THEORY OF NON-MARKOVIAN RELAXATION OF SINGLE TRIPLET ELECTRON SPINS USING TIME-AND FREQUENCY-DOMAIN MAGNETIC-RESONANCE SPECTROSCOPY MEASURED VIA OPTICAL FLUORESCENCE - APPLICATION TO SINGLE PENTACENE MOLECULESIN CRYSTALLINE P-TERPHENYL

Fluorescence-detected magnetic resonance (FDMR) allows one to monitor magnetic resonance phenomena via fluorescence. Experimental FDMR data obtained using single triplet-state chromophore guest molecules in a l ow-temperature organic host matrix are analyzed using a stochastic app roach to describe tripler electron spin dephasing resulting from frequ ency fluctuations U-t induced by host-matrix proton spin dynamics. Mod eling the fluctuations U-t by a sum of N independent random telegraph processes with the same jump rate nu but different variances sigma(k) we construct an exact set of equations for the density matrix of a fiv e-level molecule averaged over fluctuation histories U-t. These equati ons provide a basis to study non-Markovian effects of microwave- (MW-) field-dependent dephasing in the FDMR response of a molecule undergoi ng slow fluctuations U-t (sigma(2)/nu(2)greater than or equal to 1, si gma(2)=Sigma sigma(k)(2)) to a MW field that is resonant with a transi tion between triplet spin substates. Both frequency- and time-domain F DMR phenomena such as (i) power-broadened FDMR line shapes, (ii) FDMR Hahn echo signals, and (iii) FDMR free induction decay are studied, An alytical expressions for the FDMR response are obtained in the case nu much greater than k(j)(i) where k(j)(i) is an intersystem crossing ra te. Experimental data on power-broadened line shapes for a pentacene+p -terphenyl pair which demonstrate a pronounced effect of MW-field-supp ressed dephasing are explained in the context of the theory. [S0163-18 29(98)00337-3].