Photodissociation and Radiative Association of CH(+)

Previous studies have shown that rotationally quasibound levels of the A('1)(PI) state of the molecular ion CH('+) enhance the rate of formation of this ion by radiative association in diffuse interstellar clouds. These quasibound levels have been identified here in an analysis of a fast ion beam photodissociation study. Assignments of the observed transitions were made possible by the computation of eigenvalues for trial potentials, constructed using the best available theoretical and experimental information, and by the evaluation of oscillator strengths for transitions in the wavelength range surveyed by the experiment. This study resulted in an improved determination of the bond energy and of the A('1)(PI) and X('1)(SIGMA) + potential curves. The dissociation limit C('+)(('2)P) + H(('2)S) is split by the spin-orbit interaction of ionic carbon. The A('1)(PI) state of CH('+), the radiating state in the radiative association process of astrophysical interest, correlates adiabatically to the upper C('+)(('2)P(,3/2)) + H(('2)S) limit. The many rovibrational levels between the two dissociation limits are predissociated by coupling with other electronic states; for levels above the adiabatic dissociation limit, predissociation by tunneling is also possible. Calculations of predissociation rates of A('1)(PI) quasibound levels showed that radial and rotational couplings are of major importance in determining the lifetimes of these levels. A calculation of the radiative association rate coefficient showed a strong enhancement of the radiative association rate due to resonant processes at low temperatures and indicated a strong temperature dependence that was not anticipated by earlier calculations.