l- and n-changing processes in the Rydberg states of Li induced by thermal collisions with ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, CO, and ${\mathrm{D}}_2$

Collisional transfers induced in the n=6–13 Li Rydberg states by ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and CO molecules have been experimentally studied in a heat-pipe oven. The l-mixing cross sections have been deduced from the fast component of the population relaxations following pulsed laser excitation. The order of magnitude of these cross sections (${10}^3$ A${̊}^2$) is well predicted by the existing theories. For large n, the cross sections stay approximately constant. This can be partly explained by the influence of inelastic processes such as n changing. In the particular case of ${\mathrm{H}}_2$, n-changing collisions have been identified by the fluorescence starting from the n’=n±1, n-2 levels. Cross sections of the order of 10 A${̊}^2$ have been measured. The systematic study of the n-changing cross sections as a function of the transferred energy shows some evidence of resonance phenomena corresponding to an energy exchange between the Rydberg electron and the molecular rotation. This effect is also observed when ${\mathrm{H}}_2$ is replaced by ${\mathrm{D}}_2$. These resonances are well described by the theory of Petitjean and Gounand based on the e-quadrupolar interaction.