Physics 272 Laboratory Experiments

Dr. Greg Severn

Spring 2016

4  Atomic Emission Spectroscopy

4.1  Introduction:

4.2  Experimental Problem to solve:

A part of every experiment is doing the hard work of understanding how the experiment `works'. This involves knowing what the experiment is about, what physics issues are at stake in the experimental test, and how the experiment is contrived (with apparatus on the one hand, and with a theoretical model on the other) to illustrate or bring out those issues so that the doubtful point can become clear. Be ready to answer the question (how does the experiment work?) during various stages of the actual doing of the experiment, and of course, try to have an answer for this when you first enter the lab. Since `resolving power' is such an issue, a central one in the science of spectroscopy, I will include some notes on spectroscopy itself at the end. Bear in mind, that spectroscopy is for us simply an experimental technique. The deeper physics issues have to do with a model of nature that can account for the phenomena captured or observed using spectroscopic techniques. I think you will enjoy playing with a simulation of a gas discharge after you have completed the experiment (and before you write your report:).

Since we have not yet reached the chapters in Moore's text treating atoms and molecules, a quote from Dirac's monograph on Quantum Mechanics might help, especially with questions 6 and 7.

The necessity for a departure from classical mechanics is clearly shown by experimental results. In the first place the forces known in classical electrodynamics are inadequate for the explanation of the remarkable stability of atoms and molecules, which is necessary in order that materials may have any definite physical and chemical properties at all. The introduction of new hypothetical forces will not save the situation, since there exist general principles of classical mechanics, holding for all kinds of forces, leading to results in direct disagreement with observation. For example, if an atomic system has its equilibrium disturbed in any way and is then left alone, it will be set in oscillation and the oscillations will get impressed on the surrounding electromagnetic field, so that their frequencies may be observed with a spectroscope. Now whatever the laws of force governing the equilibrium, one would expect to be able to include the various frequencies in a scheme comprising certain fundamental frequencies and their harmonics. This is not observed to be the case. Instead, there is observed a new and unexpected connexion between the frequencies, called Ritz's Combination Law of Spectroscopy, according to which all the frequencies can be expressed as differences between certain terms, the number of terms being much less than the number of frequencies. This law is quite unintelligible from the classical standpoint.

4.3  Procedure:

1. The fibre optic cable can collect enough light to saturate the detector. Consult the instructor regarding the software controls that will permit desensitizing the detector. Information about this can be found in the OceanView manual which, one hopes, will be on our website by the time of the lab!
2. The spectral lamps emits a considerable amount of radiation in the ultraviolet region. This can cause severe eye damage. Do not stare at the bulbs except through the spectrometer. Avoid doing more than glancing at them otherwise.
3. The potential across the discharge tubes is about 5000 volts. Do not touch the discharge apparatus when the power is on.
4. For a given source of radiation, list the source and find the peaks. Record these in your lab notebook. Record your estimate of the experimental uncertainty.
5. Repeat this for the other sources.
6. Try to resolve the Na doublet near 589nm. At this wavelength, there are two very close lines which differ by only about 0.5 nm. See if you can resolve them. Make a sketch of the data. Certainly this sketch should go in your lab notebook. Should it go in the second section of the lab report? Well, that depends on your abstract!

4.4  Questions:

1. Complete a table of observed lines and calculated wavelengths, including columns for discrepancy and uncertainty. This isn't really a question to complete in the third section of the report so much as the indication of what should go in the `graphs and tables' section of the report. Make sure there is a table caption and that the columns have titles (with units!). Compare these observations with known spectral lines for the atom or molecule in question. Spectral lines for the elements can be found in many places, including the NIST website,

    http://physics.nist.gov/cgi-bin/AtData/main_asd
   
   

   Does your uncertainty explain or account for the discrepancy? What is the principle source of uncertainty do you think?
2. Why is it that one can see all the different colors at once? Is this what one would observe if one observed a single, isolated atom? Explain.
3. Explain in your own words what gives rise to spectral lines, from a quantum mechanical point of view.
4. For the light sources used in these experiments, would the absorption spectra of the element in question have the same lines as the observed emission spectra? Explain.
5. Answer this question even if you couldn't resolve the doublet in Na. What resolving power is necessary to resolve the famous Na doublet? Get the precise numbers from the NIST website. What resolving power does the spectrometer have in 2nd order? Finally, what causes the Na doublet? Sure, there are two close lying energy states which allow transitions to the same lower energy level, but what causes the energy separation in the two close lying states? What story about Quantum Mechanics is told by the doublet?
6. 
7. 

4.5  Notes on spectroscopy and diffraction gratings: