Room temperature detection of the (H 2 ) 2 dimer.

The hydrogen dimer, (H 2 ) 2 , is among the most weakly bound van der Waals complexes and a prototype species for first principles ab initio studies. The detection of the (H 2 ) 2 infrared absorption spectrum was reported more than sixty years ago at a temperature of 20 K. Due to the sharp decrease of the (H 2 ) 2 abundance with temperature, detection at room temperature was generally considered hardly achievable. Here we report the first room temperature detection of partly resolved rotational structures of (H 2 ) 2 by cavity ring down spectroscopy at sub-atmospheric pressures, in the region of the first overtone band of H 2 near 1.2 μm. The quantitative analysis of the absorption features observed around ten allowed or forbidden transition frequencies of the monomer provides insight on the structure of this elusive species and a benchmark for future theoretical studies.