The next step in your efforts to do film-based spectroscopy is analysis of the spectra that you have recorded. The tedious part is over! The analysis will consist of inspection of the spectra that you have previously recorded, both visually and with the aid of an instrument known as a microdensitometer. Be forewarned that the microdensitometer is a rather aged, complex and cantankerous piece of vacuum-tube-based, electro-mechanical equipment. Be careful with it and be prepared to devote some time to its rather unique personality. It is an item that was obtained from government surplus, i.e. it was someone else's junk. But it does work - after a fashion.
 

The general procedure is to identify the particular spectral feature(s) recorded on a piece of film (i.e. spectrogram) that you wish to analyze, and attach the film recording to the film holder (tape!) in such a way that the feature of interest is near the location of the beam from the light source.  Using visual detection, manually translate (x-y) the spectrogram so that the light source beam ("light slit") is parallel to the spectral feature of interest and just to one side of that feature.  Adjust the light slit height so that it is slightly smaller than the spectral feature's height.  Focus the light slit onto the film as demonstrated to you previously.  Similarly, focus the light collection optics onto the film.  Caution:  These last two focussing adjustments are somewhat interacting, thus a couple of iterations will be required for proper overall focus.  Adjust the width of the light slit so that it is less than that of the spectral feature of interest.  Switch the detection to the photomultiplier tube.  Using manual translation, adjust the sensitivity & zero of the x-y plotter.  Determine the required scan direction and speed and sweep speed of the x-y plotter.  Scan the spectral feature of interest.

PART 1: VISUAL INSPECTION.

1. Along with your lab partner(s) simply hold the film between you and a bright light source and discuss the general differences between the various recorded spectra. The gross differences are readily apparent. Make some notes as to what your impressions of the differences are. Collectively you have recorded the visible/near-UV spectra of: a) a "blackbody" source and a mercury wavelength standard, b) a simple homonuclear, diatomic molecule (nitrogen), & c) the spectra of hydrogen and the alkalis Na, K, Rb & Cs.  Each group should consult the others for recording details such as source(s), slit width(s), exposure time(s), etc., used for each spectrogram. The "blackbody" spectra along with the Hg wavelength standard will provide information relative to the relative spectral response and the wavelength sensitivity limits of the spectroscopic system used.  The molecular spectra should appear considerably more complex than any of atomic sources, and should provide information as to the spectral resolution of the spectroscopic system as a function of entrance slit width.   The hydrogen and alkali spectra will provide us with information relative to some details of atomic structure that we will investigate in-depth both in lecture and in future laboratory investigations.

In your lab write-up you should discuss in general terms, and in a clear and concise way, the basic reasons for the gross differences between the atomic and the molecular spectra. The hydrogen alkali spectra will motivate our class discussion of the Schrodinger's equation solution of the hydrogen atom problem, and the related effects of: a) simultaneous electron spin and orbital motions, b) the breaking of the "l-degneracy" apparent in hydrogen, c) the topic of shielding of the optical electron from the nuclear potential by the inner-shell electrons, d) spectroscopic notation vs. electron configuration, etc...  We'll worry more about the details of molecular spectra muc later, once we have discussed molecular structure and molecular spectroscopic notation in lecture.  For now, we'll just use molecular spectra as a tool for investigating the topic of resolution.

2. Using the travelling microscope (located just to the right of the microdensitometer in the optics lab) again view the various spectral features recorded on film. (The focus knob is located on the barrel of the microscope and a light source is available for illumination). Are there visual differences in the "sharpness" of the various emission lines in your atomic spectra? What would be the cause(s) (other than poor spectrometer or microscope focus) of any "diffuseness" in the emission lines of the atoms? Note the complexity of the nitrogen band spectrum. You should be able to see (especially with the aid of the microscope) a series of rather dark, relatively fuzzy (relative to the atomic lines) lines in the nitrogen spectrum that are accompanied by a fainter series of sharper lines trailing off to one side (or in spectroscopic jargon, degraded to the blue).  Please note and comment upon the differences in the spectra afforded by different entrance slit widths. What is the source of such structure and why does slit width affect the recording of that structure?
 

PART 2: TRANSLATION OF THE SPECTRA RECORDED ON FILM TO INTENSITY
                VS WAVELENGTH PLOTS USING THE MICRODENSITOMETER.

- CAUTION -

Do not attempt to use the microdensitometer on the high speed ranges (i.e. at speeds of 1 mm/min and higher). On high speed the densitometer drive slips, grinds, binds and otherwise does not work very well!!!!!

As mentioned earlier, this device is rather complex and cantankerous. Rather than laboriously describing its various functions and principles of operation in writing, your instructor has simply shown you how to operate the device to obtain plots of intensity vs. wavelength. The plotted spectra should reinforce the impressions you gained from the visual inspection of PART 1. In any event, some things to do and think about are:

1. Produce plots of various atomic lines. Label them according to source (Na, H, Hg, etc..). Are these lines symmetric or asymmetric about the maximum? What would you expect? Remember that these lines are due to allowed electronic transitions between well defined stationary states of the atoms. Why do they not appear as "delta functions," i.e. why do they have any large width? Hints: resolution, Doppler effect etc... What would the Doppler effect have to do with emission of light from atoms? Compare linewidths (i.e. full width at half maximum, FWHM) between the various atomic sources.

2. Produce plots of at least one emission band from the nitrogen source. Note the large peak (perhaps with some finer structure superimposed) and the smaller peaks of varying intensity that accompany them.  Note the effect of slit width on resolution.  Discuss how you could use such information to quantify the resolution of the spectroscopic system.

3)  Produce plots of at least one hydrogen line, and a few doublets from each of the alkali spectra.  We will refer to these in class to help motivate the post-Bohr approach to understanding details of atomic structure and the resulting spectra.  Discuss differences in gross wavelenghts and separations between the lines comprising doublets" in the various alkali spectra.  Can you identify any systematic differences or trends?

4. On a more limited technical note, determine (roughly) the wavelength range of the relative spectral response of the spectroscopic system used to record the spectra for this lab.  Identify individual contributors of the overall spectral response of the spectroscopic system.  Discuss their relative importance.
 

PART 3: LAB REPORT

As mentioned above you are asked to write a report for this lab. A general outline is Title, Purpose (just a few lines), Results and Discussion, and Answers to Questions, e.g. those above. You all know how to write lab reports. Each will be graded relative to the rest of the class. The only guidelines I impose, other than the general outline, are to be complete, clear, and concise. Sketches of energy level diagrams or other such sketches might help clarity and reduce the need for words.

P.S. You don't have to produce microdensitometer plots for each person in your group (unless you really want to!). Make one set and Xerox for additional needed copies.

If you need help with the microdensitometer or plotter, please contact either your instructor or Mr. Rob Klueg (TA for this course).