Photoluminescence study of the charge transfer salts (BEDT-TTF)2RbHg(SCN)4 and (BEDT-TTF)2KHg(SCN)4

We have studied the photoluminescence (PL) spectra of the charge transfer salts (BEDT-TTF)2RbHg(SCN)4 at 1.8 K and (BEDT-TTF)2KHg(SCN)4 in the 1.8-77 K temperature range. Spectra at 1.8 K for both materials display an asymmetric peak at 1.97 eV and a shoulder starting at 1.7 eV. The same features were present in the measured PL spectrum of a neutral-donor BEDT-TTF (ET) molecule crystal, which leads us to associate the peak with a 2 eV intramolecular electronic excitation in the ET and the shoulder with vibronic sidebands. The temperature dependence of the PL spectra of doped and undoped crystals is different, and (ET)2KHg(SCN)4 PL is insensitive to the antiferromagnetic (AFM) phase transition that occurs at 8 K. (orig.)