# ADF**15**

## Photon emissivity coefficients

The data sets are collections of photon emissivity coefficients, given the mnemonic PEC. Since the emission in a spectrum line $\epsilon_{i \rightarrow j}$ ( photons cm$^{-3}$ s$^{-1}$) is just the population number density ions (cm$^{-3}$) in the upper state of the transition times the spontaneous emission coefficient for the transition, the emissivity coefficients are obtained by relating the emission, via a collisional–radiative model, to the driving dominant populations. These may be the free electrons and the metastables of the ion itself via excitation, or the metastables of the parent ion, via free electron recombination. They may also be the thermal neutral hydrogen and the the metastables of the parent ion via charge exchange recombination. So the emission separates in general into three parts as \[ \epsilon_{i \rightarrow j} = \sum_{\sigma}{\rm PEC}_{\sigma,i \rightarrow j}^{\left( {\rm e}xc\right)}N_{\rm e} N_\sigma^{z+} + \sum_{\rho}{\rm PEC}_{\rho,i \rightarrow j}^{\left( {\rm rec}\right)}N_{\rm e} N_\sigma^{\left( z+1\right)+} + \sum_{\rho}{\rm PEC}_{\rho,i \rightarrow j}^{\left( CX\right)}N_{{\rm H}}N_\sigma^{\left( z+1\right)+} \] and there are three types of PEC coefficients, and ADF15 is a derived data format with PECs calculated, most completely, from a generalised collisional–radiative model.

An ADF04 data set is the input to the
collisional–radiative calculation and so the scope of processes in that data set
determines whether all of the three types of PEC are present in the resultant derived
ADF15 data set or not. There is an indexed block in the data set for each spectrum
line (and for each metastable in the metastable-resolved case and for each of the three
types in general), with the tabulation an array in electron temperature and electron
density. Sub-directories of ADF15 are according to iso-electronic sequence, with the
most complete available data being for light elements. The ADF15 data sets really go
along with the ADF11 data sets so that once the distribution of dominant ions in a
plasma model is determined using ADF11 in a transport code, all the spectral emission may be predicted along
lines-of-sight using ADF15. See Summers *et al* (2006) for more detail.