Dynamics of Hydrogen Bonded Systems
Hole Burning Experiments
Figure 1: Frequency resolved pump-probe data for methanol-d/methanol-h dissolved in CCl4. Note the shift in the signal to lower frequency with increased time delay, Dt. Transient hole burning is a spectrally resolved variant of pump-probe spectroscopy. By exciting an inhomogeneously broadened absorption line with a laser pulse with a narrower spectrum than the absorption line, a hole can be burnt in the ground state population. The position and width of this hole can then be monitored by the probe pulse. By varying the time delay between pump and probe, the spectral relaxation dynamics, often termed spectral diffusion, can be monitored. We conducted hole burning experiments on the inhomogeneously broadened OD stretch band of an isotopically mixed methanol-d/methanol-h solution in order to avoid excitation transfer as a contributor to the spectral relaxation. A hole was burned on the blue side of the d absorption band at approximately 2515cm-1. By monitoring the peak position due to both the hole (ground state bleach) and excited state contributions (stimulated emission) as a function of time, we observed that the signal red shifts with increasing delay between the pump and probe pulses.
Figure 2: Spectral peak maximum of the spectral hole vs. time delay. The green line attempts to fit the data with one timescale and clearly misses the early time data points. The red line denotes a biexponential fit resulting in two shifting times of roughly equal amplitude: ~100 fs and ~1.6 ps. To acquire the spectrum of the hole from the frequency resolved signal, the influence of the probe spectrum must be deconvolved from the signal spectrum. Figure 2 shows the time dependent peak maximum of the spectral hole. The peak shifts on two timescales with a fast component of ~100 fs and a slower component of ~1.6 ps. The time dependence of the peak shift reflects the timescales of solvation within hydrogen bonded MeOD oligomers and thus provides a direct connection to microscopic dynamics in these systems. The data was unsuccessfully fit by assuming a dielectric continuum picture for the bath. This indicates that local fluctuations of the hydrogen bonded oligomer are significant in the solvation response in these systems.
