I am trying to perform KK calculations on transient reflectivity to obtain transient absorption. KK references recommend a method to perform KK on reflectivity signal, in which the phase of the reflectivity can be obtained. If this is the way to go for transient reflectivity as well, how to overcome the problem of having negative differential reflectivity in the calculations? Moreover, KK on pump probe signal is widely described based on the susceptibility. If the later method should be used, how to obtain real susceptibility from pure reflectivity signal, and then obtain imaginary part of the susceptibility using KK calculations?