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The Journal of Physical Chemistry A

On the Isotope Effect in F + HD Reaction at Ultracold Temperatures

N. Balakrishnan, A. Dalgarno

Published May 1, 2003• 39 citations
Publisher

Abstract

Isotope branching in the F + HD reaction is investigated at ultracold temperatures. It is found that in the limit of zero temperatures, the rate coefficient for HF formation in the F + HD ( v = 0, j = 0) reaction is a factor of 5.5 greater than that of DF formation. The difference is attributed to the more efficient tunneling of the lighter atom. For HD molecules initially in the j = 1 excited rotational level, a clear preference is seen for chemical reaction leading to HF and DF products with an HF/DF branching ratio of 5.2 rather than for nonreactive rotational quenching.

DOI
10.1021/jp022654v

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It is shown that van der Waals interaction potential plays a crucial role in chemical reactions at low temperatures. By taking the Cl+HD reaction as an illustrative example, we demonstrate that quasibound states of the van der Waals potential preferentially undergo chemical reaction rather than vibrational predissociation. Prereaction occurs even when the wave functions of the quasibound states peak far out into the entrance channel, outside the region of the van der Waals well. It is found that chemical reaction dominates over nonreactive vibrational quenching in collisions of vibrationally excited HD molecules with ground state chlorine atoms at ultracold temperatures.

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Isotope branching in the F + HD reaction is investigated at ultracold temperatures. It is found that in the limit of zero temperatures, the rate coefficient for HF formation in the F + HD ( v = 0, j = 0) reaction is a factor of 5.5 greater than that of DF formation. The difference is attributed to the more efficient tunneling of the lighter atom. For HD molecules initially in the j = 1 excited rotational level, a clear preference is seen for chemical reaction leading to HF and DF products with an HF/DF branching ratio of 5.2 rather than for nonreactive rotational quenching.
We report an ab initio study of the dynamics of the reaction F + D 2 →DF + D, where D 2 is initially in a rotationally excited state ( j = 2). The possibility of obtaining ultra-cold molecules and of investigating reaction dynamics at ultra-low temperature relies on the production of molecules in a well defined quantum state and it is important to know the relative efficiency of the rotational quenching and of the chemical reaction. We examine here a reaction with an activation barrier, the reaction of F with D 2 , and we find that quenching dominates the reaction when the initial rotational level lies energetically below the barrier, so severe trap loss may occur before the reaction can take place.
Cold chemical reactions represent exquisite probes of quantum-mechanical phenomena in reaction dynamics. Here, we explore isotope effects in the full quantum-state-controlled reaction between laser-cooled Be + ions in the 2 P 3/2 excited state and rovibrationally ground-state HD molecules at 5.7 K in a cryogenic ion trap. Using secular excitation mass spectrometry, we directly measure the product branching ratio of BeD + to BeH +, yielding a value of 4.1 ± 0.2. The strong preference for BeD + formation reveals the dominant role of long-range interactions in governing reaction dynamics on the underlying potential energy surface at cryogenic conditions. These detailed experimental benchmarks are crucial for refining theoretical potential energy surfaces and advancing our quantitative understanding of reaction dynamics in the cold regime. Our approach provides a platform for quantum-state-resolved investigations of ion–neutral reactions, with potential relevance to cold chemistry and astrochemical modeling.
We report quantum dynamics calculations of F((2)P)+HCl(v,j)-->HF(v('),j('))+Cl((2)P) and F+DCl(v,j)-->DF(v('),j('))+Cl reactions at cold and ultracold temperatures. The effect of rotational and vibrational excitations of the HCl molecule on the reactivity is investigated. It is found that, in the ultracold regime, vibrational excitation of the HCl molecule from v=0 to v=2 enhances the reactivity by four orders of magnitude. The rotational excitation from j=0 to j=1 decreases the reactivity while the rotational excitation from j=0 to j=2 increases the reactivity. The overall effect of rotational excitation was found to be much smaller than vibrational excitation. The reactivity of the F+DCl system is significantly lower than that of the F+HCl case indicating the importance of quantum tunneling at low energies. For both reactions, Feshbach resonances corresponding to Fcdots, three dots, centered HCl or Fcdots, three dots, centeredDCl triatomic states occur at low energies. We also explored the validity of the coupled-states approximation for cold collisions taking the F+HCl(v=0,j=0) reaction as an illustrative example. It is found that the coupled-states approximation is generally valid for the background scattering even at low energies but it is inadequate to accurately describe the rich resonances in the energy dependence of the cross section resulting from the decay of van der Waals complexes. It is further shown that the coupled-states approximation cannot be used for scattering in the Wigner threshold regime when the molecule is initially in a rotationally excited level.
A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: $T\ensuremath{\sim}{10}^{\ensuremath{-}8}$ K. A global six-dimensional H2{}_{2}2​-H2{}_{2}2​ potential-energy surface is adopted from a previous work [Boothroyd et al., J. Chem. Phys. 116, 666 (2002)]. State-resolved integral cross sections ${\ensuremath{\sigma}}_{ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}}({\ensuremath{\varepsilon}}_{\text{kin}})$ of different quantum-mechanical rotational transitions $ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}$ in the HD molecules and corresponding state-resolved thermal rate coefficients ${k}_{ij\ensuremath{\rightarrow}{i}^{\ensuremath{'}}{j}^{\ensuremath{'}}}(T)$ have been computed. Additionally, for comparison, H2{}_{2}2​+H2{}_{2}2​ calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures.
We discuss the formation of molecular oxygen in ultracold collisions between hydroxyl radicals and atomic oxygen. A time-independent quantum formalism based on hyperspherical coordinates is employed for the calculations. Elastic, inelastic and reactive cross sections as well as the vibrational and rotational populations of the product O{sub 2} molecules are reported. A J-shifting approximation is used to compute the rate coefficients. At temperatures T = 10--100 mK for which the OH molecules have been cooled and trapped experimentally, the elastic and reactive rate coefficients are of comparable magnitude, while at colder temperatures, T < 1 mK, the formation of molecular oxygen becomes the dominant pathway. The validity of a classical capture model to describe cold collisions of OH and O is also discussed. While very good agreement is found between classical and quantum results at T = 0.3 K, at higher temperatures, the quantum calculations predict a higher rate coefficient than the classical model, in agreement with experimental data for the O + OH reaction. The zero-temperature limiting value of the rate coefficient is predicted to be about 6 x 10{sup -12} cm{sup 3} s{sup 01}, a value comparable to that of barrierless alkali metal atom-dimer systems and about a factor of five larger than that of the tunneling dominated F + H{sub 2} reaction.
It is shown that van der Waals interaction potential plays a crucial role in chemical reactions at low temperatures. By taking the Cl+HD reaction as an illustrative example, we demonstrate that quasibound states of the van der Waals potential preferentially undergo chemical reaction rather than vibrational predissociation. Prereaction occurs even when the wave functions of the quasibound states peak far out into the entrance channel, outside the region of the van der Waals well. It is found that chemical reaction dominates over nonreactive vibrational quenching in collisions of vibrationally excited HD molecules with ground state chlorine atoms at ultracold temperatures.