OPTICALLY DETECTED ZERO-FIELD MULTIPLE RESONANCE STUDIES OF n,$\Pi^{*}$ TRIPLET STATES
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Date
1973
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Ohio State University
Abstract
The optical detection of magnetic resonance technique is used to investigate some dynamic and static properties of the chemically reactive lowest triplet state of cyclopentanone. These studies have brought to light two new phenomena in excited states: 1) multiple, interconverting types of paramagnetism and 2) multiple, interconverting types of luminescence. These properties are characteristics of a single molecular state. The zero-field optically detected EPR spectrum of cyclopentanone consists of several triads of resonances. One of these, the normal resonance has already been $described.^{1}$ Two other triads, termed satellite and weak also occur. The normal triad can be identified with the normal configuration of cyclopentanone which is responsible for the bulk of the molecular phosphorescence and for the phosphorescence-modulated double resonance (PMDR) spectrum which has the same spectral profile as the phosphorescence. The PMDR spectra for the satellite and weak triads are spectrally different and thus cannot be ascribed to “normal” triplet cyclopentanone. The satellite and weak triads also show a pronounced interaction with the normal triad and with each other. This has been investigated by means of a microwave-microwave-optical triple quantum technique whereby the weak and satellite sublevel populations can be shown to be altered by saturation of any of the three resonances of the normal triad and vice-versa. Four possible mechanisms are discussed: A) capture and release of excitons by traps, B) annihilation between trapped excitations or between excitons and traps, C) magnetic cross-relaxation between traps or between exitons and traps, and D) interconversion between several electronic configurations of one and the same molecular entity. The last mechanism is capable of explaining all our experimental observations and an interpretation on the basis of electronic-nuclear coupling is given.o
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Author Institution: Department of Physics, University of Delaware