Metastable Ne Research Articles

Energy Transfer Reactions of Nitric Oxide With Rare Gas Metastable Atoms

Diatomic molecule NO is one of the most important compounds in atomospheric chemistry and environmental science. The study of its reactions, energy, transfer and dissociation process has been paid great attention to for practical importance. Much dynamical information is available concerning the E-E energy transfer reactions of NO with low energy metastable molecules N_2(A) and CO(a). Because the energy of rare gas metastable atoms He (2~3S), Ne (~3P_(0.2)) and Ar (~3P_(0.2)) is much higher than theionization energy of NO molecules,the ionization and dissociation are the major channels in the reaction of NO with Rg~*. Coxon et al. have studied the reaction ofHe(2~3S) with NO in flowing after-glow apparatus. The dissociative excitation and penning ionization channels were investigated by emission spectrometry, but the energy transfer process which is a minor channel (fraction<1%) was not discernable in their work.In this work, the authors used the electric discharge nozzle source to produce

High-resolution measurement and quantum-defect analysis for the Ne nd' J= 1, 2 and 3 autoionizing resonances

Using resonant two-photon excitation of a metastable Ne(2p53s 3P0.2) beam by two single-mode CW dye lasers, we have measured reduced widths Gamma r=n*3 Gamma n for the four Ne(2p5(2P12/) nd' (K)s) autoionizing resonance series ((K)s=(3/2)3, (3/2)2, (5/2)2, and (5/2)3). The resulting values are Gamma r=167(7), 350(7), 87(2), and 73(3) cm-1, respectively, in good agreement with predictions by a semiempirical multichannel quantum-defect analysis involving complex quantum defects.

Fine-structure transitions in metastable Ne*(3P0,2) colliding with ground state rare gases at thermal energies

The emission of XUV photons from collision-induced fine-structure transitions Ne*(3P0,2)+R→Ne*(1,3P1)+R→Ne(1S)+R+hν(74 nm) has been measured for R=He, Ne, Ar, and Kr(1S) in a beam-cell experiment. The metastable Ne* velocity ranges from 550 to 1600 m/s. No emission was found for Ne*(3P0)+R. An appropriate detector system enables the determination of absolute 3P2 → 3P1 transition cross sections. The results of Ne*(3P2)+He and Ne are compared with theoretical calculations.

The dynamics of hydroneon(1+) production in neon*(3P2,0)-hydrogen chloride collisional autoionization

The ionization of HCl molecules by thermal energy collisions with metastable Ne * atoms has been studied experimentally and theoretically. The integral elastic cross sections have been measured and analyzed, together with the total ionization cross sections, to obtain an optical potential able to describe the collision dynamics of the neutral reactants. The relative cross sections for NeH + and NeD + production in Ne * -HCl or DCl have been measured as a function of the collision energy. A theoretical calculation of the potential energy for the reactive Ne + HCl + (A)→NeH + +Cl system has been performed by SCF, CI, and CASSCF methods

Penning ionization of fullerenes: reactions of C 60 and C 70 with metastable atoms of the rare gases He, Ne, Ar and Kr

Results of flow-tube experiments are reported for reactions of metastable He, Ne, Ar and Kr with C 60 and C 70 at 294 ± 2 K in helium gas at 0.35 Torr. All eight of these reactions were observed to lead to (Penning) chemi-ionization.

Deexcitation cross sections of Ne(3P2, 3P1, and 3P) by molecules containing group-IV elements

Deexcitation cross sections of Ne(3P2), Ne(3P1), and Ne(3P0) by CH4, SiH4, GeH4, CF4, and SiF4 have been measured at a mean collisional energy corresponding to room temperature (295 K) and compared systematically with those by other molecules to understand general features of the cross section values dependent on both the electronic states of excited neon atoms and the target molecules. A small difference in the cross section values between metastable atoms, Ne(3P2) and Ne(3P0), and a resonant atom, Ne(3P1), has indicated that Penning ionization by Ne(3P1) is mainly governed by an electron exchange interaction rather than a dipole–dipole interaction. A spatial electron distribution of the outermost orbital of a target molecule is especially of great importance in the determination of the absolute cross section values in the deexcitation of excited neon atoms due to an electron exchange interaction.

Optical determination of the ^20,22Ne 3s–3p specific mass shift

By using a supersonic beam of metastable Ne* atoms and a single-mode dye laser we determine the 2p(5)3s-3p specific mass shift of the (20,22)Ne isotopes by means of optical spectroscopy. The resulting value, on the basis of a systematic measurement of transition isotope shifts at different transition energies between 15,300 and 17,000 cm(-1), is 473 +/- 2.5 MHz. This is ~ 50 MHz larger than the best-known theoretical value.

Total ionization cross sections for Ne**{(2p) 5(3p); J = 3}-molecule systems: Absolute values and polarization effects

For the molecular targets CH 4, H 2, N 2, O 2, CO, CO 2 and N 2O we have investigated the ionization cross section 3 Q for collisions with laser excited Ne**{(2p) 5(3p); J = 3} atoms in the range 100 ⩽ E ⩽ 4000 meV of collision energies (80 ⩽ E ⩽ 800 meV for H 2). A crossed-beam apparatus with well defined beam geometry is used, which allows the investigation of polarized-atom cross sections 3 Q | M| , with | M| the magnetic quantum number with respect to the initial relative velocity. Following Bussert et al., the large ionization cross section 40 ⩽ 3 Q ⩽ 80 Å 2 at thermal energies are interpreted in terms of an initial state with an attractive type interaction with a deep well ϵ/ E⪢1. An orbiting collision model predicts 3 Q⩽ Q orb = 5.9 ( C 6/ E) 1 3 with C 6 the Van der Waals parameter. At E = 100 meV we find an ionization probability 3 Q/ Q orb in the range 0.6 to 0.8 (0.3 for CH 4) which are realistic values. In the superthermal energy range the cross section 3 Q decreases to the value of 2 Q for the metastable Ne*{(3s); 3P 2} state, indicating that the repulsive branch is determined by the (2p) 5 core and the (3s) or (3p) valence electron no longer plays an important role. In the thermal energy range the polarization effect decreases from 3 Q | M| = 0,1 / 3 Q = 1.11 to 1.00 when going from CH 4 to N 2O, which should be compared to 1.22 for Ar. This decrease corresponds to an increase of the anisotropy of the long-range polarizability of the molecule. At superthermal energies the polarization effect is equal to unity for all systems including Ar. For comparison we also present experimental data on the polarization effect for intramultiplet mixing by molecular targets at thermal energies. Again we observe a strong decrease when going from He (no ionization) to ionizing targets (Ar) with an increasing anisotropy (CH 4, H 2, N 2, CO 2, N 2O).

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔIS in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔIP (γ = ΔIS/ΔIP). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s3P2), Ar* (4s3P2), and Kr* (5s3P2) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (23S, 21S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.

Four-beam laser trap of neutral atoms

An efficient spontaneous-force laser trap of neutral atoms without standing waves is demonstrated by using the Ne 1s(5) metastable state. The metastable Ne beam is decelerated by a laser by using Zeeman tuning and is trapped in a trap consisting of a quadrupole magnetic field and four laserbeams in a tetrahedral configuration. The dynamics of atomic motion from the deceleration stage to the trap is discussed.

Alignment of electronic angular momentum by stimulated raman scattering with delayed pulses

The recently developed STIRAP-technique (J. Chem. Phys. 92, 5363 (1990)) is applied to efficient population transfer and alignment of metastable Ne* atoms. Alignment parameters in the range −0.3≧A02≧−1.0 are observed. The degree of alignment can be controlled by changing the angle between the field vectors of the linearly polarized pump- and Stokes-lasers. The transfer efficiency depends also on this angle. The results of numerical calculations agree satisfactorily with the experimental data.

Energy Transfer Processes in Reaction Between Metastable Atoms Ar,He and Ne with CCl&lt;SUB&gt;4&lt;/SUB&gt; and CBr&lt;SUB&gt;4&lt;/SUB&gt;

Energy Transfer Processes in Reaction Between Metastable Atoms Ar,He and Ne with CCl&lt;SUB&gt;4&lt;/SUB&gt; and CBr&lt;SUB&gt;4&lt;/SUB&gt;

Luminescence of excited fragments produced from methyl halides in noble gas flowing afterglow

The reaction of metastable He(23S) and Ne(3P0,2) atoms with methy halides has been investigated by flowing afterglow technique. Emission spectra have been observed and formation rate constants of some fragments have been measured. The rotational and vibrational temperatures of CH(A2Δ) were also estimated. Key words: flowing afterglow, methyl halides, metastable atoms.

A crossed beam study of the ionization of molecules by metastable neon atoms

Total ionization cross sections for the collisions between metastable Ne*(3P2,0) atoms (predominantly 3P2) and several molecules (N2, CO, O2, NO, HCl, HBr, Cl2, CO2, N2O, CH4, CH3Cl, CH3Br, C2H6, C2H4, and C2H2) have been measured, in a crossed beam experiment, within the ≈ 0.03 to ≈ 0.5 eV collision energy range. The cross sections have been obtained on the same relative scale for all systems and calibrated to an absolute value by assuming a literature value for the reference system Ne*-Ar. These experimental cross sections have been integrated over the Maxwell-Boltzmann distribution to estimate the values of the ionizationrate constants. Comparison between the ionization and the quenching rate constants shows that the ionization is the main channel for the quenching of metastable neon atoms in thermal systems.

Recent results from photon stimulated desorption of rare gas solids excited with synchrotron radiation

Desorption of neutral rare gas atoms following selective excitonic excitation of rare gas solids (Ne, Ar, Kr, Xe) by VUV photons underlines the crucial role of excitons concerning (i) transport of excitation energy from the bulk to the surface, and (ii) the microscopic desorption mechanism itself. All rare gas solids yield desorption of ground state atoms which stem from relaxation of molecular-type excitons after their radiative decay. In addition, solid Ne and Ar yield desorption of 3P1, 1P1, and metastable (3P2, 3P0) atoms. Total desorption yields as well as partial yields of excited atoms will be presented. The dominating role of primary excitation of surface excitons is obvious for excited atoms. Primary creation of free electron-hole pairs favours desorption of ground state atoms. The kinetic energy distributions of metastable Ne and Ar atoms were determined by a time-of-flight technique. Maxima were found at ≅ 0.2 eV (Ne) and 0.04 eV (Ar) which establish a new desorption mechanism of excitonic origin.

Electron transfer from laser excited rydberg atoms to molecules. Absolute rate constants at low and intermediate principal quantum numbers

Using mass spectrometric detection of positive and negative ions, we have investigated ionizing reactions of Ne(ns,nd) Rydberg atoms, efficiently excited by resonant two-photon excitation of metastable Ne(3s3P2) atoms, with electron attaching moleculesBC (BC=SF6, CCl4, CS2, O2) at thermal collision energies. Absolute rate constants have been determined in the range of low and intermediate principal quantum numbersn(5≦n≲30) by utilizing the photoionization signal caused by room temperature black-body radiation and the loss of Ne(3s3P2) atoms, associated with the laser excitation. Substantially differentn-dependences of the electron transfer cross section have been found for the larger molecules (BC = SF6, CCl4) and the smaller molecules (BC = CS2, O2). Simple model calculations have been performed to gain new insight into the dynamics of the electron transfer process; forBC = SF6, our results at lown(5 ≦n ≦ 10) suggest that internal energy conversion in the Coulombic complex Ne+ — SF 6 − is important for the formation of the detected ions.

The low energy collision between metastable Ne*(3 P 0,2) and He(1 S 0): ab initio potentials, differential cross sections and polarization effects

Potential energies for molecular states dissociating into Ne*(1P1,3P0,1,2) + He(1S0) have been calculated ab initio within the distance range 4–100a0. The SCF energy (without spin-orbit interaction) is optimized on the lowest3Σ state. After CI, the four Λ-states (1,3Σ,1,3Π)are obtained. They dissociate into Ne*(1,3P) + He(1S). All of them are repulsive atR ≲ 8a0, they exhibit shallow wells around 12a0 and have a correct asymptotic behaviour (∼ -R−6). The spin-orbit interaction is introduced, using the Cohen-Schneider scheme, and adiabatic Ω-potentials are derived. The collision at low energy (E ≦ 124 meV) is described in the frame of a fragment-state basis. By means of a deflation procedure, it is shown that states dissociating into Ne*(1P1) + He can be eliminated, which lead to a 9 × 9 interaction matrix dynamically equivalent to the original 12 × 12 matrix, in the subspace of interest. Collision channels are defined by angular momenta,J (total),j (of Ne*) andl (of the relative motion). Scattering radial equations are solved by the algorithm of Gordon and theS matrix is derived. Two sets of physically meaningful scattering amplitudes (and differential cross sections) are constructed, referred to the incident axis or to the initial and final directions of the internuclear axis. Polarization effects are discussed. The case of a quantization axis perpendicular to the collision plane is also mentioned.

Emission spectra of N 2O +(Ã 2Σ +-X̃ 2Π), CS 2+(Ã 2Π u-X̃ 2Π g) and ICN +(Ã 2Σ +-X̃ 2Π) excited by penning ionization in low-pressure neon and helium afterglows

Penning ionization of N 2O, CS 2 and ICN by metastable Ne( 3P 0,2) and He(2 3S) atoms has been studied by observing the N 2O + (Ã 2Σ +-X̃ 2Π), CS 2 + (Ã 2Π u-X̃ 2Π g) and ICN + (Ã 2Σ +-X̃ 2Π) emission spectra in low-pressure flowing afterglows. Non-Franck-Condon vibrational distributions are found for the formation of N 2O + (Ã) and ICN + (Ã) by Ne( 3P 0,2) impact. The deviations are attributed to either autoionization or collisional perturbation. The Ω = 1 2 to Ω = 3 2 ratio of CS 2 + (Ã-X̃) in the Ne( 3P 0,2)/CS 2 reaction is about 1.5 times larger than that in the He(2 3S)/CS 2 reaction.

Collisions of metastable Ne*, He* atoms with ground-state He, Ne atoms studied by atomic beam and laser techniques

A crossed nozzle-beam experiment is used to investigate thermal energy collisions: Ne*(2p53s,3P0, 2)+He(1s2,1S0), almost purely elastic, and He*(1s2s,1, 3S)+Ne(2p6,1S0), in which inelastic excitation transfers occur. State and velocity selection of the scattered Ne* atoms is performed using a tunablecw dye laser frequency locked on a definite Zeeman component of the transition 1s5→2p6 (λ=614.3 nm) of20Ne or22Ne. In the purely elastic case, this technique allows the selection of one of the two final velocities, and then an unambiguous LAB-CM transformation. The differential cross section at 62 meV tallies on accords with a calculation using a single effective potential. In He* on Ne collisions, the main inelastic processes are endothermic excitation transfers from He*(21S). Experimental results obtained at different energies (62, 95, 109, 124 meV) show that the transfers essentially result in levels 3s and 4d of Ne.

Intermolecular potentials for the metastable Ne*-rare gas and Ne*-molecule systems

The absolute value and the velocity dependence of the total cross section Q( g) has been measured in a crossed beam machine for the Ne*-Ar, Kr, Xe and Ne*-O 2, N 2, CH 2 and CO 2 systems, using a mixed beam containing Ne*( 3P( 0) and Ne * ( 3P 2) fine structure states in a 1:5 ratio. The range of velocities is typically 1000 ⩽ g ⩽ 8000 m s −1, always including the interesting N = 1 glory oscillation. The results for the Ne* -rare gas systems are in excellent agreement with the predictions of the ion-atom Morse-Morse-spline-van der Waals potentials of Gregor and Siska, both with regard to the absolute value (1.5%), position of the N = 1 glory maximum (2.7%) and the amplitude of the N = 1 glory maximum (4.3%). The predictions of the potentials proposed by Hausamann are less satisfactory, most likely due to the specific switchover function used to connect the well area at R/R M ≈ 1.1 to the van der Waals long-range attractive branch at R/R M ≈ 2 ( R M is the well position). By using a semiclassical scaling method the potential parameters ϵ (well depth), R M (well position) and C 6 (van der Waals constant) have been determined for the Ne*-molecule systems, using the Gregor and Siska IAMMSV potential for the Ne*-Xe system as a reference. The well parameters are (ϵ (meV), R M (Å)) = (3.21, 5.43), (4.24, 5.17), (13.55, 4.74) and (7.08, 5.44) for the Ne*-N 2, O 2, CO 2 and CH 4, systems, respectively. For the C 6 values we observe a fair scaling with the polarisibility α of the molecule. For the Ne*-CO 2 system we observe a damping of the amplitude of the glory oscillations, which increases rapidly with decreasing velocity. This damping is interpreted in terms of the probability for ionisation along the glory trajectory, providing useful information for determining a complex potential for this system.