Microwave spectroscopy, as the name implies, is spectroscopy carried out using electromagnetic radiation in the microwave region of the spectrum (0.3 to 300 GHz). The energies associated with microwaves are relatively low, and these energies are typically a good match for energy-level spacing between the different rotational levels of molecules. Thus, microwave spectroscopy is typically used to study the rotational energy levels of molecules. The rotational energy levels of molecules are typically studied in the gas phase.