Collisions at thermal energy between metastable hydrogen atoms and hydrogen molecules: total and differential cross sections

A metastable hydrogen (deuterium) atom source in which groundstate atoms produced by a RF discharge dissociator are bombarded by electrons, provides a relatively large amount of slow metastable atoms (velocity 3–5 km/s). Total integral cross sections for H*(D*)(2s) + H₂(X ¹Σ _g ⁺ ,v=0) collisions have been measured in a wide range of relative velocity (2,5–30 km/s), by using the attenuation method. A significant improvement of accuracy is obtained, with respect to previous measurements, at low relative velocities. Total cross sections for H* and D*, as functions of the relative velocity, are different, especially in the low velocity range. H* + H₂ total differential cross sections have also been measured, with an angular spread of 3.6°, for two different collision energy distributions, centered respectively at 100 meV and 390 meV. A first attempt of theoretical analysis of the cross sections, by means of an optical potential, is presented.     

Improved study of the ionization of hydrogen atoms in thermal energy collisions with metastable He(23 S) atoms

The first electron spectrometric study of the ionizing reaction of metastable He(2³ S ₁) atoms with ground state hydrogen atoms has been carried out with sufficiently high resolution to partially resolve the rotational structure due to formation of rovibrationally excited HeH⁺ (v, J) ions at two different beam source temperatures (300 K and 90 K). The electron energy spectrum has been reproduced in model quantum calculations, using a new large scale ab initio calculation of the He(2³ S)+H(1² S)²Σ-potential. The imaginary part has been adjusted to yield a satisfactory fit to the measured spectrum. The collision energy dependence of the associative ionization electron spectra and of the total and partial ionization cross sections is discussed in some detail. No significant signs for limitations of the used local complex potential method, indicated by results of an earlier study of the He(2³ S)+H(1² S) system, have been found in the present work, in which the calculations were carried out with an improved and corrected program.     

Low-temperature inelastic collisions between hydrogen molecules and helium atoms

Inelastic H(2):He collisions are studied from the experimental and theoretical points of view between 22 and 180 K. State-to-state cross sections and rates are calculated at the converged close-coupling level employing recent potential energy surfaces (PES): The MR-PES [J. Chem. Phys. 100, 4336 (1994)], and the MMR-PES and BMP-PESs [J. Chem. Phys. 119, 3187 (2003)]. The fundamental rates k(2-->0) and k(3-->1) for H(2):He collisions are assessed experimentally on the basis of a master equation describing the time evolution of rotational populations of H(2) in the vibrational ground state. These populations are measured in the paraxial region of supersonic jets of H(2)+He mixtures by means of high-sensitivity and high spatial resolution Raman spectroscopy. Good agreement between theory and experiment is found for the k(2-->0) rate derived from the MR-PES, but not for the BMP-PES. For the k(3-->1) rate, which is about one-third to one-half of k(2-->0), the result is less conclusive. The experimental k(3-->1) rate is compatible within experimental error with the values calculated from both PESs. In spite of this uncertainty, the global consistence of experiment and theory in the framework of Boltzmann equation supports the MR-PES and MMR-PESs, and the set of gas-dynamic equations employed to describe the paraxial region of the jet at a molecular level.     

Vibrattonal excitation of molecules by collisions with “hot” hydrogen atoms from laser photolysis

Excimer laser (ArF) photolysis of diatomic and triatomic hydrides produces hydrogen atoms with translational energies in excess of 15000 cm^?1 per atom. Subsequent collisions of these “hot” atoms with CO₂ and N₂O produces vibrationally excited molecules which can be detected by their characteristic infrared emission.     

Ionizing collisions of potassium atoms with molecules: Excitation energy of the product molecular ion

For processes of the type K + M

K⁺ + M^? at eV collision energies, measurements are reported of the population of excited states of the product negative ions. For M = Br₂ and SF₆ only a small excitation (0.5 – 3 eV) is found. For M = O₂, the products can be excited up to higher energies (? 5 eV); this is ascribed to a second electronic channel of the process.     

Velocity dependence of vibrational branching ratios in electronic energy transfer collisions of metastable argon atoms with nitrogen

Measurements have been made of the vibrational branching ratio (υ′=0)/(υ′=1) in N^*₂ (C³Π_u) formed in electronic energy transfer collisions between argon metastable atoms and ground state nitrogen molecules, using crossed molecular beams. In the relative collision energy range, 0.08–0.20 eV, this ratio is 3.5±0.2.     

Dissociation of ozone by oxygen atoms and metastable oxygen molecules

The spatial distribution of O₃ in the flow tube of an O₂ dioscharge at 0.5–3 torr was mass spectrometrically determined. Using a silver oxide surface for scavenging O and O₃, [O] and [O₂^#] were calorimetrically determined. The decay processes are discussed.     

Electron screening in low energy scattering of muonic hydrogen on hydrogen atoms

Electron screening corrections to the cross sections for low energy scattering of muonic hydrogen on hydrogen atoms are calculated. It is shown that the presence of the electron influences considerably the elastic cross sections at collision energies below 1 eV. This influence is relatively small for the spin-flip and isotopic exchange processes.     

Quenching of Na resonance radiation; collisions of excited sodium with atoms,molecules and electrons in the ICP

Spatial distributions of Na resonance level lifetimes in the inductively coupled plasma (ICP) are measured by time-resolved laser-excited fluorimetry. In order to elucidate the quenching mechanism, the lifetimes obtained are compared to theoretical values estimated from the quenching rate coefficients and number densities of colliding species. As a result, it is shown that resonance level Na atoms are mainly deactivated by electrons at lower heights in the plasma, while collisions with nitrogen molecules entrained from the surrounding atmosphere dominated the quenching reaction in the tail flame region.     

Tests of semiclassical treatments of vibrational-translational energy transfer collinear collisions of helium with hydrogen molecules

Three kinds of semiclassical theory are tested against quantum mechanical results for vibrational transition probabilities and average vibrational energy transfers in collinear collisions of atoms with harmonic and Morse vibrators for the He-H₂ mass combination. The interaction potential is assumed to be a repulsive exponential function with an exponential parameter which is realistic for He-H₂ collisions. The energy range studied is total energies of 2–8 in units of ?ω_e. The uniform semiclassical approximations of classical S matrix theory are tested only for classically allowed transitions, i.e., for transition probabilities greater than about 0.2. They are accurate quantitatively for both harmonic and Morse vibrators. The integral expressions of classical S matrix theory are found to be quantitatively accurate for classically allowed and weakly classically forbidden transitions, i.e., for transition probabilities greater than about 0.01–0.05, and to be unreliable for strongly classically forbidden transitions. Quasiclassical trajectory methods yield qualitatively accurate results only for classically allowed transitions but the phase-averaged energy transfer in quasiclassical collisions may be accurate even when classically forbidden transition probabilities are important for the calculation of the average energy transfer. Forced quantum oscillator methods using a classical path whose initial velocity is the average of the initial and final velocities corresponding to the transition of interest are accurate for transition probabilities as small as 4 × 10^?8 for harmonic vibrators but do not seem to accurately account for the effect of anharmonicity.     

Vibrationally selective electronic energy transfer from metastable xe atoms to N2 and CO molecules

Formation of N₂(B) and CO(a', d) by collisions with Xe(³P_0,2) has been studied in the energy range 50–700 meV_c.m using crossed beams and beam/gas apparatus. Well-resolved product emission spectra were observed which showed high vibrational state specificity and low rotational excitation. These features are explained using semi-empirical interaction potentials.     

On long range effects in non-adiabatic collisions of atoms with molecules

Long range effects in atom-molecule collisions are considered. They are shown to have an important effect on the branching ratio between states that are asymptotically degenerate or nearly degenerate. A simple model shows that depending on the initial preparation of the reagents the occupation probability for a molecular state may be quite different. A semiclassical approach is proposed to include the coriolis interaction at long range into the scattering calculations. The model is applied to quenching collisions between Na*(3p) and diatomic molecules.     

Elastic and reactive collisions of atoms and molecules with neutral alkali clusters

We have measured absolute integral cross sections for low-energy collisions of atoms and molecules with neutral sodium clusters over a wide cluster size range (n=2–40). The cross sections are exceptionally large, reaching values of thousands of square angstroms. Consequently, the scattering involves long-range interactions. The van der Waals force, acting either alone (Na_n+N₂) or in concert with the inelastic charge-transfer “harpooning” channel (Na_n+Cl₂, Na_n+O₂) can describe the measurements. Using interaction parameters taken from spectroscopic studies of alkali clusters, we find very good agreement with the data. This provides a point of contact between beam scattering experiments and studies of cluster electromagnetic response properties.     

The interaction of metastable helium atoms with alkali atoms

Using crossed beams of ground state alkali atoms A (A = Li, Na, K, Rb, Cs) and metastable He(2³ S), He(2¹ S) atoms, we have measured the energy spectra of electrons resulting in the respective Penning ionization processes at: thermal collision energies. The data are interpreted to yield the well depthD _e ^* of the²Σ interaction potentials as follows: He(2³ S)+A:D _e ^* (A=Li)=868(20) meV;D _e ^* (Na)=740(25) meV;D _e ^* (K)=591(24) meV;D _e ^* (Rb)=546(18) meV;D _e ^* (Cs)=533(18) meV. He(2¹ S)+A:D _e ^* (Li)=330(17) meV;D _e ^* (Na)=277(24) meV;D _e ^* (K)=202(23) meV;D _e ^* (Rb)=219(18) meV;D _e ^* (Cs)=277(18) meV. The well depth for He(2³ S)+A(²Σ) is always close to 80% of the well depth for Li(2s)+A(X ¹Σ). The ionization cross sections for He(2¹ S)+A are about 3 to 4 times larger than those for He(2³ S)+A.     

Reactions of high translational energy hydrogen atoms with propane

The reactions of translationally excited deuterium atoms with propane have been investigated by use of photochemically produced atoms of 157kJmol(-1) initial energy. The reaction mechanism was tested by comparing the results for the DBr scavenged system to those where Br(2) was used as radical scavenger. Values were obtained for the fraction of atoms undergoing "hot" reaction on collision with propane, and the relative cross sections for the interaction of deuterium atoms with propane and DBr. The effect of the initial energy of the "hot" atoms is discussed.     

Red- and blue-shifted hydrogen bonds: the Bent rule from quantum theory of atoms in molecules perspective

MP2/6-311++G(3pd,3df) calculations were performed on complexes of acetylene and fluoroform acting as the proton donating systems and different Lewis bases being the proton acceptors since these complexes are linked through C-H···Y hydrogen bonds. Quantum Theory of Atoms in Molecules (QTAIM) is applied to explain the nature of these interactions. The characteristics of bond critical points are presented for these complexes. The inter-relations between energetic and geometrical parameters as well as the parameters derived from the Natural Bond Orbital (NBO) theory are analyzed here. Red- and blue-shifted hydrogen bonds are detected for the complexes investigated and the differences between those interactions are analyzed from the QTAIM perspective. It is shown that such differences are in agreement with the Bent rule. The position of the bond critical point of the proton donating bond is connected with the nature of hydrogen bonding, that is, if it is blue- or red-shifted.     

Scattering experiments with hydrogen atoms: Velocity dependence of the integral total cross section with simple molecules in the energy range 0.01–1.00 eV (c.m.)

Integral total cross sections for hydrogen atoms scattered by H₂, D₂, N₂, CO, NO, O₂, CO₂, C₂H₂ have been measured for H atom velocities in the range 1.2 to 13.4 × 10⁵ cm s^?1 (0.007 ? E_c.m. ? 0.94 eV). The data have been fitted to a Lennard-Jones (12,6) model potential for which the product of the potential well parameters, ?R_m, was determined (? is the well depth and R_m the location of the potential minimum). Separate values for ? and R_m are estimated using a combination rule procedure tested in earlier work on fi— rare gas scattering.