Ultracold Sticky Collisions: Theoretical and Experimental Status

Abstract

Collisional complexes, which are formed as intermediate states in molecular collisions, are typically short-lived and decay within picoseconds. However, in ultracold collisions involving bialkali molecules, complexes can live for milliseconds, completely changing the collision dynamics. This can lead to unexpected two-body loss in samples of nonreactive molecules. During the past decade, such "sticky" collisions have been a major hindrance in the preparation of dense and stable molecular samples, especially in the quantum-degenerate regime. Currently, the behavior of the complexes is not fully understood. For example, in some cases, their lifetime has been measured to be many orders of magnitude longer than recent models predict. This is not only an intriguing problem in itself but also practically relevant, since understanding molecular complexes may help to mitigate their detrimental effects. Here, we review the recent experimental and theoretical progress in this field. We treat the case of molecule−molecule as well as molecule−atom collisions.

Sticking Lifetime of Ultracold CaF Molecules in Triplet Interactions

“…In the case where lifetimes have been measured, they are sometimes even far larger, for reasons that remain mysterious. The current situation is reviewed in Bause et al…”

“…In the case where lifetimes have been measured, they are sometimes even far larger, for reasons that remain mysterious. The current situation is reviewed in Bause et al 2 Given a potential energy surface such as our triplet potential, the density of states can be estimated by a quasi-classical calculation derived in ref 19. For a system having N i particles, the total number of quantum states below a certain energy E, with total angular momentum J 0 and center of mass X = (0, 0, 0) is given by…”

“…The encounter between two CaF molecules, both in their 2 Σ ground state, can proceed through either singlet or triplet interactions of the four-body system Ca 2 F 2 . In a previous paper, 1 we developed the singlet scattering surface for this system, identifying the reaction 2CaF → CaF 2 + Ca as barrierless and exothermic by ∼4300 cm −1 .…”

“…Even if they are restricted to the triplet surface, spin-polarized CaF molecules may not scatter elastically in a simple way. At ultracold temperatures, diatomic molecules are often observed to vanish into long-lived complex states, a phenomenon known as “sticking” . Long dwell times in this complex can not only appear as a loss mechanism, they can also enhance even weak couplings to the singlet surface, thereby further reducing the immunity of spin-polarized CaF molecules to reaction.…”

Efficient pathway to NaCs ground state molecules

“…Ultracold molecules [1][2][3][4] are an emerging platform for the investigation of new frontiers in many-body quantum physics [5][6][7][8][9][10], quantum information [11][12][13][14][15][16], and quantum chemistry [1,[17][18][19][20][21][22][23][24]. Many of the proposed applications require large samples of ultracold molecules at high phase-space densities.…”