Photon echo measurements in liquids using pulses longer than the electronic dephasing time

Abstract

Liquid dynamics has been studied by various time-domain techniques such as photon echoes [1] and dynamic Stokes shift measurement [2], which utilize electronic absorption as a probe. Attempts to measure electronic dephasing times in solution using the standard photon echo technique have been hampered by the rapidity as well as the non-Markovian nature of the dynamics. Much emphasis has been directed toward employing shorter and shorter pulses. However, photon echoes using short pulses often simply measure the ultrafast break up of the intra-molecular vibrational wavepacket created by the large spectral bandwidth of the pulses. Recently, it has been shown that three pulse stimulated photon echo peak shift (3PEPS) measurements give accurate dynamical information on solute-solvent interaction [1,3]. In this technique, peak shifts are determined precisely by simultaneously measuring signals in the phase matching directions –k1+k2+k3 and k1−k2+k3. The peak shift reflects the ability of the system to rephase after evolving in a population state for time, T. That is, the decrease of 3PEPS mirrors the electronic transition frequency correlation function, M(t). Here we report 3PEPS studies on various polar protic and aprotic solvents using 22 fs and 90 fs pulses. It is shown the 3PEPS with pulses much longer than a typical electronic dephasing time still gives accurate information on ultrafast as well as slow dynamics in liquids. The experimental results are consistent with the numerical simulations including finite pulse duration.