Atomic quantities such as oscillator strengths, spectral line
wavelengths, energy levels and ionization potentials depend on the nuclear
charge number Z. Methods of plotting such atomic data along isoelectronic
sequences have been in use for a long time. For example, this
was how Edlén
(1942) showed that the mysterious green coronal line
originates from a
forbidden transition within the ground configuration of Fe XIV. He had the
simple but astute idea of plotting essentially the fourth root of the ground
term splitting 
as a function of Z. The resulting
reduced splittings deduced from laboratory spectra of neutral aluminium and
the first eight ions of the sequence lie on a smooth curve. When extended to
higher values of
Z the curve is seen to coincide with the coronal lines
and 
, assuming they come from Fe XIV and
Ni XVI
respectively.
Here we show how the program OMEUPS developed by
Burgess (see Burgess &
Tully 1992) can be used for exhibiting and spline
fitting atomic data along an
isoelectronic sequence. Instead of inputting the collision strength
as a function of the final collision energy E, we input
A(Z) where A is the quantity under study and Z is the
atomic, or nuclear
charge, number.
An attractive feature of OMEUPS is that it makes use of
interactive graphics.
The program transforms A(Z) to a reduced form 
where
the reduced charge number 
varies from zero when 
to unity when 
. The value assigned to 
determines the
range of elements we wish to include. In general we assume 
,
where N is the number of bound electrons, so that the range includes the
neutral atom and all positive ions.
A parameter C occurs in the definition of 
to provide a useful
degree of flexibility for modifying the plot. By varying the
value of C one
alters the way in which the data points are distributed across the figure.
In many cases 
tends to a finite limit at
which can be determined. This is often helpful when
approximating the data points by a least squares 5 point cubic spline.
The program carries out the spline fitting procedure efficiently
and rapidly.
We compare our method with Edlén's for a fine structure transition in the aluminium sequence (see Fig. 3 (click here)) and give other examples to show the usefulness and versatility of the present approach.
Figure 1: Fluorine sequence: 
, 
, C = 4.4
Preliminary versions of the examples given here were presented at the 5th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas which was held at the Observatoire de Paris (Meudon) from 28 to 31 August 1995.