Desire to control the outcome of chemical processes has been driving the efforts of chemists and physicists for many decades. The conventional methods use catalysts, temperature,
or pressure, and provide only a limited control without the desired selectivity. The development of lasers opened a perspective to prepare the reactant atoms and molecules
selectively in predetermined excited states, which evolve preferentially along the desired path. However, this approach faces limitations due to such properties of the molecular potentials
like crossings and mixing, and rapid diffusion of optical excitation into rotations and vibrations of the nuclei. It requires a precise knowledge of often highly excited molecular states, for which
exact quantum mechanical description is hardly achievable.
The coherent control schemes, which were developed during the last decade, use coherent radiation fields to drive the molecular fragmentation through the desired channel,
while the efficiency is, in principle, limited only by the conservation laws. Unfortunately such schemes are limited to the control of branching in unimolecular fragmentation. Bimolecular encounters
proceed at a broad range of impact parameters, which restricts the achievement of a controllable superposition of eigenstates to a tiny fraction of the total ensemble of reactants.
In addition, the high laser power used in the coherent control may lead to multiphoton absorption and ionisation, transforming the initially neutral media into ionised plasma.
The understanding of bimolecular encounters is hampered by the complexity of exact quantum chemical description. This difficulty is insurmountable if the molecular complex contains more than
two single colliding atoms and one of the colliding partners is excited to a high electronic (Rydberg) state. The experimental evidences indicate that simplified models viewing bimolecular encounters
as a collision of the highly excited electron of one partner with the other partner molecule may fail to explain the observations, and the formation of an intermediate bimolecular complex is likely
involved. The transfer of the highly excited electron often leads to fragmentation of the complex and the formation of chemically active radicals. In fact, the electron transfer plays a decisive
role in many processes observed in laser excited gases, low temperature plasmas, environmental sciences (e.g., stratospheric ozone depletion), and biology (e.g., strand breaking in plasmid DNA
due to electron attachment). Therefore there is a definitive need for development of understanding of processes occurring during bimolecular encounters.
Rydberg atoms and molecules are particularly interesting for studies in view of their distinct differences from atoms and molecules in ground or lower excited states (see for
general reference T. F. Gallagher, "Rydberg atoms", Cambridge UP, 1994). They feature extremely large size, small spacing between adjacent energy levels, and small binding energy of
the Rydberg electron. Naturally, one would expect them to behave differently during their interaction with other atoms and molecules than "normal" atoms and molecules would do. Indeed,
a number of novel phenomena associated with such interactions have been recently discovered. For example, in ultracold gases, Rydberg atoms have been shown to simultaneously interact with
a large number of other atoms (frozen Rydberg gases [PRL 80, 249 (1998); PRL 253 (1998)]). At sufficiently high densities, such collective effects can transform the
Rydberg gas into ultracold plasma [PRL 85, 4466 (2000)]. Intriguingly, peculiar ultra long-range Rydberg molecules have been predicted, which
feature huge permanent dipole moments for homonuclear diatomics [PRL 85, 2458 (2000); JPB 35, L199 (2002)]. Moreover, when cooled to ultracold
temperature, gases of Rydberg atoms may actually be used some day for quantum information processing [PRL 87, 037901 (2001)].
The global objective of our work is to develop understanding about bimolecular collisions involving atoms and molecules in Rydberg states and explore the possibilities to
control them by means of laser manipulation. Before this can be achieved, several associated questions must be answered: