Pulsed EPR Signals from Triplets

Abstract

Abstract Pulsed EPR experiments have proven to be an important tool for measuring EPR spectra, kinetics and relaxation rates of free radicals and triplet molecules. The EPR frequencies and selection rules from CW-EPR spectra also govern pulsed EPR-experiments, but pulsed excitation provides much greater control over spin dynamics and allows clean separation and measurement of many properties of the spin system. Most pulsed EPR measurements of triplet molecules have been made in the selective pulse limit where only one EPR transition of a molecule is excited by microwave pulses and its EPR spectroscopy behaves like that of a radical with spin of S=1/2. However, some important classes of systems with S=1, such as molecular photoconversion devices, donor-acceptor complexes, biradicaloids, biradicals and triplet states with high symmetry, can have all their EPR transitions excited simultaneously, the so-called hard-pulse limit. The pulsed EPR signals in this limit have many similarities, but also some important differences from those in the selective pulse limit or from radicals with S=1/2. The dependence of pulsed EPR signals on microwave pulse phase and turning angle is examined in both limits for S=1 systems in thermal equilibrium or with electron spin polarization.