The very first steps of condensation as studied experimentally in the simplest molecular system (para-H2) are reported. The fast time–space evolution of the nascent clusters have been measured using state-of-the-art Raman spectroscopy implemented on cryogenic supersonic jets. The time-dependent onset of condensation is presented in a non-equilibrium pressure-temperature phase diagram. Dimer and trimer formation are found to obey three-body processes whose rates have been determined.
