ENDOR amplitudes of triplet state molecules: I. Electric-circuit analogy treatment

Abstract

Abstraet. ENDOR spectra of triplet state molecules have a characteristic line fxom degenerate NMR transitions within the zero level (ZL) electron spin manifold M s = 0. The ZL line, observed at the free nuclear Larmor frequency, dominates spectra when the number of nuelei is large. This line was found to be substantially reduced in intensity at low temperature. The strong variation of the ZL line intensity is analyzed within the frame of an electric-circuit analogy modeling. The result is as follows: At low temperature the electron and nuclear spin-lattice relaxation rates become small, and the nuclear-nuclear spin flip-flop transitions between degenerate substates, M 1 = coust within the ZL manifold, become relatively strong to compete for population redistribution. This reduces the population differences between nuclear sublevels. Additional NMR irradiation can thus do very little to reduce these differences even more, and the ENDOR effect becomes suppressed. A certain enhancement of the relaxationally suppressed ZL line occurring at increased EPR saturation is explained by the coherent action of the microwave field on the selected substate within the ZL manifold that shifis its energy out of the other degenerate states thereby closing the flip-flop relaxation channel.

ENDOR amplitudes of triplet state molecules: II. Orientational dependence of the νp frequency line and S-T0 mixing

“…Sample preparation, instrumentation and measuring procedures were described elsewhere [ 1 ]. The EPR spectrum of the Zn(DBSQ) 2 complex is shown in Fig.…”

“…All the ENDOR frequencies from within the EPR-active 10) manifold equal vp, and the spectra can be treated as in [1]. Common expectation here is a dominance of the Vp frequency line ("multiplet advantage", [1]) that corresponds to spectra c and d of Fig. 2. b) For the intermediate case, IAI ~ ruin{lorA}, analysis is more complicated than for the limiting cases.…”

“…These amplitudes can be treated within the electric-circuit analogy approach of [1], however, certain modifications should be done here to aceount for the changes of transition probabilities. For instance, a decrease of EPR or NMR transition probability means an increase of the "internal source resistance" of the analog electric circuit [1]. Moreover, the spin-lattice relaxation times, which are represented by resistances within the electric network, are also affected by the mixed nature of states ~_+.…”

“…Another interesting observation has been reported in [1] concerning the orientation dependence of the amplitude of the ZL line of the diradical complex Zn(DBSQ)2 (DBSQ stands for 3,6-di-tert-butyl-o-semiquinone). This line dominates the ENDOR spectra for orientations of the complex which are detected at the perpendicular canonical components of the EPR spectrum, and vanishes for complexes oriented with their zero-field splitting (ZFS) z-axis parallel to the direction of the magnetic field, i.e., which are detected at the parallel canonical components.…”

“…In our previous work [1] the amplitudes of specific lines in the ENDOR spectra of triplet state molecules have been analyzed. These lines ate both caused by degenerate NMR transitions within the M s --0 zero-level (ZL) manifold, and by the "secondary ENDOR" effect originating in NMR transitions within the M s manifold which is not directly affected by EPR irradiation.…”