Limitations on measuring solvent motion with ultrafast transient hole burning

Abstract

A number of limitations to hole burning in the liquid phase are identified. As spectral diffusion becomes more rapid, a point is reached where the narrowest hole width no longer measures the homogeneous linewidth. Spectral congestion of the solute is also shown to limit the fastest detectable spectral diffusion rate. An optimal pulse length is found which allows observation of the most rapid spectral diffusion. In addition, the coherence effect observed when the pulses overlap is shown to have resonances at both the excited and ground state vibrational spacings. The coherence effect greatly resembles the hole burning spectrum, but contains no dynamical information. Because of these limitations, hole burning may not be observed even when the homogeneous spectrum is well resolved and spectral diffusion is slow. Experimental confirmation is found in the hole burning spectrum of iodine in hexane, which shows no hole burning despite having a narrow homogeneous linewidth.