Polarized modulated PMDR spectroscopy and the zero-field origin of phosphorescence

A new technique is demonstrated which enables the identification of the zero-field origin of each vibronic band in the phosphorescence spectrum of polyatomic molecules at very low temperatures. The method employs modulation of the microwave radiation which saturates one of the zero-field transitions while detecting the changes in the vibronic band intensity in the phosphorescence spectrum at the modulation frequency. Bands originating from either of the zero-field levels being saturated appear in the modulated spectrum with 180 phase shifts. Bands originating from both zero-field levels being saturated may show weakly in the modulated spectrum but can readily be identified by recording the modulated polarized optical spectrum.