Molecular-beam-laser studies of the states X1Σ+ and A0+ of PbS

By using a molecular-beam apparatus, laser excitation spectra and microwave-optical double-resonance spectra were observed on the electronic states X¹Σ⁺ and A0⁺ of PbS. For the microwave spectra a resolution up to 10 kHz was achieved and microwave two-photon transitions could be detected. All information was combined into single fitting routine including data from other sources. Besides the usual Dunham parameters the fit model takes account of the breakdown of the Born-Oppenheimer approximation and the isotopic field shift. From the field shift of the electronic energy, the vibrational motion and the rotational motion the electron density as a function of the internuclear distance at the Pb nucleus is derived, which allows a more detailed discussion of the electronic structure of the molecule in combination with future quantum-chemical calculations. Additionally it is shown that the variation of the electron density along the vibrational motion at low vibrational levels can be as large as the variation for the electronic transition A0⁺ -X¹Σ⁺ in the case of PbS. The magnetic hyperfine structure of ²⁰⁷Pb is resolved in a molecule for the first time. The coupling parameter of PbS is compared with that of the isovalent molecule TlCl.