Cavity-ring-down spectroscopy on the oxygenAband in magnetic fields up to 20 T

Rotationally resolved spectra of the $b{}^1{\Sigma }_g^{+}\mathrm{}(v=0\mathrm{})\mathrm{}\leftarrow X{}^3{\Sigma }_g^{-}\mathrm{}(v=0\mathrm{})\mathrm{}$ band of molecular oxygen are recorded by cavity-ring-down (CRD) spectroscopy in magnetic fields up to 20 T. Measurements are performed in a 3-cm-long cavity, placed in the homogeneous field region inside a Bitter magnet. CRD absorption spectra are measured with linearly and circularly polarized light, leading to different $\Delta M$ selection rules in the molecular transition, thereby aiding in the assignment of the spectra. Dispersion spectra are obtained by recording the rate of polarization rotation, caused by magnetic circular birefringence, using the polarization-dependent CRD detection scheme. Matrix elements for the Hamiltonian and for the transition moment are presented on a Hund’s case a basis in order to calculate the frequencies and intensities of the rotational transitions of the oxygen A band in a magnetic field. All spectral features can be reproduced, even in the highest magnetic fields. The molar magnetic susceptibility of oxygen is calculated as function of the magnetic-field strength and the temperature, and a discussion on the alignment of the oxygen molecules in the magnetic field is given.