground-state caesium atoms and molecules, have been measured. Caesium atoms were optically excited to the 5DJ states via quadrupole-allowed 6S1/2 -> 5DJ transitions, while the resonance states were populated by the radiative and collisional 5DJ -> 6PJ transitions. The relative populations of the Cs 6PJ sublevels, as well as ratio of the 5D3/2 to 6P3/2 populations, were measured as a function of the caesium ground-state number density. From these measurements we obtained the cross section of (14 ą 5)x10^-16 cm^2 at T = 585 K for the process Cs(6P1/2) -> Cs(6P3/2) induced by collisions with ground-state caesium atoms. The applied experimental approach enabled the determination of the effective spontaneous rates for the 6PJ states which are in agreement with the predictions of Holstein’s theory. The cross sections for quenching of 6PJ by caesium atoms and molecules were measured at T = 635 K and the obtained values are (1.6 ą 1.4)x10^-16 cm2 and (1210 ą 260)x10^-16 cm2, respectively. Using recently calculated Cs*+Cs potentials we performed an analysis which shows a good agreement between the measured values and the theoretical estimates.

The cross sections for the excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the and transitions. The measurements yield the following cross section values for those processes investigated: , and , at .

The cross sections for the excitation energy transfer in the collisional processes Na*(3P12/)+M(nS12/) to or from Na*(3p32/)+m(Ns32/), m being ground-state Rb (n=5) or Cs (n=6) atom, have been measured by laser fluorescence method. With either of the two sodium 3P fine-structure states optically excited, the intensity ratio of the sensitized to direct fluorescence was measured. The polarization effects due to partially polarized and anisotropic direct fluorescence were investigated. The measured cross sections (T=530 K) for the sodium 3P fine-structure excitation transfer induced by collisions with Rb are: sigma (1/2 to 3/2)=170 AA2, with an overall accuracy of 20%. The corresponding values in the Cs case are: sigma (1/2 to 3/2)=140 AA2, sigma (3/2 to 1/2)=83 AA2. The theoretical calculations reveal the decisive role of the dipole-quadrupole interaction. The obtained values are in satisfactory agreement with the experiment only if both the dipole-dipole and dipole-quadrupole interactions are taken into account, while the calculations involving the dipole-dipole term alone yield the cross section values which are as much as one order of magnitude lower than the experimentally observed ones.

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: C6eff=(4.9+or-0.3)* 10-30 cm^6 s^-1 for the D2 line. The theory predicts C6eff=4.95*10-30 cm^6 s^-1 for the D1 line and C6eff=2.37*10-30 cm^6 s(-1) for the D2 line.

A diode-laser fluorescence experiment was performed in order to study fine-structure transitions between 52P states of rubidium atoms colliding with ground-state potassium or caesium atoms. The Rb (52P32/) state was optically excited and the intensity ratio of sensitized to direct fluorescence was measured. The obtained cross sections for the excitation energy transfer Rb(2P32/) to Rb(2P12/) induced by collisions with K and Cs (T=573 K) are sigma =54 AA2+or-30% and sigma =15 AA2+or-30%, respectively. Assuming detailed balancing, the cross sections for the reverse process Rb(2P12/) to Rb(2P32/) are sigma =60 AA2+or-30%, and sigma =16.5 AA2+or-30% for K and Cs, respectively.

The authors have measured the absorption coefficient in the impact region of the first potassium resonance lines broadened by rubidium atoms. For the broadening constants gamma K*-Rb they have obtained the values (6.2+or-0.5)*10-9 s-1 cm3/atom and (7.8+or-0.5)*10-9 s-1 cm3/atom for the stronger and weaker components, respectively. The theoretical values calculated from the Lindholm formula are 6.0*10-9 s-1 cm3/atom for the stronger and 7.0*10-9 s-1 cm3/atom for the weaker component. The corresponding theoretical effective C6 constants are 21.6*10-30 s-1 cm6 and 15.3*10-30 s-1 cm6, respectively.