Atomic emission spectra experiment Contents 1 Overview 1 2 Equipment 1 3 Measuring the grating spacing using the sodium D-lines 4 4 Measurement of hydrogen lines and the Rydberg Constant 5 5 Measurement of mercury lines 6 6 Measurement of helium lines 6 A Review of Interference and Diffraction 6 1 Overview In this experiment, we will use a grating spectrometer to measure the emission spectrum of hydrogen, mercury and helium gas. We will measure the Rydberg constant, and identify as many atomic transitions as possible. Remember that you will be graded on the clarity and completeness of your report, the accuracy of your experimental data and calculations, and the insightfulness of your conclusions. 2 Equipment 1. Spectrometer (Sargent-Welch, Model S-75903-80) 2. Diffraction grating (6000 lines per centimeter, black block says “down” on one side) 3. Hydrogen, mercury and helium discharge tubes 4. Discharge tube power supply (5kV, 10 mA) 5. High intensity mercury and sodium discharge lamp 1 Figure 1: Clockwise from left: spectrometer, discharge tube power supply with hydrogen tube, spare tube box, spectrometer light shields, and diffraction grating. 2 Spectrometer components The collimator is an optical device used to direct a narrow beam of parallel light at the prism (or grating). It is rigidly attached to the base and is factory adjusted to have its optical axis perpendicular to the central vertical axis of the instrument. At the front of the collimator is an adjustable slit designed to operate smoothly and retain good parallelism of jaws at all slit openings. The telescope is the optical instrument used to view the emergent light. It is rigidly attached to a bracket supported on the central shaft in such a way that its optical axis is always at the same plane as the optical axis of the collimator, yet it can be rotated about the axis of the prism table and clamped independently. It contains a focusing eyepiece and a built in cross hair. The eyepiece tube is provided with a small opening into which light may be passed to illuminate the cross-hairs. This is particularly useful for viewing faint spectra when the background setting is too dark to make the cross-hair setting precise. The cross-hairs can be illuminated through a small hole above the eyepiece. The prism table is used to hold the prism or grating in alignment with the telescope and collimator. It can be rotated through 360 degrees or locked in position with the locking screw beneath the table. Its top surface is parallel to the optical axis of the telescope and collimator. A vertical post and spring hold the prism or grating rigidly on the table. An especially designed light shield is provided to eliminate stray light. The shield consists of an inner sleeve and an outer hood. The degree scale and the angstrom scales are mounted on the base. The outermost scale is the master angstrom scale. It is rigidly attached to the base and is immovable. The outer scale on the rotatable center dial is the vernier for the master angstrom scale. One side of the vernier is labeled for use with the prism while the other side is labeled for use with the grating. The innermost scale is the degree scale. Spectrometer setup The spectrometer is a precision instrument and rather expensive; please treat it with care. Note that the movable parts that can rotate, such as the grating table and the telescope arm, have clamping screws. DO NOT FORCIBLY MOVE ONE OF THESE PARTS WITH THE CLAMP SCREW TIGHT. IF ANY PART THAT YOU WANT TO TURN RESISTS MOVEMENT, ASK THE INSTRUCTOR FOR HELP. 1. Adjust the telescope for parallel rays (a) Point the telescope at some distant object through an open window or at the far wall of a large room (b) Focus the eyepiece on the cross hairs in the telescope by first withdrawing the eyepiece and then slowly pushing it inward with a slight turning motion until the cross hairs appear in sharp focus. 3 (c) Move the telescope sleeve out or in until the object selected is in clear focus (d) Test for parallax between cross hairs and distant object by moving the head slightly to left and right while sighting through the telescope. If any relative motion appears between the two, repeat the previous two steps until no par- allax can be observed. The telescope is now in proper adjustment and should remain so. However it is good practice to recheck the adjustment periodically by repeating the above procedure. 2. Adjust the collimator for parallel rays (a) Having first adjusted the telescope, rotate the telescope arm until the telescope and collimator tubes are aligned end to end. (b) Direct the instrument toward a light wall or white paper. Open the adjustable slit wide to admit plenty of light. (c) Sight on the slit through the telescope and adjust the slit tube in and out of the collimator tube until the slit edges are in sharp focus and parallax between cross hairs an d slit image has been eliminated. The collimator is not adjusted. In any subsequent applications, if the instrument appears to be out of focus, first readjust the eyepiece to obtain sharp focus on the cross hairs. Then, if the slit is out of focus or parallax exists, repeat all steps in the given order above. 3 Measuring the grating spacing using the sodium D-lines The diffraction grating supplied has a nominal number of lines per inch of 15,000 (or 6000 per centimeter), but this should not be considered precise. One always finds the grating spacing, d, from a known optical wavelength because this is much more accurate than the control of grating manufacture. We will use the bright doublet known as the sodium D-lines, with wavelengths λ1 = 588.996nm and λ2 = 589.593nm. First, make the room as dark as possible. Turn of the overhead lights. Use a small pen light for use in reading your notes and the dials of the spectrometer. This will allow your eyes to remain adjusted to the darkness. Use the sodium lamp as the light source for the spectrometer. After focusing the spectrometer, adjust the slit width until the sodium yellow line is resolved. Adjust the position of the tube until the lines are as brilliant as possible. Record, as shown in Tab.1, the angular settings of the sodium d-line. You should use the vernier scale so as to read the angle to within a tenth of a degree. Also record the order number, m, of each measured yellow line and the angular position of the central maxima. Use this data to compute the grating spacing. If the deviations of the line to the left and the right of the central maximum differ by more than a degree, readjust the orientation of the grating until they are made more nearly 4 order angular m position 2θm θm sin θm m λ mλ d 1 (+) 1 (−) 2 (+) 2 (−) Table 1: Data table used to compute grating spacing. equal. If the diffracted lines to the right of the central maximum are much lower or higher in the field of view than the lines to the left, the table holding the grating is not level. This condition can be corrected by using the leveling screws. What is the diffraction grating spacing? How would your pattern look different if you had used a shorter wavelength light, such as blue light? Would this change your measured values of the slit spacing and slit width? Finally, identify the atomic transition that gives rise to the sodium D-lines. Why is this a doublet (as opposed to, say, a singlet or a triplet.) 4 Measurement of hydrogen lines and the Rydberg Constant Place a hydrogen discharge tube in the power supply and the diffraction grating on the table in the spectrometer. Place the entrance slit of the spectrometer as close as possible to the hydrogen tube without touching. Find the position of the spectrometer for which the hydrogen lines are as brilliant as possible. Record the angular positions, to the left and right of the central image, of all the hydrogen lines that are visible, in both first and second orders. These lines should include a red one, a blue-green one and a violet one. A far-violet one may also be visible, but the far-violet line is very faint and requires good eyes as well as optimum adjustment of the slit width, and reduction of background light, to perceive it at all. These lines are listed in reverse order as seen when going left or right from the central maximum (i.e. you will first see the violet, then blue-green, and then red). Record the angular positions of the lines you can see from both orders as shown in Tab.2 From your data and the grating spacing which you determined from the sodium D-lines in your previous experiment, determine the wavelength of each of the observed hydrogen lines. Plot your values of 1/λ against 1/n2. What shape do you expect for this graph? Does your data fit this? Make use of the Rydberg formula 1 1 1 = R 2 − 2 (1) λ n1 n2 where n1 and n2 are the various n values of the terms, to analyze your graph. From the slope of the graph, deduce the value of the Rydberg constant, R, and compare it with the accepted value. From the intercept, compute the series limit. Compare this to the Balmer 5 color angular 2 order position 2θm θm sin θm m d d sin θm λ 1/λ 1/n 1/n (+) (−) (+) (−) (+) (−) (+) (−) Table 2: Example data table used to compute hydrogen spectrum.