Classical theory of ultrafast pump-probe spectroscopy: Applications to I_2 photodissociation in Ar solution

We apply a time-frequency response theory of pump-probe spectroscopy, due to Yan, to the experiments of Zewail and co-workers on I_2 photodissociation in Ar solution. This theory can be implemented with quantum, semiclassical or classical dynamics, thereby allowing the treatment of complex systems such as molecules in solution. The advantage of this theory is that it can take into account the actual light pulses that are used in the experiment, and we can therefore make a separation of those effects due to the physics of the system from those due to the finite temporal and spectral width of the pump and probe excitation pulses. We show here that the classical mechanical formulation of pump-probe dynamics is able to reproduce qualitatively many features of the experimental measurements. This agreement allows us to interpret much of the underlying physics of the pump-probe process in terms of the underlying microscopic dynamics.