l- andn-changing collisions during interaction of a pulsed beam of Li Rydberg atoms withCO2

The pulsed Li atomic beam produced in our experiment is based on controlled transversely-excited-atmospheric ${\mathrm{CO}}_2$ laser–induced ablation of a Li metal target. The atomic beam is propagated in vacuum or in ${\mathrm{CO}}_2$ gas at low pressure. Atoms in the beam are probed by laser-induced fluorescence spectroscopy. This allows the determination of time-of-flight and velocity distributions. Li Rydberg states (n=5–13) are populated in the beam by two-step pulsed-laser excitation. The excited atoms interact with ${\mathrm{CO}}_2$ molecules. l- and n-changing cross sections are deduced from the time evolution of the resonant or collision-induced fluorescence following this selective excitation. l-changing cross sections of the order of ${10}^4$ A${̊}^2$ are measured; they increase with n as opposed to the plateau observed for ${\mathrm{Li}}^{\mathrm{*}}$ colliding with a diatomic molecule. This behavior is qualitatively well explained in the framework of the free-electron model. n→n’ changing processes with large cross sections (10–100 A${̊}^2$) are also observed even in the case of large electronic energy change (Δ$E_{\mathrm{nn}\mathcal{’}}$ ${>10\mathrm{}}^3$ ${\mathrm{cm}}^{\mathrm{-}1}$). These results can be interpreted in terms of resonant-electronic to vibrational energy transfers between Li Rydberg states and ${\mathrm{CO}}_2$ vibrational modes.