Minimal and complete set of descriptors for IR-absorption spectra of liquid H2–D2 mixtures

The IR spectra of liquid hydrogen isotopologues (Q$_{2}$ = H$_{2}$, D$_{2}$, T$_{2}$, HD, HT, DT) are dominated by the interaction induced absorption. Therefore, the complexity tremendously increases with the number of different isotopologues in the sample. As we aim for a system independent calibration of IR absorption spectroscopy against all six isotopologues and three ortho–para ratios, we need a minimal and complete set of descriptors to predict the spectra and to decrease the needed calibration effort. For this, we grouped the absorption lines into three groups: absorption on monomers, phonons, and molecular dimers. In particular, molecular dimers contribute to the absolute number of absorption lines in the spectra of mixed isotopologues. To develop and test the set of descriptors, we make use of three spectra: a pure H$_{2}$ sample, a pure D$_{2}$ sample, and a mixed H$_{2}$–D$_{2}$ sample. We show a detailed analysis of these three spectra in the first and second vibrational branch in the range from 2000 cm$^{-1}$ to 9000 cm$^{-1}$. The set of descriptors found within this work can be used to identify and predict all lines in this range for liquid H$_{2}$–D$_{2}$ mixtures.