8.2.1 Water vapor

The rotational energy level diagram of water is shown on Fig.8.2. Each level is denoted, as usual for asymmetric top molecules, by three numbers J_K-1,K+1. J, which is a ``good'' quantum number, represents the total angular momentum of the molecule; by analogy with symmetric tops, K_-1 and K₊₁ stand for the rotational angular momenta around the axis of least and greatest inertia. Allowed radiative transitions obey the selection rules $\Delta{J}=\pm1, \Delta{K}=\pm1, 3,$ with $K_{-1},K_{+1}: odd,odd\leftrightarrow even,even$ or $o,e\leftrightarrow e,o$. The levels with K_-1 and K₊₁ of the same parity are called para levels, those of opposite parity, ortho levels. Transitions between ortho and para levels are forbidden; due to the presence of two symmetrical hydrogen nuclei, the ortho levels have a nuclear statistical weight 3 times larger than the para levels of same J (see e.g. [Townes & Schawlow 1975]). The arrows in Fig.8.2 show the transitions which will be observed by the satellite FIRST (J.Cernicharo, private communication).

  

Figure: The rotational energy level diagram of water vapor