Fig. 2: Characteristics of the radiative transition of the HBr molecule.

a Schematic energy level diagram depicting a first-order rotational–vibrational overtone absorption transition and the corresponding laser transitions; inset: the vibrational normal mode of HBr. The small, white-filled circle represents the hydrogen atom, and the larger, black-filled circle represents the bromine atom; the horizontal line between them represents the electrostatic force, and the black arrows indicate the relative motions of the atoms. b Calculated absorption spectrum of H⁷⁹Br molecules in the 2 μm spectral region from the HITRAN database⁴³. The rotational lines become closer together with increasing rotational quantum number on the R-branch side, while the P-branch lines become more widely spaced as the rotational quantum number increases. The rotational lines marked with blue arrows represent the chosen pump wavelengths. c Corresponding emission spectrum of H⁷⁹Br molecules in the 4 μm spectral region