Quantum dynamics of tunneling dominated reactions at low temperatures

Abstract

We report a quantum dynamics study of the Li + HF → LiF + H reaction at low temperatures of interest to cooling and trapping experiments. Contributions from non-zero partial waves are analyzed and results show narrow resonances in the energy dependence of the cross section that survive partial wave summation. The computations are performed using the ABC code and a simple modification of the ABC code that enables separate energy cutoffs for the reactant and product rovibrational energy levels is found to dramatically reduce the basis set size and computational expense. Results obtained using two ab initio electronic potential energy surfaces for the LiHF system show strong sensitivity to the choice of the potential. In particular, small differences in the barrier heights of the two potential surfaces are found to dramatically influence the reaction cross sections at low energies. Comparison with recent measurements of the reaction cross section (Bobbenkamp et al 2011 J. Chem. Phys. 135 204306) shows similar energy dependence in the threshold regime and an overall good agreement with experimental data compared to previous theoretical results. Also, usefulness of a recently introduced method for ultracold reactions that employ the quantum close-coupling method at shortrange and the multichannel quantum defect theory at long-range, is demonstrated in accurately evaluating product state-resolved cross sections for D + H 2 and H + D 2 reactions. offer computational advantages for reactive scattering calculations they are problematic at very low energies and have not been proven to be accurate for cold and ultracold collisions.The goal of this paper is two-fold. First, we present state-to-state reaction dynamics of the Li + HF v j ( , ) i i → LiF(v f , j f ) + H reaction using a quantum close-coupling (CC) method focusing on moderate to low collision energies in light of recent experimental data [16] that showed qualitatively different behavior at low collision energies compared to previous calculations based on a wave packet method [17]. Second, we demonstrate usefulness of a recently developed approach [18] for ultracold reactions that invoke the quantum CC method at short-range and the multichannel quantum defect theory (MQDT) at long-range. The method yields full state-resolved cross sections at comparable level of accuracy as full CC calculations but at much reduced computational expense. Results are presented for HD formation in both D + H 2 and H + D 2 reactions.The Li + HF reaction has been the topic of numerous experimental and theoretical studies over the last four decades. Experimental studies [16,[19][20][21][22][23][24][25][26][27][28] focused on measurements of its reaction cross section as well as determination of product angular distributions and alignment effects. Theoretical work included extensive investigation of its potential energy surfaces (PESs) [29][30][31][32][33][34][35][36][37][38][39] and quantum dynamics calculations based on both time-dependent and time-independent methods [17,[40][41][42]...

Quantum Dynamics of Li+HF/DF Reaction Investigated by a State-to-State Time-dependent Wave Packet Approach

“…In this collision energy range, as shown in Fig.3, there are some differences between the total reaction probabilities, obtained in Refs. [45,46,49] and in our present work. The ABC code is employed in Ref.…”

“…As shown in Fig.2 up to J=44, the ICS and DCS converge up to about 0.25 eV. Since there are quite several similar studies on this reaction, in Fig.3, our calculated total reaction probabilities for the Li+HF reaction with the total angular momentum J=0 and 10 are shown for a collision energy range of 0−0.5 eV, along with results [45,46,49].…”

“…The ABC code is employed in Ref. [49] with limited basis set, and the maximal helicity quantum number K max employed in Ref. [49] was truncated, unlike in our work all of the allowed K have been included.…”

Perspective: Ultracold molecules and the dawn of cold controlled chemistry

“…238 The low energy behavior of the experimental results indicated no energy threshold for the reaction which was confirmed by quantum calculations of Hazra and Balakrishnan. 239 As discussed previously, Janssen et al 178 have explored reactive NH-NH collisions resulting from a spin-exchange process. In this case, collisions of the two NH( 3 Σ − ) molecules with total electronic spin S NH = 1 and its projection M S NH = 1 can form dimers in high spin quintet states, |S = 2, M S = 2⟩.…”

Tunneling through a barrier with the phase–amplitude method

“…It has played an important role in the interpretation of various physical phenomena, such as the decay of a nucleus through α-particle emission, the fission [1] or fusion [2,3] of a heavy nucleus at energies below the Coulomb barrier, optical model scattering calculations [4][5][6], in the physics of ultra-low temperature atoms [7], and may have applications in other fields of physics [8] as well. Since the 1930s the Wentzel, Kramers, Brillouin (WKB) approximation provided a very useful tool in the calculation of barrier penetration probabilities.…”