Theory of femtosecond time-resolved infrared absorption spectroscopy with application to ligand dissociation in haemoglobin

In this paper we present a theory of femtosecond time-resolved infrared absorption spectroscopy utilizing the theory of nonstationary time-dependent absorption. We treat the pump-probe absorption with nonoverlapping pulses using the stochastic Liouville equation. In this case, we deal only with the linear susceptibility and obtain the generalized linear response theory due to the fact that the system is initially nonstationary because of the femtosecond pulse duration. The adiabatic approximation is introduced to derive the theoretical expressions for the time-resolved IR absorption. Theoretical results are applied to interpret the experimental fs time-resolved IR spectra reported by Hochstrasser and co-workers for the direct observations of ligand dynamics in haemoglobin.