Present day laser technology has advanced such that multiple resonance excitation can be performed using several lasers of various wavelengths. Also narrowband tunable extreme ultraviolet laser radiation is readily available, to bridge the gap between the low-lying electronic ground state and the excited singlet states in molecular hydrogen. These methods have been employed to investigate a new class of excited states of H2 that are confined to large internuclear separation.

Introduction

Molecular hydrogen, the smallest neutral chemical entity, is often considered to be the simplest molecule. For a spectroscopist, however, H2 brings about a number of complications which make it a difficult object to study. First of all, from an experimental perspective, the electronic ground state is separated from the excited states by a large energy gap, which can be bridged only by photons in the domain of the extreme ultraviolet (XUV). Furthermore hydrogen is a light molecule with a very open rotational structure; the rotational lines are often so widely spaced that it is not obvious that they form a progression. Also, as a consequence of the small mass, deviations from the Born-Oppenheimer are most prominent and strongest in H2. Non-adiabatic interactions shift the energy levels over several tens of cm−1, so that the rovibronic structure becomes confused. As a result assignment of observed spectra, even with rotational quantum numbers only, is not straightforward. This point is illustrated by the Dieke atlas (Crosswhite 1972), a compilation of spectra pertaining to transitions between excited states, recorded in the visible domain with a classical spectrometer.