Knowledge Bank

We report high resolution $(0.005cm-1)$ infrared (IR) absorption spectra of doped millimeters-thick parahydrogen $(pH2)$ solids with intentionally elevated residual orthohydrogen $(oH2)$ concentrations. In addition to the weakly-hindered rotor spectrum of isolated $H2O$ molecules, $H2O/oH2/pH2$ samples exhibit a series of sharp $(0.03cm-1$ FWHM) absorption peaks in the 1593 to $1597cm-1$ region which we show are due to $H2O-oH2$ clusters. We have also identified $CO2-oH2$ clusters as the source of the very sharp $(0.007cm-1$ FWHM) fine structure in IR spectra of $CO2/oH2/pH2$ $solids.a$ The gradual evolution of the $CO2-oH2$ features with increasing $oH2$ concentration raises the interesting notion of spectral inhomogeneity as a discrete, countable entity.