An educational perspective on the use of terahertz radiation and rotational spectroscopy of gaseous water

The aim of this work is to describe an educational approach to the rotational spectroscopy of water in the terahertz region of the electromagnetic spectrum. We describe an experimental methodology for gas phase sample preparation, coupling the sample holder with the terahertz spectrophotometer, spectral measurements, and data analysis. The molecular structure of water can be described by the quantum mechanical asymmetric rotor to understand the experimental results. Therefore, we present a detailed tutorial for data analysis, starting from the atomic coordinates of water up to the calculation of spectroscopic transitions. The experimental and theoretical results are in excellent agreement. This activity also allows the discussion of three fundamental aspects of molecular spectroscopy: water spectra in the gas versus liquid phase, the Boltzmann distribution, and the influence of nuclear spin on rotational spectra. In this way, the methodology presented here contributes to a quantitative study of molecular spectroscopy. The comparison between theory and experiment also helps in understanding fundamental concepts of the quantum mechanics of the asymmetric rigid rotor through a perspective that is accessible to students and researchers interested in this field.