OPTICAL STUDY OF THE NEAR-INFRARED EMISSION AND ABSORPTION IN THE STRONGLY EXCHANGE-COUPLED DIMER CS3MO2CL9

Highly structured near-infrared emission has been observed from single-crystal Cs3Mo2Cl9 for the first time, corresponding to the spin-allowed 3E"(3A21E) --> 3A2"(3A23A2) pair transition. The spectroscopically determined singlet-triplet ground-state exchange splitting is approximately 842 cm-1, in excellent agreement with 840 +/- 50 cm-1 obtained from earlier ground-state magnetic studies. The emission spectrum is dominated by long progressions in the nu(Mo-Mo) stretching mode of 111 cm-1, based on two static origins separated by 50 cm-1. The nu(Mo-Mo) stretching mode is reduced some 31 cm-1 from that of the 1A1'(3A23A2) ground state, indicating significant weakening of metal-metal bonding in the 3A2"(3A23A2) excited state. Band shape analysis results in Huang-Rhys parameters of approximately 2.8 and 3.6 for the two progressions, corresponding to lengthening of the Mo-Mo bond by approximately 0.09 and 0.10 angstrom, respectively. The large zero-field splitting of 50 cm-1 observed for the 3A2"(3A23A2) ground pair state is consistent with calculated values based on a perturbation treatment of the pair configuration in the Mo-Mo sigma-bond limit. A temperature-dependence study of thick single crystals of Cs3Mo2Cl9 indicates that the entire 8000-cm-1 absorption band is due to the 1A1'(3A23A2) --> 3E"(3A21E) pair transition only, and this is consistent with the moderate Mo-Mo pi-interaction observed previously for the double-excitation region between 12 500 and 15 500 cm-1. From the spectroscopic analysis, the A1' + A2', E", and E' spin-orbit levels of the 3E"(3A21E) multiplet were located with A1' + A2' established as the emitting level. At low temperatures, evidence exists for a small departure from strict D3h selection rules.