For more details, check out our paper in JQSRT.
### The Basics

You can run up to 3 simulations, each with 3 species (for a total of 9), before you must clear the plot.

Absorption simulations of atomic spectra are performed using the NIST Atomic Spectra Database (ASD). Information on NIST’s ASD can be found here

All wavelengths and frequencies correspond to vacuum values.

Unique to SpectraPlot and NIST simulations, we require the user to specify an electronic temperature, T_{elec}, and a kinetic temperature, T, which is used to calculate the number density of the gas and the Doppler width. T_{elec} is used to calculate the electronic partition function, the population in the absorption state, and the transition linestrength. If the gas is in equilibrium, T_{elec} should be set equal to T.

NIST’s ASD does not provide collisional-broadening coefficients for any transitions. However, γ(To) and its temperature exponent, n, can be specified by the user. More information is provided below.

Please don't hesitate to get in touch using the 'Contact' link in the bottom right!

### The Details

Absorbance is defined using Beer's Law: I/I_{o} = exp(-α) = exp(-nσL).

Spectra for ionization levels greater than X^{2+} (e.g., Na^{3+}) are not provided in SpectraPlot, but spectroscopic parameters for ionization states greater than 2+ are available from NIST in most cases.

If γ(T_{o}) and n are specified by the user, γ is calculated at the user-specified temperature and applied to all transitions in the simulation range according to: γ (T) = γ (T_{o})(296/T)^{n} with T_{o} = 296 K. γ(T) is the collisional-broadening coefficient of the mixture and defined according to: Δv_{c}=2Pγ(T) where Δv_{c} is the collisional (i.e., Lorentzian) full-width at half-maximum in units of cm^{-1} and P is the gas pressure in units of atm.

A Voigt lineshape is used when the Lorentzian-to-Doppler width ratio (L/D) is from 0.01 to 100. Gaussian and Lorentzian lineshapes are used when L/D < 0.01 and > 100, respectively.

The electronic partition function, Q_{elec}, is calculated at the user-specified electronic temperature, T_{elec}, according to the energy levels and degeneracies provided by NIST’s ASD. If the degeneracy of a particular state is not given by NIST, an arbitrary degeneracy of 3 is used for that level in the partition sum. At T_{elec} > 10 million Kelvin, Q_{elec} is linearly extrapolated from a look-up table calculated by Team SpectraPlot from 0.1 Kelvin to 100 million Kelvin.

The simulation width (wing_{eval}) for all absorption features varies as a function of pressure. For pressures less than 0.5 atm, wing_{eval} = 2 cm^{-1}. For pressures between 0.5 and 5 atm, wing_{eval} = 10*P cm^{-1}, and for pressures greater than 5 atm, wing_{eval} = 50 cm^{-1}.

Please consult the NIST ASD website for proper bibliographic sources and references.