Rydberg constant from atomic spectra of gases

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1 Page 1 of 8 Rydberg constant from atomic spectra of gases Objective - Calibrating a prism spectrometer to convert the scale readings in wavelengths of spectral lines. - Observing the Balmer series of atomic hydrogen spectral lines and measuring the wavelengths. - Calculating the Rydberg constant R H. - Identifying an "unknown" gas by measuring its spectral lines. CALIBRATION The apparatus is essentially a spectrometer plus a variety of spectral "tubes" which are the sources of the line spectra to be studied. Calibrating the spectrometer means determining the relationship between the wavelength of a spectral line and the scale reading on the instrument. Once this relationship is obtained, you can use it to convert the scale reading of an unknown line to the corresponding wavelength. Use the helium lamp and the helium spectral line tables from Appendix 2 to calibrate the spectrometer using one of the methods outlined below. THE SMOOTH CURVE METHOD In this method you draw the simplest of calibration curves. Make a plot of wavelength against scale reading. If you have correctly identified the wavelength of the lines you chose to measure you should obtain a smooth curve. If it is not smooth, go back and check both your measurements and the wavelengths you assigned each line. The advantage of this method is simplicity. The disadvantage is the difficulty in drawing the correct smooth curve between the calibration points which limits the accuracy of your results. THE HARTMANN RELATION METHOD The relationship between the scale reading y and the wavelength λ is given by the Hartmann dispersion relation: m y = + b (1) λ λ 0 where λ0, m and b are constants. For your convenience, the constant λ0, which is a function of the optics of your spectrometer, has been measured for each spectrometer and is marked on the side of the instrument. By plotting y vs. (λ λ0) 1, a straight line can be obtained, and m and b can be estimated. The advantage of this method is that since the calibration curve is a straight line, you need fewer calibration points, it is easier to draw and it is more accurate. The disadvantage is that you must rely on the observations made by someone else.

2 Page 2 of 8 THE RYDBERG CONSTANT Use your calibration curve to measure the wavelengths of the bright lines in the visible spectrum of hydrogen known as the Balmer series. Since hydrogen is a molecule you will see a number of molecular bands in the visible range as well but these will in general be much fainter than the bright atomic lines. In 1885 Balmer showed that the wavelengths of the atomic lines in the visible part of the Hydrogen spectrum satisfied the relation: = R H 2 2 (2) λ 2 n for values of n > 3, where R H is the Rydberg constant. There are four lines that you should see in the hydrogen spectrum that satisfy the Balmer relation. These include: a red line, a blue green line and two violet lines. Measure their wavelengths, verify that equation (2) is satisfied and determine a "best value" for R H. What are the units of this number? Hint: Use Δλ = 3 nm as the uncertainty in λ. GAS IDENTIFICATION SLEUTHING You can now test your skills by measuring the wavelengths of the spectral lines of "unknown" substances contained in the spectral tubes placed near the spectrometers. Identify a couple of "unknowns" by using Appendix 2 which lists the spectral lines of many common substances. Also, the colour of the light given off by the spectral light source often gives a hint to the gas contained. Other calculations Evaluate hcr H (h = Planck's constant, c = velocity of light). This has units of energy. Convert to ev (electron volts) and compare it to the ionization energy of atomic hydrogen. What is the physical significance of hcr H? Hint: the lines of the Lyman series which are in the ultraviolet obey the relation: = R H, n (3) λ 1 n

3 Page 3 of 8 APPENDIX 1 USE OF THE SPECTROMETER DESCRIPTION The tri prism optical spectrometer is capable of resolving to about 0.1 nm (1 Angstrom). The instrument reads the wavelength of the light in arbitrary units. The visible spectrum is contained in 20 of the 50 divisions, with resolution by the vernier to 0.01 divisions. Repeatability is about ±0.02 divisions. A calibration chart is required to convert scale readings to wavelength. Setting of the instrument is accomplished by aligning a cross hair precisely on an image of the spectral line seen in the eyepiece. PRINCIPLE OF OPERATION The light to be analyzed enters the slit, and due to diffraction, it diverges from the slit opening. The slit is situated at the focal point of the collimator lens, so after passing through it, the light is in a parallel beam. It then passes through the tri prism. The central prism is the one which disperses the light. The first and third prisms are employed to straighten the beam and thus make the spectrometer easier to handle. After leaving the prism the dispersed beam passes into the

4 Page 4 of 8 measuring telescope. Real images of the slit are created in the plane of the cross hairs, and these are examined using the eyepiece as a simple magnifier. OPERATING INSTRUCTIONS Slit adjustment and line focus: Use a sodium lamp as source and find the bright yellow double line. Focus the spectrometer optics on the spectral line by moving the eyepiece assembly in or out of the black main frame with the knurled knob. Reduce the slit width until the sodium line resolves into two. (Refocus if necessary). When the line is clearly focused, tighten the focusing clamp. The paint dot on the slit adjustment should be kept on the upper side of the spectrometer to maintain calibration. The spectrometer is now ready for use. When observing spectra the slit may be opened or closed, as desired. Opening the slit will increase the brightness but decrease the resolution. Measure the wavelength of lines by precisely splitting the line with the centre of the cross hairs, and then reading the scale and vernier. Precaution: The vernier has only 50 divisions. Remember to add 50 to the vernier reading when the point is in the second half of a main division. When making observations in the violet it may be necessary to slightly push in the eyepiece in order to keep the cross hairs in focus. APPENDIX 2 L WAVELENGTH TABLES Remarks on the use of the tables 1) Use the intensity indications with caution. They are meant only as a general guide. Relative intensities vary widely depending on the mode of excitation and so the source that you are using may have lines with different intensities. However, the very bright lines should appear bright in all sources. 2) These tables give most of the lines that you will be able to see (and many that you won't be able to see if you are using a narrow slit width). However, the tables are not complete and lack some faint lines. 3) Lines separated by less than one nanometre will not be resolved if the slit is wide. If the slit is too narrow, weak lines won't be seen. 4) You may assume that the errors in these values are negligible in comparison to the other errors in this experiment.

5 Page 5 of 8 HELIUM WAVELENGTH RELATIVE nm INTENSITY COLOUR RED RED RED RED YELLOW GREEN GREEN GREEN GREEN BLUE BLUE VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET

6 Page 6 of VIOLET ARGON KRYPTON WAVELENGTH RELATIVE WAVELENGTH RELATIVE nm INTENSITY COLOUR nm INTENSITY COLOUR GREEN RED GREEN RED GREEN RED GREEN ORANGE GREEN ORANGE GREEN ORANGE GREEN ORANGE GREEN YELLOW BLUE YELLOW BLUE GREEN BLUE GREEN VIOLET GREEN VIOLET GREEN VIOLET GREEN VIOLET GREEN VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET Argon has many faint lines in the red and yellow which vary in intensity depending on the source and because of the confusion that this can lead to only wavelengths less than 580 nm are given. In this region there are a very large number of lines. Only relatively brighter ones are listed. Fainter ones may provide a haze in the background.

7 Page 7 of 8 MERCURY XENON WAVELENGTH RELATIVE WAVELENGTH RELATIVE nm INTENSITY COLOUR nm INTENSITY COLOUR RED RED RED RED RED RED RED RED RED RED RED RED RED RED RED RED ORANGE RED ORANGE ORANGE ORANGE ORANGE YELLOW ORANGE YELLOW ORANGE YELLOW YELLOW YELLOW YELLOW YELLOW YELLOW GREEN YELLOW GREEN YELLOW GREEN GREEN GREEN GREEN GREEN GREEN GREEN GREEN GREEN GREEN GREEN GREEN BLUE GREEN BLUE GREEN VIOLET BLUE VIOLET BLUE VIOLET BLUE VIOLET BLUE VIOLET BLUE VIOLET VIOLET VIOLET VIOLET VIOLET VIOLET

8 Page 8 of 8 NEON NITROGEN WAVELENGTH RELATIVE WAVELENGTH RELATIVE nm INTENSITY COLOUR nm INTENSITY COLOUR RED GREEN RED GREEN RED GREEN RED BLUE RED BLUE RED BLUE RED BLUE RED BLUE RED VIOLET ORANGE VIOLET ORANGE VIOLET ORANGE VIOLET ORANGE VIOLET YELLOW VIOLET YELLOW VIOLET GREEN VIOLET VIOLET VIOLET Many orange and yellow lines have been omitted as well as lines of wavelength less than 540 nm (hundreds). Most of these are faint but some overlap to give the appearance bright lines. Since nitrogen is a molecule, the all spectrum consists of bands rather than lines. This is due to rotation of the molecules. In the visible the most prominent structure is the First Positive of series with about 30 regularly spaced bands in the region nm. Only the band heads of the Second Positive series are tabled above. The bands trail off to shorter wavelengths. As indicated by the relative intensities on a scale of 10, the Second Positive series is less intense than the First Positive series. This exercise was revised in 2014 by Ruxandra M. Serbanescu. Previous version: TK 1998 Revised for EngSci students in September 2016 by N. Krasnopolskaia