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Helium capillary dielectric barrier discharge driven by the square wave-shaped high voltage was investigated spatially and temporally by means of optical emission spectroscopy. The finding of the previous investigation conducted with the sinusoidal-like high voltage was confirmed, i.e., the plasma in the jet and the plasma in the capillary constitute two temporally separated events. The plasma in the jet occurs prior to the discharge in the capillary and exists only during the positive half period of the applied high voltage. The time delay of the capillary discharge with respect to the discharge in the jet depended on the high voltage, and it was between 2.4 and 8.4 μs for the voltage amplitude change in the range from 1.96 to 2.31 kV, respectively. It was found that, compared to sinusoidal-like voltage, application of the square wave high voltage results with stronger (~6 times) He line emission in the jet, which makes the latter more favorable for efficient soft ionization. The use of the square wave high voltage enabled comparison of the currents (~1 mA) flowing in the capillary during the positive and negative high voltage periods, which yielded the estimation for the charge dissipated in the atmosphere ((4 ± 20 %) × 10(-11) C) through the plasma jet.

The populations of the lowest excited helium states 2s 3S1, 2s 1S, 2p 3P0J, and 2p 1P0 created in an atmospheric helium capillary dielectric barrier discharge were determined by means of optical emission spectroscopy. The emitted intensities of 388, 501, 587, and 667 nm lines were measured side-on and end-on with respect to the discharge axis. The comparison of optically thin side-on spectra with end-on spectra, which exhibited the absorption effects in the line kernels, enabled the determination of the average values of the number densities n1 in the considered He states along the plasma length L. The field of the theoretical profiles for a series of the n1L parameters pertinent to the experimental conditions was calculated for each line. By introducing the experimental data into the field of calculated curves, n1L corresponding to the particular state could be obtained. The measurements of the emission profiles were done as a function of the discharge voltage in the range covering homogeneous as well as filamentary DBD operation mode. Due to nonuniformity of the excited atom density distribution along the plasma, the values of n1 could be obtained only in the homogeneous operation mode where the nonuniformity was small. The following maximum values were found for the number densities in the investigated states: n1av (2s 3S1) = (2.9 ± 1.1) × 10^13 cm^–3, n1av (2s 1S) = (1.4 ± 0.5) × 10^13 cm^–3, n1av (2p 3P0J) = (1.1 ± 0.4) × 10^13 cm^–3, n1av (2p 1P0) = (4.2 ± 1.6) × 10^12 cm^–3, and they represent the average populations along the plasma column in the capillary.

Among different applications of dielectric barrier discharge (DBD) plasma, the soft ionization ability is certainly one of the most interesting. In this paper the helium plasma jet, produced by a capillary DBD, penetrating in the ambient atmosphere, has been spectroscopically investigated in dependence on applied voltage and helium flow. It was found that the change of the applied voltage leads to different discharge modes. Based on the measurements of the emission spectra of atomic He and N 2 + and N 2 molecules in the capillary and in the plasma jet with high spatial resolution, it can be assessed in which mode, i.e. under which conditions the plasma jet is expected to be most effective for soft ionization of molecules.

A capillary He dielectric barrier discharge was investigated with respect to its performance as a soft or dissociative ionization source. Spatiotemporal measurements of the plasma emission showed that in one voltage duty cycle the plasma evolved from a soft to dissociative ionization source. At the earliest time, the soft plasma was generated between the electrodes as well as outside the capillary forming the plasma jet. It was characterized by significant radiation arising only from He and N2(+), which are known to be important in the process of the soft ionization of the analyte. Later in time, the plasma capable of dissociating molecules develops. It is characterized by appreciable radiation from analyte dissociation products and is restricted to the interelectrode region in the capillary. Thus, for the soft ionization purposes, it is feasible to introduce the analyte exclusively in the plasma jet. For elemental analysis, the interelectrode plasma is appropriate.

The present work is a systematic experimental study of the plasma formation in cesium vapor induced by a continuous laser tuned to the resonance transition 6S1/2–6P3/2. Taking into account the measured absolute population densities of Cs ground and excited state atoms as well as the electron densities derived from Stark broadening of the Cs lines, complete local thermodynamic equilibrium in the laser-produced plasma was found for laser power densities ≈ 10 Wcm− 2 at cesium ground state number densities of about 1017 cm− 3. Direct conversion of the excitation energy or parts of the excitation energy in exothermic collisions of laser-excited atoms is concluded to be the major process for atomic vapor heating and subsequent formation of LTE plasmas.

The collisional broadening and shift rate coefficients of the “ forbidden“ 6p2 3P0 -> 6p2 3P1 transition in lead were determined by diode laser absorption measurements performed simultaneously in two resistively heated hot-pipes. One hot-pipe contained Pb vapor and noble gas (Ar or He) at low pressure, while the other was filled with Pb and noble gas at variable pressure. The measurements were performed at temperatures of 1220 K and 1290 K, i.e., lead number densities of 4.8x10^15 cm^(-3) and 1.2x10^16 cm^(-3). The broadening rates were obtained by fitting the experimental collisionally broadened absorption line shapes to theoretical Voigt profiles. The shift rates were determined by measuring the difference between the peak absorption positions in the spectra measured simultaneously in the heat pipe filled with noble gas at reference pressure and the one with noble gas at variable pressure. The following data for the broadening and shift rate coefficients due to collisions with Ar and He were obtained: gammaB-Ar= (3.4 ± ; ; 0.1)x10^(-10) cm^3 s^(-1), gammaB-He= (3.8 ± ; ; 0.1)x10^(-10) cm^3 s^(-1), gammaS-Ar= (-7.3 ± ; ; 0.8)x10^(-11) cm^3 s^(-1), gammaS-He= (-6.5 ± ; ; 0.7)x10^(-11) cm^3 s^(-1).

A simple and accurate way of determining atom number densities in dense rubidium vapors is presented. The method relies on the experimental finding that the reduced absorption coefficients of the Rb triplet satellite bands between 740 nm and 750 nm and the triplet diffuse band between 600 nm and 610 nm are not temperature dependent in the range between 600 K and 800 K. Therefore, the absolute values of the reduced absorption coefficients of these molecular bands can provide accurate information about atomic number density of the vapor. The rubidium absorption spectrum was measured by spatially resolved white-light absorption in overheated rubidium vapor generated in a heat pipe oven. The absolute values for the reduced absorption coefficients of the triplet bands were determined at lower vapor densities, by using an accurate expression for the reduced absorption coefficient in the quasistatic wing of the Rb D1 line, and measured triplet satellite bands to the resonance wing optical depth ratio. These triplet satellite band data were used to calibrate in absolute scale the reduced absorption coefficients of the triplet diffuse band at higher temperatures. The obtained values for the reduced absorption coefficient of these Rb molecular features can be used for accurate determination of rubidium atomic number densities in the range from about 5 × 10 16 cm − 3 to 1 × 10 18 cm − 3 .

The spatially and temporally resolved optical emission from the helium capillary dielectric barrier discharge driven by a square wave high voltage was measured simultaneously with the discharge currents during the positive and negative voltage periods. The He and N 2 + emission intensities were monitored at 501 nm and 391 nm, respectively. The plasma jet was found to be formed only during the positive voltage period, and it preceded the discharge ignition in the capillary. Thus, the measurements of the difference between the capillary discharge currents during the positive and negative voltage periods enabled quantification of the charge, which was transported through the plasma jet. The analysis of the spatiotemporal behavior of the He emission intensities in the plasma jet yielded the velocity of the helium excitation propagation and the effective plasma jet length. The combination of these measurements enabled determination of the absolute values of the plasma jet currents. The linear relationship between the He emission intensities in the plasma jet and the plasma jet currents was found to exist over the emission intensity range of two orders of magnitude. The N 2 + emission intensity was also found to be linearly dependent on the plasma jet current. The fast electrons, producing the excited He atoms, were found to be located mostly within the estimated effective plasma jet length. In this region a cascade of processes involving excited He atoms, N 2 + and H 2 O molecules lead to establishment of the conditions necessary for the plasma jet to be used as soft ionization source. The production of reactive species can be related to the plasma jet current, which can serve as a measure of the soft ionization efficiency.

A helium capillary dielectric barrier discharge was investigated by means of time-resolved optical emission spectroscopy with the aim of elucidating the process of the formation of the plasma jet. The helium emission line at 706 nm was utilized to monitor spatial and temporal propagation of the excitation of helium atoms. The discharge was sustained with quasi-sinusoidal high voltage, and the temporal evolution of the helium atomic emission was measured simultaneously with the discharge current. The spatial development of the plasma was investigated along the discharge axis in the whole region, which covers the positions in the capillary between the electrodes as well as the plasma jet outside the capillary. The high voltage electrode was placed 2 mm from the capillary orifice, and the distance between the ground and high voltage electrode was 10 mm. The complete spatiotemporal grid of the development of the helium excitation has shown that during the positive half-period of the applied voltage, two independent plasmas, separated in time, are formed. First, the early plasma that constitutes the plasma jet is formed, while the discharge in the capillary follows subsequently. In the early plasma, the helium atom excitation propagation starts in the vicinity of the high voltage electrode and departs from the capillary towards the ground electrode as well as several millimeters outside of the capillary in the form of the plasma jet. After relatively slow propagation of the early plasma in the capillary and the jet, the second plasma starts between the electrodes. During the negative voltage period, only the plasma in the capillary between the electrodes occurs.

Excited states population distributions created by two-step 6S 1/2 → 6P 3/2 → 6D 5/2 laser excitation in room temperature cesium vapor were quantitatively analyzed applying absorption and saturation spectroscopy. A simple method for the determination of the excited state population in a single excitation step that is based on the measurements of the saturated and unsaturated absorption coefficients was proposed and tested. It was shown that only ≈ 2% of the ground state population could be transferred to the first excited state by pumping the Doppler broadened line with a single-mode narrow-line laser. With complete saturation of the second excitation step, the population amounting to only ≈ 1% of the ground state can be eventually created in the 6D 5/2 state. The fluorescence intensity emerging at 7P 3/2 → 6S 1/2 transition, subsequent to the radiative decay of 6D 5/2 population to the 7P 3/2 state, was used to assess the efficiency of the population transfer in the chosen two-step excitation scheme. The limitations imposed on the sensitivity of such resonance fluorescence detector caused by velocity-selective excitation in the first excitation step were pointed out and the way to overcome this obstacle is proposed.

The cross-sections for collisional excitation transfer Cs ⁡ ( 6 ⁢ D ) → Q D ⁢ 3 Cs ( 7 P 1 / 2 ) and Cs ⁡ ( 6 ⁢ D ) → Q D ⁢ 4 Cs ( 7 P 3 / 2 ) , and fine structure mixing Cs ( 6 D 5 / 2 ) → Q 65 Cs ( 6 D 3 / 2 ) due to collisions with helium and argon atoms at temperatures in the range between 333 and 367 K were determined using atomic fluorescence. The cross-sections for the processes Cs(6D) + He, Ar → Cs(7PJ) + He, Ar were found to be QD3He = (0.9 ± 0.4) × 10− 16 cm2, QD4He = (2.6 ± 1.2) × 10− 16 cm2, QD3Ar = (1.2 ± 0.6) × 10− 18 cm2 and QD4Ar = (2.6 ± 1.3) × 10− 18 cm2. These values are consistent with the results obtained at higher temperatures and reported previously. The cross-sections for the He and Ar-induced 6DJ fine-structure mixing were determined to be Q65He = (54 ± 15) × 10− 16 cm2 and Q65Ar = (17 ± 5) × 10− 16 cm2 and were found to fit very well into the existing body of cross-section data for intra-multiplet (nP1 / 2 → nP3 / 2; nD5 / 2 → nD3 / 2) mixing in cesium induced by collisions with helium and argon.

A small, low-pressure dielectric barrier discharge used as a detector for the analysis of halogenated hydrocarbons was studied by diode laser absorption spectroscopy of excited plasma atoms. The distribution, as well as diffusion of the excited atoms, was measured with high spatial and temporal resolution. The major part of the excited atoms was found in a very narrow discharge volume, where the maximum gas temperature and electron density, determined from broadening of the absorption line profiles, were approximately 1000 K and greater than 10 15 cm −3 , respectively.

The capillary helium dielectric barrier discharge (DBD) operating at atmospheric pressure was investigated by means of optical emission spectroscopy with the aim to determine the dominant broadening mechanism of the helium spectral lines, gas temperature and electron number density. The dependence of emission profiles of helium 388 nm, 501 nm, 587 nm, 667 nm, 706 nm and 728 nm lines on discharge voltage, helium pressure and position along the DBD capillary was investigated. Also, the pressure and voltage dependence of the profiles of hydrogen H-alpha and H-beta lines was examined. The Lorentzian width4 s of the normalized helium line profiles were found to be constant with respect to the applied voltage and the position along the capillary. The dominant broadening mechanism for all investigated lines was identified to be collisions with ground-state helium atoms, with the Stark broadening being negligible. It was determined that the temperature of the gas was constant along the capillary and independent of the voltage applied on the DBD electrodes and that its value coincided with the room temperature. The measurements of the dependence of the Lorentzian width of hydrogen H-alpha and H-beta lines on helium pressure, combined with gas temperature determined in the experiment, yielded the following values for the broadening parameters due to broadening by neutral helium: gamma He(Halpha)=1.56 x 10^(-9)cm^3 s ^(-1) and gamma He(Hbeta)= 3.16 x 10^(-9)cm^3 s ^(-1). From the analysis of the measured H-alpha and H-beta line profiles the upper limit of the electron number density in the investigated plasma was obtained as ne < 1.4 10^12 cm^(- 3).

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.

The collisional broadening and shift rate coefficients of the 283.39 nm (6p 2 3P0 → 7s 3P 1 ), 364.06 nm (6p 2 3P1 → 7s 3P 1 ), 368.45 nm (6p 2 3P1 → 7s 3P 0 ) and 405.90 nm (6p 2 3P2 → 7s 3P 1 ) Pb lines by He and Ar have been measured by fitting the experimental absorption line shapes to theoretical Voigt profiles. The absorption measurements were performed in a resistively heated, Pb loaded oven with an integrated dc noble gas discharge to produce also Pb atoms in the 6p 2 3P1 and 6p 2 3P2 metastable states. The diffusion of the metastable atoms out of the discharge zone into the neutral noble-gas atmosphere enabled the line-shape and shift measurement of the lines involving the metastable states without the influence of the discharge plasma.

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 total rate coefficient κ for the energy pooling process Cs * (6P) + Cs * (6P) → Cs(6S) + Cs ** (6D) has been measured in dependence on the ratio of the number densities of the Cs 6P fine-structure sublevels. The measurements were performed applying the laser fluorescence method. From the set of the obtained data for k, the rate coefficients for the particular Cs * (6P J ) + Cs * (6P J' ) collisions were derived. The obtained values for the Cs * (P 1/2 ) + Cs * (P 1/2 ), Cs * (P 1/2 ) + Cs * (P 1/2 ) and Cs * (P 3/2 ) + Cs * (P 1/2 ) collisions at T = 600 K are κ 1 = 2 x 10 -9 cm 3 s -1 , κ 2 = 3.1 x 10 -10 cm 3 s -1 and κ 3 < 3 x 10 -10 cm 3 s -1 , respectively. The value for κ 1 and the estimate for κ 3 are consistent with the data previously published by other authors. The rate coefficient κ 2 is reported for the first time.

The energy-pooling rate coefficient for the \(\)\(\) process has been measured. The barium atoms were excited by a cw diode laser tuned to the frequency of the 791.3 nm intercombination line and the metastable atoms in the 6s5d \(\) state were produced due to radiative and collisional depopulation of the laser-excited 6s6p \(\) state. The measurements were performed at \(\) number densities of about \(\) and at 30 mbar argon as the buffer gas. Most of the barium ground state atoms in the excitation zone were transferred to the trip let metastable state at the laser pump power applied. The energy pooling rate coefficient was determined by comparing the fluorescence intensity of the barium 553.6 nm resonance line and the fluorescence intensity of the intercombination line 791.3 nm. In addition, the populations of the metastable atoms were probed with low intensity laser radiation from a single mode ring dye laser. The rate coefficient was found to be \(\) at \(\).

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 section for the fine-structure excitation transfer Cs(6P1/2) -> Cs(6P3/2), induced by collisions with the ground state potassium atoms, has been measured by resonant Doppler-free two-photon spectroscopy. The population densities of caesium 6PJ (J = 1/2, 3/2) levels were probed by thermionic detection of the collisionally ionized caesium atoms from the Cs(6PJ) -> Cs(10S1/2) excitation channel. The cross section for the transfer process at the temperature T = 503 K has been found to be sigma(1/2 -> 3/2) = 45*10(-16) cm(2) +- 20%. The result is compared with previously published experimental cross sections for fine-structure transfer in resonance states of other alkali elements perturbed by potassium and a theoretical value of the Li(2PJ) - K system calculated in a simple approach.

Excitation transfer between the barium low lying excited states 6s6p 3 P 1 0 , 6s5d 1 D 2 and 6s5d 3 D J by collisions with He,Ar,Xe and Ba has been investigated. The population densities in all levels involved were probed by absorption or by fluorescence usingcw lasers. The depopulation cross sections of the Ba3 P 1 0 state by collisions with noble gases were found to be σHe(3 P 1 0 )=5.5·10−16 cm2, σAr(3 P 1 0 )=4.6·10−16 cm2, and σXe(3 P 1 0 )=1.7·10−16 cm2. For Ar, the collisional depopulation of the3 P 1 0 level is exclusively due to the transition to the1 D 2 state. Under the assumption that the3 D J metastable states are populated collisionally by1 D 2 →3 D J transfer only, we have deduced the upper limit for the corresponding cross section σ 13 Ar =1.5·10−18 cm2. From the Ba1 D 2 and Ba3 D J steady-state diffusion distributions, collisional relaxation rates of the1 D 2 and3 D J levels were evaluated. The collisional relaxation rates by Ar and Ba yielded total cross sections for the depopulation of metastable levels: σAr(1 D 2)=1.5·10−17 cm2, σBa(1 D 2)℞1·10−13 cm2, σAr(3 D J)=7·10−21 cm2, and σBa(3 D J)=1·10−15 cm2. Furthermore, it was found that the main contribution of the collisional depopulation of the1 D 2 state by Ar is related to back transfer to the3 P 0 state, whereas the deactivation of the3 D J metastable state is due to back transfer to the1 D 2 state. Taking into account other cross sections reported in literature we can conclude that collisional deactivation of both metastable levels by Ba ground state atoms can be attributed to their mutual collisional mixing.

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.

An atmospheric pressure microplasma ionization source based on a dielectric barrier discharge with a helium plasma cone outside the electrode region has been developed for liquid chromatography/mass spectrometry and as ionization source for Ion mobility spectrometry. It turned out that dielectric barrier discharge ionization could be regarded as a soft ionization technique characterized by only minor fragmentation similar to atmospheric pressure chemical ionization (APCI). Mainly protonated molecules were detected. In order to characterize the soft ionization mechanism spatially resolved optical emission spectrometry (OES) measurements were performed on plasma jets burning either in He or in Ar. Besides to spatial intensity distributions of noble gas spectral lines, in both cases a special attention was paid to lines of N2+ and N2. The obtained mapping of the plasma jet shows very different number density distributions of relevant excited species. In the case of helium plasma jet, strong N2+ lines were observed. In contrast to that, the intensities of N2 lines in Ar were below the present detection limit. The positions of N2+ and N2 distribution maxima in helium indicate the regions where the highest efficiency of the water ionization and the protonation process is expected.

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.

The “forbidden” Pb 6p2 3P 0 ↦6 p 2 3P1 line at 1278.9 nm is measured by diode laser absorption in a resistively heated hot-pipe filled with a lead vapour and argon as buffer gas. The measurements performed at a temperature of 1170 K and a lead number density of 2.4×10 15 cm-3 yield the oscillator strength f F = (4.5±1.1)×10 -7 which corresponds to a radiative transition probability A F = (6.1±1.5) s-1. Within the error bars, the result is in agreement with theoretical data published by different authors.

The measurement of the collisional cross section for the process Li*(2P)+Cs(6S)→Li(2S)+Cs*(5D) are reported. The technique of resonant Doppler-limited two-photon laser excitation with thermionic detection is applied. The population density of the Cs*5D state is probed by photoionization, and the signals of the Cs(6S)→Cs*(5D) and the Li(2S)→Li*(2P) transitions are compared. The value for cross section of 30 A2 is measured, with an accuracy of 45%.

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.

Elemental fractionation in femtosecond laser ablation is studied by ICP-MS by applying successive single laser shots to binary metallic and semiconductor samples as well as to multi-component glasses. Fractionation can be observed in the first laser shots in particular if the laser fluence is near the ablation threshold of the sample. However, the element ratio in the laser-sampled masses changes from shot to shot until it reaches an asymptotic fluence-independent value representing stoichiometric sampling. The asymptotic stoichiometric ratios can be obtained with fewer shots if higher laser fluences are applied. It is shown by electron probe X-ray analysis that different elemental ablation probabilities modify the element compositions in the surface layers of the laser craters until equilibrium conditions are obtained. These conditions can be reached by applying many shots of low laser fluence or with one high-fluence laser shot only. The experimental data reveal that in most cases the elemental ablation probability can be correlated with the respective ionization energies of the elements, i.e., the elements with lower first ionization energy have higher ablation probability. No or only very weak fractionation was observed when elements with nearly the same ionization energies were sampled.

Experimental studies of the absorption spectrum of the ${\mathrm{Rb}}_{2}$ dimer are performed in the $600--1100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength range for temperatures between 615 and $745\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The reduced absorption coefficient is measured by spatially resolved white light absorption in overheated rubidium vapor with a radial temperature gradient, which enables simultaneous measurements at different temperatures. Semiclassical and quantum spectral simulations are obtained by taking into account all possible transitions involving the potential curves stemming from the 5 $^{2}S+5$ $^{2}S$ and 5 $^{2}S+5$ $^{2}P$ asymptotes. The most accurate experimental potential curves are used where available, and newly calculated potential curves and transition dipole moments otherwise. The overall consistency of the theoretical model with the experimental interpretation is obtained only if the radial dependence of both the calculated transition dipole moments and the spin-orbit coupling is taken into account. This highlights the low-resolution absorption spectroscopy as a valuable tool for checking the accuracy of molecular electronic structure calculations.

Quantum efficiency improvement of a cesium based resonance fluorescence detector (RFD) was achieved by enhancing the transfer in a particular channel of the RFD excitation scheme with noble gas-induced collisional excitation energy transfer (CEET). The influence of Cs–Ar and Cs–He collisional mixing between the 6D and 7P states in cesium on the quantum efficiency of the 6S → 6D → 7P → 6S excitation scheme was investigated by fluorescence measurements at relevant transitions. Ar-induced CEET was found to have little effect on the fluorescence response and quantum efficiency of the Cs RFD excitation scheme. However, a 35 fold quantum efficiency increase in the cesium resonance fluorescence detector response at only moderate He pressures was observed.

This paper is a review (with a few original additions) on the radiative transport and collisional transfer of energy in laser-excited cesium vapors in the presence of argon or helium. Narrow-band excitation of lines with Lorentz, Doppler and Voigt profiles is studied in order to calculate effective rates for pumping of spectral lines with profiles comprising inhomogeneous broadening components. The radiative transport of excitation energy is considered, and a new, simple and robust, but accurate theoretical method for quantitative treatment of radiation trapping in relatively optically thin media is presented. Furthermore, comprehensive lists of experimental values for the excitation energy transfer cross-sections related to thermal collisions in Cs–Ar and Cs–He mixtures are given. Within the collected cross-section data sets, specific regularities with respect to the energy defect, as well as the temperature, are discerned. A particular emphasis is put on the radiative and collisional processes important for the optimization of resonance–fluorescence imaging atomic filters based on Cs–noble gas systems.

We report an experimental study of enegy pooling collisions involving Cs atom in the 6P and 5D states. The 5D state was populated by a cw dye-laser tuned to the cesium dipole-forbidden transition 6S->5D at 685.0 nm. The 6P state was populated by subsequent radiative relaxation of the 5D state. The 6P population density was determined from the absorption of a cw diode-laser probe beam. The population densities of the 5D state and the higher, by energy pooling excited states, were determined by measuring the corresponding fluorescence intensities relative to the fluorescent intensity form the optically thin quasi-static wings of the cesium D2 line. The rate coefficient for the process Cs(6P) + Cs(6P) -> Cs(6D) + Cs(6S) is found to be (4.2+- 0.13)x10(-10) cm(3) s(-1) at T = 570 K. In addition, estimates of the rate coefficients for the processes Cs(6P) + Cs(5D) -> Cs(7D) + Cs(6S) and Cs(5D) + Cs(5D) -> Cs(7F) + Cs(6S) are given.

Applying diode-laser resonant fluorescence method, the cross sections for the excitation energy transfer of the collisional process K*(4 2P1/2)+Cs(6 2S1/2)K*(4 2P3/2)+Cs(6 2S1/2) have been measured. The values we have obtained are sigma(1/2->3/2)=77 Å ; 2 and sigma(3/2->1/2)=48 Å ; 2. These results complete the sequence of data for the fine-structure mixing of the first-resonance states of alkali atoms colliding with the ground-state caesium atoms.

Applying resonant Doppler-free 2-photon laser spectroscopy with thermionic diode detection, the cross sections for the excitation energy transfer of the collisional process7Li*(2P 1/2+Cs(6S 1/2)→7Li*(2P 3/2)+Cs(6S 1/2) have been measured. The experimental cross sections, σLi-Cs (1/2→3/2)=890 Å2 and σLi-Cs (3/2→1/2)=430 Å2, are compared with theoretical data obtained by a sudden impact approximation approach taking into account the long-range interaction potentials only. The calculated cross sections show an excitation mixing process at large internuclear distances where Li-Cs dipole-dipole and dipole- quadrupole interaction forces are predominant.

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.

The authors have measured the absorption efficient in the quasi-static wings of the first potassium resonance lines in various K-Cs mixtures. At sufficiently high concentrations of caesium atoms both components of the potassium first resonance doublet have extended blue wings of the van der Waals type. The corresponding effective C6 constants determined from the experimental data are (1.5+or-0.3)*10-29 s-1 cm6 and (2.4+or-0.5)*10-29 s-1 cm6 for the stronger and weaker components, respectively. The calculated values of the effective C6 constants are 1.78*10-29 s-1 cm6 for the stronger component and 2.35*10-29 s-1 cm6 for the weaker component.