Bath-induced correlations and relaxation of vibronic dimers

We consider a vibronic dimer bilinearly coupled through its two vibrational monomer modes to two harmonic reservoirs and study, both analytically and numerically, how correlations of the reservoir-induced fluctuations affect dimer relaxation. For reservoirs with fully correlated fluctuations, we derive an exact quantum master equation for the density matrix of the symmetric vibronic dimer. We demonstrate that reservoirs with fully correlated or anticorrelated fluctuations do not allow for complete vibrational relaxation of the dimer due to the existence of decoherence-free subspaces. For reservoirs with partially correlated fluctuations, we establish the existence of three different mechanisms of vibrational relaxation. Weak inter-monomer couplings, as well as predominantly correlated or anticorrelated fluctuations, render two of these mechanisms relatively inefficient, leading to slow decays of the populations and coherences of the dimer density matrix. The analytical results are illustrated and substantiated by numerical studies of the relaxation behavior of photoexcited dimers.