Cedomil Vadla

Elemental fractionation and stoichiometric sampling in femtosecond laser ablation

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.

Population and deactivation of lowest lying barium levels by collisions with He, Ar, Xe and Ba ground state atoms

Excitation transfer between the barium low lying excited states 6s6p3P10, 6s5d1D2 and 6s5d3DJ 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 Ba3P10 state by collisions with noble gases were found to be σHe(3P10)=5.5·10−16 cm2, σAr(3P10)=4.6·10−16 cm2, and σXe(3P10)=1.7·10−16 cm2. For Ar, the collisional depopulation of the3P10 level is exclusively due to the transition to the1D2 state. Under the assumption that the3DJ metastable states are populated collisionally by1D2 →3DJ transfer only, we have deduced the upper limit for the corresponding cross section σ13Ar=1.5·10−18 cm2. From the Ba1D2 and Ba3DJ steady-state diffusion distributions, collisional relaxation rates of the1D2 and3DJ levels were evaluated. The collisional relaxation rates by Ar and Ba yielded total cross sections for the depopulation of metastable levels: σAr(1D2)=1.5·10−17 cm2, σBa(1D2)℞1·10−13 cm2, σAr(3DJ)=7·10−21 cm2, and σBa(3DJ)=1·10−15 cm2. Furthermore, it was found that the main contribution of the collisional depopulation of the1D2 state by Ar is related to back transfer to the3PJ0 state, whereas the deactivation of the3DJ metastable state is due to back transfer to the1D2 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.

Radiative transport and collisional transfer of excitation energy in Cs vapors mixed with Ar or He

Abstract 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.

Capillary Dielectric Barrier Discharge: Transition from Soft Ionization to Dissociative Plasma.

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 non-Lorentzian wings of alkali resonance lines: the determination of the atom number density in pure and mixed alkali vapours

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 C6 constants for Rb*-Cs interaction: C6eff=(4.9+or-0.3)* 10-30 cm6 s-1 for the D2 line. The theory predicts C6eff=4.95*10-30 cm6 s-1 for the D1 line and C6eff=2.37*10-30 cm6 s(-1) for the D2 line.

The collision cross sections for excitation energy transfer in Rb*(5P3/2) + K(4S1/2) → Rb(5S1/2) + K*(4P J ) processes

The collisional excitation transfer for the processes Rb*(5P3/2)+K(4S1/2) → Rb(5S1/2)+K*(4PJ),J=1/2, 3/2, was investigated using two-photon laser excitation technique with a thermionic heat-pipe diode as a detector. The population densities of the K 4PJ levels induced by collisions with excited Rb atoms as well as those produced by direct laser excitation of the potassium atoms were probed through the measurement of the thermionic signals generated due to the ionization of the potassium atoms emerging from the K(4PJ) → K(7S1/2) excitation channel. The measurements of the thermionic signals in addition to the spectroscopic determination of the potassium number density yield the following values for the excitation transfer cross sections: σ1(Rb 5P3/2 → K 4P1/2)=8 Å2 and σ2(Rb 5P3/2 → K 4P3/2)=11 Å2. The accuracy of the presented results is ∓15%. The obtained results are compared to those for the opposite processes K*(4PJ)+Rb(5S1/2) → K(4S1/2)+Rb*(5P3/2).

Caesium 6P fine-structure mixing and quenching induced by collisions with ground-state caesium atoms and molecules

Applying the cw laser fluorescence method, the cross sections for the fine structure mixing and quenching of the Cs 6P state, induced by collisions with ground-state caesium atoms and molecules, have been measured. Caesium atoms were optically excited to the 5DJ states via quadrupole-allowed 6S1/25DJ transitions, while the resonance states were populated by the radiative and collisional 5DJ6PJ 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±5) × 10-16 cm2 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 Holsteins 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±1.4) × 10-16 cm2 and (1210±260) × 10-16 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 collision cross sections for the fine-structure mixing of caesium 6P levels induced by collisions with potassium atoms

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 temperatureT=503 K has been found to be σ(1/2 → 3/2)=45 Å2 ± 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 thoeretical value of the Li(2PJ)-K system calculated in a simple approach.

Collision cross sections for excitation energy transfer in Na* (3P1/2)+K(4S1/2)⇔Na*(3P3/2)+K(4S1/2) processes

The cross sections for the excitation energy transfer between the 32PJ states of sodium atoms by collisions with ground-state potassium atoms have been measured by resonant Doppler-free two-photon spectroscopy, where the population densities of directly pumped and collisionally excited Na(3PJ)(J=1/2, 3/2) levels were probed by counter-propagating Na(3PJ) → Na(4D3/2, 5/2) excitation and detected with the thermionic diode. Cross sections of σ(3P1/2 → 3P3/2)=190 Å2±20% and σ(3P3/2 → 3P1/2)=100 Å2±20% were found. The theoretical calculations taking into account the long-range interaction termsR−6,R−8 andR−10 yield a value σ(3P1/2 → 3P3/2)=165 Å2. On the basis of these long-range interaction potentials the differential cross section has been calculated and compared with recently published experimental data. Very good agreement between the theoretical and experimental data was found.

THE TEMPERATURE DEPENDENCE OF THE CROSS SECTION FOR THE ENERGY POOLING PROCESS NA(3P) + NA(3P) NA(4D) + NA(3S)

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.