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Self-bound droplets and droplet arrays are observed in an ultracold gas of strongly dipolar sodium–caesium molecules, establishing ultracold molecules as a system for the exploration of strongly dipolar quantum matter.

Collisions between atoms and molecules are largely understood; however, our understanding of collisions between two molecules is lacking because they are significantly harder to study, Now, correlated rotational excitations have been observed in inelastic collisions between NO and O₂ molecules. It is shown that the energy-gap law that governs atom–molecule collisions does not generally apply to bimolecular excitation processes.

Calculations at the theoretical gold standard generally yield accurate results for a variety of energy-transfer processes in molecular collisions. Using anti-seeding methods in a crossed-beam inelastic scattering experiment, a resonance structure is clearly resolved for NO–H₂ collisions, pushing the required accuracy for theoretical potentials beyond the gold standard.

A transient intermediate complex in a chemical reaction—formed from collisions between molecules with a few atoms—is observed under ultracold conditions. Its lifetime can be directly measured after suppression of the photo-excitation process.

Molecular energy transfer is thought to follow a simple rule of thumb: high energy transfer requires hard collisions that result in backscattering. However, now it has been observed that an unexpected forward scattering occurs in NO–CO and NO–HD collisions even for high energy transfer. This is attributed to ‘hard-collision glory scattering’, a mechanism that appears to be ubiquitous in molecule–molecule collisions.

Bose–Einstein condensate of sodium–caesium molecules is observed by means of evaporative cooling and collisional shielding.

The high inelastic loss rate in gases of bosonic molecules has so far hindered the stabilization needed to reach quantum degeneracy. Now, an experiment using microwave shielding demonstrates a large reduction of losses for bosonic dipolar molecules.

A type of universal scattering resonance between ultracold microwave-dressed polar molecules associated with field-linked tetramer bound states in the long-range potential well is observed, providing a general strategy for resonant scattering between ultracold polar molecules.

A general and efficient approach to evaporatively cool ultracold polar molecules through elastic collisions to create a degenerate quantum gas in three dimensions is demonstrated using microwave shielding.

Stereodynamics describes how the vector properties of molecules affect the probabilities of specific processes in molecular collisions. Measurements of irregular diffraction patterns for NO radicals colliding with rare-gas atoms reveal a previously unrecognized type of quantum stereodynamics and a ‘propensity rule’ for the magnetic quantum number (m) of the molecules.

Molecular collisions can lead to the absorption of incident light even for transitions that are spectroscopically forbidden for the isolated molecules. Now the electronic–vibrational transitions of O₂ have been theoretically studied and, contrary to textbook knowledge, it is shown that the absorption mechanism and the spectral line shape depend on the collision partner, oxygen or nitrogen.

Ultracold molecules and ion–neutral systems offer unique access to chemistry in a coherent quantum regime. This Review charts the progress of studies of quantum chemistry in such platforms, highlighting the synergy between theory and experiments.