Phosphorescence amplitude modulated

Figure 6.2. (I) Conventional phosphorescence spectrum of 2,3-dichloroquinoxa-line in durene at 1.6°K. (II) am-PMDR spectrum, obtained by amplitude modulation of microwave radiation that pumps the tv-t, (1.055 GHz) zf transition with the detection at the modulation frequency. Only bands whose intensities change upon microwave radiation (1.055 GHz) and thus originate from tv or rz appear in the am-PMDR spectrum. Transitions from r and rv appear with opposite sign (phase-shifted by 180°). (Hb, lie ) Polarization of the am-PMDR spectral transitions, relative to the crystal axes. The band at 0,0-490 cm-1 originates from both the r and t spin states its intensity does not change upon the 1.055-GHz saturation (no band in II) however, its polarization does rhanp. (bands in Hb and IIc ). (Reproduced with permission from M. A. El-Sayed.tt7W)...

The method of phosphorescence microwave double resonance (PMDR) spectroscopy is based, like the two other methods discussed above, on c.w. excitation of the Pd(2-thpy)2 compound at low temperature. Additionally, micro-wave irradiation is applied, whereby the frequency is chosen to be in resonance with the energy separation between the two substates I and III of 2886 MHz. With this set-up, one monitors the phosphorescence intensity changes in the course of scanning the emission spectrum. Technically, the phosphorescence spectrum is recorded by keeping the amplitude-modulated microwave frequency at the constant value of 2886 MHz and by detecting the emission spectrum by use of a phase-sensitive lock-in and signal averaging procedure (e.g. see [61, 75,90]). 

Slow-passage ODMR signals frequently are observed by the continuous wave method in which the optical effect is monitored using broadband detection. On the other hand, if the triplet state decay constants are sufficiently large, the microwave power may be amplitude modulated at an audio frequency which results in modulated phosphorescence when the microwave frequency is at resonance. The phosphorescence is then monitored with narrow-band phase-sensitive detection, for a great improvement in the signal/noise ratio. The latter detection method is frequently used to produce a magnetic resonance-induced phosphorescence spectrum by a technique referred to as phosphorescence-microwave double resonance (PMDR). The microwave frequency is fixed at resonance,... 

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