Dynamics of Hydrogen Bonded Systems
Anisotropy Relaxation Dynamics in Methanol Solutions
The decay of polarization anisotropy in liquids can be determined by measuring the probe polarization dependence of the pump-probe signal. By measuring the pump-probe anisotropy decay for both isotopically pure and isotopically mixed solutions of methanol, the rates of vibrational excitation transfer and molecular reorientation can be separately investigated. The experiments were performed on both a 26 mol % methanol-d solution and a 0.8 mol % methanol-d 23 mol % methanol-h in CCl4 solution. In the sample with low methanol-d concentration, vibrational excitation transfer does not occur because of the very large separation between methanol-d molecules, as will be discussed further below. Therefore, any anisotropy decay results from orientational relaxation. The different rates of anisotropy decay between the dilute and concentrated methanol-d samples result from vibrational excitation transfer.
Excitation of the sample by the linearly polarized pump pulse results in a cos2q distribution of vibrationally excited molecules and the corresponding orientational hole in the ground state distribution, where q represents the angle between the transition dipole moment and the pump polarization vector. For excitation of a hydroxyl stretch, the transition dipole moment will be directed along the OD bond. When the probe pulse has the same linear polarization as the pump pulse, reorientation of the hydroxyl group of methanol-d or excitation hopping, which transfers the excitation to another hydroxyl group with a different transition dipole orientation, will result in a diminished signal. For a probe polarization rotated 54.7º from the polarization of the pump, orientational relaxation is eliminated from the signal. The parallel, S||(t), and magic angle, Sma(t), signals can be utilized to determine the time dependent anisotropy,
which has a theoretical maximum value of 0.4 that decays to zero.
The anisotropy decay in the isotopically mixed methanol solution is a bi-exponential characterized by 1.7 ± 0.7 ps and 17 ± 3 ps time constants, as shown in figure 1. The bi-exponential anisotropy decay has been analyzed with a restricted orientational diffusion model that involves fast orientational diffusion within a cone of semi-angle qc, followed by slower, full orientational relaxation. The fast orientational relaxation occurs within a cone semi-angle of qc = 45º ± 5º, with a diffusion coefficient of Dc-1 = 12 ± 5 ps. The slower anisotropy decay results from the full orientational diffusion and occurs with a diffusion coefficient of Dq-1 = 100 ± 20 ps. The anisotropy decay for isotopically pure methanol-d in CCl4, as shown in figure 2, is much faster because of vibrational excitation transfer in addition to the orientational relaxation. The excitation transfer has been successfully analyzed as transition dipole – transition dipole mediated transfer using a theory developed for randomly distributed chromophores.
Figure 1: Anisotropy decay of isotopically mixed methanol-d/methanol-h dissolved in carbon tetrachloride. The multi exponential decay indicates that relatively fast, ~2 ps, orientational relaxation occurs within a restricted orientational range, while the full range orientational diffusion occurs much more slowly, ~20 ps. Figure 2: Anisotropy decay of isotopically pure methanol-d dissolved in carbon tetrachloride. The significantly faster orientational relaxation results from excitation transfer, where orientational randomization occurs via the hopping of the excitation from molecule to molecule. Note the different time axis.
