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This radiation is known as h2 fluorescence the h2

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Unformatted text preview: The following are then possible: • Absorption of a photon with electric field parallel to the internuclear axis can € excite one electron into the lowest σu orbital. This puts the molecule into the B1Σ + electronic state. The associated absorption lines are called the Lyman u band and have a minimum energy of 11.2 eV. € 3 Absorption of a photon with electric field perpendicular to the internuclear axis can excite one electron into the lowest πu orbital. This puts the molecule into the C1Π u electronic state. The associated absorption lines are called the Werner band and have a minimum energy of 12.3 eV. The electronically excited H2 molecules will decay back to the X 1Σ + state, and g € in doing so emit UV radiation. There may also be IR radiation since the molecule generally does not decay back into the ground rotation ­vibration state. This radiation is known as H2 fluorescence. € The H2 molecule may also decay back into an unbound vibrational state. In such a case it is destroyed. The net reaction is: H 2 ( X 1Σ + ) + γ → H 2 ( B1Σ + or C1Π u ) → 2H(1s1/2 ) + γ . g u The rate of this reaction is determined by the frequency ­averaged cross section of the Lyman and Werner lines, the probability p to dissociate the molecule, € and the intensity of the radiation field. This is kUV ~ 4×10−11G0 s−1. In a PDR with G0 ~ 104, an H2 molecule may “live” for only a month before being destroyed. The abundance of H2 relative to H0 is determined by the rate at which H2 can be re ­formed: n (H 2 ) k = . 02 [ n (H )] k UV Thus we see that the radiation intensity G0 (denominator), and the temperature (numerator), as well as the density, determine the balance of atomic € versus molecular hydrogen. In particular, the combination G T1 / 2 γ≡ 0 ne controls the ratio of H2 to...
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This document was uploaded on 03/08/2014 for the course AY 102 at Caltech.

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