Collisional excitation and forbidden

Collisional excitation and forbidden - there are additional...

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Collisional excitation and forbidden-line emission . The collisional excitation cross-section for a transition from state 1 to 2 is given by in which the electron's velocity and mass are used. The v 2 factors out the strongest dependence explicitly to make the Ω term better behaved with electron energy, and easier to interpolate. The Ωs require direct quantum calculations; most astronomers take the easy way out and look them up. The rate of collisional excitations between states 1 and 2 per unit time and volume is if Ω(E) is constant over the relevant range. As in the recombination case, a thermal (Maxwellian) distribution of electron velocity will generally be expected; significant deviations are possible if
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Unformatted text preview: there are additional energy sources (beyond photoionization) for the electron population. The quantity in square brackets is often denoted by q 12 . There exists a relation between Ω for a single term and that for a set of closely spaced levels if S=0 or J=0 . This means that certain line ratios are fixed by statistical weights in the atoms - the ratio of photon numbers in the lines is the ratio of statistical weights, and the ratio of line intensities is this value modified by the ratio of frequencies. This accounts for the (nearly) 3:1 ratios in line pairs such as [O III] λ4959,5007 Å, [N II] λ 6548,6583 Å, and [O I] &lambda 6300,6363 Å....
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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