RYDBERG TRANSITIONS IN BIS(ETA(6)-BENZENE)CHROMIUM - A STUDY OF ISOTOPIC EFFECTS AND VIBRONIC STRUCTURES

Gas-phase electronic absorption spectra of (eta(6)-C6H6)(2)Cr 1 and (e ta(6)-C6D6)(2)Cr 2 have been measured with 30 cm(-1) resolution. Both spectra show 3d(a(1g)) --> Rnp(e(1u))(n = 4-12) and 3d(a(1g)) --> Rnp( a(2u))(n = 5-8) series. The first ionization potentials, determined as a convergence limit of the Rnp(e(1u)) series, are 5.459 +/- 0.004 and 5.446 +/- 0.004 eV for 1 and 2, respectively. The Rydberg np 0(0)(0) bands display a ca. 100 cm(-1) shift on going from 1 to 2. The decreas es in ionization potential and Rydberg frequencies have vibrational na ture and indicate lower zero-level vibrational energy for bis(eta(6)-b enzene)chromium in the ground electronic state in comparison with that of the molecule in the Rydberg np states and the ground-state cation. A complete assignment of the vibronic components of the Rydberg trans itions in 1 and 2 has been made on the basis of comparison of the spec tra. In addition to vibrational progressions on the totally symmetric metal-ring stretching mode, the R4p(e(1u)) and R5p(e(1u)) transitions show components arising from excitation of CH vibrations. The correspo nding absorption features are shifted towards the 0(0)(0) bands on goi ng from 1 to 2.