ESR study of optically enhanced phase transition in (BEDT-TTF) ₃Ta₂F₁₁

We have identified an optically enhanced magnetic phase transition in the newly synthesized organic molecular charge transfer salt, (BEDT-TTF) 3Ta2F11 (BEDT-TTF≡bis- ethylenedithiolotetrathiafulvalene) by ESR absorption measurements in the X band microwave region. At room temperature, only a doublet state ESR absorption is observed, but below 30 K several triplet ESR absorptions appear. The orientation dependence of the ESR absorption under illumination at energies near the band gaps in the material (640 nm, T=12 to 5 K, H0<0.34 T) indicates that there are rapid spin exchange processes with times τe< 10-8 s near 7 to 5 K along certain crystallographic directions with a temperature dependence suggesting spin-lattice relaxation times which proceed via Van Vleck "direct processes." This, to our knowledge, is the first case where the magnetic properties of a charge transfer salt are altered by the interaction with photons of energy equal to the band gaps in a low dimensional solid, providing a new, interesting way to investigate these materials. © 1987 American Institute of Physics.