Electron Impact Excitation Of He Research Articles

194 nm microplasma lamps driven by excitation transfer: optical sources for the 199Hg ion atomic clock and photochemistry

A series of miniature, microcavity plasma lamps emitting predominantly at 194 nm has been successfully developed and tested as the optical pump for the microwave 199Hg-ion atomic clock (40.507 GHz), replacing low pressure, RF-powered Ar/Hg discharges. Intense fluorescence on the 6p 2P1/2 → 6s 2S1/2 transition of the singly-charged 202Hg ion at 194.23 nm has been generated in arrays of cylindrical microplasmas through electron-impact excitation of He, followed by three-body formation of He2(a 3 ) and Penning ionization of Hg. Emission spectroscopy and kinetic modeling of He/Hg vapor plasmas demonstrate that the population of the Hg+(62P1/2) radiating state (16.82 eV), produced by direct or two-step electron impact processes, is 1% of that generated by excitation transfer to Hg by H. Flat, fused silica lamps having emitting areas as small as 4 mm2 and containing several mg of 202Hg and 50–800 Torr of He have been fabricated and serve as optical drivers for the Hg+ atomic clock cycle. Based on small arrays of 500 μm–1 mm diameter microcavities, these lamps produce peak and average intensities at 194 nm greater than those associated with the Hg resonance transition at ∼ 254 nm, despite the factor of 3 difference between the energies of the 6p 3P1 and 6p 2P1/2 states of neutral Hg and Hg+, respectively. These lamps are unique in the sense that the desired radiating species is an excited ion and the background He gas pressure can reach 1 atm, both of which contribute to a dense glow plasma placing severe demands on E/N, power conditioning, and materials selection. Nevertheless, with proper attention given to design, vacuum processing, and preparation of the lamps, lifetimes above 1500 h have been realized to date. When these lamps drive Jet Propulsion Laboratory Hg+ clocks, the stability floor has been measured to be −14. The implications of this lamp for gas-phase and solid-state photochemistry are also discussed.

Triplet Excitons in Small Helium Clusters.

An electron traveling through liquid helium with sufficient kinetic energy can create a low-lying triplet exciton via inelastic scattering. Accompanying repulsion between the exciton and nearby atoms results in bubble formation. That is not all, however. Repulsion compresses an "incipient He2* exciton", pushing it into a region where an He2* moiety commences evolution toward its potential energy minimum. The above picture follows from ab initio calculations of the two lowest adiabatic potential energy surfaces for collinear three-atom systems and dynamics studies launched on the lowest adiabat that calculate said surface on the fly. The timescale for launching trajectories toward the He2* moiety is significantly shorter than the timescale for pushing helium away from the exciton in large systems, making results with three atoms relevant to liquid helium. This explains how He2* might be created in the aftermath of electron-impact excitation of He*. Interplay between the lowest adiabats is discussed, underscoring the importance of nonadiabatic processes in such systems. Results with eight-atom systems further illustrate the critical role of nonadiabatic transitions.

Reaction pathways for bio-active species in a He/H2O atmospheric pressure capacitive discharge

Helium/trace gas atmospheric pressure radio-frequency (rf) capacitive discharges have increasing biomedical applications. We have performed a principal pathway analysis for a chemically complex, bounded He/H2O atmospheric pressure, planar capacitive discharge, with a discharge gap of 0.5 mm and a power of 0.85 W cm−2 at 13.56 MHz (ne ≈ 1.6 × 1017 m−3). The discharge is embedded in a larger volume in which the H2O fraction is controlled to be 0.001. The generation and loss pathways for eleven species of interest for discharge maintenance and biomedical applications have been determined. The production and consumption pathways of He*, H2O, and electrons are found to be tightly coupled. The metastable He* generated by electron impact excitation of He is mostly consumed by Penning reactions with H2O, followed by subsequent three-body association reactions with H2O, to form the dominant positive ion, . The main loss pathways for are ion cluster fragmentations at the wall, which are important generation pathways for H2O. The generation and loss pathways for electrons are almost the same as for . OH and H2O2 generation and loss are strongly coupled, and they are important intermediate species in the generation pathways for the purely O-containing bio-active species: O2(a), O, O3 and O*. The generation and loss pathways for the latter four species were found to be strongly coupled by volume and surface processes, with O2 as an important precursor. The generation of O2 from H2O involves H2O2 as a key long-lived intermediate.

Calculation of differential cross sections for electron impact excitation of H and He+

We present the distorted wave Born approximation (DWBA) for electron impact excitation and a method to calibrate the DWBA. With the calibrated DWBA, the differential cross sections (DCS) for excitation of H and He + from 1s to 2s and 2p are calculated and the results are compared with the absolute experimental measurements for H at incident energies of 50 eV and 100 eV. It has been found that the theoretical results are in very good agreement with the experiment, which confirms the validity of the calibration procedure. This work prepares an efficient theoretical method for numerical simulations of non-sequential double ionization of He in strong laser pulse in which laser-induced electron impact excitation of He + is involved.

Comment on “Near-threshold behavior of electron-impact excitation of He+(2s)and He+(2p)”

Xu and Shakeshaft [Phys. Rev. APLRAAN1050-294710.1103/PhysRevA.84.024701 84, 024701 (2011)] presented theoretical cross sections for excitation of the 2s and 2p states of He+ at low collision energies, pointing out in particular the presence of cusps at the excitation threshold. We update their discussion of the comparison between theory and experiment, and confirm that results obtained using standard R-matrix and two-dimensional R-matrix propagation techniques also show such cusps.

Near-threshold behavior of electron-impact excitation of He+(2s)and He+(2p)

We present results for cross sections summed over partial waves $L=0$--2 for electron-impact excitation of the $2s$ and $2p$ states of He${}^{+}$ in the energy range of 40.84 to 45.66 eV. We find that these cross sections exhibit cusps at the excitation threshold of 40.81 eV.

Electron-impact excitation of He and H2in the doubly excited state energy region observed using a momentum transfertechnique

We report evidence for the electron-impact excitation of doubly excited states in helium andH2which are stable against autoionization. By detecting only long-lived (metastable)excited neutral states created by near-threshold electron scattering our techniquediscriminates against directly excited metastable states and is highly sensitive.

Electron-impact excitation of He 3 1D:channel-coupling effects on the orientation, charge cloud, and cross section

A violation of the propensity rule for positive angular-momentumtransfer at small scattering angles in He 1 1S → 3 1Delectron-impact excitation (Cvejanović D, McLaughlin D T and Crowe A 2000 J. Phys.B: At. Mol. Opt. Phys. 33 3013) has been investigated. The analysis of results fromnumerical models with an increasing number of coupled states in a close-couplingexpansion shows that only the most sophisticated ‘convergent close-coupling’ and‘R-matrixwith pseudo-states’ models can reproduce the experimental data. This points tochannel-coupling effects as the principal reason for violation of the propensityrule, a conclusion supported by comparing predictions for the angle-integratedcross section from these models. Finally, the component of the charge cloudlocated along the direction perpendicular to the scattering plane becomesincreasingly important with increasing energy. This can also be seen in theaccompanying QuickTime movies.

Resonance contribution to 1s - 2s electron-impact excitation of He+ion

In the frame of quantum defect theory, the resonance excitation cross sections of 1s - 2s electron-impact excitation of He+ ion are calculated by a simple relativistic configuration interaction method. The present theoretical results are in agreement with the previous experimental measurements.

EUVE Observations of Jupiter During the Impact of Comet Shoemaker-Levy 9

The Extreme Ultraviolet Explorer (EUVE) satellite conducted extensive observations of the jovian system before, during, and after the impact of the fragments of comet Shoemaker-Levy 9 in July 1994. About 2 to 4 hours after the impacts of several of the larger fragments, the brightness of the neutral helium (He I) resonance line at 58.4 nanometers temporarily increased by a factor of about 10. The transient 58.4-nanometer brightenings are most simply explained by resonant scattering of sunlight from the widespread high-altitude remnants of the larger impact plumes. Other possible sources of emission, such as electron impact excitation of He or radiative recombination of He(+), may contribute to the observed signal.

Electron impact excitation of He 21P in the presence of a nonresonant laser field

We report experimental differential cross sections for inelastic scattering of electrons from helium atoms in the presence of an intense (~108 W cm−2) carbon dioxide laser. The cross sections for excitation of the 21P state of helium with the simultaneous absorption or emission of one laser quantum were measured over the incident electron energy range of 36–70 eV and scattering angles of 13–31°.

Polarization of rare-gas radiation in the vacuum-ultraviolet region excited by electron impact: Helium and neon.

Vacuum-ultraviolet (vuv) polarization data are presented for the integrated radiation emitted following electron-impact excitation of He and Ne in the energy range from threshold to 500 eV. The data represent an improvement both in accuracy and energy range over those available previously. They can be used as secondary standards for calibrating vuv polarization analyzers and other optical devices.

Collisional properties of atoms in high-lying doubly excited states.

Using the hyperspherical wave functions of the high-lying doubly excited $^{1}\mathrm{S}^{\mathrm{e}}$ states of He, we have calculated the generalized oscillator strengths between these excited states and the Born cross sections for electron-impact excitation of He in these excited states within the manifold of $^{1}\mathrm{S}^{\mathrm{e}}$ states. Based on the rovibrator model of the doubly excited state, we predict theoretically that for these excitation processes a He atom tends to enlarge its size while keeping its shape, as a limber ``e-${\mathrm{He}}^{2+}$-e'' linear molecule, the same as that in the initial states.

Plasma effect on electron-helium ion scattering

The effect of plasma on the electron impact excitation of He+ is investigated. The collision strengths for 1s-2s, 1s-2p and 2s-2p transitions are calculated in the framework of a two-potential approach along with the Debye-Huckel model for plasma screening. Comparison is made with the other calculation.

Electron impact excitation of He

A new approximation is developed for electron collisions with two-electron atoms and is applied to the excitation of He. The core orbital is taken to be that given by equivalent frozen 1s2 orbitals and the valence wavefunction is taken to be that given by a Hartree or Hartree-Fock frozen-core approximation. The resulting orthogonality of the core and valence wavefunctions enables the exact three-electron collision equations to be simplified to a form similar to that for a two-electron problem. The explicit dependence on the core wavefunction being removed, the effect of choosing a non-equivalent core is investigated within the Hartree frozen-core approximation. The collision equations are solved using a number of five-state exchange distorted-wave (EDW) approximations which include various multipole exchange distorting potentials exactly. The authors calculate total collision strengths for all transitions between the 11S, 23S, 21S, 23P and 21P states of He for incident electron energies from just above threshold up to 15.0 Ryd.

The 1s–2s and 1s–2p excitation of helium ions by electron impact

The electron impact excitation of He+ to the n = 2 level is studied using two variants of the polarized orbital method, one due to Daskhan et al. and the other due to Temkin and Lamkin. The present results for 1s–2p total excitation cross sections are in fair agreement with the measured values. For the 1s–2s excitation, our values fail to reproduce the more elaborate theoretical prediction by Morgan near the threshold energies.

Electron-impact excitation of He: Threshold region

Differential cross sections for excitation to the $n=2$ states of He are studied with an electron-impact spectrometer from threshold to 4 eV above. We observe pronounced angle-dependent structures in threshold excitation to the $2^{3}P$ state and in near-threshold excitation in the $2^{3}S$ state. Together, these experimental findings and recent theoretical considerations suggest the existence of a new $^{2}P$ resonance near 20.8 eV.

Benchmark cross sections for electron-impact excitation ofn S1levels of He

Absolute total emission cross sections have been measured for electron-impact excitation of He giving radiation from the $6^{1}S\ensuremath{-}2^{1}P$, $5^{1}S\ensuremath{-}2^{1}P$, $4^{1}S\ensuremath{-}2^{1}P$, and $3^{1}S\ensuremath{-}2^{1}P$ transitions. Electron energies were 50, 100, 500, 1000, and 2000 eV. Particular care was taken to make the measurements accurate and to make knowledgeable assessments of uncertainties, so that the results serve as reliable benchmarks for calibration of other excitation apparatus as well as for comparison with theory. The most accurate results are for 500-eV electron energy, where the mean uncertainty is only 3.5% high confidence level. The measured emission cross sections are modified to account for branching and cascade to give level-excitation cross sections, and results are compared with other experimental data and theoretical predictions. At 2000 eV, the measurements average 2.5% below Born-approximation calculations.

Negative ions in electron impact excitation of He near 60 eV

An experimentally verifiable effect on the excitation cross section in helium due to the broad (2s2p 2) 2S e He - state predicted by Fano and Cooper in 1965, but hitherto unidentified, and other He - states, are predicted.

Second Born Zero Angle Differential Cross Sections in the High Energy Limit for the Electron Impact Excitation of He: The 23S ← 11S and 23P ← 11S Transitions

The high energy limit of the inelastic differential cross section for the excitation of the 23S and 23P states of He from the ground state in the forward direction is investigated by expanding the T matrix in powers of V about zero scattering angle and considering all terms proportional to V and V2 where V is the interaction potential. It is shown that the leading terms in a power series in the reciprocal of the incident energy in this second Born infinite channel close coupling theory can qualitatively account for the observed enhancement of the exchange cross section in the forward direction over the predictions of the first Born theory. The ratio of the square of the leading second Born term to the square of the leading first Born term is shown to be proportional to the incident energy for the 23S ← 11S transitions and of the same order in the incident energy as the first Born form in the case of the 23P ← 11S transition. The value of the ratio at 500 eV for the 23S ← 11P transition is estimated to be close to 100 and is relatively independent of the choice of an effective energy loss parameter introduced to carry out the sum over intermediate states.