Towards chemistry at absolute zero

Abstract

The prospect of cooling matter down to temperatures that are close to the absolute zero raises intriguing questions about how chemical reactivity changes under these extreme conditions. Although some types of chemical reaction still occur at 1 µK, they can no longer adhere to the conventional picture of reactants passing over an activation energy barrier to become products. Indeed at ultracold temperatures, the system enters a fully quantum regime, and quantum mechanics replaces the classical picture of colliding particles. In this Review, we discuss recent experimental and theoretical developments that allow us to explore chemical reactions at temperatures that range from 10 K to 500 nK. Although the field is still in its infancy, exceptional control has already been demonstrated over reactivity at low temperatures.ultracold temperatures, the classical description of the collision of particles needs to be replaced with a quantum description and the picture of a ball rolling over a hill needs to be replaced by that of a wave propagating across a barrier.The quantum regime is a physical regime in which our basic understanding of how chemical processes happen, in terms of molecules bumping into each other and breaking and remaking bonds, needs to be challenged. Strictly speaking, the dynamical system of moving and colliding molecules needs to be described as a superposition of partial waves, the components of which represent the different angular momentum states associated with the relative motion of the colliding particles. As the temperature is lowered, the system moves towards a regime in which it can be described by just a few quantised collisional angular momentum states (and, ultimately, just a single state). We denote the collisions as 's-wave' or 'p-wave' collisions (known as the partial-wave description) to represent those with zero or one unit of collisional angular momentum, respectively. Furthermore, in the quantum regime the population of molecular quantum states is distributed over just a few rovibrational states and ultimately collapses to a single quantum state at the lowest temperatures (under conditions of thermal equilibrium). At low temperatures, quantum effects on the rate coefficient and other properties of the reaction, such as collision energy resonances, quantum reflection and geometric phase (described in detail in the 'Reactions in the ultracold regime' section), are most likely to be observable. This is in contrast to what we observe at room temperature, where we understand the overall rate of a process to be an average of the behaviours of individual collisions of molecules with vastly varying sets of quantum numbers and a distribution of energies.When the temperature reaches sub-µK, the translational wavelength typically exceeds the average separation of molecules in the sample, even for a low density gas, and ultimately the system may enter the regime of quantum degeneracy (for example, bosonic particles that form a Bose-Einstein condensate, in which all particles are in the same quan...

Multipole-moment effects in ion–molecule reactions at low temperatures: part I – ion-dipole enhancement of the rate coefficients of the He+ + NH3 and He+ + ND3 reactions at collisional energies Ecoll/kB near 0 K

“…The article series addresses fundamental aspects of chemical reactivity in low-energy ion-molecule collisions which play a crucial role in the understanding and modelling of ion-molecule reactions at low temperatures. [1][2][3][4][5][6][7][8][9][10] The focus is placed on the measurement and theoretical analysis of the deviations of the collision-energy-dependent reaction rates k(E coll ) from the Langevin rate k L at very low collision energies resulting from the electrostatic interaction between the charge of the ion and the electric dipole, quadrupole and octupole moments of the neutral molecule.…”

Multipole-moment effects in ion–molecule reactions at low temperatures: part II – charge–quadrupole-interaction-induced suppression of the He+ + N2 reaction at collision energies below kB·10 K

“…Experimental studies of ion–molecule reactions at low temperatures and low-collision energies are difficult because ions are easily heated up by stray electric fields and space-charge effects. Experiments combining cold ions in ion-traps and Coulomb crystals with cold molecules in slow beams are promising approaches to reach very low temperatures and collision energies in studies of ion–molecule reactions 1–3 but have not yet been broadly applied. Measurements of reaction rates down to ≈10 K are possible in ion guides and traps 4–7 and in uniform supersonic flows 8–10 but hardly any measurements have been performed below 10 K. It is, however, below 10 K that the effects of the electrostatic interactions between the charge of the ion and the multipole moments of the molecule become dominant and are expected to strongly affect the reaction rates.…”

Molecule–molecule and atom–molecule collisions with ultracold RbCs molecules

“…This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom-molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.Ultracold polar molecules have been proposed for applications in the fields of quantum computing [1-7], quantum simulation [8-14], quantum-state-controlled chemistry [15][16][17][18], and precision measurements [19][20][21][22][23][24][25][26]. Experiments are now able to produce a wide variety of ultracold polar molecules in the ground state by association of atom pairs in a pre-cooled atomic mixture [27][28][29][30][31][32][33][34][35][36] or by direct laser cooling of the molecules [37][38][39][40][41][42][43][44][45][46][47].…”