6 - The excitation of fine structure transitions

Summary

Introduction

An important cooling process in interstellar clouds is the excitation of fine structure transitions in abundant atoms and ions, followed by radiative decay. The relevant transitions are those between the fine structure components of ground terms, such as C⁰ 2p^{2 3}P, O⁰ 2p^{4 3}P and C⁺ 2p ²P^o. By term is meant the LS-coupling state denoted by ^2S+1L, where L is the total electronic orbital angular momentum quantum number and S is the total electronic spin angular momentum quantum number. Only the outer (valence) electrons (e.g. 2p²) need to be listed, as the inner shells and sub-shells are closed and have zero resultant angular momenta. Spectrosopic notation is used to denote the orbital angular momentum: ‘s’ for l = 0, ‘p’ for l = 1, ‘d’ for l = 2, …, with upper case letters indicating resultant angular momenta (vector sums of the contributions of the individual valence electrons). Departures from LS-coupling, owing to the spin—orbit interaction, result in the states with different values of J, the total electronic angular momentum (the vector sum of the orbital and spin angular momenta) having slightly different energies; these fine structure states are denoted ^2S+1L_J. Thus the ground ³P term of C⁰ and O⁰ is a triplet, comprising the three fine structure components with J = 0, 1, 2, and the ²P^o ground term of C⁺ is a doublet with J = 1/2, 3/2.