Effects of Local Liquid Structure on Orientational Relaxation: 2-Ethylnaphthalene, Neat and in Solution

The rotational dynamics of 2-ethylnaphthalene have been studied using subpicosecond transient grating optical Kerr effect techniques. Both the neat liquid and a series of dilutions in carbon tetrachloride were examined, The neat liquid was studied over a temperature range of 2-100 OC, and the 1 M solution was studied from -30 to +25 OC. After ultrafast librational dynamics have decayed, there remains (>2 ps) in the neat liquid a triple-exponential decay of the orientational anisotropy. The slower two decays exhibit hydrodynamic temperature dependences, corresponding to rotational diffusion. The fastest decay is not hydrodynamic, being temperature independent from 2 to 40 OC. This decay is attributed to the relaxation dynamics of local structures in the liquid and is not present in all but the highest concentration ( 5 M) solution.