Site selective electron paramagnetic resonance study of photoexcited chromium doped forsterite

Time resolved electron paramagnetic resonance ~EPR! measurements on a photoexcited chromium doped forsterite ~Cr:Fo! single crystal are reported. The spectral changes with time, magnetic field, crystal orientation, microwave power and, in particular, photoexciting wavelength, provide a selective picture of the various chromium dopants and the absorption–relaxation cycle associated with each optical excitation. Both Cr ions lodged at tetrahedral ~Td! and octahedral ~Oh! sites as well as Cr ions are detected. In particular, the laser-EPR technique enabled us to monitor the spin dynamics associated with the lasing center ~Cr/Td! in the time regime of 200 ns–100 ms following a selective photoexcitation of the crystal between 532 and 1064 nm. The transient EPR signals associated with the lasing Cr/Td ions, exhibit a noticeable dependence on even small changes ~;0.5 nm! in the exciting wavelengths that correspond to the visible A2→T1 and the near infrared A2→T2 transitions. The transient magnetization associated with each absorption–relaxation cycle is quantitatively analyzed in terms of site selectivity due to the narrow band ~i.e., low intensity! microwave detection following a narrow band optical excitation. Given this observed selectivity, it is suggested that laser-EPR may be employed to study intersite interactions and site structure versus optical function relationships in forsterite as well as other solids doped with transition metal ions. © 1995 American Institute of Physics.