Microwave Hotspots: Thermal Nonequilibrium Dynamics from the Perspective of Quantum States

The observed microwave effects include thermal effect, superheating or hotspots, and selective heating. These phenomena are almost impossible in classical heating, and the existence of nonthermal effect is still a controversial topic. Hotspot effect is a phenomenon that is often observed in microwave-assisted reaction and is significantly different from the traditional heating reaction. We use the quantum-state specified master equation model of microwave-assisted reaction proposed in 2016 to study the possible mechanism of microwave hotspots. We divide the hotspots into space hotspots and intramolecular hotspots, which correspond to thermal conduction and luminous behavior, respectively. For the model system in the microwave field, the microwave hotspot cannot be generated at a very low temperature of 100 K, and it is possible to generate the microwave hotspot above 300 K. Moreover, the probability of generating the microwave hotspot at 500 K is about 75 times higher than that at 350 K. The appearance of this nonlinear phenomenon is related to the uneven distribution of temperature and microwave intensity in the macroscopic level and directly related to the nonequilibrium behavior caused by microwave absorption in the quantum-state level. It is suggested that microwave hotspots can be induced by heating the given regions in the reaction vessel in advance. In addition, the formation of intramolecular hotspots can also be induced by pre-exciting the local groups in specific molecules.