Raman spectra of rhodium trimers in argon matrices

Resonance Raman spectra of rhodium trimers (Rh-3) in argon matrices have been obtained for the first time. Our samples are prepared by neutralizing a mass-filtered beam of trimer ions. Although the absorption spectrum of our optically thin samples failed to show any obvious transitions, a rich Raman spectrum was found for laser excitation wavelengths between 545 nm and 593 nm. Three fundamentals, corresponding to normal modes of a bent (C-2 upsilon) molecule can be assigned with up to four overtones in the symmetric stretch. Using standard techniques, we obtained for this mode omega (e)=322.4(6) cm(-1), with omega (e)x(e)=0.49(10) cm(-1). The resulting stretching force constant is k(e)=2.10 +/-0.01 mdyn/Angstrom. Assuming a Morse potential governs the symmetric stretching normal coordinate, we obtain an atomization energy of D-e=6.6 +/-1.4 eV. The asymmetric stretch and bend are found at 259 cm(-1) and 247.9(8) cm(-1), respectively. A line at 400 cm(-1) is observed, which does not fit the normal mode scheme of the ground state. This is assigned as the origin of a low lying electronic state. (C) 2000 American Institute of Physics. [S0021-9606(00)00941-7].