New experimental insight into the ground state potential surface of triatomic hydrogen

Continuous UV spectra emitted by the triatomic hydrogen molecules ${\mathrm{H}}_3$, ${\mathrm{D}}_3$, ${\mathrm{H}}_2$D, and ${\mathrm{D}}_2$H have been measured between 200 and 400 nm and exhibit two pronounced maxima. The two maxima in the spectra are interpreted as arising from the decay to both sheets of the ground state potential surface which form a conical intersection at the point of equilateral symmetry. This is confirmed by a detailed computer simulation. The fact that the ${\mathrm{H}}_3$ ground state potential surface has two sheets is, to our knowledge, observed here experimentally for the first time.