in a deep cryogenic regime

We demonstrate the first cavity-enhanced spectrometer fully operating in a deep cryogenic regime down to 4 K. Not only the sample but the entire cavity, including the mirrors and cavity length actuator, is uniformly cooled down ensuring the thermodynamic equilibrium of the gas sample. The setup is designed in a way that efficiently attenuates both external vibrations and those originating from the cryocooler itself ensuring stable operation of the optical cavity. Thermodynamic equilibrium and high tunability of pressure, temperature as well as high tunability of wavelength by implementation of an optical parametric oscillator, opens the way to a variety of fundamental and practical applications.

We performed accurate Doppler-limited spectroscopy of the S(0) 1-0 line in molecular hydrogen, achieving accuracy at the level of 10^-6 cm^-1. We demonstrate the first optical realization of three SI units in the challenging 4 - 20 K temperature regime: temperature (kelvin), amount of substance per unit volume (mole · meter^-3) and pressure (pascal). We optically measured  a part of the H₂ phase diagram covering over three orders of magnitude in pressure. This considerably improves the accuracy of the previous best p(T) curve. Using optical methods, we also determined the conversion dynamics of the ortho-para H₂ spin isomers with high temporal resolution.