Polarization labeling spectroscopy of NO2

Polarization-labeling spectroscopy is used to simplify the visible spectrum of nitrogen dioxide. Nonlinear polarization effects are used to label and probe molecules in selected rotational-vibrational levels. Those labeled molecules can be detected by their depolarizing effect on a probe laser. The resulting spectra are considerably less complicated than normal absorption spectra. P- and R-branch transitions can be readily identified, and approximate excited-state rotational constants are determined. With more powerful lasers, it may be possible to use polarization labeling to unravel the very complicated vibrational structure of the NO₂ excited states.