Nonthermal Effect of Microwave Irradiation on the Molecular Level: Emergence of Coherent Subterahertz Vibrations of Hydration Water in Reverse Micelles

Abstract

It is believed that the effect of microwave irradiation on samples has a thermal origin. Nevertheless, the nonthermal effect is often proposed and remains controversial because of incomplete experimental results such as the lack of local temperature around reactant molecules. We have conducted a new method of in situ terahertz (THz) time domain spectroscopy under microwave (MW) irradiation at 2.45 GHz for water in nanoscale reverse micelles, allowing one to determine the temperature of this water and its low-frequency vibrations simultaneously. Comparison with the results of temperature-dependent THz spectroscopy without MW irradiation has demonstrated that the water temperature in the reverse micelle is almost the same as the sample’s value measured by a thermographic camera under MW irradiation. In contrast, distinct sub-THz vibrational peaks of hydration water emerge nonthermally. Our findings provide a new aspect for understanding MW effects on the molecular level and may have biological implications.