Experimental benchmarking of relative potential energy minima internuclear distances with femtometer length accuracy

The two-dimensional photon-excitation photon-emission (PhexPhem) map of the archetypal $H_{2}$ molecule, constructed from dispersed-fluorescence measurements after energy-scanning excitation by small-bandwidth photons, displays complete systems of its Condon diffraction bands. They originate from spontaneous radiative dissociation of individual bound electronically excited rovibronic levels, one of the most relevant processes for the destruction of $H_{2}$ in space. For the 𝐵−𝑋 system of electronic states, we show that specific measured spectral characteristics of individual bands are extremely sensitive to the difference Δ⁢𝑅 of the internuclear distances, where the two potential energy curves each have their minimum. Using data from recorded $H_{2}$-PhexPhem maps, it is possible to experimentally validate the calculations of Δ⁢𝑅 down to an accuracy of at least 50⁢f⁡m. This accuracy may be used as a sensitive experimental test to calculational accuracy. It is well feasible to improve the currently achieved experimental accuracy, for an even more accurate benchmark of advanced potential energy curve calculations.