Microwave spectroscopy

Spectroscopy is an experimental discipline that studies the emission, scattering and absorption of electromagnetic radiation by atoms or molecules.

Experimental methods in spectroscopy began in the most accessible area of the electromagnetic spectrum, the visible, where the eye could be used as a detector. In 1655, Newton began his experiments on the dispersion of white light using a glass prism. However, it wasn't until around 1860 that Bunsen and Kirchhoff built the prism spectroscope for use as an instrument.

In 1885, Balmer empirically established a mathematical relationship that made it possible to calculate the wavelengths of the visible lines in the spectrum of the hydrogen atom. Thus began the close relationship between experiment and theory in spectroscopy, with experiment providing the results and appropriate theory attempting to explain them and predict results in related experiments.

However, the theory encountered increasing difficulties as long as it was based on Newton's classical mechanics, until Schrödinger's development of quantum mechanics in 1926. By this time, the data from spectroscopic experiments, except those carried out on very simple atoms, exceeded the predictions of the theory, which was limited by the approximations made so that the calculations could succeed.

The first experiment in which microwave frequencies were used to study a molecule was carried out by Cleeton and Williams in 1934. They had built custom magnetron oscillators to carry out a study of the vibrational mode inversion of the molecule NH ₃ .

Microwave spectroscopy is used in the field of physical chemistry to determine the structure of molecules in the gas phase with great precision. The difficulty of using spectra to determine the geometric structure of a molecule increases with its size and complexity. Rotational transition frequencies could already be measured with high precision at that time, but researchers were unable to provide information on the structure of molecules with a precision corresponding to that obtained from experiment. This situation improved with advances in numerical methods. From the 1960s onwards, with the advent of powerful computers enabling approximations to be reduced, theory began to predict spectroscopic properties with an accuracy comparable to that obtained experimentally.

Improvements in electronic instrumentation and vacuum equipment after 1970 led to further improvements in experimental technique. In 1979, Balle and Flygare designed a Fourier Transform Microwave Spectrometer (FTMW). The principle behind this type of spectrometer is to excite molecules with a microwave pulse and measure the...