This lab is intended to further your appreciation of the differences between atomic and molecular spectra and to give you some experience with technologies for recording such spectra that is more advanced than the prism spectrometer used in SPECTROSCOPY I. Although you will still see the prism-based instrument in action, this time around it is fitted with a camera that converts it into a recording (film) spectrograph. You will not be required to use the prism spectrograph instrument, though you are certainly free to give it a try if you wish. You will be asked to make film recordings of various atomic spectra and the spectrum of molecular nitrogen, as well as the Fraunhofer spectrum of Sol, using the large grating-based spectrograph located in Room 1 Culler.  (NOTE:  Each lab group will make separate film recordings of different sets of spectra and will collectively share the resulting spectrograms).  Also, you will be asked to be quite a bit more quantitative here in that the results will be, eventually, in the form of plots of intensity vs wavelength. The primary quantitative goals here will be to investigate the isotope shift in spectra emanating from a hydrogen/deuterium discharge lamp, and to identify the origins of some of the more prominent lines in the solar spectrum.

This lab will be considerably more complex than was SPECTROSCOPY I. For example, you will have to cut the film to size, mount it in the plate holder, transport it to the spectrograph in Rm 1, expose it, develop it, process the spectra that are recorded on the film using a travelling microscope and a device known as a micro-densitometer and, finally, analyze the spectra. However, that rather long process will afford you the opportunity to: 1) gain some experience with the technology involved, 2) obtain high-resolution recordings of a number of interesting atomic spectra that will be discussed in lecture, 3) measure some interesting properties of spectra from different isotopes of the same element (hydrogen), & 4) develop a better qualitative appreciation of the complexity (rotational, vibrational and electronic structure) of "busy molecular spectra" which will be dealt with from comprehensive theoretical and quantitative experimental points of view much later in this course.
 

A 1.5 m spectrometer is rather imposing in size and massive in construction. The reason is that the limiting resolution is 0.02-0.03 nm (using the Rayleigh criterion) when used in first order. By the way, that resolution is available with the particular spectrograph that you will use and would be at least an order of magnitude better in a state-of-the-art instrument of the same size. Once again we are using an antique that is older than most all Miami students (and many of the junior faculty as well). A resolution of 0.02 nm means that transition wavelengths separated by that amount can be considered to be far enough apart for accurate spectroscopic identification. As you will see, high resolution grating spectrometers must necessarily be quite large in size.

The basic layout of the spectrograph we will use is shown in Figure 1. The entrance slit, s1, is variable in width (via micrometer adjustment) and will perform the same function as the slit on the prism spectrometer of SPECTROSCOPY I. Slit height is also adjustable. A light source placed in front of this slit will have some portion of its light output enter the spectrograph. The light that is incident on mirror M1 will be reflected and focused upon the diffraction grating G-1. (When the grating is at the position indicated by G-0 the spectrometer will transmit the zeroth order.) The grating is rotatable simply by turning a knob on the side of the spectrometer. The diffraction grating will act to disperse the incident light. Though the mechanism is very different, the effect of the dispersion is much the same as with the Pellin-Broca prism used in SPECTROSCOPY I. The grating is a reflection type which means that, in addition to dispersing the incident light into its constituent wavelengths, it will also reflect the dispersed light onto mirror M2. The optics of the spectrograph are designed such that the light reflected from the second mirror is focused on a flat plane. It is obvious that the photographic emulsion on the recording film must coincide precisely with the position of the focal plane if the instrument is to achieve its limiting resolution. This will not be possible with our spectrograph due to the rather crude (homemade) film holder that must be used. It will nevertheless be sufficient for our purposes.

The heart of the instrument is the diffraction grating. Diffraction grating details will be handled by a handout as they are only a means to our end. A brief description of what a diffraction grating is and how they work is as follows: The grating used in our spectrometer is, again, a reflection type. It consists basically of an extremely flat block of glass upon which a coating that is highly reflective over a very broad band of wavelengths (190-1100 nm) has been deposited (aluminum in our case). Although there exists other methods of grating manufacture, the grating you will use was prepared by etching a series of very closely spaced, parallel grooves (1180 grooves/mm) into the reflective coating using a fine-pointed tool. A top view of a grating appears in figure 2a which depicts light incident from the top-right, and the dispersed light (several orders) reflected towards the lower right. The mathematical relation that "explains" the dispersion characteristics of a grating is most simply written as,

where m is the spectrum order,  represents wavelength,  is the angle through which the "m^th" order spectrum will be deviated, and a is the distance between grooves. This grating equation is clearly wavelength dependent and implies, except for the uninteresting zeroth order, that each individual wavelength of input light will exit (reflect is really not a good word) the grating at a different angle. The greater the distance at which you view this diffracted light, the greater the separation between adjacent wavelengths, i.e. the higher the resolution. (Thus the need for large size for high resolution grating spectrometers and spectrographs.) The optical path length between grating and focal plane for our instrument is 1.5 m. You should also note that observing the spectrum of a source in second order (accomplished by rotating the grating through a larger angle while viewing at the some focal plane) will double the resolution, and so on for higher orders. Viewing in higher order will in general result in a smaller transmitted signal, but there are ways around that difficulty too (blaze!).

What is presented above is an extremely condensed version of the optics of diffraction grating spectrographs. It is nevertheless sufficient for our purposes at this time. We will encounter several other versions of grating-based spectroscopy instruments later on. In the meantime, feel free to explore the interior of the spectrometer.

- CAUTION -

-Do not touch any of the optical surfaces (mirrors, grating). Doing so will seriously degrade their quality.
 

You should record the spectra of a number of atomic sources (details to be supplied later) and the spectrum of molecular nitrogen. The same - CAUTIONS - stated in the SPECTROSCOPY I handout should be kept in mind when dealing with the light sources. An outline of the general procedure that you should follow is given below:
 

PART 1: SPECTROMETER PREPARATION

1. Set slit height to approximately 0.5 cm.

2. With the film holder in place and the mercury source in front of the entrance slit, rotate the grating such that most all the visible spectrum can be seen through the open door of the film holder. Arrange it so that the violet lines are on the far right side of the open door.

3. Note the settings for film holder height that will place the mercury spectrum at the top and the bottom of the open door. You will want to record several spectra on a single piece of film so prior knowledge of the film holder height setting will be important.

4. Take the film holder to the darkroom to prepare the film.
 

PART 2: FILM PREPARATION (DARKROOM!!!)

The film must be cut to the proper size to fit the film holder. This must be done in total darkness. This will constitute something of a problem to inexperienced darkroom users. Therefore, familiarize yourself thoroughly with the layout of the darkroom prior to turning off the lights, as well as the helpful hints provided by your instructor. It will not be possible to turn on the lights to check your notes!!!

1. Arrange the film cutter, film bag, film box, film holder and film template in a convenient order within the dark room and memorize their locations.

2. Remove the film box tops and place them where you will be able to find them and replace them with a minimum of searching in the dark!!

3. Remove the film envelope from the box, take out a piece of film and return the film envelope to the box.

4. Find the edge of the film that has a series of notches cut into it. When the notches are in the upper right hand corner the photographic emulsion is facing away from you. You will eventually want to place the emulsion side in the film holder such that the emulsion side faces mirror M2 in the spectrometer (see figure 1).

5. With the aid of the pre-cut template and using any procedure you find safe, cut a piece of film and place it in the holder with the emulsion side away from the film holder "door." Cut the edge opposite the notches so that the next person cutting a piece of film can ascertain which is the emulsion side. Note: If you end up not getting the emulsion side toward M2, all is not lost. It just means that you cannot record any wavelengths that the plastic film will not transmit. That's no real problem for visible wavelengths, but it may affect the spectral response of the film.

6. Return any unused portion of film to the film box and put it away.

7. Place the film holder (with film securely mounted) in the film bag and either zip both zippers shut or otherwise ensure that no light falls upon the film (e.g. "strangle the bag").

8. Turn on the lights and take the film bag to the spectrograph in Rm. 1.
 
 

PART 3: EXPOSING THE FILM - THE TRICKY PART!!!

1. Place the film bag inside the spectrometer and zip up the cover. Be sure the spectrometer's entrance slit is covered.

2. Reach into the spectrometer through the hand holes, remove the film holder from the film bag and mount it on the spectrometer being careful that it seats properly. Set film holder height so that you will expose the top portion of the film. (Setting noted earlier). Close the spectrometer door.

3. Set the slit width (setting to be provided by your instructor).

4. Place a source directly in front of the slit and as close to the slit as possible. Uncover the slit to expose the film. Exposure times will be given later.

5. Move the film holder height to allow access to an un-exposed portion of the film.

6. Uncover the entrance slit and record the spectrum of the next source.

7. Move the film holder to a new setting, set up the next source and, and so on, and so on...

8. When the spectra of all the sources for which your lab group is responsible have been recorded, remove the film from the holder, place it inside the film bag, zip open the spectrometer cover and move back the darkroom. (NOTE:  You can remove the entire film holder if you so choose before returning to the darkroom).
 

PART 4: DEVELOPING THE FILM

1. Familiarize yourself with the locations of the timer, the developing baths and the sink, fill one side of the sink with a reasonable depth of tap water & Turn the lights off!!!

2. Remove the film (or film holder plus film) from the bag

3. Place the film in the developer bath for the time noted on the index card in front of it. Agitate periodically.

4. When timer buzzes, remove the film from the developer bath and shake the excess liquid off the film (over the sink!!!).

5. Place the film in the stop bath (middle bath) for the indicated time. Agitate constantly.

6. Repeat 4 & 5 for the third bath (fixer solution) for the indicated time, turn on the lights and wash the film for the indicated time in a pool of water in the sink. Admire your work, or try to figure out what went wrong and repeat the relevant portions of the procedure above.

Dealing with the microdensitometer and analysis of your spectra will be discussed in a following handout.