Ar Collisions Research Articles

Energy Transfer between Polyatomic Molecules II: Energy Transfer Quantities and Probability Density Functions in Benzene, Toluene, p-Xylene, and Azulene Collisions

Collisional energy transfer, CET, is of major importance in chemical, photochemical, and photophysical processes in the gas phase. In Paper I of this series (J. Phys. Chem. B 2005, 109, 8310) we have reported on the mechanism and quantities of CET between an excited benzene and cold benzene and Ar bath. In the present work, we report on CET between excited toluene, p-xylene, and azulene with cold benzene and Ar and on CET of excited benzene with cold toluene, p-xylene, and azulene. We compare our results with those of Paper I and report average vibrational, rotational, and translational energy quantities, <DeltaE>, transferred in a single collision. We discuss the effect of internal rotation on CET and the identity of the gateway modes in CET and the relative role of vibrational, rotational, and translational energies in the CET process, all that as a function of temperature and excitation energy. Energy transfer probability density functions, P(E,E'), for the various systems are reported and the shape of the curves for various systems and initial conditions is discussed. The major findings for polyatomic-polyatomic collisions are: CET takes place mainly via vibration-to-vibration energy transfer assisted by overall rotations. Internal free rotors in the excited molecule hinder energy exchange while in the bath molecule they do not. Energy transfer at low temperatures and high temperatures is more efficient than that at intermediate temperatures. Low-frequency modes are the gateway modes for energy transfer. Vibrational temperatures affect energy transfer. The CET probability density function, P(E,E'), is convex at low temperatures and can be concave at high temperatures. A mechanism that explains the high values of <DeltaE(a)> and the convex shape of P(E,E') is that in addition to short impulsive collisions there are chattering collisions where energy is transferred in a sequence of short encounters during the lifetime of the collision complex. This also leads to the observed supercollision tail at the down wing of P(E,E'). Polyatomic-Ar collisions show mechanistic similarities to polyatomic-polyatomic collisions, but there are also many dissimilarities: internal rotations do not inhibit energy transfer, P(E,E') is concave at all temperatures, and there is no contribution of chattering collisions.

SIMION ion optics simulations at atmospheric pressure

A method for simulating the motions of charged particles in atmospheric pressure conditions in electrostatic and magnetic fields has been developed and implemented in a user program for SIMION 7.0 and the predictive capability of the model tested against experiment. The statistical diffusion simulation (SDS) user program avoids the computationally intensive issues of high collision rates by employing collision statistics to simulate the effects of millions of collisions per time step. Ion motions are simulated by a combined viscous ion mobility and random ion jumping approach. Comparison of the model predictions against measurement of Cs + transport through room air, N 2, Xe, Ar, and He collision gases in a simple drift cell at pressures from 6 to 640 Torr are favorable and provide confidence that the approach is viable.

Fermi-shuttle type multiple electron scattering in atomic collisions

We present three-body classical trajectory Monte-Carlo calculations for ionization in slow (1–50 keV/u) N + on Ar collisions. According to the calculations, at smaller projectile energies ionization occurs mostly due to multiple scattering processes. At 1 keV/u impact energy, we found rather long (6–14-fold) scattering sequences.

B-spline Breit–Pauli R-matrix calculations for electron collisions with argon atoms

We have applied the Breit–Pauli B-spline R-matrix method described earlier (J. Phys. B: At. Mol. Opt. Phys. 37 (2004) 2173) to the treatment of e–Ar collisions. An individually optimized, term-dependent set of non-orthogonal valence orbitals is used to account for the strong term dependence in the one-electron orbitals. Concentrating on the near-threshold regime for excitation of the 3p54s manifold, we obtain excellent agreement with benchmark experimental data for the production of metastable atoms and for excitation of the VUV-emitting J = 1 levels. A detailed partial-wave resolved resonance analysis shows that many of the structures listed by Buckman and Clark (Rev. Mod. Phys. 66 (1994) 539) are composed of overlapping resonances. These, together with their principal components and decay channels, are identified and, in some cases, alternative classifications are suggested.

Transfer ionization of the isocharge sequence ion and Ar collisions

The ratios of transfer ionization to single capture for isocharge Cq+, Nq+, Oq+, Neq+ ions on Ar atoms are measured by using position-sensitive and time-of-flight techniques. It is found that the ratio R1 increases with nuclear charge Z in a q=4 sequence and the ratio R1 sequences have the minimum values at Z=7 and Z=8 in q=5 and q=6 sequences, respectively and the ratio R1 decreases with the increase in the nuclear charge Z in a q=7 sequence. The results may be explained within the frame of the statistical model. It may be concluded that the transfer ionization depends remarkably upon the electronic structure of the projectile. The projectiles used in this study have the same charge state and velocity, but different electronic structures. The dependence of the ratio R1 on q is studied for Neq+-Ar collisions. The ratio R1 is found to increase as q increases for Neq+–Ar collisions. The measured dependence of the ratio R1 on q is compared with the calculation using the molecular Coulomb over-barrier model.

Peculiarities of atom–quasidiatom collision complex formation: classical trajectory study

General peculiarities in the formation of quasibound complexes (QC) in collisions of an atom with a rigid diatomic or linear polyatomic molecule are studied using the method of classical trajectories. The CO2–Ar system is taken as an example. Statistical distributions of parameters characterizing either the formation of QC or those appropriate to rotational relaxation cross-sections are simulated and analyzed. Various factors are examined as far as their role in the formation of the QC is concerned: (1) the complexity of a trajectory approach (exact classical, planar, coplanar); (2) the selection of intermolecular potential energy surface (ab initio Parker–Snow–Pack potential; empirical Berreby–Dayan potential); (3) the departure of the initial CO2 rotational distribution from equilibrium. The cross-sections and bimolecular thermal rates for QC formation are obtained using the modified Sorbie–Murrell model. The use of derived analytical expressions makes the computations of QC fractions in CO2–Ar collisions easier both in equilibrium and in rotational non-equilibrium conditions.

Franck-Condon effects in collision-induced electronic energy transfer: I2(E; v = 1,2) + He, Ar.

Collisions of I2 in the E electronic state with rare gas atoms result in electronic energy transfer to the D, beta, and D' ion-pair electronic states. Rate constants for each of these channels have been measured when I2 is initially prepared in the J = 55, nu = 1 and 2 levels in the E state. The rate constants and effective hard sphere collision cross sections confirm the trends observed when nu = 0 in the E state is initially prepared: He collisions favor population of the D state, while Ar collisions favor population of the beta state. Final state vibrational level distributions are determined by spectral simulation and are found to be qualitatively consistent with the trends in the Franck-Condon factors. The experimental distributions are also compared to the recent quantum scattering calculations of Tscherbul and Buchachenko.

State-to-state rate constants for rotational relaxation of CO in collisions with Ar: a quantum study

Integral cross-sections and thermal rate constants for the CO( v=0, j) + Ar collisions are calculated within the coupled states method on two recent ab initio potential energy surfaces. The vibrationally elastic approximation is shown to provide an excellent agreement with three-dimensional quantum results whose sensitivity to the interaction potential is also examined. It is demonstrated that at j=0, both the energy corrected sudden scaling procedure and semiclassical method are unable to reproduce oscillations in the rotational product state distributions which is in accordance with recent benchmark close coupling calculations.

Binding energy enhancement in the isocharge sequence of ions in collisions with Ne

The dependence of the ratio R 1 for transfer ionization to single capture for C q+ , N q+ , O q+ , Ne q+ ions on Ne target upon the electronic structure of the projectile is studied by using position sensitive and time-of-flight techniques. It is found that for A q+ –Ne collisions the ratio R 1 decreases as the atomic number of the projectile increases for q=4,5,6,7 sequences which differs markedly from the results we obtained for A q+ –Ar collisions. The difference is explained by using a statistical model and the orbital penetration picture. Comparison of the R 1 ratios of A 4+–Ne and A 4+–Ar collisions provides strong evidence for the increase of the binding energy of the target valence electron after single electron capture. The increase in binding energy depends both upon the atomic number of the projectile Z 1 and the target atom Z 2. The larger Z 1, the larger the increase in binding energy, while the larger Z 2, the smaller the increase in binding energy.

Coupled mean-field description of electron removal processes in He+–Ne and He+–Ar collisions

Ion–atom collisions with active projectile and target electrons are considered in an approach which combines mean-field models for both types of electrons. A previous calculation is improved by taking into account the nonorthogonality of the propagated orbitals. Results for net recoil ion and free electron production in He+–Ne and He+–Ar collisions are shown to be in good agreement with experiments.

A Chemical Timing Method for Absolute Vibrational Relaxation Rate Constants in the Vibrational Quasi-Continuum Region of S1 p-Difluorobenzene

A new method has been developed to measure absolute rate constants for gas-phase collisional vibrational relaxation in high S 1 p-difluorobenzene (pDFB) vibrational energy regions where the levels overlap to form a quasi-continuum. The heavily mixed vibrational character of these levels produces S 1 → S 0 fluorescence without the vibrational structure that is needed to measure rate constants. Following well-studied chemical timing procedures, the addition of large pressures (in the kTorr range) of O 2 to a low pressure pDFB sample yields a structured fluorescence spectrum that proves to be suitable for measurement of absolute rate constants for vibrational relaxation induced by argon added to the pDFB + O 2 mixture. The first application concerns S 1 pDFB with ∈ v i b = 3700 cm - 1 and a high vibrational state density (ρ v i b > 4000 per cm - 1 ) that creates a quasi-continuum. The absolute rate constant for vibrational energy transfer in Ar collisions from the initially pumped region into the surrounding vibrational field is k Α r v = (9.4 ′ 1.5) x 10 6 Torr - 1 s - 1 = 2.9 × 10 - 1 0 cm 3 molecule - 1 s - 1 . This rate constant is within the experimental error of rate constants determined previously for three lower levels with ∈ v i b ≥ 2887 cm - 1 . The value is roughly 50% of the Lennard-Jones rate constant usually assumed for the vibrational activation/deactivation step of thermal unimolecular reactions. A rate constant analogous to k A r v but for O 2 collisions is determined to have approximately the same value as k A r v. An approximate intramolecular vibrational redistribution (IVR) rate constant for S 1 pDFB with ∈ v i b = 3700 cm - 1 corresponds to an IVR lifetime in the range of 45-75 ps.

Deactivation of the 6s and 6s' states of a xenon atom in collisions with helium, argon, and xenon atoms

A series of publications devoted to the study of collisional deactivation of Xe atoms in the 6s and 6s' states in high-pressure Ar — He and He — Xe mixtures with a low content of Xe is summarised. The processes of quenching of the *P1, *P0, *P1, and *P2 levels are studied in two-particle Xe*+Ar(He) → products+Ar(He), three-particle Xe*+ 2Ar(2He) → ArXe*(HeXe*+Ar(He), and three-particle Xe*+Xe+Ar(He) → Xe*2+Ar(He) collisions. The gases were excited by a fast-electron beam. The measurements were performed by the method of absorption probing by analysing the time dependence of the concentration of the excited states in the afterglow of the electron beam. The rate constants of 24 plasma-chemical reactions were measured, 17 of which — for the first time.

State-selective electron-capture calculations forp-Ar collisions in an independent many-electron model

We extend previous calculations of many-electron processes in ion-atom collisions to the case of state-selective electron capture by protons from the K,L,M shells in argon at 1-200 keV/amu collision energies. The results are based on a time-dependent density functional theory framework and the basis generator method for the propagation of atomic orbitals. Impact-parameter-dependent probabilities are obtained from a statistical analysis. The results for capture to the H(K,L,M) shells are compared with experiments, and are found to agree with them much better than previous calculations. This serves as an indication that both a proper account of atomic structure and flexibility in the propagation of atomic orbitals are important.

Electron loss from 0.74- and 1.4-MeV/u low-charge-state argon and xenon ions colliding with neon, nitrogen, and argon

Absolute total-, single-, and multiple-electron-loss cross sections are measured for $({\mathrm{Ar}}^{+}\ensuremath{-},$ ${\mathrm{Ar}}^{2+}\ensuremath{-},$ ${\mathrm{Xe}}^{3+})\ensuremath{-}(\mathrm{Ne},$ ${\mathrm{N}}_{2},$ Ar) collisions at 0.74 and 1.4 MeV/u. In addition, a many-body classical trajectory Monte Carlo model was used to calculate total- and multiple-electron-loss cross sections for ${\mathrm{Ar}}^{+}$ impact. For ${\mathrm{N}}_{2}$ and Ar targets, excellent agreement between the measured and calculated cross sections is found; for the Ne target the experimental data are approximately 40% smaller than the theoretical predictions. The experimental data are also used to examine cross-section scaling characteristics for electron loss from fast, low-charge-state, heavy ions. It is shown that multiple electron loss increased the mean charge states of the outgoing argon and xenon ions by 2 and 3 respectively. The cross sections decreased with increasing number of electrons lost and scaled roughly as the inverse of the sum of the ionization potentials required to sequentially remove the most weakly bound, next most weakly bound, etc., electrons. This scaling was found to be independent of projectile, incoming charge state, and target. In addition, the experimental total loss cross sections are found to be nearly constant as a function of initial projectile charge state. As a function of impact energy, the theoretical predictions yield an ${E}^{\ensuremath{-}1/3}$ behavior between 0.5 and 30 MeV/u for the total loss cross sections. Within error bars, the data are consistent with this energy dependence but are also consistent with an ${E}^{\ensuremath{-}1/2}$ energy dependence.

The effect of hydrogen-bonding and terminal group structure on the dynamics of Ar collisions with self-assembled monolayers

Gas–surface scattering experiments are used to probe the dynamics of energy exchange in collisions of 80 kJ/mol Ar atoms with self-assembled monolayers (SAMs) composed of HS(CH 2) 11OH, HS(CH 2) 10COOH, HS(CH 2) 11CH 3, and HS(CH 2) 9CHCH 2 alkanethiols on gold. Strong similarities in scattering from the methyl and alkene-terminated SAMs indicate that end-group torsional motion and vibration of the terminal C–C bond play only a minor role in the gas–surface energy transfer dynamics. Both of the purely hydrocarbon monolayers proved to be highly effective at dissipating the Ar incident energy. In scattering experiments performed at a specular angle of 30°, over 60% of the incident Ar atoms reached thermal or near-thermal equilibrium before desorbing from the CH 3- and CH 2-terminated monolayers. For those atoms that do scatter directly from the surfaces after an impulsive collision, less than 13% of their incident energy is retained. In comparison, the OH- and COOH-terminated SAMs also yield nearly identical scattering results, but they appear to be much more rigid than the hydrocarbon monolayers. For the OH and COOH surfaces, only 43% of the atoms scatter at 30° off the surface normal after approaching thermal equilibrium and the atoms that scatter impulsively from the surface retain approximately 17% of their collision energy. Since these energy exchange characteristics do not appear to be the result of alterations in the structure or mass of the four monolayers, and the energy modes in the terminus of the monolayers have only minor influences on energy exchange, the differences in the scattering dynamics are attributed to the hydrogen-bonding nature of the COOH- and OH-terminated alkanethiol self-assembled monolayers.

Saturated Absorption Spectra of Metastable Argon Atoms in a Thin Discharge Cell

Saturated absorption spectra of the 1s 3 –2p 2 (Paschen notation) line of metastable argon (Ar * ) atoms in a 1 mm thick glow discharge cell have been observed at room temperature. We examined the pressure dependence of the spectra from 0.025 to 0.5 torr. The profiles of the spectra were compared with the velocity-changing collision kernels of the Ar * (1s 3 )–Ar collisions. The results suggest that the saturated absorption spectroscopy with the thin cell is useful to detect collision kernels of metastable atoms.

Investigating the gas-phase chemical reactions of molecular dications

Download options Please wait... Article information DOI https://doi.org/10.1039/B301957H Article type Invited Article Submitted 19 Feb 2003 Accepted 26 Mar 2003 First published 02 Apr 2003 Download Citation Phys. Chem. Chem. Phys., 2003,5, 1717-1729 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Investigating the gas-phase chemical reactions of molecular dications S. D. Price, Phys. Chem. Chem. Phys., 2003, 5, 1717 DOI: 10.1039/B301957H To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Stephen D. Price

Particle-in-cell/Monte Carlo simulation of a capacitively coupled radio frequency Ar/CF4 discharge: Effect of gas composition

A one-dimensional particle-in-cell/Monte Carlo model is developed to study a capacitively coupled radio frequency discharge in a gas mixture of argon and CF4. The simulation takes into account the following charged particles: electrons, two kinds of positive ions (Ar+, CF3+), and two kinds of negative ions (F−, CF3−). The model considers electron–Ar collisions, electron−CF4 collisions, various kinds of collisions of CF3+, F−, CF3−, or Ar+ with Ar or CF4, and positive–negative ion recombination. The probability for the positive–negative ion recombination is determined from a recombination rate constant. The ion–neutral elastic and reactive collisions are simulated by an ion–molecule collision model for endothermic reactions. The typical results of this model are electron and ion densities, fluxes and energy distributions, collision rates, and electric field and potential distributions. The simulation is performed for 0.1/0.9, 0.5/0.5, and 0.9/0.1 ratios of a Ar/CF4 mixture, as well as for pure Ar and pure CF4 discharges at a pressure of 200 mTorr. It is observed that at high CF4 concentration the discharge behaves as a typical electronegative discharge and that CF3+ is the major positive ion. At low CF4 concentration, keeping the other operating parameters the same, the double layer structure and the electron density maxima at the bulk–sheath interface, which are representative for an electronegative discharge, disappear and the Ar+ density exceeds the CF3+ density by more than 1 order of magnitude. The results show that the F− ions are the dominant negatively charged species for all Ar/CF4 ratios investigated.

Rotational energy transfer in collisions of jet-cooled NO(X2Π1/2, v = 0) with Ar and with He: energy and angular momentum constraints

Using a crossed molecular beam apparatus, we have examined rotational energy transfer in collisions of NO(X2Π1/2, v = 0, ji = 0.5 and 1.5) with Ar and with He. Relative values of the populations transferred into specific final states (2Π1/2, v = 0, jf) are reported for the following collision energies: NO–Ar, 265, 514, 720 and 980 cm−1, and NO–He, 260, 530, 784 and 963 cm−1. In all cases, the distribution over final rotational states is wide with the cross-sections decreasing with increasing Δj. The average energy transferred to rotation in NO is approximately independent of collision energy and is ca. 17% in the case of NO–Ar, and ca. 11% for NO–He. In NO–Ar collisions, transfer takes place to essentially all energetically accessible jf. This is not so for NO–He, where the smaller range of orbital angular momentum leads to a lower limit to the highest jf state. The results are compared with those from model calculations in which energy is transferred impulsively. The agreement between the results of the experiments and these calculations in respect of the highest rotational levels that are accessed is good, confirming that in collisions where the spin-orbit state of the NO is preserved, energy is transferred through the action of repulsive forces and that the difference between the results for NO–Ar and NO–He can be put down to the operation of momentum constraints in the latter case. More detailed comparisons suggest that in the case of NO–Ar collisions, the distribution of NO molecules over jf states might be significantly modified by secondary encounters.

Theoretical study of cusp-electron emission from O8+ + Ar collisions

Cusp-electron emission via processes electron capture to the continuum (ECC) and transfer ionization (TI) has been investigated theoretically in 0.3–2 MeV/amu O8+on Ar collisions. Doubly differential cross-sections for ECC and TI were calculated by using the classical trajectory Monte Carlo method within the framework of the independent particle approximation. The TI/ECC cusp-intensity ratio was found to have a maximum at about 1 MeV/amu impact energy, in accordance with the previous experimental observation. The maximum is due to the increasing contribution of the L shell of argon to the cusp-electron production with increasing collision velocity.