Thermal Energy Collisions Research Articles

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.

Accurate collision-induced line-coupling parameters for the fundamental band of CO in He: Close coupling and coupled states scattering calculations

We present here the first accurate theoretical values for off-diagonal (i.e., line-coupling) pressure-broadening cross sections. Calculations were done for CO perturbed by He at thermal collision energies using an accurate ab initio potential energy surface. Converged close coupling (CC), i.e., numerically exact values, were obtained for coupling to the R(0) and R(2) lines. These were used to test the coupled states (CS) and infinite order sudden (IOS) approximate scattering methods. CS was found to be of quantitative accuracy (a few percent) and has been used to obtain coupling values for lines to R(10). IOS values are less accurate, but, owing to their simplicity, may nonetheless prove useful as has been recently demonstrated.

Internal-energy-selected measurements of acetylene ions with acetylene at thermal energies

A photoelectron-photoion coincidence technique is used to measure the breakdown curve of the primary acetylene ion and the internal-energy dependence of the reaction of C 2H + 2 ions with C 2H 2 molecules, to form C 4H + 2 or C 4H + 3 ions. Both the electronic ground state and the first electronically excited state are included. A possible decay mode of C 2H + 2 in the first excited state, which reconciles seemingly contradictory published experimental data, is proposed. The reaction is studied at thermal collision energies. A statistical model is compared with the data. As a result of this comparison absolute cross sections are derived.

Radiative relaxation of vibrationally excited ions

A triple cell ICR spectrometer analogous to a three-stage mass spectrometer (MS/MS/MS) but capable of studying thermal energy collisions is described. The facility to store ions in a good vacuum allows the measurement of radiative lifetimes using a chemical reaction to probe the energy contents of the ions as a function of their storage time. Vibrational lifetimes have been measured for two diatomic ions in their electronic ground state: NO + and HCl +. NO + and NO have similar lifetimes, in very good agreement with calculated values. HCl + ( v = 1) has a lifetime ten times shorter than HCl (3 ± 1 ms compared with 29 ms), in fairly good agreement with the calculated value of 4.6 ms. This is the first experimental confirmation of the very short lifetimes predicted theoretically for diatomic hydride ions. Qualitative results have been obtained for the radiative lifetime of NH 3 +. The shape of the decay curves indicates the presence of long-lived (more than 300 ms) and short-lived (10–20 ms) states.

Penning and associative ionization of mercury by collision with metastable rare gas atoms

The ionization of mercury atoms by thermal energy collisions with metastable neon, argon, and krypton has been studied in a crossed beam experiment. The production of both Hg+ and RHg+ (R=Ne, Ar, Kr) ions has been observed and the collision energy dependence of the cross section for each ionization channel has been measured. The experimental relative cross sections have also been analyzed in terms of an optical model. In all cases the main ionic product is the Hg+ ion. The average relative yield of RHg+ appears to increase going from neon to krypton. The different role of the two spin–orbit states of the metastable krypton atoms has been assessed: The Kr(3P0) atoms can produce both Hg+ and KrHg+ in the whole energy range investigated, while the Kr(3P2) atoms can ionize the mercury only for a collision energy larger than ∼0.15 eV, producing KrHg+ in the energy range between ∼0.15 and ∼0.5 eV, and producing also Hg+ at energies larger than ∼0.5 eV.

Thermal energy collisions between metastable Ne⋆(2p 53s,3 P 0, 2) and H2(X,1Σ g + )

A crossed beam experiment is used to investigate the Ne*(2p5 3s,3P0, 2) − H2(1Σ g + ) collision at thermal energy (67 meV). The H2 beam is supersonic, the Ne* beam is thermal. Different collision processes have been analyzed separately by means of a double chopping technique combined with a time of flight measurement. Ions produced by Penning effect and chemi-ionization have been separated from scattered metastable atoms by an accelerating electric field small enough to preserve a reasonable angular resolution: δϑ(ions)=±5.5°, δϑ(Ne*)=±1°, which allows a determination of differential cross sections. The attenuation method, combined with an absolute measurement of the total H2 flux, has been used to measure the total cross section: σ t =940±220a 0 2 . Differential cross sections have been obtained, in arbitrary but unique unit, for the following processes: (1) elastic collisions, for a mixture (1:3) of para- and ortho-hydrogen; (2) rotationally inelastic collisions:J=0→2; (3) Penning ionization resulting into H 2 + ions; (4) chemiionization yielding NeH+ ions.

Collision-induced dissociation of laser-excited Br2[B 3Π(0+u);v′, J′]: Formation of Br*(2P1/2)+Br(2P3/2) at energies 1–5 k T below dissociation

Absolute rate constants are measured for the dissociation of initially excited Br2[B 3Π(0+u);v′,J′] into Br*(2P1/2)+Br(2P3/2) fragments by thermal energy collisions with Br2(X 1Σ+g), Xe, and Ar at 297 K. A 0.04 cm−1 étalon-narrowed pulsed dye laser populates specific rovibrational levels of isotopic Br2 which are 1–5 kT below the B-state dissociation limit; in addition the laser directly photodissociates molecules which are in thermally excited vibrational levels. The method used to determine the absolute rate constants combines four sets of experimental determinations, which include infrared detection of Br*, visible Br2(B) fluorescence lifetimes, absorption spectroscopy of Br2 (B←X), and transient gain-vs-absorption spectroscopy on Br/Br*. At 1 kT below the dissociation limit, the absolute rate constants for collisional dissociation to Br*+Br are 2.9×10−10, 1.2×10−10, and 5.4×10−11 cm3 molecule−1 s−1 for Br2, Xe, and Ar, respectively. The results represent an average of some rotational and vibrational states grouped around a particular energy since rotational and vibrational energy transfer occur on a competitive time scale. The other major process that contributes to the total quenching rate is collision-induced predissociation to Br+Br. At 1 kT below the B-state dissociation limit, ∼75% of the quenching proceeds upward in energy to form Br*+Br, while at 5 kT this fraction is ∼8%. The efficiency of upward collisional dissociation correlates with the relative translational energy available.

Ionisation in thermal-energy collisions of a resonance-state atom with a ground-state atom. The role of quasimolecular term structure peculiarities

The inelastic collisions of He(1s2p 1P1) atoms with ground-state noble-gas atoms are studied using the kinetic plasma spectroscopy method. Quenching rate constants are found to be (25+or-5)*10-10, (27+or-5)*10-10 and (32+or-5)*10-10 cm3s-1 for Ar, Kr and Xe, respectively (T=600 K). A detailed theoretical interpretation taking the specific features of quasimolecular terms into account is developed. Good agreement between theory and experiment is obtained for He-Ar and He-Kr. Possible reasons for the discrepancy for the He-Xe system are discussed.

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 * of the2Σ interaction potentials as follows: He(23 S)+A:D * (A=Li)=868(20) meV;D * (Na)=740(25) meV;D * (K)=591(24) meV;D * (Rb)=546(18) meV;D * (Cs)=533(18) meV. He(21 S)+A:D * (Li)=330(17) meV;D * (Na)=277(24) meV;D * (K)=202(23) meV;D * (Rb)=219(18) meV;D * (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.

The interaction of metastable Ne(3s 3 P 2,3 P 0) atoms with alkali atoms

Using crossed beams of alkali atoms (Li, Na, K) and state-selected metastable Ne(3s3P2,3P0) atoms, we have measured the energy spectra of electrons resulting in the respective Penning ionization processes at thermal collision energies. The spectra are very different for Ne(3P2) and Ne(3P0): those for Ne(3P2) are broad due to a strongly attractive interaction potential with a well depth of 798 (30) meV (Li), 672(20) meV (Na), and 561(20) meV (K), those for Ne(3P0) are narrow and compatible with van der Waals type attraction (well depth <50 meV). The Ne(3P2) cross section exceeds the one for Ne(3P0) by about an order of magnitude.

Nonadiabatic theory of fine-structure branching cross sections for Na-He, Na-Ne, and Na-Ar optical collisions

The nonadiabatic close-coupled theory of atomic collisions in a radiation field is generalized to include electron spin and is used to consider the weak-field Na--rare-gas (RG) optical collision Na${(}^{2}$${S}_{1/2}$)+RG+nh\ensuremath{\nu} \ensuremath{\mu}Na${(}^{2}$${\mathrm{P}}_{\mathrm{j}}$)+RG+(n-1)h\ensuremath{\nu}. The effects of detuning and incident energy on the branching into the atomic Na 3${p}^{2}$${P}_{3/2}$ and 3p $^{2}P_{1/2}$ states are examined. The cross sections \ensuremath{\sigma}(j) are found to have a strong asymmetry between red and blue detuning as well as a complex threshold and resonance structure dependence on energy. A partial cross-section analysis of \ensuremath{\sigma}(j) shows a significant difference between contributions from states of e and f molecular parity. The theoretically calculated detuning dependence of the branching ratio into each fine-structure state is in good agreement with available experimental data for Na-Ar, Na-Ne, and Na-He, as well as the total absorption coefficient for the production of Na 3p atoms. The fine-structure branching ratio for thermal energy collisions shows considerable variation with a rare-gas collision partner, due to the different interaction potentials. For sufficiently high collision energy, the branching approaches a recoil limit which is independent of collision partner.

Ionization of mercury by thermal energy collisions with metastable krypton atoms

In a crossed-beam experiment the cross sections for Hg+ and KrHg+ production in collisions between metastable krypton and mercury atoms have been measured in the 0.07–0.50 eV collision energy range. The 3P0 and 3P2 states of Kr* show a different role in these ionization processes: the 3PO state produces both the atomic and diatomic ion in the whole energy range investigated, while the 3P2 state produces only the diatomic ion after a threshold energy around 0.15 eV.

Semiclassical analysis of the anisotropic total cross section in thermal energy collisions between a non-spherical and a spherical atom

Thermal energy atomic collisions of the typeA(2P3/2)+B(1S0) are discussed to clarify the dependence of the orientational anisotropy Δσ in the total cross section σ on the relative velocityv. Simple expressions for Δσ/σ are derived by semiclassical treatments designed to be appropriate for large spin-orbit interactions. The Δσ/σ oscillates withv around its average value. The velocities giving the extrema in the oscillating curve are almost solely determined by the isotropic part\(\bar V\left( R \right)\) in the interaction potential, while the amplitude of the oscillation depends sensitively on the anisotropic interaction nearRmi, whereRmi is the location of the minimum in\(\bar V\left( R \right)\). The oscillation-averaged Δσ/σ is independent of the relative velocity and proportional to the anisotropy in the van der Waals interaction.

The influence of internal energy on the cross section of the ion-molecule reaction C 3H +4 (C 3H 4, H)C 6H +7

A coincidence technique is used to study the influence of the internal energy of the reactant ion on the relative cross section of the reaction C 3H + 4 + C 3H 4 → C 6H + 7 + H. The reaction is studied at thermal collision energies, and for two isomeric collision systems. From an analysis of the results in terms of a statistical model for the reaction, it is concluded that the reactant isomeric structure is maintained in an intermediate collision complex as well as at a rate determining reaction barrier.

Spectroscopy of neon Rydberg states detected by electron transfer to SF 6 molecules

We have studied the ionization of laser excited 20Ne( ns, md) Rydberg states in thermal energy collisions with SF 6 molecules as a means of sensitive detection of electronically excited states with binding energies from zero up to at least 0.4 eV. Effective collisional ionization was observed for n⩾8, m⩾6, but not for the Ne(5s)-, the Ne(4d)-, and other lower-lying Ne-states. A precise value for the ionization limit 20Ne +( 2P 3 2 ) has been derived from Doppler free laser spectroscopy of the unperturbed 20Ne( md J = 4) series ( m = 4, 6–72).

Internal-energy-selected measurements on the relative cross section of the ion-molecule reaction NH3+(NH3, NH2)NH4+

Abstract A photoelectron-photoion coincidence technique is used to measure the internal-energy dependence of the ion-molecule reaction NH 3 + ( E int +NH 3 → NH 4 + + NH 2 at thermal collision energy. The range in which the internal energy is varied, is enlarged by including in the experiment the electronically excited state of the NH 3 + ion. Special attention is paid to the possible influence of the product's kinetic energy on the measurements. The experimental results are analysed using a modified statistical model and compared with previous data.

Symmetry constraints in energy transfer between state-selected Ar*(3P2, 3P0) metastable atoms and ground state H atoms

The excitation-transfer reaction in thermal energy collisions of state-selected metastable Ar*( 3 P 2 ) and Ar*( 3 P 0 ) atoms with ground state H atoms, giving excited H*( n = 2) atoms, has been studied with the stationary afterglow technique. The rate constant for the excitation of H atoms by Ar*( 3 P 2 ) has been found to be more than one order of magnitude larger than in excitation by Ar*( 3 P 0 ). This difference in the reactivity of two metastable species is explained to be a consequence of the attractive nature of the D( 2 II) and E( 2 Σ + ) potentials that develop from the Ar*( 3 P 2 )+H entrance channel and which give curve crossing with the B( 2 II) and C( 2 Σ + potentials, respectively, leading to the Ar+H*( n =2) exit channel, whereas only a repulsive 4 II (Ω= 1 2 ) potential develops from the Ar*( 3 P 0 +H entrance channel.

Polarization effects in ionizing thermal energy collisions of laser-excited Ne(3p 3 D 3) atoms with Ar atoms

Using transverse and longitudinal excitation of a collimated metastable Ne(3s 3 P 2.0) beam with average velocities of 500, 800, and 1,200 m/s by means of a single mode dye laser on the20Ne(3s 3 P 2→3p 3 D 3) transition, we have investigated ionizing collisions of polarized Ne(3s 3 P 2) and Ne(3p 3 D 3) atoms with Ar atoms. The product electrons were energy analyzed with high resolution (9–25 meV). The resulting Ne(3p 3 D 3) electron spectra exhibit a strong dependence on the three types of laser polarization (π ∥,π⊥, σ⥮), chosen to prepare the excited atoms. In contrast, the Ne(3s 3 P 2) spectra are only weakly dependent on polarization. Detailed model calculations have been carried out for the Ne(3p)+Ar cross sections, using computed excited-state potential curves, semi-empirical ionic potentials, and local autoionization width functions. A semiclassical closecoupling method is applied to describe the evolution of the polarized collision system in the coupled entrance channels. It is found that a single autoionization widthΓ(R) is not sufficient to describe the measured polarization effects properly. The dependence ofΓ on the initial and final state is expressed in terms of few reduced electronic transition matrix elements, which are determined by comparison of measured and calculated total cross sections and Ar+(2 P 3/2)/Ar+(2 P 1/2) branching ratios for ionizing collisions of the various Ne(3pJ=1,2,3) multiplet states with Ar. The matrix elements corresponding to Ar(3pσ)→Ne(2pσ) electron transfer during autoionization are found to dominate, but Ar(3pπ)→Ne(2pπ) transfer has also to be included. The resulting calculated electron spectra reproduce the measured polarization effects in a semi-quantitative way.

IONIZING COLLISIONS OF LASER-EXCITED RARE GAS ATOMS

Using mass spectrometry and electron spectrometry, we have studied ionization of Ar and Kr atoms in thermal energy collisions with laser-excited short-lived Ne(2p5 3p J=1,2,3) atoms. The cross sections show a strong state- dependence, which is reproduced in calculations based on theoretical poten- tial energy curves and a single exponential autoionization width. Significant polarization effects have been observed in the total and partial ion yields and in the electron spectra; they can only be explained with R-dependent au- toionization probabilities favouring o + o electron transfer.

A molecular beam study of electronic to electronic, vibrational, and rotational energy transfer in the collision of two step laser excited sodium with N2

The quenching of excited Na* (4d,5s,4p,4s) by N2 has been studied in a crossed atomic and molecular beam apparatus at thermal collision energies. The sodium atoms are excited by two laser beams of different wavelengths to either the 42D5/2 or 52S1/2 state, via the intermediate 32P3/2 state. For both excitation schemes optical relaxation processes lead to a population in the 42P3/2 and 42S1/2 states of several percent. The relative densities of the excited states have been calculated from rate equations using stationary conditions. The measured energy transfer spectra show high scattering intensities at low kinetic energies together with some structure at medium energies. This structure can be partially disentangled using the results of the previously studied Na*(32P3/2)+N2 quenching process. The main conclusion is that collisional deexcitation to the Na(3s) ground state is negligible, whereas among the higher levels the collisional energy transfer cross sections are between 0.5 and 7.5 times the magnitude of the 3p–3s quenching cross section and they are strongly forward peaked in the same way. Relative values for the differential quenching cross sections are reported.