Communication: Probing non-equilibrium vibrational relaxation pathways of highly excited CN stretching modes following ultrafast back-electron transfer

Fifth-order nonlinear visible-infrared spectroscopy is used to probe coherent and incoherent vibrational energy relaxation dynamics of highly excited vibrational modes indirectly populated via ultrafast photoinduced back-electron transfer in a trinuclear cyano-bridged mixed-valence complex. The flow of excess energy deposited into four C N stretching (nu(CN)) modes of the molecule is monitored by performing an IR pump-probe experiment as a function of the photochemical reaction (tau(vis)). Our results provide experimental evidence that the nuclear motions of the molecule are both coherently and incoherently coupled to the electronic charge transfer process. We observe that intramolecular vibrational relaxation dynamics among the highly excited nu(CN) modes change significantly en route to equilibrium. The experiment also measures a 7 cm(-1) shift in the frequency of a similar to 57 cm(-1) oscillation reflecting a modulation of the coupling between the probed high-frequency nu(CN) modes for tau(vis) < 500 fs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731882]