This work reports an experimental methodology to study the connection between ultrafast optical Kerr effect spectroscopy and heat transfer and fluid mechanics from femtoseconds to real time. The methodology was applied to hexane and o-dichlorobenzene, two molecular liquids with contrasting thermal diffusion behavior. We show that the ultrafast spectroscopic signals are markedly different under equilibrium (off-resonant) and nonequilibrium (resonant) conditions. By varying the intensity of the resonant pump pulses, we modulate the photothermal effect and show how transient and cumulative heating effects are manifested in the ultrafast signals. We vary the rotation rate of a magnetic stirrer to show how turbulent flow influences the signatures of heating in single-shot ultrafast measurements. We show how the time delay between pump and probe pulses can be used to characterize real-time conductive and convective modes of heat transfer as well as microscopic heat dissipation rates on the femtosecond timescale. The methodology is generally applicable to other molecular liquids and also to transparent liquids containing absorbing chromophores, which have the advantage of allowing control of the amount of heat generated after light absorption across different fluids. • Ultrafast spectroscopy measurements of liquids coupled to heat transfer and fluid mechanics. • Systems studied at equilibrium and in non-equilibrium steady-state thermal gradients. • Ultrafast measurements under heat conduction, convection, and turbulence.
Honorato et al. (Sun,) studied this question.