lecture notes6

This is how the interstellar cyanopolyynes hc3n hc5n

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Unformatted text preview: e most important molecular vibration lines are those of H2. In particular, the H2 1—2 S(1) line at 2.247 μm falls in a wavelength range observable from the ground. It is therefore of great interest to observations of molecular gas (although a source of excitation is needed to see the vibrational lines). C. ELECTRONIC SPECTRA Physics very similar to that for vibrational spectra occurs for electronic spectra. The upper and lower states of the molecule are of different electronic states, and there are no exact vibrational selection rules. The electronic states do however obey the usual selection rules from atomic physics: |ΔΛ|≤1; no Σ+ Σ−; and for homonuclear molecules, the parity (g or u) must change. Every molecule has a very large number of electronic transition lines, since there are many possible combinations of rotation and vibration quantum numbers involved. Interpretation of molecular electronic spectra (e.g. UV absorption by H2) therefore requires detailed modeling to determine which lines will be most important. D. HYPERFINE STRUCTURE Finally, nuclear spins introduce the possibility of hyperfine splittings in non ­ closed ­shell molecules. The classic case is OH, whose ground term symbol is 2Π. Precession of the electron spin around the internuclear axis splits the Σ=1/2 and 3/2 levels (Hund case a) and the Σ=3/2 case has lower energy. The state is doubled so that both + and − parity are present for the lowest angular momentum (J=3/2); it turns out that the − parity has lower energy. Finally, the proton spin couples with the orbital + electron spin angular momentum to give F=1 (lower) or 2 (upper). The + and − parity states of OH are connected by an electric dipole transition that in circumstella...
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This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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