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I. Atomic Physics Experiment (ATM)

II. Pre-Labs must be printed separately.ATM Pre-Lab.PRINT, FILL THIS OUT, Get it Signed by 111-Staff, turn in your Signed Pre-Lab Sheet with your report, to the 111-Lab Staff.

III. Appendix I: ARC Model AM-505 Atmospheric Monochromator

IV. Optics Tutorial

V. Camera Polaroid Camera Currently Not Used

Reprints and other information can be found on the Physics 111 Library Site

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Contents 1 Before The Lab 2 Prerequisite Reading Materials 3 Introduction 4 Preparation: Balmer Series 4.1 Procedure: Balmer Series 4.2 Wavelength Calibration 4.3 Additional Questions: Balmer Series 5 Preparation: Zeeman Effect 5.1 Apparatus: Zeeman Effect 5.2 Procedure: Zeeman Effect 5.3 Acton 0.5 Meter Monochromator

Before The Lab

View the Atomic Theory Video, Balmer Series Video,and the Zeeman Effect Video, discuss the pre-lab questions with an instructor, and have the ATM Pre Lab Questions and Staff Sign Off Sheets.....PRINT, FILL THIS OUT, Turn it in.

Discuss the Physics about this experiment with the faculty or the GSI's in the 111-Lab before starting.

You should keep a laboratory notebook. The notebook should contain a detailed record of everything that was done and how/why it was done, as well as all of the data and analysis, also with plenty of how/why entries. This will aid you when you write your report.

You may find it useful to review principles of optics in the Optics Tutorial and view the Optical Tutorial Video, Energy Level (part 1) Video and Energy Levels (part 2) Video.

Prerequisite Reading Materials

Index to the 111-LAB Reprints: [1]

1. Herzberg, Gerhard. Atomic Spectra & Atomic Structure, Dower Publisher, 1944, New York. Preface pg. 1-257. \#QC451.H43; This is an old text, but gives a good perspective on how the theory developed.

2. Scherberger, R.F. "Ultraviolet Radiation, "Black" and Otherwise", Eastman Organic Chemical Bulletin: vol. 49, No. 2, 1977, pp. 1-2.

3. Harnwell, G.P., and Livingood, J.J., "Ch. 7: Line Spectra", Experimental Atomic Physics, 1st edition, McGraw-Hill, pp.224-288. \#QC173.H38.

4. Jenkins, F.A. and White, H.E. ""Ch. 14: Interference Involving Multiple Reflections", Fundamental of Optics, 4th ed., McGraw-Hill, 1976, pp.286-314. \#QC355.2.J461.

5. Fowles, Grant R. "Ch. 7.5: Quantum Mechanics of the Hydrogen Atom", Introduction to Modern Optics, pp. 235-242. \#QC356.F65

6. Melissinos, Adrian C. "Ch. 7: High-Resolution Spectroscopy", Experiments in Modern Physics cp.1966; pp. 280-327. \#QC33.M4

7. Jenkins, F.A. and White, H.E. "Ch. 32: Magneto-Optics and Electronics", Fundamental of Optics, 4th ed., 1957. Pp. 679-686. \#QC356.J4

8. RCA-Photo-Multiplier-Tube-PMT-Manual

Other References:

1. "Introduction to Atomic Spectra", H. E. White,. This classic text was written by one of our own faculty members, and is still useful after 65 years.

2. "Chapter 4 The Propagation of Light and Chapter 5 Geometric Optics -- Paraxial Theory",Hecht and A. Zajac, Optics.

3. "Atomic Energy Levels and Grotrian Diagrams", Bashkin and J. M. Stoner,

4. " Atomic Energy Levels", C. M. Sitterly, (1949). Easiest place to find exact values of energy levels and transition energies for the red and yellow lines of helium.

5. "An Introduction to Error Analysis", John Taylor, , 2nd Edition Essential for data analysis.

6. "Experiments in Modern Physics", A. C. Melissinos and Jim Napolitan 2nd Edition

7. Spectral Tubes Energy Level data

Introduction

Atomic spectroscopy is the field of physics that was the proving ground for the quantum mechanics of atoms and their energy level structures. In this experiment we observe the atomic spectra of hydrogen, helium, and mercury, and correlate the data with known energy levels.

It is important that you become familiar with light sources, geometrical optics (how to get light into the spectrometer), learn a little about diffraction, actually see what the spectra look like, and record each spectrum photographically and photoelectrically. See Optics Tutorial [[2]] alone or below on this page.

Your report-written or oral-should include enough physics to make the experiment understandable to one of your classmates who has not done the experiment, and enough experimental details to show that you have done all the right things. Include photographs, recorder traces, diagrams, and whatever else is needed for clarity and completeness. Your written report should include answers to all the Pre-Lab questions.

Spectroscopy predates quantum theory, so some of the diagrams you see in early texts are not labeled correctly. Look for simplicity and correctness which you can check with current quantum mechanics books.

We think of an isolated atom as having an energy level structure described by the quantized energies that its outermost or optical electron can assume. When this electron is excited by collisions or by electromagnetic radiation, the atom emits radiation of characteristic wavelengths when the electron returns to its unexcited state. These characteristic wavelengths are called spectral lines, because they appear as sharp lines when the radiation is examined with a spectrometer. We say that when an atom makes a transition between two energy levels it emits a spectral line. In principle we can use quantum mechanics to calculate these energy levels if we can write down the proper Hamiltonian.

When the atom is placed in a magnetic field a new term is added to the Hamiltonian. It is a perturbation which splits each energy level into several levels, called Zeeman levels after a Dutch physicist. The optical electron circulating around the nucleus is like a magnetic dipole in a magnetic field, and the energy added by an external magnetic field is therefore . Each different orientation results in a different energy, and a single level is split into as many levels as there are orientations of the dipole in the field. The splitting energy is much less than the energy difference between the two energy levels which produce a spectrum line. A classical description would have a continuous distribution of orientations, but a quantum mechanical description allows only a finite number of possible orientations. When an atom in an external magnetic field is excited and then radiates energy, a single spectral line becomes several spectral lines spaced very close together, with spacings which are characteristic of the magnetic moment of the electron and the magnetic field strength. This splitting of spectral lines in a magnetic field is called the Zeeman effect. The radiation of these lines is elliptically polarized. When the radiation is viewed perpendicularly to the magnetic field, some lines are linearly polarized parallel to the field direction, and the others perpendicularly to the field direction. When viewed parallel to the field, some lines are right circularly polarized, the others left circularly polarized, and some are missing entirely. Linear and circular polarizations are special cases of elliptical polarization.

From your experience with quantum mechanics you will note that perturbations are only approximately linear, and are not always simply calculable as implied above, especially when the perturbation energy becomes comparable to other energies in the Hamiltonian. We will only work with the linear Zeeman effect here. Also, in the early days of the Zeeman effect, the terms "normal" and "anomalous" were used, but not any longer except in an historical approach in physics texts.

The key to the explanation of the Zeeman effect lies in how the magnetic moment of the atomic electron is related to its mechanical angular momentum. Qualitatively, the magnetic moment is given by , where is the total angular momentum and g is the Lande g-factor which relates the magnetic moment to the angular momentum. The energy levels are then split into as many levels as the quantum mechanical scalar product can have. The product looks like μ₀gm_jB, where is called the Bohr magneton. The symbol m_j represents the magnetic quantum number, and has half-integral values between -j and +j. The extra energy resulting from the magnetic field is ΔE=μ₀gm_jB.

Many atoms, such as helium, have more than one optical electron, in which case the angular momentum symbol is an upper case J, and the quantum number M_J can be integral or half-integral, depending upon whether the number of optical electrons is even or odd.

Preparation: Balmer Series

1. Draw an energy level diagram of hydrogen and show the Balmer series transitions with their corresponding wavelengths and energies. Show what the spectrum ought to look like. You can construct the principal levels from the simple energy expression for hydrogen which you learned in quantum mechanics class, but other texts show more complete diagrams with the angular momentum states designated. References are books by Bashkin & Stoner, Herzberg, and White, and the tabulation by Sitterly (see Prerequisite Reading Materials). Refer to the CRC Handbook for wavelengths and relative intensities. You may need to review spectroscopic notation.
2. Draw an energy level diagram of mercury and show the transitions likely to be observable (yellow, green, blue, and violet lines).
   You could make a copy of all these diagrams out of books, but they have so much detail you would lose sight of only the few lines it is possible to observe in a simple experiment. Keep your diagrams simple.
3. Read carefully the operating instructions for the spectrometer in Appendix I. This is a very expensive piece of equipment, and the instructions are simple enough that there should be no reason for causing damage! Be sure to pay particular attention to the cautions listed. If you have to force something the slightest bit on this piece of equipment, something is not right. Ask someone how to turn a knob if it doesn't want to turn. Also note that our spectrometer has a grating of 300 grooves/mm. Multiply the dial number by 6 to get the correct wavelength.
4. Read about the function and operation of the photomultiplier tube (PMT) in any text on radiation detection and measurement. (See, for example, Knoll Chapter 9 [Physics 111 Library Site] and [RCA PMT Manual]

Procedure: Balmer Series

1. Look over the equipment and locate the spectrometer, light sources, photomultiplier tube (PMT), power supply, and digital multimeter.
   • The spectrometer disperses the light into its various colors and forms an image of the entrance slip, in light of each color in the source. The spectrometer has two sets of optics: one set forms images, and the other disperses the light - spreads it out with each color of light traveling in a different direction from all the others. The dispersing element in our spectrometer is a diffraction grating. The image -forming optics are two concave mirrors. Light from an entrance slit which acts as the source of light is collimated by the first mirror (the slit is at the focal plane of the mirror) and directed toward the grating. The grating disperses the light and reflects it back to the second mirror, which in turn focuses the light on the exit slit at the photomultiplier detector or onto the Polaroid film, depending on which way the plane mirror is positioned.
   • Now be very careful, and take the top off the spectrometer - unscrew the black knobs; they don't come off of the top, but get very loose. Lift off the top. Do not touch or breathe directly on the mirrors or grating - your breath is full of noxious chemicals which stick to glass and aluminum surfaces and make them cloudy. Locate the entrance and exit slits, collimating and camera mirrors, and grating. Observe how the grating turns as the wavelength region is changed. Replace the top and DO NOT over tighten the screws. It may help to draw a sketch to see just where the light is going, and why.
2. Turn on the scope that is set up to monitor the output of the photomultiplier tube (PMT).On the Power Supply make sure that the HIGH VOLTAGE switch is on STDBY, that the POLARITY switch is set to NEGATIVE, and that the voltage is set to 1000 volts. You may have to change this voltage later, but do not exceed 1500 volts! Turn on the Power Supply, and when the STDBY/RESET light comes up, switch the voltage on.
3. Wear the plastic safety goggles for the UV light, although none of the radiation is dangerous.
   • Turn on the mercury lamp and place it about 30 cm from the slit. Open the entrance slit to about 1 mm. Open the shutter by turning the shiny elbow until it points toward the source.
   • Set the spectrometer wavelength to zero (for zeroth order diffraction). Remember that in order to change the wavelength setting manually, you must set the SPEED CONTROL KNOB between any two settings. To do this, pull the SPEED CONTROL KNOB out and then turn it half way between any two settings.
   • Set the diverter mirror to the film holder position "S" (to the right) and look at the diffracted light - put your eye where the polaroid film will be placed. You may have to move your head around in front of the film holder in order to bring the line into view. You should see a blue line at the center of the film holder (change the grating angle if it is not).
   • Position the light source on the spectrometer axis by moving it sideways until the blue line is in the center of the diffraction grating. If there is not enough light to see the grating outline, take off the spectrometer cover.
   • Put up a lens between the source and the slit and focus the light from the source on the slit. The blue line should now be expanded to cover the entire grating. If it is not, ask an instructor to help you. The source is now properly positioned for observing spectra.
   • To see a spectrum line, you must move your eye back away from the film holder until you can focus on the position where the film will be. Or, you can use another lens as a magnifier by placing it between your eye and the film holder. It may help to draw a picture of the optics, to be sure you understand what is happening.
   • Observe the effect of varying the entrance slit width by using the micrometer on the top of the entrance housing, but don't worry about setting it for now.
4. Switch the diverter mirror to the PMT position ("O", to the left), and look at the output of the PMT. The source lamps run on AC, and therefore the light output is modulated at 120 Hz; we can see the output of the PMT modulated as well.
   • You may have to decrease the wavelength setting slightly to get the light into the PMT. What does this tell you about the calibration of the spectrometer? Observe the effects of varying the position of the light source, lens, slit width, and high voltage (not exceeding 1500V). Everything is positioned properly when your output is a maximum, but is not "clipped."
5. Switch the diverter mirror back to the film holder position, and increase the wavelength setting until you see sharp lines in the visible spectrum of mercury corresponding to first order diffraction. The counter should read about 610. Now choose the proper slit width. There is a way to calculate it on the basis of theory, but in practice you simply want narrow yet bright lines. Too large a slit gives bright lines that have a rectangular profile, while too narrow gives the proper line shape (Gaussian) but with reduced intensity. Observe the effects of a slit width that is too narrow or too wide, but end up with a choice that will give you good pictures. Once you have made a choice, you should set the micrometer on the PMT housing to the same width.

1. Use the LabView automatic scan drive and chart recorder to record the mercury spectrum. The output of the PMT should be fed into the two wire sens "HI" and "LO" inputs of the DMM. Be sure that the terminator is attached on the BNC tee at the DMM input. Open the ATM chart recorder VI for Labview. The computer does not control the power to the drive motor on the spectrometer. You will need to enter the initial wavelength counter reading on the spectrometer into the program and select the appropriate scan speed. It is recommended that you start the spectrometer scan a few Angstroms away from your intended starting wavelength. As the spectrometer reaches your wavelength, press the Start button to begin collection data. Before you actually take a recording, look at the output of the PMT on the scope. Remember that your signals should not be clipped, and that your resolution should be good enough to make out the features of the spectrum. Make sure that your signal does not exceed the maximum range for the DMM. Identify the lines with their wavelengths, and determine a correction factor for the dial readings.

Wavelength Calibration

The wavelength scale on the spectrometer is only good to a few nanometers at worst. You can use mercury lines to correct for scale errors. Set up the mercury lamp; get a strong mercury line and adjust the spectrometer until the line is centered in the exit slit; the signal on the oscilloscope will be a max. Record the scale reading, and compare with the wavelengths given below. Repeat for other mercury lines.

When you do the Balmer lines, set each one in turn to a max on the oscilloscope. You can also look at the strip chart recorder graph output on the computer, and calibrate it. This method is probably better for the very weak lines.

Traditionally, wavelengths in air are measured. To use the Balmer series equation you must convert the wavelengths you measure into wavelengths in vacuum. You can do this by using the index of refraction of air, given by

n = 1 + 6432.8 x 10-8 + 2929810/(146 x 108 - σ²) + 25540/(41 x 108 - σ²)

where sigma is the reciprocal of the wavelength in vacuum, called the wavenumber, in units of reciprocal centimeters cm-1.

Another way is to use a table converting air wavelengths to vacuum wavenumbers (?). Table of Wavenumbers, Volume 1, NBS Monograph 3, May 2, 1960.

Think before you do anything, because some errors are so small as to be insignificant.

Mercury wavelengths in air and wavenumbers in vacuum

Red	6149.50	16 256.986
Yellow	5790.66	17 264.401
Yellow	5969.60	17 327.439
Green	5460.74	18 307.479
Blue	4358.33	22 938.156
Blue-violet	4046.56	24 705.376
Helium
Red	6678.15	14 970.074
Yellow	5875.62	17 013.752
Hydrogen
alpha	6562.72	15 233.377
beta	4861.33	20 564.758
gamma	4340.47	23 032.505
delta	4101.74	24 373.020
epsilon	3970.072	25 181.336
	3889.049	25 705.931
	3835.384	26 065.615

1. Set up the hydrogen source. Record the lines in the spectrum (photograph and chart), and measure their wavelengths by using the spectrometer readout calibrated as mentioned above. You should be able to see at least SIX lines in the Balmer series. Check with an instructor if you cannot.
2. Identify lines of the Balmer series and make a plot of 1/? vs. 1/n². Fit the plotted points to a line, determine the Rydberg constant and the series limit, and estimate their errors. Follow the example given in Section 2.9, page 63, of Lyons. Do not neglect this error-analysis treatment of your data.

Additional Questions: Balmer Series

1. What elements in the equipment set upper and lower limits on the wavelengths that can be observed? What are these limits?
2. Calculate the dispersion and resolution of the spectrometer when used in the first order at a wavelength of 546.1 nm (mercury green line). The parameters of the spectrometer are given in the manufacturer's manual. Compare to the dispersion on your photograph, and to the dispersion and resolution on the strip chart record.

Preparation: Zeeman Effect

Let us take a specific example of the Zeeman effect. We will draw an energy level diagram, show the splittings of the levels, show the spectral lines emitted and their polarizations when viewed perpendicular to the magnetic field. Refer to figure 1 on the following page.

At this point we need to get an idea of the magnitude of the Zeeman effect. It is derived from the following: νλ=c energy E=hν=hc/λ = hcσ, where σ =1/λ is called the wavenumber, given in reciprocal centimeters, 1/cm or cm^-1. Because E∝σ, it is an accepted practice to use cm^-1 as the unit of energy. An increment of energy is then Δσ, and the energy of Zeeman splittings is Δσ =gm_jLB. Here L is called the Lorentz unit and is approximately 5 x 10^-5 cm^-1/gauss. One electron volt is approximately 8000 cm^-1. [Look up all the exact numbers before you make any calculations]. For example, if we have a field of one tesla, a g-factor of 1.5, and a J-value of 2, we have a level split into 5 components, each one 0.75 cm-1 from its neighbor. For practice, compute the separations of the Zeeman lines shown in the Figure below. You will see that the line separations are less than the level separations, because it is the difference in g-factors that is important.

The mercury green line at 546.1 nm is an example of such a transition. [after Melissinos, p. 294]

Can we observe the Zeeman lines, in a practical case? Yes, if the lines are separated by more than their widths, and if we have a spectrometer of high enough resolution. A spectrum line has finite width (a frequency spread) because the energy levels are not infinitesimally sharp (natural width) and because the atoms are in thermal motion (doppler width). The doppler width is by far the larger in ordinary light sources, and has an approximate magnitude of 7x10^-7, where T is the absolute temperature and M is the gram atomic weight. For example, the green line mercury at 546.1 nm in a discharge tube at room temperature has a doppler width of 0.016 cm-1. (Run through this calculation yourself-it will give you good practice in juggling units).

Your goals for this lab are to see the Zeeman effect in action and to learn something about the Fabry-Perot interferometer. Experimentally this is one of the easiest labs to perform, but treat it as an opportunity to learn more about the realities of quantum physics. Take the time to do a good job and to understand fully what is going on. And don't leave your calculations and write-up until the last minute just because you think they will be easy; many students err here and receive poor grades on this easy lab just because they are rushed at the end. The same applies to presenting your oral report. There are lots of good quantum questions that can be asked.

Apparatus: Zeeman Effect

1. To observe the Zeeman effect we must put a light source in a magnetic field and observe the radiation with a spectrometer which will resolve the Zeeman lines. We are going to use a discharge tube, an ordinary electromagnet with iron pole pieces, a lens, a Fabry-Perot interferometer, and a telescope, arranged as shown in Figure 2.
   
   
   Fig 2: Apparatus
   
   The lens is present merely to get light into the interferometer; it plays no part in resolving the spectrum lines. The interferometer forms fringes at infinity, as will be discussed below, and the telescope is used to magnify and view these fringes. It is possible to dispense with the telescope if you can focus your eyes at infinity but the fringes will look smaller and be harder to see.
2. The Fabry-Perot interferometer (see Ref. 3). Also look at Video \#27 Optical Instruments.

The Fabry-Perot interferometer works on the principle of multiple amplitude division of a wavefront and recombination of these wavefronts, each of which has traveled a different optical path length. We can represent these wavefronts by rays, as shown in figure 3.

The interferometer itself is nothing but two plane parallel partially-reflecting films. In our case they are multi-layer dielectric coated mirrors supported on two flat quartz discs, called plates, or substrates.

The optical path difference between two adjacent rays is σ = 2tcosθ, and the order of interference n is this path difference divided by the wavelength. When all the rays are added together, the intensity distribution is

is the phase difference between adjacent rays, and R is the reflectance of a single coating. The interference fringes are circles. The smallest wavenumber difference (energy) that can be resolved is given by

There is also a wavenumber interval called the free spectral range, the wavenumber difference in two lines which will give a ring of exactly the same radius (fringes at the same angle theta, but differing in order by 1. It is also approximately the reciprocal of the path difference between adjacent fringes of the same wavelength.

The F-P interferometer used in this experiment is a very precise and expensive piece of equipment. Do not touch its optical surfaces or attempt to clean it-if you think it's dirty, ask the staff for assistance. The only adjustments you need make are are its horizontal and vertical orientations, controlled by the two large graticulated knobs. Its spacing "t" is 8.11 mm, and its reflectance R is 0.90.

Procedure: Zeeman Effect

1. Before you turn on the helium lamp, turn on the cooling air. If the lamp gets too hot it breaks. The air valve is located in 286 LeConte on the east wall next to the BSC parts-it should be adjusted so that the indicator ball is between 40 and 50 on the scale.
2. Make sure that the black POWERSTAT is set to zero, and then turn it on.
3. Then turn on the box marked 'HIGH VOLTAGE'. Increase the voltage on the POWERSTAT until the He tube begins to glow brightly.
4. The Fabry-Perot interferometer is already adjusted. The plates are flat and parallel to a fraction of a wavelength .
5. Set up the optical system with the helium lamp as shown above, but without the red filter, and adjust the position of the lens until you see sharp circular fringes in the telescope. Make sure that the lamp cooling air is on and watch the lamp carefully; if it gets too hot, it will melt.
6. Observe, record, and explain what happens when you do the following, using your eye without the telescope; move the position of the lens; tilt the F-P plates; raise and lower the interferometer. With the telescope in place, change the position of the telescope; shift the telescope sideways.
7. Put in the red glass filter; adjust the system until the fringes are sharp. Turn on the magnetic field. To control it, use the LAMBDA DC POWER SUPPLY. Turn the OUTPUT VOLTAGE VDC knob to zero, and the CURRENT LIMITER IDC knob to 2 Amps. Turn on the power supply. Increase the VDC control until you see the fringes start to split. If the lamp flickers or goes out, increase the voltage on the POWERSTAT. Can you explain why this happens? The lamp is just a tube filled with He gas, and electrons are accelerated through the tube by an electric field from the high voltage. As they travel through the tube, they collide with He atoms, and excite them. When these de-excite, they emit the characteristic He lines. Now what happens when we add a transverse magnetic field? Rotate the polarizer and note the behavior of the lines. Don't be discouraged if you see fuzzy fringes and bizarre behavior. With a little experience your skill will improve until you can produce and see the proper Zeeman effects.
8. Rotate the polarizer until only the sigma components are observed; increase the field strength until the fringes show equal spacings between the rings. What fraction of an order did the magnetic field shift the frequencies of the Zeeman components of the line? \[1/2, 1/3, 1/4, or ? We take one order to be the spacing between two unsplit lines - no magnetic field.\] Repeat several times, so you can make some estimate of the error in your measurements. Measure the magnetic field strength with the gaussmeter. The He discharge tube is mounted on a track so that you can slide it back out of the center region of the magnet and position the meter probe where the tube usually sits.
   • Before taking any readings with the gaussmeter, zero it by using the Zero Gauss Chamber located in the unit, and the Zero Adjust and Differential Zero Balance knobs, and then calibrate it using the 1000 gauss Probe Reference Magnet and the Calibrate and Differential Zero Balance knobs. You may have to repeat both steps several times in order to have the meter both zeroed and calibrated correctly. Make sure that you calibrate the meter on the scale that you will use to make your measurements.
9. Turn the magnet through 90 degrees, and look at the lamp through the hole in the pole piece. What is the effect of removing the Polaroid?
   
   Setup the CCD Camera:
10. After you have the Zeeman apparatus set up, you can get an even better view of the fringes (and take some snapshots for your write-up) by replacing the telescope with a zoom lens and video camera. Attach the camera to the FireWire cable and run Prosilica Viewer on the PC from the start menu. (You will need to turn the computer monitor around 180 degrees to see what you are doing.) Temporarily remove the filter, lens, polarizer, and interferometer and line the camera up on the window in the center of the electromagnet. Then put all of the other optics back in place and align each component vertically with the aid of the camera. Zoom out all the way and set the focus to infinity. You can use the micrometer adjustment knobs on the Fabry-Perot mount to center the fringes in the field of the camera.
    • Select Camera->Controls from the Prosilica Viewer menu. Click the 'Auto Expose' button to have the computer set a reasonable exposure time. If you select the 'Auto' button in the exposure controls, the computer will continually adjust the exposure for a decent picture. (This is often useful, but sometimes produces an annoying flicker. Use manual exposure if the image flickers badly.) You can increase the gain of the camera by selecting the 'ADC' tab of the camera control window. It is best to leave the gain as low as possible since camera noise increases with gain.
    • To take a snapshot, select Camera->Snapshot and then select the snapshot window from the View menu. Save the snapshot as a tiff file in your 'My Documents' folder by selecting File->Save.

Calculations

1. Using your observations, calculate the ratio of energy level splitting to magnetic field, expressed in cm^-1/gauss. Always use these units. While others such as joules/gauss are correct, they are never used by practicing spectroscopists. Compare to the value of the Bohr magneton based on universal constants (m, e, h, etc.). The thickness of the spacer is 8.11 mm, and its reflectance R is 0.90
2. Something more complex and interesting (and yes, a required part of this lab)
3. Use the spectrometer to see what lines are actually present in helium. Set up the system to use the prism spectrometer as a filter to isolate the red, yellow, and blue lines, and observe the splitting with the interferometer.

The trick here is to focus the light from the lamp on the slit of the prism spectrometer and opening the slit up all the way to let in as much light as possible. Then closing it down to see the yellow line in the center of the eyepiece.

Setup using the Prism Spectrometer

Find the levels which give rise to the 587.56 nm lines of helium. Work out the Zeeman pattern thoroughly enough to show you understand the problems. You will need to check out the magnitudes of the triplet splittings. Use the spectrometer together with the interferometer to observe the Zeeman effect in this line, and compare to the calculated pattern. Why don't you see a clear pattern? Can you still see polarization effects? Originally Zeeman did not have enough resolution or magnetic field to see components clearly, but only slight polarization effects in the wings of the line.

APPENDIX I

Acton 0.5 Meter Monochromator

Sections of the Operating Instructions ARC Model AM-505 Atmospheric Monochromator

SECTION III: OPERATION

Bilateral Slit Assemblies Slit Width: The slit width of each bilateral slit assembly is adjustable from 0.005 mm to 3 mm (5 to 3,000 microns), by a micrometer knob located on the slit housing. The knob is graduated in 0.01 millimeter (10 micron) increments. One counterclockwise revolution of the micrometer knob increases the slit width 0.25 mm (250 microns). For maximum reproducibility the slit width should be set in a counterclockwise direction (increasing slit widths) each time it is changed. The micrometer knob should not be rotated below a reading of 0.05 or above a reading of 3.00. A micrometer setting of less than 0.005 mm (5 microns) should not be used, because a stop is provided to prevent the jaws from touching each other. Wavelength Indicator A five-digit mechanical counter is mounted on the side of the instrument housing, and indicates the wavelength at the exit slit in Angstrom units when a 1800 G/mm grating is used. For other grating groove spacings, simply factor the counter reading in an inverse proportion to the change in groove spacing. For example, if a 300 G/mm grating is installed, the counter reading must be multiplied by six to obtain the correct wavelength at the exit slit. [***Note: the grating installed in the monochromator in 111-Lab has 300 G/mm.***]. Manual Scanning Drive A manual scanning knob is located in the side of the instrument housing, adjacent to the Wavelength Indicator. One revolution of this knob changes the wavelength by 20 Angstroms when a 1800 G/mm grating is installed. NOTE: The speed control knob must be disengaged - placed between any two speed positions - before turning the manual scan knob. To change the speed control knob, first pull it out and then turn it. CAUTION: Do not force Manual Scanning Knob. Do not scan below a counter reading of 99990 (equivalent to -10), or above a counter reading of 8000. Synchronous Motor Scanning Drive A synchronous motor with a ten-speed transmission provides scanning speeds from 1 to 1000 Angstroms/minute. The various speeds are controlled by a speed control knob located on the side of the instrument housing. The speed control knob is labeled with the scanning speed directly in Angstroms per minute with a 1800 G/mm grating installed. For other grating groove spacings, factor the speeds in the same manner as the wavelength counter. To set the desired speed knob position, pull the speed control knob out and rotate until the desired position is in line with the dot on the instrument housing; release the knob to engage the transmission. To scan manually, set the speed control knob between any two speed positions. A scan power and direction switch is also located on the side of the instrument housing. The switch is labeled "H" (higher), "OFF" and "L" (lower). To scan to lower wavelengths move the scan switch to the "L" position, and to scan toward higher wavelengths move to the "H" position. Scans should be made to higher wavelengths for maximum reproducibility and accuracy. Moveable Diverter Mirror A moveable diverter mirror directs the beam either straight through, "O" position, to the PMT, or to the side slit in the "S" position. A knob on the top of the instrument indexes the mirror to either the "O" or "S" position. To change the mirror position, gently rotate the knob to the desired "S" or "O" position; a click will be heard and felt when the mirror indexes into position.

• • • FOR FURTHER INFORMATION, SEE THE COMPLETE MANUAL ON THE 111-LAB Library Site at [Monochromator Manual] Physics Library Site online***