Autoionizing Rydberg states of Li2: Analysis of electronic–rotational interactions

Highly excited Rydberg states of 7Li2 have been measured by two-step excitation with two narrow band pulsed dye lasers in a collimated lithium beam. A complex many-line two-photon ionization spectrum has been observed which consists of sharp lines whose natural widths are smaller than the laser line width (0.03 cm−1). The spectrum has been analyzed with the help of multichannel quantum defect theory. Radial electronic dipole transition moments for the second excitation step were calculated and provided the key to the analysis. The spectra correspond to the excitation of s and d Rydberg series with n∼18–40. The series are affected by strong rotational l uncoupling as well as electronic s–d mixing. The analysis yields accurate values for the vibrational and rotational constants for the X 2Σ+g state of Li+2 and an improved value of the ionization potential of Li2.