Looking forward to my visit to Toronto next week I have been reading a couple of beautiful ultrafast spectroscopy papers from a collaboration between the Miller (Toronto) and Nibbering (Berlin) groups. In a 2005 Nature paper, Cowan et al. showed that in pure water excitation of the OH stretch lost its memory, within about 50 fsec, faster than in any other liquid. They suggested that librational motions (restriction rotations due to hydrogen bonding) are key to understanding this.
In a more recent PNAS article, Kraemer et al., measured the temperature dependence from (274 to 340 K) and found how the polarisation anisotropy decay did not vary with temperature whereas the population lifetime increased by about 50 per cent.
The main questions I have are:
Is the population lifetime largely due to intermolecular Forster Resonant Energy Transfer?
What lifetimes do Forster's expressions give?
Can the spectral diffusion and dephasing be largely described by the expressions below together with the spectral density J(omega) from my previous post?
This can be tested by comparing the isotope and temperature dependence of the frequency dependent dielectric constant of bulk water (and particularly the features associated with librations).
The spectral density from the dielectric continuum model mentioned in the previous post is
The real part of the phase of the off-diagonal part of the density matrix (for the nu=0,1 vibrational states) describes quantum decoherence:
The spectral diffusion is described by the time derivative of the imaginary part of the phase of the off-diagonal part of the density matrix: 
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