The critical contribution to vibrational dephasing

It is shown that the divergence in the amplitude of the low-frequency density fluctuations which occurs in a liquid as the critical point is approached will also give a divergence in the rate of vibrational dephasing (or vibrational linewidth) observed in that liquid. When the ‘fast-motion’ relaxation regime applies and the lineshape is lorentzian, the critical increase will depend on whether the lifetime of the critical density fluctuations (ϕ-1) is long or short compared to the spectral linewidth (θ). When θϕ-1 < 1 the critical exponent is predicted to be -2γ + v + a (∼ -1·3) and when θϕ-1 > 1 this exponent will increase to -3γ/2 + 2v + a/2 (∼ -0·3). In the limit of extremely slow motion (the ‘rigid lattice’ limit), the critical linewidth anomaly is expected to vanish.