Paramagnetic exciton dynamics studied by measurements of nuclear relaxation times Tsub(1D) and Tsub(1P)

To date, Tsub(1D) and Tsub(1P) measurements have been used to study slow molecular motions. We show how these quantities can be used in electronic spin dynamics studies. While Tsub(1N) provides informations at the electronic and nuclear Larmor frequencies, Tsub(1D) and Tsub(1P) allows one to reach the very low part of the electronic frequency-correlation functions (approximately 104s-1). The theoretical expressions of Tsub(1delta) and Tsub(1D) relaxation due to electronic spins are derived from a thermodynamical formalism. We report experimental results concerning the salt (PHI3 CH3 As)+(TCNQ)2-. Measurements of Tsub(1D) and Tsub(1delta) as a function of temperature demonstrate the existence of a new type of exciton slow motion: sharp minima appear at about 300 K while no effect is observed on Tsub(1N). We attribute the effect to an exciton creation-annihilation process. The activation energy of the process and the exciton life time are then obtained. We precise also the type of transfer motion of the excitons across the crystal: it is a random walk process along the linear chains of TCNQ molecules. (author)