Sources of the infrared radiation generated by the interaction of fast O(3P) with H2O in space

Recent broad band-pass infrared (2.3–3.6 μm) observations of the interaction of the exhaust plumes from space shuttle ram firing dual primary reaction control system (PRCS) engines with atmospheric atomic oxygen, O(3P), are analyzed. The results from previous laboratory and space-based experiments that observed the IR emissions from the high-velocity interaction of O(3P) and H2O are reexamined. Specifically, emission in the 2.7-μm region is reinterpreted in terms of the reactive process O(3P) + H2O → OH(ν) + OH rather than the collisional excitation mechanism O(3P) + H2O → H2O (ν1 and ν3) + O(3P), which was the focus of the previous analyses. A spectral analysis is presented of a previously reported molecular beam experiment that obtained spectrally resolved emission measurements for a relative collision velocity of 8 km/s between O(3P) and H2O and that shows that OH(ν) is the dominant source of emission in the 1 to 5-μm region. The effect of rotational radiative relaxation is discussed, and it is shown that for highly rotationally excited molecules the timescale for this process can be much faster than vibrational radiative decay, thus resulting in much narrower spectral distributions than might ordinarily be expected.