Water Dimer Rotationally Resolved Millimeter-Wave Spectrum Observation at Room Temperature

Water dimers $({\mathrm{H}}_2\mathrm{O}{)}_2$ are believed to affect Earth’s radiation balance and climate, homogeneous condensation, and atmospheric chemistry. Moreover, the pairwise interaction which binds the dimer appears to be of paramount importance for expounding a complete molecular description of the liquid and solid phases of water. However, there have been no secure, direct observations of water dimers at environmentally relevant temperatures despite decades of studies. We report the first unambiguous observation of the dimer spectrum recorded in equilibrium water vapor at room temperature.

Figure 1

Observed water vapor spectrum after monomer contribution subtraction (broken line), calculated water monomer spectrum (smooth dotted line), and model dimer absorption (smooth solid line). Vertical bars show positions of the $J+1\leftarrow J$, $K=0$, $E_1$-type dimer rotational lines.

Figure 2

The observed dimer spectrum (a). The ab intio calculated [21] dimer spectrum [(b) upper trace]. The water vapor continuum absorption measured in moist nitrogen [28] at atmospheric pressure (c). The calculated collision-induced absorption [32] [(b) lower trace], the absorption is shown multiplied by factor of 1000 to make it visible). All spectra correspond to water vapor at 13 Torr and 296 K.