Classical dynamics of state-resolved hyperthermal O(3P) + H2O(1A1) collisions

: We present global potential energy surfaces for the three lowest triplet states in O(3P) + H2O(X1A1) collisions and present results of classical dynamics calculations on the O(3P) + H2O(X1A1) yields OH(X2Pi) + OH(X2Pi) reaction using these surfaces. The surfaces are spline-based fits of approx. 20,000 fixed geometry ab-initio calculations at the CASSCF+MP2 level with a O(4s3p2d1f)/H(3s2p) one electron basis set. Computed rate constants compare well to measurements in the 1,000-2,500 K range using these surfaces. We also compute the total, ro-vibrationally resolved, and differential angular cross sections at fixed collision velocities from near threshold at approx. 4 km/s (16.9 kcal/mol collision energy) to 11 km/s (122.5 kcal/mol collision energy), and we compare these computed cross sections to available space-based and laboratory data. A major finding of the present work is that above approx. 40 kcal/mol collision energy ro-vibrationally excited OH(X2Pi) products are a significant and perhaps dominant contributor to the observed 1-5 micron spectral emission from O(3P) + H2O(X1A1) collisions. Another important result is that OH(X2Pi) products are formed in two distinct ro-vibrational distributions. The active' OH products are formed with the reagent O-atom, and their ro-vibrational distributions are extremely hot. The remaining spectator' OH is relatively ro-vibrationally cold. For the active OH, rotational energy is dominant at all collision velocities, but the opposite holds for the spectator OH. Summed over both OH products, below approx. 50 kcal/mol collision energy, vibration dominates the OH internal energy, and above approx. 50 kcal/mol rotation is greater than vibrational energy. As the collision energy increases, energy is diverted from vibration to mostly translational energy. We note that the present fitted surfaces can also be used to investigate direct collisional excitation of H2O(X1A1) by O(3P) and also OH(X2Pi) + OH(X2Pi) collisions.

Absolute state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of HF with He, Ne, and Ar at mean center-of-mass collision energies of 480, 390, and 350 cm−1, respectively. HF seeded in Ar diluent gas is cooled into the J=0 ground rotational state in a pulsed supersonic expansion, followed by single collision rotational excitation with rare gas atoms from a second pulsed supersonic jet. The column-integrated densities of HF in both the initial and final scattering states are probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm−1), continuously tunable, color center laser. Total inelastic cross sections for collisional loss out of J=0 and collisional excitation into J>0 states are determined in absolute units from the dependence of infrared absorption signals on collider gas concentration. Full close coupling scattering calculations are performed on several ab initio and empirical potential energy surfaces for each of the three HF+rare gas systems. Agreement for He+HF and Ar+HF integral cross sections is remarkably good, but significant discrepancies are noted for the less accurately determined Ne+HF surface. Photoelastic polarization modulation of the IR laser is used to probe for rotational alignment in the scattered HF flux; the measurements set an upper polarizance limit for collisionally populated J=1 HF molecules [probed on P(1)] of |P|<2%. High resolution IR laser Dopplerimetry reveals velocity structure in the collisionally excited J=1 Doppler profiles, which is in excellent qualitative agreement with theoretical predictions of rainbow features in the J=1←0 state-to-state differential cross section.

Molecular anions have recently been detected in the interstellar and circumstellar media. Accurate modeling of their abundance requires calculations of collisional data with the most abundant species that are usually He atoms and H2 molecules. In this paper, we focus on the collisional excitation of the first observed molecular anion, C6H(-), by He and H2. Theoretical calculations of collisional cross sections rely generally on ab initio interaction potential energy surfaces (PESs). Hence, we present here the first PESs for the C6H(-)-H2 and C6H(-)-He van der Waals systems. The ab initio energy data for the surfaces were computed at the explicitly correlated coupled cluster with single, double, and scaled perturbative triple excitations level of theory. The method of interpolating moving least squares was used to construct 4D and 2D analytical PESs from these data. Both surfaces are characterized by deep wells and large anisotropies. Analytical models of the PESs were used in scattering calculations to obtain cross sections for low-lying rotational transitions. As could have been anticipated, important differences exist between the He and H2 cross sections. Conversely, no significant differences exist between the collisions of C6H(-) with the two species of H2 (para- and ortho-H2). We expect that these new data will help in accurately determining the abundance of the C6H(-) anions in space.

Differential cross sections and escape plots for low-energy solitonic SU(2) BPS magnetic monopole dynamics

Rotational excitation in collisions of atoms with diatomic molecules is investigated using classical mechanics. The structure of the fully resolved cross sections with respect to the final molecular angular momentum, its projection onto the quantization axis, and the scattering angle are studied numerically using simple model potentials. In particular the influence of isotropic and anistropic attractive forces is investigated. In the first case the structure of the cross section is still similar to that for repulsive scattering. Anistropic attraction introduces new phenomena whose relations to the properties of the potential are explored.

Four different ab initio calculations were used to determine four sets of potential energies at a common set of geometries for the Li+H2 collision system. The four calculations involved choices between a moderate and a large basis set and between the Hartree–Fock and the multiconfiguration-self-consistent field formalisms. Only the nonreactive ground electronic state surface was examined. An analytic functional form was least squares fit to each set of potential energies to produce four analytic potential energy surfaces. Classical trajectories at energies below the reaction threshold were run on each surface. The trajectories were analyzed to obtain the inelastic cross sections, differential cross sections, and average energy loss as a function of angle. The comparison of these collision observables from surface to surface was used to characterize the chemical accuracy of each surface. All four surfaces, from the least to the most accurate, produced significant amounts of rotational excitation.

The cross section of a given process fundamentally quantifies the probability for that given process to occur. In the quantum regime of low energies, the cross section can vary strongly with collision energy due to quantum effects. Here, we report on a method to directly measure the atom-ion collisional cross section in the energy range of 0.2-12 mK$\cdot$ k$_B$, by shuttling ultracold atoms trapped in an optical-lattice across a radio-frequency trapped ion. In this method, the average number of atom-ion collisions per experiment is below one such that the energy resolution is not limited by the broad (power-law) steady-state atom-ion energy distribution. Here, we estimate that the energy resolution is below 200 $\mu$K$\cdot$k$_B$, limited by drifts in the ion's excess micromotion compensation and can be reduced to the 10's $\mu$K$\cdot$k$_B$ regime. This resolution is one order-of-magnitude better than previous experiments measuring cold atom-ion collisional cross section energy dependence. We used our method to measure the energy dependence of the inelastic collision cross sections of a non-adiabatic Electronic-Excitation-Exchange (EEE) and Spin-Orbit Change (SOC) processes. We found that in the measured energy range, the EEE and SOC cross sections statistically agree with the classical Langevin cross section. This method allows for measuring the cross sections of various inelastic processes and opens up possibilities to search for atom-ion quantum signatures such as shape-resonances.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Hiroaki Matsumoto; Monte Carlo Simulation of Hypersonic Rarefied Gas Flow using Rotationally and Vibrationally Inelastic Cross Section Models. AIP Conference Proceedings 20 May 2011; 1333 (1): 1245–1250. https://doi.org/10.1063/1.3562814 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAIP Conference Proceedings Search Advanced Search |Citation Search

An apparatus is described for measuring the inelastic differential cross section for vibrational excitation in collisions of diatomic molecules with monoenergetic ions at laboratory energies between 10 and 50 eV. The method consists of measuring the time of flight of single ions with a time amplitude converter and displaying the results on a 100 channel pulse height analyzer. From the shift in the time of flight relative to that expected for elastic scattering the final state of the molecule excited in a single collision is identified. By studying only central collisions with almost zero impact parameter rotational excitation is strongly suppressed. Measured times of flight after collisions of monoenergetic Li+ ions with H2 show that with increasing energy the most probable vibrational quantum jump increases from 0→1 to 0→2,0→3 etc. Contrary to the usual assumption of a small steric factor for vibrational excitation the results show that the inelastic cross section is larger than the elastic cross section. Using reported potential parameters the energy dependence of the most probable excited state is compared with the calculations of Secrest and Johnson for a one-dimensional collinear collision. The satisfactory agreement suggests that the steric factor is close to 1. From measurements at different scattering angles at 10 eV the integral inelastic cross section is found to be about 0.2 A2 corresponding to a differential cross section of 0.4 A2/sr. Measured values of integral and differential total cross sections for Li+-He andLi+-H2 are reported and compared with theory. Direct dissociation of D2 by Li+ in the energy range from 25 to 55 eV was not observed, yielding an upper limit for the cross section of 4 · 10−4 A2/sr.

The cross section for ionization of Rydberg atoms in collision with electrons at low and moderate energies is calculated using classical trajectories. The dependence of the total cross section on the initial angular momentum of the target is revealed. Low-discrepancy sequences of (quasirandom) numbers are compared with plain random numbers to confirm that they can lead to improved convergence of calculation in certain cases. A phenomenological parametrization using only two parameters is shown to describe well the classical cross section over a wide range of energies and angular momenta. Using the classical scaling law, the results are extended to any principal quantum number.

The photodissociation of NO 2 in the second absorption band: Ab initio and quantum dynamics calculations

A semi-classical molecular dynamics approach is used to model the dissociation of water to form the hydroxyl radical. The unimolecular dissociation of water is used to calculate the probability of reaction as well as determine the product OH translational, vibrational, and rotational energy distributions. The molecular dynamics probabilities of reactions and product distributions are used in the direct simulation Monte Carlo method to model spatial distribution and temperatures of OH in the bow-shock of a 5 km/sec vehicle at 80 and 100 km. Ultraviolet spectra of the OH(X→A) transition are computed and compared with data from the Bow Shock Ultraviolet Flight Experiment 2. Excellent agreement of the spectral features between theory and experiment is obtained.

The reliability of five N2-Ar potential-energy surfaces in representing the N2-Ar interaction has been investigated by comparing their abilities to reproduce a variety of experimental results, including interaction second viral coefficients, bulk transport properties, relaxation phenomena, differential scattering cross sections, and the microwave and infrared spectra of the van der Waals complexes. Four of the surfaces are the result of high-level ab initio quantal calculations; one of them utilized fine tuning by fitting to microwave data. To date, these four potential-energy surfaces have only been tested against experimental microwave data. The fifth potential-energy surface, based upon the exchange-Coulomb potential-energy model for the interaction of closed-shell species, is developed herein: it is a combination of a damped dispersion energy series and ab initio calculations of the Heitler-London interaction energy, and has adjustable parameters determined by requiring essentially simultaneous agreement with selected quality interaction second viral coefficient and microwave data. Comparisons are also made with the predictions of three other very good literature potential-energy surfaces, including the precursor of the new exchange-Coulomb potential-energy surface developed here. Based upon an analysis of a large body of information, the new exchange-Coulomb and microwave-tuned ab initio potential-energy surfaces provide the best representations of the N2-Ar interaction; nevertheless, the other potential-energy surfaces examined still have considerable merit with respect to the prediction of specific properties of the N2-Ar van der Waals complex. Of the two recommended surfaces, the new exchange-Coulomb surface is preferred on balance due to its superior predictions of the effective cross sections related to various relaxation phenomena, and to its reliable, and relatively simple, representation of the long-range part of the potential-energy surface. Moreover, the flexibility still inherent in the exchange-Coulomb potential form can be further exploited, if required, in future studies of the N2-Ar interaction.

We report real wave packet (WP) calculations of reaction probabilities, cross sections, rate constants, and product distributions of the reaction N(4S)+O2(X 3∑g−)→NO(X 2∏)+O(3P). We propagate initial WPs corresponding to several O2 levels, and employ reactant coordinates and a flux method for calculating initial-state-resolved observables, or product coordinates and an asymptotic analysis for calculating state-to-state quantities. Exact or J-shifting calculations are carried out at total angular momentum J=0 or J>0, respectively. We employ the recent X 2A′ S3 potential energy surface (PES) by Sayós et al. and the earlier a 4A′ PES by Duff et al. In comparing S3 results with the WP ones of a previous X 2A′ S2 PES, we find lower S3 energy thresholds and larger S3 probabilities, despite the higher S3 barrier. This finding is due to the different features of the doublet PESs in the reactant and product channels, at the transition state, and in the NO2 equilibrium region. We analyze the effects of the O2 initial level and show that tunneling through the S3 barrier enhance the room-temperature rate constant by ∼3.7 times with respect to the previous S2 WP rate. The agreement with the room-temperature experimental result is thus notably improved. The NO vibrational distribution is inverted and the rotational ones are strongly oscillating. We explain these nonstatistical results showing that the reaction partners approach each other with a large impact parameter. The WP vibrational distribution is however different from that observed, which is oscillating. WP calculations show that the new S3 PES describes accurately several features of the X 2A′ state, although a lowering of its barrier height by ∼0.56 kcal/mol should bring calculated and observed rate constants in full agreement.

Accurate quantum mechanical close coupling (CC) integral cross sections are reported for rotationally inelastic Ar–N2 collisions at room temperature using an empirical intermolecular potential. These cross sections are used to test the infinite order sudden (IOS) approximation and the results of several methods [classical trajectories (CT), full sudden (SA), generalized phase shift (GPS), and classical limit generalized phase shift (CGPS)] as reported in the preceding paper by Pattengill. The IOS approximation works very well for cross sections involving low rotational states. The CT method works well for all the cross sections examined, justifying the popular belief that the rotational motion of N2 is essentially classical at room temperature.