Autoionizing Rydberg levels of Li2 molecules in a supersonic molecular beam are populated by stepwise excitation with two tunable pulsed dye lasers. The observed autoionization spectra show severe perturbations. Based on calculations of quantum defects and a perturbation treatment of l-uncoupling a tentative assignment of Rydberg series up to n = 32 is proposed. The convergence limits of these series yield a value of IP = 41475 cm−1 for the adiabatic ionization potential and a vibrational constant ωe = 263 cm−1 for the X2Σ+g ground state of Li+2. The experimental results are compared with ab initio calculations combined with a core polarization potential, which yield the potential curve. the dissociation energy, the quadrupole moment and the vibrational frequency for the X2Σ+g ground state of Li+2, in the excellent agreement with experimental findings.

Sequential two-photon ionization of Li2 in a supersonic beam with two pulsed tunable dye lasers yields the adiabatic ionization potential IP(Li2) = 41475 ± 8 cm− and the dissociation energy of the ion D0(Li+22Σ+g) = 10353 ± 25 cm−1. The influence of electric fields on the measured value of the ionization potential and the appearance of numerous autoionization lines are discussed.

Three different techniques of laser spectroscopy with sub-Doppler resolution are discussed which have been applied to the investigation of diatomic and triatomic molecules. These are linear laser spectroscopy in collimated molecular beams, polarization spectroscopy and a combination of both methods with optical-optical double resonance techniques. The methods are illustrated by high resolution spectra of the molecules NaK, Cs2 and NO2. A section on time resolved spectroscopy, applied to lifetime measurements and to the deterniination of collision induced relaxation processes concludes the paper.

Excitation of NaK molecules in a collimated molecular beam by a single mode tunable argon laser with sub-Doppler resolution allows selective population of definite vibrational rotational levels (v′, J′) in the D 1Π-state. The resultant fluorescence consists of a singlet spectrum in the 470–570 nm range and a triplet spectrum in the 625–715 nm range. The analysis of the triplet fluorescence, which consists of discrete lines and of a modulated continuum, allows the determination of the a 3Σ+ potential and yields very precise values for the dissociation energies in the a 3Σ+ and X 1Σ+ states, which are measured as De(3Σ+) =204±4 cm−1 and De(1Σ+) =5269±6 cm−1. For some excitation wavelengths a fluorescence spectrum with discrete lines and with a double-modulated continuum is observed which is assigned to ’’ Condon internal diffraction’’ bands terminating close below and above the dissociation limit of the X 1Σ+ state. Its relevance for the determination of the upper and lower potential curves is discussed.