Prof. Kurt Dressler (Professur für Molekularspektroskopie 1968-1994)
Spectra of diatomic molecules in gas phase and in low-temperature solids

The main research interest in K. Dressler's group was the interaction between electronic and nuclear motions in diatomic molecules in the gas phase and in molecular solids.
Every neutral molecule has (a) excited states which form Rydberg series and (b) excited states of valence, ionic, or charge-transfer character with electronic energy curves that would, in the absence of configuration interaction, intersect those of the Rydberg states. The avoided crossings between potential curves of Rydberg and non-Rydberg states of like symmetry are associated with strongly non-adiabatic behavior of electronic and vibrational motions. The entanglement is most intricate when the characteristic frequency of the electronic resonance between different electronic configurations is of comparable magnitude with vibration frequencies, because then neither the adiabatic nor the diabatic description of electronic states is a good approximation.

Such effects have been studied in the ultraviolet spectra of NO, N2, und H2. Many assignments of previously ill understood spectroscopic structures could thus be established. For H2 and its isotopomers HD, D2, HT, and T2, the highly accurate ab initio calculations by frequent academic guest Prof. L. Wolniewicz, Torun, aided the spectroscopic assignments. The comparison of ab initio theory with experiment is of fundamental interest in the case of molecular hydrogen. New assignments in the molecular hydrogen emission spectra were also possible through application of robust statistics to Dieke and Crosswhite’s old and famous spectral measurements.

The VUV absorption spectra of solid CO and of CN- ions were explained by the application of the theory of strong exciton-vibron-phonon coupling, while many interesting features in emission spectra of activated solid N2 could be shown to arise from vibron- and phonon-induced dipole transition moments in forbidden electronic transitions of matrix isolated N atoms. These features had been a long standing puzzle in the spectrum of the glow of low-temperature deposits of active nitrogen.