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The next issue to be addressed is the role of the predissociative XY states. It turns
out that they both open up the new dissociation channels, XY ! X C Y ,and
affect the characteristics of transitions to the ionization continuum. The contribution
of predissociation to the anomalous vibrational distribution of H 2 C ions produced
by the photoionization of molecular hydrogen has been discussed (Hickman 1987 ;
Dixit et al. 1989 ; O'Halloran et al. 1987 ).
One low-temperature exothermic reaction of the particular importance is that of
dissociative recombination (DR) of O 2 C ions, responsible for the rate of charge
loss processes in the upper atmosphere. The DR cross section can be increased by
several orders of magnitude by choosing suitable values of the laser parameters,
which makes the laser-assisted reaction feasible (Golubkov et al. 2002 , 2003 , 2011 ;
Adamson et al. 2009 ). The problem reduces to finding the optimal conditions for the
laser control of low-temperature DR and formulating recommendations for future
Laser control efficiency depends on the incident electron energy, the field
strength and frequency, and the angle between the electron and laser beams. The
laser pulse duration here is much longer than the transition times between the
intermediate molecular states. For example, the predissociation times for high-
lying states of XY are of the order of 10 11 s. Note that the tunable lasers
widely used in various photo-induced processes are characterized by strengths of
10 12
10 14 W=cm 2 and pulse repetition rates of 10 3 s. The corresponding
saturation and rotational widths do not exceed 10 1 cm -1 , and the Doppler broaden-
ing is routinely eliminated by splitting the laser beam (Letokhov 1987 ). Because the
energy dependence of the DR cross section cannot be measured to accuracy better
than E 10 3 eV, the spread in the pulse frequency ! f is negligible and the
beam can be considered monochromatic.
The problem of quantitative description of DR reaction in the framework of
MQD theory directly stems from the following feature of MQD theory: it is highly
sensitive to the variation of quantum defects and configuration interactions as
functions of internuclear distance. The analysis of partial and total low-temperature
DR rate constant as a function of parameters of the theory was carried out by
Golubkov et al. ( 2000a ). It was demonstrated that even an insignificant variation of
quantum defect may lead to the variation (of the of one to two orders of magnitude)
in the rate constant. The dependence of rate constant on the configuration interaction
is smoother. Because quantum chemistry is incapable of providing the necessary
accuracy, the further development of the theory is impossible without invoking
additional modern high-resolution experiments (for example, REMPI, REMPD).
Restoration of MQD theory parameters from the experimental data represents the
reverse problem, which is incorrect by definition (Golubkov et al. 2000b ). It was
shown that it can be solved because (1) the direct and reverse problems both
are solved in the framework of universal unitary MQD theory; (2) intensities of
the photo-processes are analytical functions; (3) the solution is unique only if a
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