Near-Threshold Photodissociation of ColdCH+in a Storage Ring

Photodissociation of ${\mathrm{CH}}^{+}$ molecules ( ${\mathrm{CH}}^{+}+h\nu \to {\mathrm{C}}^{+}+\mathrm{H}$) is studied by collinear laser spectroscopy on a rotationally relaxed fast molecular ion beam in a storage ring. We have detected the resonances predicted between the thresholds related to the two fine-structure levels of the ${\mathrm{C}}^{+}$ fragment for low initial rotations, and found that no heating processes hinder the complete rovibrational thermalization of the fast stored beam in a room temperature environment.