Characterization of the S=9 excited state in Fe8Br8 by electron paramagnetic resonance

High-frequency electron paramagnetic resonance has been used to observe the magnetic dipole, DeltaM(s)=+/-1, transitions in the S=9 excited state of the single-molecule magnet Fe8Br8. A Boltzmann analysis of the measured intensities locates it at 24+/-2 K above the S=10 ground state, while the line positions yield its magnetic parameters D=-0.27 K, E=+/-0.05 K, and B-4(0)=-1.3x10(-6) K. D is thus smaller by 8% and E larger by 7% than for S=10. The anisotropy barrier for S=9 is estimated as 22 K, which is 25% smaller than that for S=10 (29 K). These data also help assign the spin exchange constants (J's) and thus provide a basis for improved electronic structure calculations on Fe8Br8.