  The hydrogen dimer, (H2)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 (H2)2 infrared absorption spectrum was reported more than
thirty years ago at a temperature of 20 K. Due to the sharp decrease of the
(H2)2 abundance with temperature, a detection at room temperature was generally
considered as hardly achievable. Here we report the first room temperature
detection of partly resolved rotational structures of (H2)2 by cavity ring down
spectroscopy at sub-atmospheric pressures, in the region of the first overtone
band of H2 near 1.2 micron. 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.
