Knowledge Bank

Radiative association of $C++H$ is Che principal formation mechanism of $CH+$ in diffuse interstellar clouds. Current description of this process relies on model calculations using theoretical potential energy curves for the $X1\Sigma +$ and $A1\pi$; states of $CH+$, but cannot account for the observed abundance of this $ion.1$ We are attempting to determine experimentally the potential curves for these states in the regions relevant to radiative association by observing the reverse process: predissociation of $CH+$ through shape resonances In the $A1\pi$ state. A mass selected beam of $CH+$ ions is merged with a laser beam over a distance of 60 cm. The appearance of fragment $C+$ ions from the photodissociation of $CH+$ is studied as a function of both wavelength and production kinetic energy. The wavelength dependence is studied either by employing a tunable dye laser or by Doppler tuning a fixed ion laser frequency by variation of the ion beam velocity. Our Initial measurements of the predissociation of $CH+$ using ion laser lines in the region of 3500 {\AA} are described in Ref. 2. We have extended these measurements to 6471 {\AA} using both ion laser lines and a tunable dye laser. Predissociations of rotationally quasi-bound levels of the $A1\pi$ state are observed throughout this wavelength range. Assignment of the transitions has been made on the basis of the observed transition frequencies, Photofragment kinetic energies, and predissociation lifetimes. Several significant discrepancies are found between the photodissociation measurements and the predictions of theory, demonstrating that the potential energy curve of the $A1\pi$; state is less attractive at large internuclear distances and somewhat shallower than predicted.