Molecular hydrogen interacts more strongly when rotationally excited at low temperatures leading to faster reactions.

Abstract

The role of internal molecular degrees of freedom, such as rotation, has scarcely been
explored experimentally in low-energy collisions despite their significance to cold 
and ultracold chemistry. Particularly important to astrochemistry is the case of the 
most abundant molecule in interstellar space, hydrogen, for which two spin isomers 
have been detected, one of which exists in its rotational ground state whereas the 
other is rotationally excited. Here we demonstrate that quantization of molecular 
rotation plays a key role in cold reaction dynamics, where rotationally excited 
ortho-hydrogen reacts faster due to a stronger long-range attraction. We observe 
rotational state-dependent non-Arrhenius universal scaling laws in chemi-ionization 
reactions of para-H2 and ortho-H2 by He(2³ P2), spanning three orders of magnitude in 
temperature. Different scaling laws serve as a sensitive gauge that enables us to 
directly determine the exact nature of the long-range intermolecular interactions. 
Our results show that the quantum state of the molecular rotor determines whether or 
not anisotropic long-range interactions dominate cold collisions.