The physics of heating by time-dependent fields: microwaves and water revisited

Heating samples by microwave radiation is a particular example of the more general phenomenon where materials absorb energy from an external time-dependent field of an electric, magnetic, or mechanical nature. How this compares with conventional heating is a question of continued interest. Here, we show that the origin of the absorptivity determines whether energy accumulates in the slower configurational degrees of freedom or transfers rapidly to the phonon bath, where only the latter situation is equivalent to conventional heating. Based upon time-resolved measurements of the configurational temperatures, evidence is provided for simple liquids displaying nonthermal behavior if heated by external fields, with molecules being more mobile than expected on the basis of the actual temperature. However, water and related materials are the exception regarding absorptive heating, because energy is transferred to the phonons more rapidly than it is absorbed from the field, and nonthermal effects thus remain absent.