HCl Collisions Research Articles

Rainbow scattering in rotationally inelastic collisions of HCl and H2.

We examine rotational transitions of HCl in collisions with H2 by carrying out quantum mechanical close-coupling and quasi-classical trajectory (QCT) calculations on a recently developed globally accurate full-dimensional ab initio potential energy surface for the H3Cl system. Signatures of rainbow scattering in rotationally inelastic collisions are found in the state resolved integral and differential cross sections as functions of the impact parameter (initial orbital angular momentum) and final rotational quantum number. We show the coexistence of distinct dynamical regimes for the HCl rotational transition driven by the short-range repulsive and long-range attractive forces whose relative importance depends on the collision energy and final rotational state, suggesting that the classification of rainbow scattering into rotational and l-type rainbows is effective for H2 + HCl collisions. While the QCT method satisfactorily predicts the overall behavior of the rotationally inelastic cross sections, its capability to accurately describe signatures of rainbow scattering appears to be limited for the present system.

Vibrational relaxation time measurements in shock-heated oxygen and air from 2000 K to 9000 K using ultraviolet laser absorption

Vibrational relaxation times of oxygen (O2) were measured behind reflected shocks in shock-tube experiments with O2 and nitrogen (N2) collision partners. To determine relaxation times, a tunable ultraviolet laser absorption diagnostic probed time-histories involving the fourth (v″ = 4), fifth (v″ = 5), and sixth (v″ = 6) vibrational levels of the ground electronic state of O2. Taking the ratio of two absorbance time-histories involving different vibrational levels yielded vibrational temperature time-histories that were fit to isolate the relevant vibrational relaxation times. Pure O2 experiments were used to isolate the vibration–translation (VT) relaxation time of O2 with O2. Results for τVTO2–O2 agree with the Millikan and White correlation at temperatures below 4000 K. However, high-temperature data deviate from the Millikan and White correlation, exhibiting a reduced temperature dependence—an observation that remains consistent with previous experimental studies. Additional experiments in 10% and 21% O2 in N2 mixtures were used to isolate both the VT and vibration–vibration (VV) relaxation times of O2 with N2. The data for τVTO2–N2 exceed the Millikan and White correlation by 70% but show reasonable agreement with previous data below 5000 K. High-temperature results again show a reduced temperature dependence, but this study shows longer relaxation times than the previous work. The data for τVVO2–N2 exceed the semi-empirical relation developed by Berend et al. [“Vibration-vibration energy exchange in N2 with O2 and HCl collision partners,” J. Chem. Phys. 57, 3601–3604 (1972)] by 70% but overlap with previous measurements. Due to insensitivity of the chemical system to VV transfer at high temperatures, results for τVVO2–N2 were only measured below 6000 K.

Stereodynamic control of overlapping resonances in cold molecular collisions

Stereodynamic control of resonant molecular collisions has emerged as a new frontier in cold molecule research. Recent experimental studies have focused on weakly interacting molecular systems such as HD collisions with H$_2$, D$_2$ and He. We report here the possibility of such control in strongly interacting systems taking rotational relaxation in cold collisions of HCl and H$_2$. Using explicit quantum scattering calculations in full six dimensions it is shown that robust control of the collision dynamics is possible even when multiple (overlapping) shape-resonances coexist in a narrow energy range, indicating that cold stereochemistry offers great promise for many molecules beyond simple systems. We demonstrate a striking case where two prominent peaks in overlapping resonances are switched-off simultaneously by suitable alignment of the HCl molecule.

Energy Transfer and Bond Dissociation in the o‐chlorotoluene + H2/Cl2/HCl Collisions

Energy flow and C–H and C–Cl bond dissociations in o‐chlorotoluene (OCT) in collisions with H2, Cl2, and HCl are studied using classical trajectory methods. The energy loss by the excited OCT is small, but it increases with increasing total vibrational energy content ET of OCT between 5000 cm−1 and 30000 cm−1. The energy loss is larger in OCT + HCl collision than in OCT + H2 or OCT + Cl2. The difference is mainly due the near‐resonant conditions between the vibration of the incident molecule and the C–H or C–Cl vibrations. Intermolecular energy transfer occurs through vibration–translation (V–T) and vibration–vibration (V–V) pathways. The intramolecular energy flow between C–H and C–Cl in highly excited OCT (60000–60300 cm−1) leads to bond dissociation. The probabilities of C–H and C–Cl dissociation are 10−5–10−1 and increase exponentially with increasing vibrational excitation of OCT. The probabilities are found to increase when the incident molecule becomes heavier.

Vibrational energy transfer near a dissociative adsorption transition state: State-to-state study of HCl collisions at Au(111).

In this work we seek to examine the nature of collisional energy transfer between HCl and Au(111) for nonreactive scattering events that sample geometries near the transition state for dissociative adsorption by varying both the vibrational and translational energy of the incident HCl molecules in the range near the dissociation barrier. Specifically, we report absolute vibrational excitation probabilities for HCl(v = 0 → 1) and HCl(v = 1 → 2) scattering from clean Au(111) as a function of surface temperature and incidence translational energy. The HCl(v = 2 → 3) channel could not be observed-presumably due to the onset of dissociation. The excitation probabilities can be decomposed into adiabatic and nonadiabatic contributions. We find that both contributions strongly increase with incidence vibrational state by a factor of 24 and 9, respectively. This suggests that V-T as well as V-EHP coupling can be enhanced near the transition state for dissociative adsorption at a metal surface. We also show that previously reported HCl(v = 0 → 1) excitation probabilities [Q. Ran et al., Phys. Rev. Lett. 98, 237601 (2007)]-50 times smaller than those reported here-were influenced by erroneous assignment of spectroscopic lines used in the data analysis.

Quantum State Resolved 3D Velocity Map Imaging of Surface-Scattered Molecules: Incident Energy Effects in HCl + Self-Assembled Monolayer Collisions

Thermal and hyperthermal HCl (v = 0, J = 0) collision dynamics at the surface of methyl-terminated self-assembled monolayers (SAMs) are probed by state-selective ionization followed by velocity-map imaging (VMI) to yield a full 2π steradian map of final 3D velocity distributions (vx, vy, vz) as a function of rovibrational (v, J) quantum state. “DC slicing” of the scattered HCl flux normal to the surface (vz) provides a powerful tool for eliminating incident beam contamination, as well as access to fully correlated, 3D flux weighted rovibrational quantum state + translational scattering dynamics in unprecedented detail. At low collision energies (Einc ≈ 0.7(1) kcal/mol), the scattering dynamics are completely dominated by trapping-desorption (TD) events, for which both external (i.e., translational) and internal (i.e., rovibrational) degrees of freedom quantitatively track the SAM surface temperature (TS). Hyperthermal scattering data at high collision energies (Einc ≈ 17(1) kcal/mol) provide direct eviden...

Gas–Microjet Reactive Scattering: Collisions of HCl and DCl with Cool Salty Water

Liquid microjets provide a powerful means to investigate reactions of gases with salty water in vacuum while minimizing gas-vapor collisions. We use this technique to explore the fate of gaseous HCl and DCl molecules impinging on 8 molal LiCl and LiBr solutions at 238 K. The experiments reveal that HCl or DCl evaporate infrequently if they become thermally accommodated at the surface of either solution. In particular, we observe minimal thermal desorption of HCl following HCl collisions and no distinct evidence for rapid, interfacial DCl→HCl exchange following DCl collisions. These results imply that surface thermal motions are not generally strong enough to propel momentarily trapped HCl or DCl back into the gas phase before they ionize and disappear into solution. Instead, only HCl and DCl molecules that scatter directly from the surface escape entry. These recoiling molecules transfer less energy upon collision to LiBr/H2O than to LiCl/H2O, reflecting the heavier mass of Br(-) than of Cl(-) in the interfacial region.

Temperature and collision energy effects on dissociation of hydrochloric acid on water surfaces.

Collisions of HCl at the air-water interface modelled by a 72 molecule water slab are studied for a range of various impact energies and temperatures using ab initio molecular dynamics with density functional theory. A range of short-timescale events can follow the collision, from direct scattering to nondissociative trapping on the surface. In most cases, HCl dissociation occurs within a few picoseconds, followed by the formation of a solvent-separated ion pair, or rarely, the reformation of HCl. With increasing impact energy and/or system temperature, dissociation occurs more rapidly, with Cl(-) tending to diffuse deeper into the slab. At temperatures corresponding to the frozen water regime, dissociation is seen only once out of the five thermal collisions, but with the addition of a total of 4kT or more of kinetic energy to HCl, it occurs in all our trajectories within a few ps.

Estudo teórico da estrutura eletrônica e da dinâmica induzida por lasers da molécula de HCl

Potential energy and dipole moment curves for the HCl molecule were computed. Calculations were performed at different levels of theory (DFT, MRCI). Spectroscopic properties are reported and compared with experimental data, for validating the theoretical approaches. Interaction of infrared radiation with HCl is simulated using the wave packet formalism. The quantum control model for population dynamics of the vibrational levels, based on pi-pulse theory, is applied. The results demonstrate that wavepackets with specific composition can be built with short infrared laser pulses and provide the basis for studies of H + HCl collision dynamics with infrared laser excitation.

Theoretical Study of the Dynamics of Collisions Between HCl and ω-Hydroxylated Alkanethiol Self-Assembled Monolayers

We present a classical-trajectory study of collisions of HCl with hydroxylated alkanethiol self-assembled monolayers. The potential-energy surface used in the trajectory propagation is a combination of the standard OPLS force field to describe the surface and an analytical potential for the gas/surface interaction developed in this work. The gas/surface potential has been derived based on high-quality electronic-structure calculations of model HCl−alcohol systems in the gas phase and includes a flexible Buckingham term and a Coulombic term. The results of the trajectories calculations are in good agreement with recent molecular-beam experiments on the same system, thereby lending support to the accuracy of the calculations. The collision dynamics differ vastly from prior scattering studies involving rare gases and CO, primarily because the gas/surface attraction governed by hydrogen bonding dramatically increases the ability of the gas molecules to trap on the surface for extended times. The properties of...

Crossed-Beams and Theoretical Studies of Hyperthermal Reactions of O(3P) with HCl

The reaction of O((3)P) with HCl at hyperthermal collision energies (45-116 kcal mol(-1)) has been investigated with crossed-molecular beams experiments and direct dynamics quasi-classical trajectory calculations. The reaction may proceed by two primary pathways, (1) H-atom abstraction to produce OH and Cl and (2) H-atom elimination to produce H and ClO. The H-atom abstraction reaction follows a stripping mechanism, in which the reagent O atom approaches the HCl molecule at large impact parameters and the OH product is scattered in the forward direction, defined as the initial direction of the reagent O atoms. The H-atom elimination reaction is highly endoergic and requires low-impact-parameter collisions. The excitation function for ClO increases from a threshold near 45 kcal mol(-1) to a maximum around 115 kcal mol(-1) and then begins to decrease when the ClO product can be formed with sufficient internal energy to undergo secondary dissociation. At collision energies slightly above threshold for H-atom elimination, the ClO product scatters primarily in the backward direction, but as the collision energy increases, the fraction of these products that scatter in the forward and sideways directions increases. The dependence of the angular distribution of ClO on collision energy is a result of the differences in collision geometry. Collisions where the H atom on HCl is oriented away from the incoming reagent O atom lead to backward-scattered ClO and those where the H atom is oriented toward the incoming O atom lead to forward-scattered ClO. The latter trajectories do not follow the minimum energy path and involve larger translational energy release. Therefore, they become dominant at higher collision energies because they lead to lower internal energies and more stable ClO products. The H-atom abstraction and elimination reactions have comparable cross sections for hyperthermal O((3)P) + HCl collisions.

Improved nonlocal resonance model for electron – HCl collisions

We present an improved nonlocal resonance model for electron-HCl collisions. The short-range part of the model is based on ab-initio electron scattering eigenphase sums calculated using the Schwinger Multichannel method and the long-range part on the potential curve of the bound HCl− anion. We have calculated cross sections for vibrational excitation and dissociative electron attachment. For all collision processes where quantitative experimental data are available, the new model agrees with experiments better than the previous model.

Formation of Complex XeHCl+in the Xe++ HCl Collision

The formation of complex <TEX>$XeHCl^+$</TEX> in the collision-induced reaction of <TEX>$Xe^+$</TEX> with HCl has been studied by use of classical dynamics procedures using the London-Eyring-Polanyi-Sato empirical potential energy surfaces. A small fraction of trajectories on the <TEX>$Xe^+$</TEX> + HCl and Xe + <TEX>$HCl^+$</TEX> surfaces lead to the formation of complex <TEX>$XeHCl^+$</TEX> with life-times of 1-2 ps which is long enough to survive many rotations before redissociating back to the reactant state. The formation of complex <TEX>$XeHCl^+$</TEX> occurs mainly from collision angle of <TEX>$\Theta$</TEX> = <TEX>${45^{\circ}}$</TEX>.

Classical trajectory study of the formation of XeH+ and XeCl+ in the Xe++HCl collision

The collision-induced reaction of Xe+ with HCl has been studied by use of classical dynamics procedures at collision energies 2-20 eV using empirical potential parameters. The principal reaction pathway on the potential energy surface is the formation of XeH+ with the maximum reaction cross section, 1.2 A2, occurring at E=9 eV. At lower energies, the cross section for the charge transfer process Xe++HCl-->Xe+HCl+ is comparable to that for XeH+ formation, but at higher energies, it is larger by a factor of 2. The cross section of the XeCl+ formation is an order of magnitude smaller than that of XeH+. For both XeH+ and XeCl+ formations, the reaction threshold is approximately 2 eV. The XeH+ formation takes place immediately following the turning point in a direct-mode mechanism, whereas an indirect-mode mechanism operates in the formation of XeCl+. Both XeH+ and XeCl+ formations come mainly from the perpendicular configuration, Xe+...HCl, at the turning point. Product vibrational excitation is found to be strong in both XeH+ and XeCl+.

Observation of a Change of Vibrational Excitation Mechanism with Surface Temperature: HCl Collisions with Au(111)

We have measured the vibrational excitation probability (Pv) of HCl incident on a Au(111) surface at kinetic energies (Ei) of 0.59 eV to 1.37 eV and surface temperatures (Ts) of 273 K to 1073 K. For all energies, the slope of the Pv as a function of Ts exhibits a sharp increase above Ts approximately 800 K. We show this change in slope and the threshold behavior of Pv to be consistent with a change in excitation mechanism from an electronically adiabatic mechanical mechanism to an electronically nonadiabatic mechanism involving excited electron-hole pairs.

Dynamics of HCl Collisions with Hydroxyl- and Methyl-Terminated Self-Assembled Monolayers

Molecular beam scattering techniques are used to explore the energy exchange and thermal accommodation efficiencies of HCl in collisions with long-chain OH- and CH(3)-terminated self-assembled monolayers (SAMs) on gold. Upon colliding with the nonpolar methyl-terminated SAM, HCl (E(i) = 85 kJ/mol) is found to transfer the majority, 83%, of its translational energy to the surface. The extensive energy loss for HCl helps to bring the molecules into thermal equilibrium with the monolayer. Specifically, 72% of the HCl approaches thermal equilibrium prior to desorption. For the molecules that do not thermally accommodate, but scatter after an impulsive collision with the surface, the final translational energy is observed to be directly proportional to the surface temperature as the thermal surface energy and gas translational energy exchange during the collision. For the OH-terminated SAM, the impulsively scattered HCl escapes from the surface with slightly more average energy. The rigid nature of the OH-terminated SAM is due to the extended intra-monolayer hydrogen-bonding network that restricts some of the low-energy modes of the surface. However, despite the rigid nature of this system, the extent of thermal accommodation for HCl on these two surfaces is remarkably similar. It appears that the potential energy well between the impinging HCl and the polar surface groups is sufficient enough to trap HCl molecules that would otherwise scatter impulsively from this rigid SAM.

Quantum/classical studies of O(3P)+Ar⋅HCl collision dynamics

The dynamics of the O(3P)+HCl and Ar–HCl reactions is investigated using a multiple configuration quantum/classical approach. In this work the dynamics of the hydrogen atom is propagated quantum mechanically in the three Cartesian coordinates of the atom, while the dynamics of the other atoms is propagated classically, in a center-of-mass frame. It is found that the introduction of the argon atom affects the reaction probability through two mechanisms. For nearly collinear O+Ar–HCl collisions, the argon atom blocks the transition state for the O+HCl reaction and inhibits the reaction. On the other hand when the collision geometry is such that the oxygen atom does not collide with the argon atom, the reaction probability is increased. These results are analyzed in terms of the shape of the ground state Ar–HCl wave function. The energy transfer dynamics from the oxygen atom and to the argon atom is also investigated.

Collisions of HCl with Rare Gas and Molecular Colliders

Collisions of HCl (j = 0) with rare gas and molecular colliders were studied using velocity-mapped ion imaging. For the rare gas colliders, Ne and Kr, the observed scattering behavior is very similar to that previously observed for HCl + Ar, and any differences are primarily due to the rule of corresponding states. For the molecular colliders, N2 and CH4, very different scattering behavior was observed, especially for high ΔjHCl transitions. When HCl is scattered into jHCl = 2 or 3, the collision results in more forward-scattering than is observed for rare gas colliders, whereas when HCl is scattered into jHCl = 4 or 5, the collision results in more backscattering. This implies that to generate HCl in a highly rotationally excited quantum state following collision with a diatomic or polyatomic collider, the collision must have a smaller impact parameter than would be required for an atomic collision.

Collisions of HCl, DCl, and HBr with Liquid Glycerol: Gas Uptake, D → H Exchange, and Solution Thermodynamics

Gas−liquid scattering, D → H exchange, and time-dependent uptake measurements are used to follow HCl, DCl, and HBr as they strike the surface of the neutral, hydrogen-bonding liquid glycerol. In this and the following paper, we report studies of gas−liquid energy transfer and trapping, the nature of HCl and HBr interfacial and bulk phase dissociation and recombination, and the thermodynamics of HCl solvation. We find that most HCl and HBr molecules readily dissipate their excess kinetic energy and thermalize at the surface of glycerol, even at impact energies up to 100 kJ mol-1 and glancing angles of incidence up to 60°. Nearly all thermally accommodated HBr molecules dissolve in glycerol for longer than 10 s. HCl is less acidic and dissolves reversibly for times of 0.1 to 1 s at incident HCl fluxes of (2 to 0.2) × 1015 cm-2 s-1, respectively. Experiments utilizing DCl show that this reversible solvation is accompanied by dissociation into D+ and Cl-, D+ → H+ exchange, and HCl desorption. The residence times of HCl in glycerol and their temperature dependences yield = −19 ± 2 kJ mol-1, = −67 ± 4 kJ mol-1, and = −165 ± 20 J mol-1 K-1 for HCl(g) ⇌ H+(gly) + Cl-(gly) at 294 K.

On the role of interaction anisotropy in vibrational relaxation of HF and HCl by Ar

A coupled states quantum scattering study of vibrational relaxation in Ar + HF and Ar + HCl collisions is presented. The energy transfer in these systems exhibits a quasiresonant character resulting in a strongly inverted rotational population of the diatomic molecules after collision. The high anisotropy of interaction is shown to be determining the quasiresonant vibrational energy transfer by favoring the population of high rotational energy levels. The role of interaction anisotropy in atom-diatomic molecule collisions is discussed.