Collisional Excitation Transfer Research Articles

Method for Determining Cross Sections of Excitation Transfer in Collisions with Atoms of Rare-earth Metals. 2. Application of the Method

Method for Determining Cross Sections of Excitation Transfer in Collisions with Atoms of Rare-earth Metals. 2. Application of the Method

Method for Determining Cross Sections of Excitation Transfer in Collisions with Atoms of Rare-earth Metals. 1. Description of the Method

Method for Determining Cross Sections of Excitation Transfer in Collisions with Atoms of Rare-earth Metals. 1. Description of the Method

Quantum dynamics of the energy transfer for vibrationally excited HF (v = 7) colliding with D2 (v = 0): Theory assessing experiment.

It is still challenging to accurately qualify the rate coefficients for vibrationally excited molecules in experiment. In particular, for the energy transfer between HF (v = 7) and D2 (v = 0), which is a prototype for near resonant collisional transfer of vibrational excitation from one molecule to the other, the two available experimental results of rate coefficients contradict each other by a factor of nearly 20. In order to benchmark these data, in this work, the rate coefficients of vibration-vibration energy transfer processes of this system at temperatures ranging from 100 to 1500 K were calculated by employing the coupled-states approximation based on our recently developed full-dimensional ab initio intermolecular potential energy surface. The state-to-state rate coefficients were found to follow the general energy gap law. The calculated total vibration-vibration energy transfer rate coefficients decrease with the increase in the angular momentum of HF at most temperatures. The vibrational relaxation rate coefficient decreases monotonously with the temperature, and the calculated result of 8.1 × 10-11 cm3 mol-1 s-1 at room temperature is in very good agreement with the experimental value reported by Dzelzkalns and Kaufman [J. Chem. Phys. 77, 3508 (1982)].

Technique for determining cross sections of excitation transfer in collisions with atoms of rare-earth metals. 2. Application of the method

Technique for determining cross sections of excitation transfer in collisions with atoms of rare-earth metals. 2. Application of the method

Method for determining cross sections of excitation transfer in collisions with atoms of rare-earth metals. 1. Description of the method

Method for determining cross sections of excitation transfer in collisions with atoms of rare-earth metals. 1. Description of the method

Studies of Thallium Line Spectra in Thallium – Mercury Discharge

In this paper, we present a study of broadening of thallium emission spectral line shapes in the Tl-Hg discharge. The spectral lines were emitted from high frequency electrodeless lamps, containing Tl, Hg, Ar mixtures and measured by means of Fourier transform spectrometer. The deconvolution procedure, by means of ill posed inverse task solution, was performed to obtain the real (without instrumental function) profiles for further analyze. The solution was implemented using Tikhonov regularization algorithm. The Tl 276.8 nm, 351.9 nm, 352.9 nm spectral lines were analyzed in detail in dependence on the discharge power. The additional broadening of Tl 276.8 nm and 351.9 nm lines were observed due to the excitation transfer in collisions of ground state Tl atoms with excited Hg atoms.

Measurements of cesium mixing and quenching cross sections in methane gas: understanding sources of heating in cesium vapor lasers.

Accurate modeling of the operation of diode-pumped alkali lasers is a critical step toward the design of high-powered devices. We present precision measurements for the Cs-CH4 62P3/2 → 62P1/2 mixing cross section and the 62P3/2,1/2 → 62S1/2 quenching cross section, which are important parameters in understanding the operation and, in particular, the heat generated in a cesium vapor laser. Measurements are carried out using ultrafast laser pulse excitation and observation of fluorescence due to collisional excitation transfer in time is done using the technique of time-correlated single-photon counting. Mixing rate measurements are acquired over methane pressures of 10 - 40 Torr, resulting in a Cs-CH4 62P3/2 → 62P1/2 mixing cross section of (1.40 ± 0.08) × 10-15 cm2, while quenching rate measurements are carried out over methane pressures of 500 - 4000 Torr, resulting in a 62P3/2,1/2 → 62S1/2 quenching cross section of (1.57 ± 0.03) × 10-18 cm2. These results suggest only a slight contribution to the heating of a cesium vapor laser is due to Cs 62P quenching, contrary to previous studies. We also discuss additional possible sources of energy transfer from upper excited states of Cs.

Collisional excitation transfer and quenching in Rb(5P) -methane mixtures

We have examined fine-structure mixing between the rubidium $5^{2}P_{3/2}$ and $5^{2}P_{1/2}$ states along with quenching of these states due to collisions with methane gas. Measurements are carried out using ultrafast laser pulse excitation to populate one of the Rb $5^{2}P$ states, with the fluorescence produced through collisional excitation transfer observed using time-correlated single-photon counting. Fine-structure mixing rates and quenching rates are determined by the time dependence of this fluorescence. As Rb($5^{2}P$) collisional excitation transfer is relatively fast in methane gas, measurements were performed at methane pressures of $2.5 - 25$ Torr, resulting in a collisional transfer cross section ($5^{2}P_{3/2} \rightarrow 5^{2}P_{1/2}$) of $(4.23 \pm 0.13) \times 10^{-15}$ cm$^{2}$. Quenching rates were found to be much slower and were performed over methane pressures of $50 - 4000$ Torr, resulting in a quenching cross section of $(7.52 \pm 0.10) \times 10^{-19}$ cm$^{2}$. These results represent a significant increase in precision compared to previous work, and also resolve a discrepancy in previous quenching measurements.

Non-LTE line formation of Fe in late-type stars – IV. Modelling of the solar centre-to-limb variation in 3D

Our ability to model the shapes and strengths of iron lines in the solar spectrum is a critical test of the accuracy of the solar iron abundance, which sets the absolute zero-point of all stellar metallicities. We use an extensive 463-level Fe atom with new photoionisation cross-sections for FeI as well as quantum mechanical calculations of collisional excitation and charge transfer with neutral hydrogen; the latter effectively remove a free parameter that has hampered all previous line formation studies of Fe in non-local thermodynamic equilibrium (NLTE). For the first time, we use realistic 3D NLTE calculations of Fe for a quantitative comparison to solar observations. We confront our theoretical line profiles with observations taken at different viewing angles across the solar disk with the Swedish 1-m Solar Telescope. We find that 3D modelling well reproduces the observed centre-to-limb behaviour of spectral lines overall, but highlight aspects that may require further work, especially cross-sections for inelastic collisions with electrons. Our inferred solar iron abundance is log(eps(Fe))=7.48+-0.04.

Calcium-hydrogen interactions for collisional excitation and charge transfer.

The accurate highly correlated ab initio calculations for ten low lying covalent Σ+2 states of CaH molecule, and one ionic Ca+H- state, are performed using large active space and extended basis set, with special attention to the long-range (6-20 Å) region where a number of avoided crossings between ionic and covalent states occur. These states are further transformed to a diabatic representation using a numerical diabatization scheme based on the minimization of derivative coupling. This results in a smooth diabatic Hamiltonian which can be easily fit to an analytic form. The diagonal elements of the diabatic potentials were then empirically corrected to reproduce experimental dissociation energies. Though the emphasis is on the asymptotic region, the obtained spectroscopic constants are in good agreement with available experimental and theoretical data. The resulting analytical Hamiltonian, after back transformation to adiabatic representation, is used to obtain cross sections for different inelastic processes using both the multichannel and the branching probability current approaches. It is shown that while for most intense transitions both approaches provide very close results, the multichannel approach underestimates the cross sections of weak transitions, as a consequence of the short-range avoided crossings that are accounted for only in the branching probability current method.

Breakdown development in helium in high voltage open discharge with subnanosecond current front

High-voltage open discharge experiments in helium [, ] showed a possibility to control the electron avalanche development on a subnanosecond timescale. The controlled current growth rate was of 500 A (cmns) for an applied voltage of 20 kV and gas pressure of 6 Torr. Here, a kinetic model of the breakdown is developed to analyze the mechanism of current growth, which takes into account the kinetics of electrons, ions, fast atoms and photons with a Doppler shift (DS). Using particle-in-cells simulations, we show a critical role of DS photons in the secondary electron emission during the breakdown. DS photons appear in discharge due to (a) electronic excitations in the background gas by heavy particles (ions and fast atoms) and (b) in collisional excitation transfer reactions between fast atoms (He f ) and atoms excited by electron impact (He), He f He He He. The latter, newly analysed, mechanism is dominant in the DS photons production during the breakdown in high-voltage open discharges. Our experimental and calculation results show a decrease of the breakdown timescale with increasing gas pressure from 3 Torr to 16 Torr.

Mechanism of formation of subnanosecond current front in high-voltage pulse open discharge.

The mechanism of subnanosecond current front rise observed previously in the experiment in high-voltage pulse open discharge in helium is studied in kinetic particle-in-cell simulations. The Boltzmann equations for electrons, ions, and fast atoms are solved self-consistently with the Poisson equations for the electrical potential. The partial contributions to the secondary electron emission from the ions, fast atoms, photons, and electrons, bombarding the electrode, are calculated. In simulations, as in the experiment, the discharge glows between two symmetrical cathodes and the anode grid in the midplane at P=6 Torr and the applied voltage of 20 kV. The electron avalanche development is considered for two experimental situations during the last stage of breakdown: (i) with constant voltage and (ii) with decreasing voltage. For case (i), the subnanosecond current front rise is set by photons from the collisional excitation transfer reactions. For the case (ii), the energetic electrons swamp the cathode during voltage drop and provide the secondary electron emission for the subnanosecond current rise, observed in the experiment.

Collisional excitation transfer between Rb(5P) states in 50–3000 Torr of 4He

Measurements of the mixing rates and cross sections for collisional excitation transfer between the 5P1/2 and 5P3/2 states of rubidium (Rb) in the presence of 4He buffer gas are presented. Selected pulses from a high repetition rate, mode-locked femtosecond laser are used to excite either Rb state with the fluorescence due to collisional excitation transfer observed by time-correlated single-photon counting. The time dependence of this fluorescence is fitted to the solution of rate equations which include the mixing rate, atomic lifetimes and any quenching processes. The variation in the mixing rate over a large range of buffer gas densities allows the determination of both the binary collisional transfer cross section and a three-body collisional transfer rate. We do not observe any collisional quenching effects at 4He pressures up to 6 atm and discuss in detail other systematic effects considered in the experiment.

Cross sections of collisional excitation transfer in collisions of rare-earth metal atoms in screened excited states with atoms of inert gases

We present and apply a method to determine the collisional excitation transfer (CET) cross sections in collisions of rare-earth metal (REM) atoms in the screened excited states 4fN − 15d6s2 with ground-state atoms of inert gases. The method is based on the fact that the upper laser levels are collisionally populated from the close-lying resonant levels, which are excited by electron impact, in REM vapour lasers. An experimental measurement of only one laser parameter (average lasing power) is required to determine the cross sections. The CET cross sections from the screened level 4f12(3H5)5d3/26s2, with energy E = 22 791.176 cm−1, to the unscreened 4f12(3H6)6s26p1/2 (E = 22 468.046 cm−1) and screened 4f13(2F07/2)5d6s(3D) (E = 22 559.502 cm−1) levels of thulium atoms in the collisions with helium atoms are estimated as an example.

Peculiarities of collisional excitation transfer with excited screened energy levels of atoms

We report an experimental discovery of deviations from the known regularities in collisional excitation transfer processes for metal atoms. The collisional excitation transfer with excited screened energy levels of thulium and dysprosium atoms is studied. The selecting role of the screening 6s shell in collisional excitation transfer is shown.

Production of O2(Δ1) in flowing plasmas using spiker-sustainer excitation

In chemical oxygen iodine lasers (COILs), oscillation at 1.315μm in atomic iodine (P1∕22→P3∕22) is produced by collisional excitation transfer of O2(Δ1) to I2 and I. Plasma production of O2(Δ1) in electrical COILs (eCOILs) eliminates liquid phase generators. For the flowing plasmas used for eCOILs (He∕O2, a few to tens of torr), self-sustaining electron temperatures, Te, are 2–3eV whereas excitation of O2(Δ1) optimizes with Te=1–1.5eV. One method to increase O2(Δ1) production is by lowering the average value of Te using spiker-sustainer (SS) excitation where a high power pulse (spiker) is followed by a lower power period (sustainer). Excess ionization produced by the spiker enables the sustainer to operate with a lower Te. Previous investigations suggested that SS techniques can significantly raise yields of O2(Δ1). In this paper, we report on the results from a two-dimensional computational investigation of radio frequency (rf) excited flowing He∕O2 plasmas with emphasis on SS excitation. We found that the efficiency of SS methods generally increase with increasing frequency by producing a higher electron density, lower Te, and, as a consequence, a more efficient production of O2(Δ1).

Experimental evaluation of the cross-sections for the Cs(6D) → Cs(7P J) and Cs(6D 5 / 2 ) → Cs(6D 3 / 2 ) collisional transfer processes induced by He and Ar

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(7P J ) + He, Ar were found to be Q D3 He = (0.9 ± 0.4) × 10 − 16 cm 2, Q D4 He = (2.6 ± 1.2) × 10 − 16 cm 2, Q D3 Ar = (1.2 ± 0.6) × 10 − 18 cm 2 and Q D4 Ar = (2.6 ± 1.3) × 10 − 18 cm 2. These values are consistent with the results obtained at higher temperatures and reported previously. The cross-sections for the He and Ar-induced 6D J fine-structure mixing were determined to be Q 65 He = (54 ± 15) × 10 − 16 cm 2 and Q 65 Ar = (17 ± 5) × 10 − 16 cm 2 and were found to fit very well into the existing body of cross-section data for intra-multiplet ( nP 1 / 2 → nP 3 / 2 ; nD 5 / 2 → nD 3 / 2 ) mixing in cesium induced by collisions with helium and argon.

Excitation in low-current discharges and breakdown in He at low pressures and very high electric field to gas density ratiosE∕N

We investigate optical emission from low-current discharges in He at very high electric field to gas density ratios E/N between parallel plate electrodes. We also determine the electrical breakdown and the voltage-current behavior at low currents. The E/N are 300 Td to 9 kTd (1 Td = 10(-21) V m2) at pressures times electrode separations p(0)d from 3 to 0.9 Torr cm. Absolute optical emission probabilities versus distance are determined for the 501.6 nm line (3 (1)P -->2 (1)S) and for the 587.6 nm line (3 (3)D -->2 (3)P) by reference to Boltzmann calculations at our lowest E/N and to published pressure dependent electron beam experiments. At E/N below 1 kTd, the emission follows the exponential growth of the electron density, while at above 7 kTd heavy particle excitation is evident near the cathode. Collisional transfer of excitation from the singlet to the triplet system dominates the 587.6 nm excitation. Comparisons of models with experiments show the importance of excitation and of electron production at the cathode by fast He atoms produced by charge transfer collisions of He+ with He. The breakdown voltage versus p(0)d is multivalued for p(0)d approximately 1.5 Torr cm. At currents below 100 microA and our lower E/N, the discharge voltage decreases linearly with current as expected for an increasing electron yield with ion energy and E/N at the cathode.

Lasing on the erbium atomic transitions in an Er-Tm-He mixture

We report for the first time on the laser radiation generated in erbium vapor, whereby the upper laser level in Er atom is populated due to collisional excitation transfer from the resonance levels of Tm atom.

Laser induced fluorescence in Ar and He plasmas with a tunable diode laser

A diode laser based laser induced fluorescence (LIF) diagnostic that uses an inexpensive diode laser system is described. This LIF diagnostic has been developed on the hot helicon experiment (HELIX) plasma device. The same diode laser is used to alternatively pump Ar II and He I transitions to obtain argon ion and atomic helium temperatures, respectively. The 1.5 MHz bandwidth diode laser has a Littrow external cavity with a mode-hop free tuning range up to 14 GHz (≈0.021 nm) and a total power output of about 12 mW. Wavelength scanning is achieved by varying the voltage on a piezoelectric controlled grating located within the laser cavity. The fluorescence radiation is monitored with a photomultiplier detector. A narrow band interference filter is used to eliminate all but the plasma radiation in the immediate vicinity of the fluorescence wavelength. Lock-in amplification is used to isolate the fluorescence signal from noise and electron-impact induced radiation. For the Ar ion, the laser tuned at 668.43 nm is used to pump the 3d 4F7/2 Ar II metastable level to the 4p 4D5/2 level. The 442.60 nm fluorescence radiation between the 4p 4D5/2 and the 4s 4P3/2 levels is captured by the photomultiplier tube. For atomic He, the laser is tuned at 667.82 nm to pump a fraction of the electron population from the 21P state to the 31D upper level. Although the 21P level is not a metastable, the close proximity of 21S metastable makes this new He I LIF scheme possible. In this scheme, a fraction of the laser-excited electrons undergo collisional excitation transfer from the 31D to the 31P level. In turn, the 31P state decays to the metastable 21S by emitting 501.57 nm fluorescence photons.