An experimental study of triplet spin-lattice relaxation in the system naphthalene-pentacene

This work concerns experimental studies of spin-lattice relaxation of short-living photo-excited triplet states like those of pentacene by means of pulsed electron spin resonance (ESR). It addresses an experimental issue particular for such studies: after photo-excitation the decay of the ESR signals depends on relaxation between the triplet sub-levels, as well on decay from these levels to the singlet ground state. As a result the observed decay is generally bi-exponential, and a full analysis requires fitting two decay rates and two amplitudes to the observed decays and extracting the relaxation and decay rates from the fit parameters. However, often the data lack sufficient precision for this procedure to yield reliable results. To solve this issue, this article introduces an alternative method for the analysis, in which one just extracts the initial decay rate by means of a linear fit to the initial part of the decay. It is shown that - provided the magnetic field is sufficiently strong - the temperature dependent component of this initial decay rate is equal to three times the triplet spin-lattice relaxation rate. By recording decay curves as a function of temperature and extracting that component, one thus obtains the desired triplet relaxation rate as a function of temperature. Results are obtained for pentacene in a naphthalene host in the temperature range 100≤T_L≤240 K for two orientations of the magnetic field and as a function of the orientation of the magnetic field in the crystalline ac-plane at 220 K. We argue that the observed relaxation rates are too fast to be interpreted as due to modulation of the zero-field splitting, but that they can be well understood as due to a Raman process involving acoustical phonons.