Detection of Nuclear Spin Isomers of Water Molecules by Terahertz Time-Domain Spectroscopy

Nuclear spin isomers of water molecules, i.e., ortho- and para-H<formula><tex>${}_{2}$</tex></formula>O, are quantum states of one of the most important molecules. Presently, our understanding of the water isomer properties, especially in the gas phase, remains insufficient. Scientific and practical applications of water spin isomers require the development of efficient methods of their detection in gaseous samples. In this paper, we apply the method of wideband terahertz time-domain spectroscopy to study rotational absorption spectra of ortho and para water isomers and measure their relative content in the ambient atmosphere. We find the ortho&#x002F;para ratio to be 3.03 &#x00B1; 0.03 close to the expected value of 3 when using the measurement spectral range 0.15&#x2013;1.05&#x00A0;THz.