Spectra of Gas Discharges

Color spectra of elements undergoing electrical discharge excitation.

Click on an link in table to run Java program

This Java program reads a file containing a list of emission line wavelengths and their corresponding strengths then simulates the appearance of the spectrum in a good visual spectroscope.

Hydrogen

Helium

Lithium

Oxygen

Carbon

Nitrogen

Neon

Magnesium

Silicon

Sulfur

Iron

Aluminum

Calcium

Argon

Sodium

Krypton

Xenon

Barium

Strontium

JPEG screen grabs of an applet which computes and plots the spectra in a web browser window. The above images aren't dithered. They may appear so if your display doesn't have enough colors to represent the entire color range. Displays limited to 256 colors or less don't produce acceptable spectra. Try increasing your color resolution to 16 or 24 bits (16 million colors).

Note: This program generates deep 24 bit color plots, therefore you may need to increase the color depth of your system to view subtle details in these spectra.

Warning: There may be a small delay as the Applet loads its element emission line file and computes the spectra...

• Click on an element name in the first column of the table below to launch the spectra viewer
• If you prefer not to run the Applet click corresponding JPG in the next to last column to obtain an image.

The element, wavelength range and line width are all controlled by applet parameter (PARAM) tags in the HTML source for this page. There are other options such as width and height of spectra in pixels and contrast which can also be controlled. There are also options to overlay a continuous blackbody spectrum of varying strength and to limit the wavelength range.
For example here are the parameters for Neon :

<APPLET CODE=discharge.class WIDTH=784 HEIGHT=64>
<PARAM NAME=element VALUE=neon.txt>
<PARAM NAME=startWavelength VALUE=4000>
<PARAM NAME=endWavelength VALUE=7000>
<PARAM NAME=lineWidth VALUE=2.5>
<PARAM NAME=contrast VALUE=10>
<PARAM NAME=continuum VALUE=0.3>
</APPLET>

The simulated gas discharge spectrum is synthesized by assigning each emission line to a gaussian and each point in the spectra is computed as a mathematical sum of all the emission lines.

• Contrast:
  • Range: 1 to 10000 (the upper limit comes from intensity of the weakest line)
  • Default: 1 (maximum of strongest line assigned to maximum intensity, no distortion of line profile)
  • Should be adjusted to 'burn out' the strongest lines and boost the intensity of the weaker lines. Should not be too high or some line blending may occur, also the relative difference between the various emission line strengths is lost. A compromise should be reached.
• Line Width:
  • Range: 0 to 100
  • Default: 3
  • The width of emission lines is user controlled and should be adjusted so that the line profile covers at least one pixel; too small a width causes undersampling to occur and some lines may 'disappear'! However too broad a line will cause blending of lines that are closer together. Again, a compromise must be reached.
• Continuum:
  • Range: 0 to 1
  • Default: 0.3
  • Physics: In many plasma environments some residual broadband background light 'pollutes' the spectrum, either by scattering from an external white source or internal transitions involving the ion continuum. This causes a smooth background to appear as weak 'rainbow' below the level of most of the emission lines. This creates a 'pleasing' colored background which 'fills' in the empty gap between the emission lines. Here is an example of the Continuum parameter set to maximum : Continuum=1 (white light).

This applet was successfully run under the following browsers :

• NetScape Navigator 3.01
• NetScape Communicator 4.01, 4.03, 4.04
• HotJava 1.0
• Microsoft Internet Explorer v3.01, 4.01

An upcoming version of this Applet will include a more graphical user interface for controlling these parameters.

This Applet was created by John Talbot. Source code is available : discharge.java (Currently limited to 200 emission lines, however this limit can easily be removed by changing the source code and recompiling. There are more details on the color encoding subroutine)

You can also download the source file, class file, these HTML pages and all the element data as :
discharge.zip (76 kBytes)

I am also working on several other Java Applets.

Physics Background

There are two basic line broadening mechanisms; instrumental and intrinsic :

• Instrumental Broadening
  • The first is due to finite spectroscopic resolution and can be controlled by the researcher. Often higher resolution can be achieved by larger gratings or coarse gratings operated in higher order or longer path lengths for fourier transform spectrometers.
• Intrinsic Broadening
  • The second is fundamental line broadening which can be caused by at least three factors:
    1. Doppler Broadening
       • Related to special relativity: Motion components of a particle along the line of sight causes a shift in radiation frequency. Since particles generally have a distribution of velocities, this creates a gaussian blurring in the spectral lines.
    2. Lifetime Broadening
       • Quantum mechanical in origin : Allowed transitions have a short lifetime and this translates to some uncertainty in the frequency of atomic oscillators creating a Lorentzian line profile.
    3. Density Broadening
       • Combination of quantum mechanical and electromagnetic effects: Ions are bombarded by transient electric and magnetic fields of high speed electrons zipping nearby, these electric fields split and shift the energy levels. This constant perturbation within the plasma environment depends most strongly on density and causes strong line broadening in higher density plasmas. Temperature, ionization level and the particular quantum transition involved also plays a role.

In most thin plasmas one sees a combination of Doppler and Lorenztian broadening called Voigt profiles. The Lorentzian component affects mostly the low intensity 'wings' of the emission lines so line profiles can be approximated as gaussians, especially considering the dynamic range limitations of computer screens. Most of the time spectra taken by researchers do not fully resolve the intrinsic line profile so the lines are broadened mainly by instrumental imperfection.

The data for these spectra is courtesy of the Astronomical Data Center, and the National Space Science Data Center through the World Data Center A for Rockets and Satellites.

References

• Links to stellar spectra data
• Color spectra of emission lines in planetary nebula [Java]
• Spectra of neutral and singly ionized elements from the Astronomical Data Center (ADC) catalog A6016, by Reader J., Corliss Ch.H. :1981, 'Line Spectra of the Elements', CRC Handbook of Chemistry and Physics; NSRDS-NBS 68

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.