The competition between multiphoton ionization and fragmentation in the diatomic molecule hydrogen chloride is reviewed. Emphasis is laid on recent experimental results employing chemical imaging methods in order to obtain kinetic energy distributions and angular distributions of photoproducts. The energy range considered is 15 to 20？eV, equivalent to the absorption of three or four photons in the ultraviolet wavelength range. The role of Rydberg states as resonantly excited intermediate states in the ionization/fragmentation processes is assessed. Mixing among states gives rise to peculiarly shaped double minimum potential energy curves which allow for the production of hydrogen and chlorine atomic and ionic fragments via several competing pathways, in addition to the production of molecular HCl+ ions. States with different electronic properties show a qualitatively different behaviour from states. Accidental resonances between states of differing orbital angular momentum or multiplicity serve to override these differences and cause subtle as well as significant deviations from the unperturbed behaviour. 1. Introduction Hydrogen halides are molecules of fundamental interest in chemistry and physics and have extensively been studied theoretically as well as experimentally. In many respects, they can be regarded as prototypes of heteronuclear diatomic molecules. While the electronic ground states of the hydrogen halides are in general well understood, their Rydberg and valence electronic structure still bears some surprises for the experimentalist and for the theoretician. For example, competing multiphoton dissociation and ionization processes are observed as a consequence of Rydberg-valence interactions at large internuclear distances, a behaviour which is primarily associated with and intermediate states. The focus of the work summarized here lies in the investigation of the photoionization of HCl following two-photon excitation of its Rydberg states. For HBr [1–3], and to a lesser extent for HI [4], similar studies and similar conclusions have been reported, whereas much less is known about HF, most probably due to the difficulty of state-selective fluorine atom detection [5] and to the more problematic chemical properties of HF making gas handling significantly more demanding. Three main issues are associated with resonance enhanced multiphoton ionization of HCl. First, if the state is accessed as resonantly intermediate state, its peculiar double-well structure opens the door for subsequent photoabsorption at very large internuclear distances,
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