The mechanisms underlying dissociative recombination of H-2(+), D-2(+) and
HD+ have been a matter of considerable debate. Superexcited states belongin
g to two different categories, leading to the so-called direct and indirect
processes, are usually considered. The direct process involves doubly exci
ted repulsive Rydberg states, which, at lower energies, possess configurati
ons (2p sigma (u))(n/lambda), forming Rydberg series converging upon the (2
)Sigma (+)(u)(2p sigma (u)) ionic limit, or, at higher energies, have confi
gurations (2p pi (u))(n/lambda), leading to series converging upon the (2)P
i (u)(2p pi (u)) ionic state. In the indirect process, vibrationally excite
d levels of singly excited bound Rydberg states with configurations (1s sig
ma (g))(n/lambda), which form Rydberg series converging upon the X (2)Sigma
(+)(g)(1s sigma (g)) ionic threshold, are thought to play a key role. Expe
rimental studies of dissociative recombination are fraught with difficultie
s. In this contribution we shall explore the uses and advantages of resonan
ce-enhanced multiphoton ionization, either with kinetic-energy-resolved ele
ctron detection (termed laser photoelectron spectroscopy), or with mass-res
olved ion detection, to investigate the role of these superexcited states.
Experiments via the B (1)Sigma (+)(u) intermediate state were carried out o
n H-2 and D-2 with (3 + 1) one-colour laser photoelectron spectroscopy, and
on H-2 with a (1+1') two-colour scheme employing ion detection. In our exp
eriments, competition between direct molecular ionization, autoionization a
nd (pre)disscsciation was apparent. Above the dissociation threshold at whi
ch atoms in the ground and n=3 excited states are formed, the formation of
molecular ions and of excited n=2 fragments was virtually terminated, to be
replaced by the generation of n=3 fragments, Our observations can be expla
ined by invoking a mechanism, which so Far has not received much attention,
namely the direct excitation of vibrational continua of singly excited Ryd
berg states.