The photodissociation of molecules near the ionisation threshold

The photodissociation (PD) of the moleculeXY near the ionisation threshold is considered within the framework of the multichannel quantum defect theory. The general analytical dependence, determining the relation between the PD amplitude, the adiabatic amplitudes of dipole transitions and elements of the collisions matrix describing the associative ionisation (X+Y→e ^?+XY ⁺) and resonance scattering of atoms (X+Y→X′+Y′), has been obtained. The collisions matrix is determined by the system of algebraic equations and is expressed via the parameters which can be reconstructed from the molecule terms adiabatic picture. The obtained expressions can be used in case of a random number of Rydberg and dissociative channels. The role of the resonance mechanism of the molecules intermediate predissociative and autoionisational states population has been investigated. The peculiarities of the PD spectra structure for different physical situations have been studied. It has been shown that the Rydberg continuum states population increases the process general efficiency (the PD cross-section averaged over resonances increases). The NO molecule near-threshold PD, proceeding through thenpπ Rydberg states is calculated.