Observation of collisional ionization of chromium atoms in high Rydberg states and the N4 scaling relation

We have used mass selected multiphoton dissociation/excitation/collisional ionization (MPD/MPE/CI) spectroscopy to study benzene chromium tricarbonyl (BCT) in an effusive molecular beam under collisionless and collisional conditions. Under collisionless conditions, our results are entirely consistent with, but not identical to, our earlier nonmass selected published experiments under bulk gas conditions. The spectroscopic results confirm that we can produce chromium atoms in a variety of well defined Rydberg states (7S3[Ns3d5], N=30--43). Following the state selective preparation of the chromium Rydberg atoms, by MPD of BCT followed by MPE of the ground state chromium atoms, they can be allowed to collide with ground electronic state benzene chromium tricarbonyl molecules producing ground state chromium ions and other products. We observe the scaling of the cross section for this collisional ionization process as a function of the Rydberg electron principal quantum number. The cross sections are a strong function of the Rydberg electron principal quantum number N, even when N is very large. Scaling as N4, the cross sections suggest that dipolar interactions with molecular colliders can be quite efficient at inducing collisional ionization of Rydberg atoms