Emission Spectra

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Background

In order for an emitter to release energy in the form of light that object must lose energy. In an incandescent object this is continuous and due to the radiative cooling of an object. Gasses have characteristic colours which are very finite in their wavelengths. These can be excited by an external voltage and, at breakdown, the gasses are ionised and conduct electricity. Investigating the light of these has given rise to the spectroscopic discipline in science and is of great importance to the fields of chemistry and physics.

You should be familiar with a continuous spectrum and how to project one.
If not try: Spectrum
And Colour for some information on colour.

In order to investigate the nature of the spectra we should consider the simplest atom first.

Hydrogen emission spectrum

The Balmer and Rydberg Equations
In 1885 Balmer came up with a simple empirical formula for predicting the wavelength of any of the visible Hydrogen spectrum. It was only three years later that Rydberg generalised the equation for the *whole Hydrogen Spectrum.

Energy levels.jpg

Rydberg’s equation:

1/λ= RH =((1/n2)2 – (1/n1)2))

RH or R is a constant known as the Rydberg constant= 1.0973731 x 107m. n1 and n2 are integers (whole numbers).n2 has to be greater than n1. In other words, if n1 is, say, 2 then n2 can be any whole number between 3 and infinity. Dropping to n=1 gives the Lyman (UV)spectrum series, n=2 Balmer (mostly visible), n=3 Paschen (IR), n=4 Brackett, n=5 Pfund and n=6 Humphreys series. As the energy the electron releases is smaller for the outer orbits,the energy of the photon is lower and the frequency smaller (longer wavelength).
These "n" values are the primary quantum numbers in the Bohr atom. These can be thought of as the allowable orbits an electron can fit into around a Hydrogen nucleus. It is often easier to visualise these as seperate orbits, although it may be more correct to consider them as a single orbit with different allowable states. One important situation is when an electron is removed completely from the atom. Here "n" is infinite and the atom is "ionised"
This can be shown in the classroom as an analogy with a vibration generator and wire loop (or a loop of thick packing band). Only certain values of frequency will fit in the band so the excitation states of the loop are quantised.For a 30cm diameter loop n= 3,4,5 and 6 can easily be shown. Varying the diameter or stiffness of the loop will effect the nodes shown.

Due to the limitations of assumptions in the Bohr atom the heavier atoms (even Helium) do not follow this formula. It does however allow interpretation of the more complex spectra.

  • The Ionistion of a gas can be shown by the "Frank-Hertz" experiment, often reffered to as the "Critical Potentials" experiment using "Teltron Tubes".

The energy of these levels and their changes are often quoted in eV -electron Volts. 1eV = 1.6x10-19J. This is for historical reasons as it enables calculations to be made easier.

  • Spectral discharges can be easily seen using a Diffraction Grating or a direct vision spectroscope;
    • Spectrum (geisler) Tubes: gas tubes requiring EHT to be supplied. These should be run at their lowest "on voltage" And used for short durations only. As a guide 30 seconds on 30 seconds off. This will significantly increase their working life. These are available as Hydrogen, Helium, Neon, Oxygen, Krypton, Argon, Nitrogen, Carbon Dioxide and water vapour.
    • Spectral Lamps requiring their own "ballasted" supplies. Available as Sodium, Mercury, Helium and Cadmium. These are more robust but may require heating to become fully functional.
    • Nalamp.jpg
      A (yellow) street lamp can act as a sodium light source. High pressure lamps have spectral broadening and lines are not so clear. You should just notice mercury lines as the start mechanism involves mercury.
    • A flourescent tube will show a strong mercury background due to the method it uses to work. The phosphor will give band spectra.
    • Salts of metal compounds in ethanol. These will require a dark room and a trustworthy class. It may be better to demonstrate and not to observe through spectroscopes etc.
    • Salts using a traditional flame test.

See also

Absorption Spectra

Showing that exciting atoms in a gas can cause characteristics lines of emission leads to investigation of what happens when light passes through an gas. A quick demo to show the absorption of Sodium can be made using a sodium lamp and lens with some chloride salts in ethanol and some watch glasses. Focussing an image of the sodium lamp onto a screen if a salt solution is placed in the watch glass between the lens and the screen the shadow from a sodium salt flame, when lit, will be significantly darker than, say, a copper or lithium salt.

Note horizontal lines are from a diffuser

When using ethanol based salts be careful with the type of watchglass used (boroscilicate) and place on a heatmat. Ethanol has a blue flame that is often nearly invisible. The watchglass will get hot.

A Spectrometer is a useful tool in Physics. This apparatus is used for accurate angle measurements particularly relating to spectrometry through either diffraction gratings or prisms.


Use In Astronomy

The Characteristic lines of absorption spectra (Fraunhoffer lines) of stars, especially the Sun, can be compared. Our Sun is used as the reference, or base line. From this the nature of the stars can be found and, by noting any wavelength changes, their relative motion can be determined. This is a case of "Doppler Shift" and often referred to as Red Shift as all stars are moving away from each other, and their spectral lines appear to be displaced to the red end of the spectrum.

External links for more detailed explanation

  • [1] wikipedia entry on the Bohr atom- useful mathematics
  • [2] wikipedia entry on H energy spectra
  • [3] electron configuration of atoms
  • [4] Emission Spectra of elements in the periodic table- click to see visible atomic spectra of elements
  • [5] A shockwave version of the Franck-Herz experiment
  • How to build a home made Vision Spectroscope

Some Trivia

The Quantum numbers for electron shells are sharp, principle, diffuse and fine. These were given the names by early spectrometers and have stuck. The nucleus has similar energy considerations for X-ray emissions and these are given letters K, L, M- again history making life complicated!

--D.B.Ferguson 13:36, 11 April 2007 (BST)

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