Spin-Orbit Coupling Drives Femtosecond Nonadiabatic Dynamics in a Transition Metal Compound

Transient absorption measurements conducted using broadband, 6 fs laser pulses reveal unexpected femtosecond dynamics in the [IrBr6](2-) model system. Vibrational spectra and the X-ray crystal structure indicate that these dynamics are not induced by a Jahn Teller distortion, a type of conical intersection typically associated with the spectral features of transition metal compounds. Two-dimensional electronic spectra of [IrBr6](2-) contain 23 cross peaks, which necessarily arise from spin orbit coupling. 'Real-valued 2D spectra support a spectroscopic basis where strong nonadiabatic coupling, ascribed to multiple conical intersections, mediates rapid energy relaxation to the lowest energy excited state. Subsequent analysis gives rise to a more generalized description of a 'conical intersection as a degeneracy between two adiabatic states having the same total angular momentum.