Search

Tugging the hydrogen atoms A simple and much-studied example of vibrationally inelastic collisions is the crashing of a hydrogen atom into a deuterium molecule. The conventional view of the process is that transient compression of the D-D bond gives rise to vibrational excitation, then the colliding partners scatter backwards. Surprisingly, experiments now reveal a different inelastic scattering mechanism. Greaves et al. observed vibrational excitation even in collisions where the two species merely graze each other. They attribute this to extension of the D-D bond through interaction with the passing H atom. This tug-of-war mechanism should be at play whenever attraction can develop between colliding partners. A simple and much-studied example of vibrationally inelastic collisions is the crashing of a hydrogen atom into a deuterium molecule. This experiment reveals a different inelastic scattering mechanism: it observed vibrational excitation even in collisions where the two species merely graze each other, and which is attributed to extension of the D-D bond through interaction with the passing H atom. This tug of war mechanism should be at play whenever attraction can develop between the colliding partners. Vibrationally inelastic scattering is a fundamental collision process that converts some of the kinetic energy of the colliding partners into vibrational excitation.sup.1.sup.,.sup.2. The conventional wisdom is that collisions with high impact parameters (where the partners only 'graze' each other) are forward scattered and essentially elastic, whereas collisions with low impact parameters transfer a large amount of energy into vibrations and are mainly back scattered.sup.3. Here we report experimental observations of exactly the opposite behaviour for the simplest and most studied of all neutral-neutral collisions: we find that the inelastic scattering process H + D.sub.2(v = 0, j = 0, 2) [right arrow] H + D.sub.2(v' = 3, j' = 0, 2, 4, 6, 8) leads dominantly to forward scattering (v and j respectively refer to the vibrational and rotational quantum numbers of the D.sub.2 molecule). Quasi-classical trajectory calculations show that the vibrational excitation is caused by extension, not compression, of the D-D bond through interaction with the passing H atom. However, the H-D interaction never becomes strong enough for capture of the H atom before it departs with diminished kinetic energy; that is, the inelastic scattering process is essentially a frustrated reaction in which the collision typically excites the outward-going half of the H-D-D symmetric stretch before the H-D.sub.2 complex dissociates. We suggest that this 'tug of war' between H and D.sub.2 is a new mechanism for vibrational excitation that should play a role in all neutral-neutral collisions where strong attraction can develop between the collision partners., Author(s): Stuart J. Greaves [sup.1] , Eckart Wrede [sup.2] , Noah T. Goldberg [sup.3] , Jianyang Zhang [sup.3] , Daniel J. Miller [sup.3] , Richard N. Zare [sup.3] Author Affiliations: [...]

Deuterium bromide (DBr) is expanded from a pulsed jet into a vacuum and a synchronized pulsed laser causes photodissociation of some of the DBr molecules to produce primarily (∼85%) ground-state bromine atoms (2P3/2) and fast D atoms. The latter collide with the cold DBr molecules and react to produce molecular deuterium (D2) via two possible channels, the adiabatic channel D2+Br(2P3/2) and the nonadiabatic channel D2+Br*(2P1/2), which are asymptotically separated in energy by the spin-orbit splitting (0.457 eV) of the bromine atom. Ion images are recorded for D2(v′=1, J′=16, 18–21), D2(v′=2, J′=6,7, 10–12, 14–16), and D2(v′=3, J′=2–5) for various collision energies. For the nonadiabatic production of spin-orbit-excited Br* in the D+DBr reaction for the conditions studied we estimate that this channel contributes 1% or less. [ABSTRACT FROM AUTHOR]

We have measured differential cross sections (DCSs) for the HD (v′=1,j′=2,6,10) products of the H+D2 exchange reaction at five different collision energies in the range 1.48≤=Ecoll≤=1.94 eV. The contribution from the less energetic H atoms formed upon spin-orbit excitation of Br in the photolysis of the HBr precursor is taken into account for two collision energies, Ecoll=1.84 and 1.94 eV, allowing us to disentangle the two different channels. The measured DCSs agree well with new time-dependent quantum-mechanical calculations. As the product rotational excitation increases, the DCSs shift from backward to sideward scattering, as expected. We also find that the shapes of the DCSs show only a small overall dependence on the collision energy, with a notable exception occurring for HD (v′=1,j′=2), which appears bimodal at high collision energies. We suggest that this feature results from both direct recoil and indirect scattering from the conical intersection. [ABSTRACT FROM AUTHOR]

We describe a new instrument based on a delay-line detector for imaging the complete three-dimensional velocity distribution of photoionized products from photoinitiated reactions. Doppler-free [2+1] resonantly enhanced multiphoton ionization (REMPI) of H and D atoms formed upon photolysis of HBr and DBr in the range 203 nm≤=λphotolysis≤=243 nm yields radial speeds measured to be accurate within 1% of those calculated. The relative speed resolution is about 5% and limited by photoionization recoil broadening. A relative speed resolution of 3.4% is obtained for [3+1] REMPI, which minimizes the ionization recoil. We also determine the branching ratio between ground-state and spin-orbit-excited product channels and their associated anisotropies. We find that DBr photolysis dynamics differs slightly from its HBr counterpart [ABSTRACT FROM AUTHOR]

Product rotational distributions for the reaction H+D2→HD(ν′=1,j′)+D have been measured for 16 collision energies in the range of 1.43≤Ecoll≤2.55 eV. Time-dependent quantum-mechanical calculations agree well in general with the experimental results, but they consistently yield slightly colder distributions. In terms of the average energy channeled into rotation, the differences between experiment and theory amount to approximately 10% for all collision energies sampled. No peculiarity is found for Ecoll=2.55 eV at which the system has sufficient energy to access the first HD2 electronically excited state. [ABSTRACT FROM AUTHOR]

We have measured differential cross sections (DCSs) for the vibrationally inelastic scattering process H + o-D2(v = 0,j = 0,2) → H + o-D2(v' = 1-4, j' even). Several different collision energies and nearly the entire range of populated product quantum states are studied. The products are dominantly forward-scattered in all cases. This behavior is the opposite of what is predicted by the conventional textbook mechanism, in which collisions at small impact parameters compress the bond and cause the products to recoil in the backward direction. Recent quasiclassical trajectory (QCT) calculations examining only the o-D2(v' = 3, j') products suggest that vibrationally inelastic scattering is the result of a frustrated reaction in which the D-D bond is stretched, but not broken, during the collision. These QCT calculations provide a qualitative explanation for the observed forward-scattering, but they do not agree with experiments at the lowest values of j'. The present work shows that quantum mechanical calculations agree closely with experiments and expands upon previous results to show that forward-scattering is universally observed in vibrationally inelastic H + D2 collisions over a broad range of conditions. [ABSTRACT FROM AUTHOR]

The differential cross section (DCS) for the reaction H + D2 --> D + HD (v' = 3, j' = 0) exhibits particularly rich dynamics; in addition to the expected direct recoil backscattering feature, a surprising time-delayed forward scattering feature appears that has been attributed to glory scattering arising from nearside and farside interference. This fact leads to a complex DCS that depends strongly on the collision energy. Its accurate calculation requires a fully quantum mechanical (QM) treatment. We report improved measurements of this DCS over the collision energy range 1.55 < or = E(coll) < or = 1.82 eV. Previous measurements using the core extraction method, while generally in agreement with theory, lacked sufficient resolution to capture all of the noteworthy behavior of the system; in the present work, we use ion imaging to observe many previously unresolved features of the DCS, particularly in the forward-scattered region. Agreement with QM calculations is found at all collision energies, reconciling an earlier discrepancy between experiment and theory near E(coll) = 1.54 eV.

We have measured differential cross sections (DCSs) for the HD (v(')=1,j(')=2,6,10) products of the H+D(2) exchange reaction at five different collision energies in the range 1.48< or =E(coll)< or =1.94 eV. The contribution from the less energetic H atoms formed upon spin-orbit excitation of Br in the photolysis of the HBr precursor is taken into account for two collision energies, E(coll)=1.84 and 1.94 eV, allowing us to disentangle the two different channels. The measured DCSs agree well with new time-dependent quantum-mechanical calculations. As the product rotational excitation increases, the DCSs shift from backward to sideward scattering, as expected. We also find that the shapes of the DCSs show only a small overall dependence on the collision energy, with a notable exception occurring for HD (v(')=1,j(')=2), which appears bimodal at high collision energies. We suggest that this feature results from both direct recoil and indirect scattering from the conical intersection.

Product rotational distributions for the reaction H + D2 --> HD(nu'=1,j') + D have been measured for 16 collision energies in the range of 1.43 < or = E(coll) < or = 2.55 eV. Time-dependent quantum-mechanical calculations agree well in general with the experimental results, but they consistently yield slightly colder distributions. In terms of the average energy channeled into rotation, the differences between experiment and theory amount to approximately 10% for all collision energies sampled. No peculiarity is found for E(coll)=2.55 eV at which the system has sufficient energy to access the first HD2 electronically excited state.