Cold atomic and molecular collisions: approaching the universal loss regime

Abstract

We investigate the behaviour of single-channel theoretical models of cold and ultracold collisions that take account of inelastic and reactive processes using a single parameter to represent shortrange loss. We present plots of the resulting energy-dependence of elastic and inelastic or reactive cross-sections over the full parameter space of loss parameters and short-range phase shifts. We then test the single-channel model by comparing it with the results of coupled-channel calculations of rotationally inelastic collisions between LiH molecules and Li atoms. We find that the range of cross-sections predicted by the single-channel model becomes increasingly accurate as the initial LiH rotational quantum number increases, with a corresponding increase in the number of open loss channels. The results suggest that coupled-channel calculations at very low energy (in the s-wave regime) could in some cases be used to estimate a loss parameter and then to predict the range of possible loss rates at higher energy, without the need for explicit coupled-channel calculations for higher partial waves., the long-range wavefunction may be expressed in terms of the analytical solutions for the long-range potential [11,12]. The model of Idziaszek and Julienne [6] successfully explained the temperature dependence of reactive KRb+KRb collisions at temperatures below 1 μK, and was later extended [13] to handle the additional − r 3 dipole-dipole potential that exists when the KRb molecules are oriented with an external electric field. More recently, Jachymski et al [14,15] have extended similar models up to the high-temperature limit, and used them to interpret merged-beam experiments on Penning ionization in collisions of metastable He with Ar [4].

Universal Scattering of Ultracold Atoms and Molecules in Optical Potentials

“…For rotation-less molecules, the van-der-Waals dispersion potential is isotropic. In a number of cases [23][24][25] these QDT results have been validated with comparisons with "exact" quantum simulations.…”

Prospects of Forming High-Spin Polar Molecules from Ultracold Atoms

“…The amplitudes of the oscillations decrease with increasing m j , as the number of open channels increases. For short-range anisotropies, the decay is substantially stronger; the resonances are progressively washed out as m j increases, and β approaches the value ofā expected in the "universal" limit of 100% loss at short range [85,86]. In this case, there are visible resonances in α and β at low field even for the uppermost channel with m j ¼ j; these arise from quasibound states trapped behind centrifugal barriers for channels with m j < j and L > 0 [87].…”

Collisions between Ultracold Molecules and Atoms in a Magnetic Trap

“…These collisions lead to rapid trap loss either due to chemical reactions [2] or, when reactions are energetically forbidden, due to the formation of long-lived complexes that are subsequently excited by the trapping laser [23,24]. The loss rate coefficients are found to be close to those predicted by a single-channel model with universal loss, in which molecules are lost with unit probability once they reach short range [25,26]. Recently, reactive losses of KRb molecules have been suppressed by using an electric field and confining the molecules to two dimensions, and this has led to the formation of a stable quantum degenerate gas of these molecules [4,27,28].…”

“…Inset: histogram of Γ Rb-CaF estimates using the method described in [43], together with a fit to a normal distribution. To interpret the experimental loss rates, we compare to calculations with a single-channel nonuniversal model based on quantum defect theory [26,43]. This models the long-range interaction potential by its asymptotic form −C 6 R −6 and takes full account of temperature dependence, which is important under the conditions of the experiment.…”