Solid State Communications, Vol. 85, No. 5, pp. 455-459, 1993. Printed in Great Britain. 0038-1098/93 $6.00 + .00 Pergamon Press Ltd STUDIES OF PHOTOEXCITATIONS IN C79 BY OPTICALLY DETECTED MAGNETIC RESO/'~ANCE X. Wei, S. Jeglinski, O. Paredes and Z.V. Vardeny Department of Physics, University of Utah, Salt Lake City, UT 84112, USA and D. Moses, V.I. Srdanov, G.D. Stucky, K.C. Khemani and F. Wudl Institute of Polymers and Organic Solids and Physics and Chemistry Departments, University of California, Santa Barbara, CA 93106, USA (Received 11 September 1992 by J. Tauc) Photoexcitations in C70 both as a dispersion in polystyrene (PS) glass and as films produced by evaporation, have been studied by photomodulation, photoluminescence and their respective versions of optically detected magnetic resonance. In C70 : PS glass at 4 K we observed metastable triplet excitons with a distribution of lifetimes and a correlated distribution of magnetic powder patterns. In C70 films, on the other hand, in addition to triplet excitons we observed photogenerated charged excitations with spin ½ and a correlated photoinduced electroabsorption; we identify the charged excitations as C~0. C70 IS THE SECOND most stable abundant member of the fullerene family [1]. However, compared to C60, substantially fewer extensive experimental studies have been carried out so far. The C70 molecule has a Dsh symmetry; its structure was recently determined [2] to be of prolate spheroid ("rugby ball"), where the long axis is 7.80 A and the diameter of the equator is 6.94/k. Recent optical studies include optical absorp- tion of the ground [3, 4] and excited states [5, 6], photoluminescence [7, 8] and light induced ESR [7] of solutions and glasses, where the role of the triplet excitons has been emphasized. The nature of photoexcitations in C70 solids, however, where the intermolecular interaction is more important, was never addressed. In this work we study photoexcitations in C70 both as a dispersion in polystyrene (PS) glass and in films produced by evaporation, using the following optical measurement techniques: photomodulation (PM), photo-luminescence (PL), and their respective versions of optically detected magnetic resonance: the absorption detected magnetic resonance (ADMR) and the PL detected magnetic resonance (PDMR). We measured in CT0:PS glass metastable triplet excitons with photoinduced-absorption (PA) bands in the triplet manifold at 1.3eV and 1.8eV, respectively. From the magnetic resonance spectrum we obtained for the excitons a zero-field splitting (ZFS) parameter D = 0.0055 cm -1, showing exciton delocalization over the entire C70 molecule. In C70 films (similar as in C60 films [9]), on the other hand, we measured both neutral and charged excitations. The neutral excitations are more delocalized triplet excitons, whereas the charged excitations are spin ½ carriers with PA bands at 0.6 eV and 1.8 eV, identified as due to optical transitions of C7o and C7~, respectively. Correlated with the photogenerated carriers, we observed photoinduced electroabsorp- tion due to the induced electric fields accompanying the localized charged carriers [9]. The PM and PL spectra have been excited by an Ar + laser beam at 458 nm (pump) with intensity IL of 400mWcm -2, which was modulated at frequency f ranging from 20 Hz to 50 kHz, by an acousto-optical modulator. The PL from the sample and the transmission of a c.w. lamp (probe beam) were dispersed by a ¼met. monochromator and measured by various solid state detectors in the spectral range of 0.3-2.7eV. For the ADMR measurements the pump and probe beams constantly illuminate the sample mounted in a high Q microwave cavity at 3 GHz equipped with optical windows and a super- conducting magnet producing a field H up to 3 tesla. Microwave (#-wave) resonant absorption, modu- lated at a frequency ~ between 20 Hz and 40 kHz and power of 100mW, leads to small changes 6T in the 455