High Resolution Spectroscopy below the Doppler Width

Microscopic insights into elementary molecular processes are only feasible through the observation of unique quantum states and the study of their couplings and time‐dependent behavior. In the case of larger chemically relevant molecules, the corresponding transitions lie so close that they mostly overlap and lead to band structures. For example, electronic band spectra of large molecules cannot be resolved into single lines even by high resolution UV‐spectrometers. The reason for this is the Doppler broadening, arising from the random velocity of the molecules in the gas phase. In this article, we want to demonstrate that modern laser spectroscopic methods make it possible to overcome this “Doppler barrier” and to resolve the line structure in band spectra. The selective excitation of unique quantum states yields precise information on the mechanisms responsible for energy redistribution in molecules. Energy redistribution processes represent a crucial first step preceding chemical reactions, which they influence decisively. Very high resolution spectroscopy elucidates the influence of molecular rotation on the energy redistribution. Through Coriolis forces in rotating molecules, vibrations can couple, causing a flow of energy from the selectively excited vibrations to the many other vibrational degrees of freedom. Weak interactions, namely van der Waals interactions and hydrogen bonding, determine the structure of large molecules (biomolecules) and supermolecular systems. Weak intermolecular interactions, given their selectively resolved quantum states, can be analyzed and understood at the microscopic level. “Doppler‐free spectroscopy” with its resulting line spectra furnishes information on the structure, including bond lengths, of van der Waals complexes. With this information, Doppler‐free spectroscopy provides the model parameters indispensable for the quantitative description of complex systems.