Vlasta Horvatic

Characterization of single Au and SiO2 nano- and microparticles by ICP-OES using monodisperse droplets of standard solutions for calibration

The masses of single SiO2 and Au nano- and microspheres were measured by introducing monodisperse droplets of diluted Au and SiO2 particle suspensions into an ICP-OES system and calibrating the element line intensities by respective intensities obtained with monodisperse droplets of Si and Au standard solutions with known concentrations and diameters. Commercially available SiO2 (diameters between 470 nm and 2.06 μm) as well as 250 nm Au particles, both with small size dispersions, were used in the investigations. Evaluation of the ICP-OES signals revealed good agreement between the atomic line intensities recorded with particles and droplets from standard solutions having the same analyte mass as the particles. Detection limits of spherical solid particles by ICP-OES were found to be ∼200 nm and ∼470 nm for Au and SiO2 particles, respectively. The corresponding analyte masses at the detection limits were ∼80 fg (Au) and ∼50 fg (Si). The general applicability of the suspension technique for measurements of nanoparticles having different sizes and masses is demonstrated analyzing a suspension containing spherical SiO2 particles of three different sizes.

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.

Atmospheric helium capillary dielectric barrier discharge for soft ionization: broadening of spectral lines, gas temperature and electron number density

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 widths 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 due to 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: γνHe(Hα) = 1.56 × 10−9 cm3 s−1 and γνHe(Hβ) = 3.16 × 10−9 cm3 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× 1012 cm−3.

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).

Atmospheric Helium Capillary Dielectric Barrier Discharge for Soft Ionization: Determination of Atom Number Densities in the Lowest Excited and Metastable States

The populations of the lowest excited helium states 2s 3S1, 2s 1S, 2p 3P0 J, 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: n1 av (2s 3S1) = (2.9 ± 1.1) × 1013 cm−3, n1 av (2s 1S) = (1.4 ± 0.5) × 1013 cm−3, n1 av (2p 3P0 J) = (1.1 ± 0.4) × 1013 cm−3, n1 av (2p 1P0) = (4.2 ± 1.6) × 1012 cm−3, and they represent the average populations along the plasma column in the capillary.

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.