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 part of the electromagnetic spectrum, the visible range, where the eye could be used as a detector. Thus, in 1655, Newton began his experiments on the dispersion of white light using a glass prism. However, it was not until around 1860 that Bunsen and Kirchhoff built the prism spectroscope that could be used as an instrument.

In 1885, Balmer empirically established a mathematical relationship that allows the wavelengths of the visible lines of the hydrogen atom spectrum to be calculated. This marked the beginning of the close relationship between experiment and theory in spectroscopy, with experiment providing the results and an 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 developed quantum mechanics in 1926. At that time, data from spectroscopy experiments, except those performed on very simple atoms, exceeded the predictions of the theory, which was limited by the approximations made to enable the calculations to be completed.

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

Microwave spectroscopy is used in the field of physical chemistry to determine the structure of molecules in the gas phase with high 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 the experiment. This situation improved thanks to advances in numerical methods. Starting in the 1960s, with the advent of powerful computers that reduced approximations, theory began to predict spectroscopic properties with a precision comparable to that obtainable experimentally.

Improvements made to electronic instrumentation and vacuum equipment after 1970 further enhanced experimental techniques. In 1979, Balle and Flygare designed a Fourier transform microwave spectrometer (FTMW). The principle behind this type of spectrometer is to excite molecules using a microwave pulse and measure...