THE TUNNELING CONTRIBUTIONS TO OPTICAL COHERENCE IN FEMTOSECOND PUMP-PROBE SPECTROSCOPY OF A 3 LEVEL SYSTEM

We present theory and observations to show that the proper description of the interaction of femtosecond light pulses resonant with both tra nsitions of a three level system must consider additional excitation p athways which do not include population creation on the intermediate e lectronic level. This leads to the extra tunneling contributions to th e pump-probe signal around zero delay time when the light pulse is cen tered close to the isosbestic point, i.e., the wavelength where the ab sorption from the lower and higher transitions are equal. We use the d ensity matrix approach in the perturbative Limit to simulate the pump- probe signal under various conditions. In the conventional calculation of transient anisotropy as measured by pump-probe spectroscopy, the a ssumption is made that the system is excited by the pump pulse only. H owever, this is not the case at zero delay time when pump and probe pu lses overlap. The mutual coherence of pump and probe pulses derived fr om the same laser source makes it necessary to include nonsequential c ontributions to the signal. We show that some of these terms can have distinct anisotropic properties and lead to noticeable deviations of t he zero delay anisotropy from the long time value. Effects of the puls e wavelength and the relative orientation of the transition dipole mom ents on the transient anisotropy are examined in detail. Application i s made to our earlier femtosecond pump-probe experiments on the light harvesting pigment proteins of Rhodobacter sphaeroides, whereby we pre sent a more elaborate explanation of the short lived induced absorptio n signal at negative delay times. (C) 1996 American Institute of Physi cs.