DOI: 10.6084/m9.figshare.1012700.v1
title: Analysis of the 17.6 GHz hydrogen spectrum from Orion A [7, 8]

description: <b>Table 5.</b> Analysis of the 17.6 GHz hydrogen spectrum from Orion A [7, 8]. The given data for each transition are the corrected central profile intensity ('temperature' <em>T<sub>a</sub></em> in mK) and frequency FWHM (\Delta f_{1/2}^{{\rm tot}}) in MHz. Corresponding derived quantities are: the electron temperature (<em>T<sub>e</sub></em>), Doppler FWHM (\Delta f_{1/2}^D) on the assumption that the atomic kinetic temperature is equal to that of the electrons, the Lorentzian FWHM (\Delta f_{1/2}^L), the 'relative intensity' (f\;I_{nn^{\prime} }^{{\rm rel}} /f_0) calculated for the derived value of <em>T<sub>e</sub></em> on the assumption of Saha–Boltzmann equilibrium, the reconstructed line profile 'area' ({\scr A}_{nn^{\prime} }) in mK MHz, the two 'intensity' decrements and the electron density (<em>N<sub>e</sub></em>) deduced from the electron impact broadening theory in [3, 14]. The frequency <em>f</em><sub>0</sub> = 17.992 56 GHz is that of the leading (most intense) member of the series. <strong>Abstract</strong> Since highly excited atoms, which contribute to the radio recombination spectra from Galactic H II regions, possess large polarizabilities, their lifetimes are influenced by ion (proton)–induced dipole collisions. It is shown that, while these ion–radiator collisional processes, if acting alone, would effectively limit the upper principal quantum number attainable for given plasma parameters, their influence is small relative to that of electron impacts within the framework of line broadening theory. The present work suggests that ion–permanent dipole interactions (Hey <em>et al</em> 2004 <em>J. Phys. B: At. Mol. Opt. Phys.</em> <strong>37</strong> 2543) would also be of minor importance in limiting the occupation of highly excited states. On the other hand, the ion–induced dipole collisions are essential for ensuring equipartition of energy between atomic and electron kinetic distributions (Hey <em>et al</em> 1999 <em>J. Phys. B: At. Mol. Opt. Phys.</em> <strong>32</strong> 3555; 2005 <em>J. Phys. B: At. Mol. Opt. Phys.</em> <strong>38</strong> 3517), without which Voigt profile analysis to extract impact broadening widths would not be possible. Electron densities deduced from electron impact broadening of individual lines (Griem 1967 <em>Astrophys. J.</em> <strong>148</strong> 547; Watson 2006 <em>J. Phys. B: At. Mol. Opt. Phys.</em> <strong>39</strong> 1889) may be used to check the significance of the constraints arising from the present analysis. The spectra of Bell <em>et al</em> (2000 <em>Publ. Astron. Soc. Pac.</em> <strong>112</strong> 1236; 2011 <em>Astrophys. Space Sci</em>. <strong>333</strong> 377; 2011 <em>Astrophys. Space Sci</em>. <strong>335</strong> 451) for Orion A and W51 in the vicinity of 6.0 and 17.6 GHz are examined in this context, and also in terms of a possible role of the background ion microfield in reducing the near-elastic contributions to the electron impact broadening below the predictions of theory (Hey 2012 <em>J. Phys. B: At. Mol. Opt. Phys.</em> <strong>45</strong> 065701). These spectra are analysed, subject to the constraint that calculated relative intensities of lines, arising from upper states in collisional–radiative equilibrium, should be consistent with those obtained from Voigt profile analysis. It is shown that the experimental technique yields an excellent temperature diagnostic for the H II regions. On the other hand, strong evidence is not obtained, from those spectra which satisfy the above constraint on intensity, to indicate that the electron impact broadening theory requires substantial correction. The main grounds for attempting a revision of theory to allow for the influence of the ion microfield during the scattering processes on the upper and lower states of each line thus still appear to have a stronger theoretical (Hey 2007 <em>J. Phys. B: At. Mol. Opt. Phys.</em> <strong>40</strong> 4077) than experimental basis.