Geometric phase effects in the ultracold H + H2 reaction

Abstract

The H system has served as a prototype for geometric phase (GP) effects in bimolecular chemical reactions for over three decades. Despite a large number of theoretical and experimental efforts, no conclusive evidence of GP effects in the integral cross section or reaction rate has been presented until recently [B. Kendrick et al., Phys. Rev. Lett. 115, 153201 (2015)]. Here we report a more detailed account of GP effects in the H + H(v = 4, j = 0) → H + H(v', j') (para-para) reaction rate coefficients for temperatures between 1 μK (8.6 × 10 eV) and 100 K (8.6 × 10 eV). The GP effect is found to persist in both vibrationally resolved and total rate coefficients for collision energies up to about 10 K. The GP effect also appears in rotationally resolved differential cross sections leading to a very different oscillatory structure in both energy and scattering angle. It is shown to suppress a prominent shape resonance near 1 K and enhance a shape resonance near 8 K, providing new experimentally verifiable signatures of the GP effect in the fundamental hydrogen exchange reaction. The GP effect in the D + D and T + T reactions is also examined in the ultracold limit and its sensitivity to the potential energy surface is explored.

Geometric phase effects in the ultracold D + HD $ \rightarrow $ D + HD and D + HD $\leftrightarrow $ H + D2reactions

“…Several partial wave shape resonances occur for higher collision energies between 0.5 and 10 K. The GP is shown to dramatically alter the predicted resonance features. The large GP effects are explained in terms of a new quantum interference mechanism which originates from the unique properties associated with ultracold collisions [64][65][66][67][68]. This novel quantum interference mechanism can lead to maximum constructive or destructive interference for ultracold collisions which effectively turns the reaction on or off, respectively.…”

“…The difference between these two phases ( )p p D = -= n n m 2 1 is still an integral multiple of π which in general is either even or odd. To our knowledge this is a new kind of quantum interference mechanism which is general and independent of the GP [64][65][66][67][68]. If the two interfering pathways encircle a conical intersection, then the associated GP leads to an additional π phase shift which changes the sign on the interference term in equation (1).…”

“…The methodology is numerically exact for a given Born-Oppenheimer PES (i.e., no dynamical approximations are used) and the computer code has been parallelized to run efficiently on a variety of computational platforms from a single workstation to massively parallel supercomputers [51][52][53]98]. The methodology can also be used to compute molecular spectra [36][37][38]104] and is well suited for treating ultracold reactions [64][65][66][67][68][79][80][81][82][83].…”

“…In the ultracold regime, a single partial wave (i.e., the l=0 angular momentum partial wave or s-wave for bosons and distinguishable particles, and l=1 or p-wave for identical fermions) contributes to the scattering cross sections and the ultracold reaction rate coefficients obey the well known Bethe-Wigner threshold laws [74][75][76][77][78]. For exoergic processes these rate coefficients approach finite measurable values comparable to or even larger than their values at thermal energies [64][65][66][67][68][79][80][81][82][83]. The tiny kinetic energy associated with ultracold collisions makes them amenable to control via external electric or magnetic fields [69][70][71][72][84][85][86][87].…”

“…Until recently, [64][65][66][67][68] all of the previous studies of GP effects in molecular scattering were done at thermal energies. At cold and ultracold collision energies (which correspond to temperatures below 1 K (´-8.6 10 eV 5 ) and 1 mK (´-8.6 10 eV 8 ), respectively) the collision outcome is governed by enhanced quantum mechanical effects which include tunneling, resonances, symmetry, and interference.…”