R. A. Ashby and H. W. Gotthard The New South Woles lnst~tuteo f Technology, Sydney, N.S.W., Australia
The Absorption Spectrum of Sodium Vapor
I
It is common practice in a first course in spectroscopy for undergraduates to study the emission spectra of atoms, particularly hydrogen ( I ) and the alkali metals (2). Such studies introduce the student to the basic concepts of atomic spectra and atomic structure, and underpin further work on the finer details of atomic structure and applications in the analytical chemistry of atomic emission spectroscopy. A typical experiment (2) is the observation and measurement of the wavelength of the visible-near ultraviolet spectral lines emitted by a sodium vapor lamp with consequent description of the spectrum in terms of related series of lines. an enerw - . selection rules. -. level diamam. etc. The first ionization potential can he determined and if desired. the auantum (Rvdhern) defects and the exnlanation of'their ;ariation among sTp, and d states in t&ms of radial probability diagrams. In the teaching situation less attention seems to he paid to the absorption spectra of atomic vapors except in regard to their direct use in the powerful analytical technique of atomic absorption spectrometry. It is true that a more complete picture of the energy level diagram, structure, and behavior of an atom is obtained if hoth the emission and absorption spectrum of that atom is available. With this principle in mind we ensure that our students make observations on the absorption spectrum of an atomic vapor as well as on the emission spectrum of the same vapor. Sodium vapor is a convenient example for study and following is the description of a simple, inexpensive (providing a spectrograph is already available), and informative experiment involving the measurement of its absorption spectrum.
the spectrum throughout the visible and ultraviolet one can extract those parts appropriate to one's spectrographic (spectroscopic) facilities. In our case, light from a hydrogen (uv) or quartz-iodine (visible) continuum lamp, or a sodium vapor lamp, is focussed through a 25-cm absorption cell, containing sodium vapor a t 300-5WSC, onto the slit of a 1.5-m focal length concave grating spectrograph (Bauscb and Lomb, Model 11: dispersion 15 A/mm first order; resolution 40,000 first order). The spectrum is detected photographically or visually, as appropriate, and wavelength calibration, as required, is accomplished using a quartz enveloped mercury lamp.
Construction of the Absorption Cell The sodium vapor absorption cell (Fig. I) can be easily constructed providing a workshop possessing a small lathe and hrazing facilities is available. The cell (25-em path length, 2.5-em id.) is made from a copper tube onto the ends of which stainless steel flanges are hrazed. These flanges accommodate the "0" ring grooves and serew-on brass caps for alignment of the quartz end windows and the application of slight pressure to the Neoprene "0" ring seals. Four turns of %in. 0.d. copper tubing (to provide water cooling far protection of the "0" rings against heating) are wound around the tube a t each end, just behind the steel flanges,
Experimental General Remarks Sodium vapor absorbs in hoth the visible and ultraviolet regions (3) and hence a spectrograph operating in both these regions is required to obtain the full spectrum. However, if a spectrograph operating only in the visible is available a great deal of useful information is still obtainable and one should not be deterred from setting up the experiment. This point is still valid even if observations can only be made by eye (i.e., photographic detection of the spectrum is not available). While the following description assumes the availability of photographic detection of
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Figure 1. The Absorption Cell (sectioned); G, gas inlet/outlet; W, coaling water inlet/outiet; H, heating coil: A, asbestos rope lagging; T, thermole I, insulation: R, retaining cap: S , silica c o ~ p l e ,P, t h e r m ~ C o ~ ppocket: window: 0, Neoprene " 0 ring: M, mica insulation: C, copper cell body. and F, flange.
and brazed in position. T m copper tubes (%in. o.d., &in. long) are let in through the walls of the cell body to allow for evacuation, admission of controlled gaseous atmospheres, etc. in conjunction with a small vacuum line. A further short piece of copper tubing is brazed to the center of the cell to serve as a packet for a thermocouple (Chromel-Alumel). Heating of the cell, to ohtain sodium in the vapor state, is accomplished by a resistance wire wound around the cell body over a thin sheet of mica or asbestos (for electrical insulation) and covered by asbestos rope lagging to some 1-in. thickness. The winding should he capable of delivering up to about 2000 W when coupled to the mains via an appropriate variable out-out transformer. The maximum temoerature ohtainahle from the cell constructed in this n a y is -5W'C wth the rmling water flow m u ndjustcd u, keep the end window* nt 3040°C and t h e o u t l e t water temperatureat -60DC. ~
~
Operation of the Cell To operate the cell a few pieces of sodium metal (-1-2 g) are placed inside, the cell evacuated, and -25 cm of Hg pressure of nitrogen1 admitted. The nitrogen acts as a heat transfer medium within the cell hut, more importantly, it reduces the rate at which sodium is transferred from the hot to the cold zones (particularly the windows) in the cell. In fact, we run the cell for about 2 hr three times per week and the minimum time for removal of sodium condensed on the windows is about 4 wk. However, cleaning of the windows is easily accomplished by removing them, while the cell is being continuously flushed with nitrogen, and washing them in ethanol or isopropanol. Between each run we admit nitrogen to atmospheric pressure to ensure against gmss leakage of airlwater vapor into the cell. We recharge the cell with fresh sodium metal about twice per semester. Observations and Results
Atomic Absorption
Figure 3. The absorption spectrum of sodium vapor in the visible-near ultraviolet. Spectra are shown for absorption cell temperatures between 525 and 27S°C in the visible and, 575 and 325% in the near ultravioiet. The temperature decreases from top to bottom in 50'C intervals. Note the convergence of the atomic lines near 240 nm and the molecular absorption near 6 0 0 . 5 0 0 . 3 3 0 , and 300 nm.
(sodium "D" lines) in the yellow, the rest occurring in the ultraviolet down to -240 nm (transitions PI, Pz, etc. of Fig. 2). In the visible region the absorption spectrum is obtained a t a number of temperatures of the cell; say, 300, 350, 450, and 500°C. At the lower temperatures the doubling of the yellow absorption line is evident hut a t >400"C the ahsorption is so strong that a single broad absorption gap replaces the double line (Fig. 3). The absorption in the yellow can be observed by eye. An interesting series of observations can he made by removing the continuous source and replacing it with a sodium vapor lamp (Philips, Type SOX, 50 w ) . Ohsewations can again he made by eye. With a cold absorption cell (no sodium vapor), the red (5%
With the cell operating a t -500°C (sodium vapor pressure -5 mm of Hg (4)) and using the continuous sources some 2&25 members of the principal series
-
(nZP~i2.,i2 3%, of sodium can he photographed in absorption (Fig. 3), starting with the
"2 Pll*.'h
" 2 Qn/zl,.*h
- GROUND STATE Figure 2 . The Energy Level Diagram of the Na Atom. Transitions P , (yellow). S (red), and D (green) are the most intense of the emission lines in the visible. The series of transitions P,,P2 . . . . P, (the principal series) is the only one seen in absorption.
-
3zp8~2,,,2) yellow
(32p3~2,,,9
-
3%J
and green ( ~ ' Q I ~ ~ I ~
3ZP3t3,1iZ)
emission douhlets of sodium can he seen but as the cell temperature is raised (increasing Na vapor pressure) the yellow doublet steadily decreases in intensity until a t about 350°C it is completely removed from the spectrum. No noticeable change in the intensity of the red and green doublets occurs up to this temperature. Reference to the energy level diagram for the sodium atom (Fig. 2) shows clearly what is happening in this part of the experiment. In the visible region the sodium vapor lamp emits most strongly the transitions indicated by PI, D, and S with the intensity concentrated in the transitions PI. Even a t 500°C the hulk of the sodium atoms in the absorption cell will he in the ground state, 32&12; hence, when the wavelengths of light characterized by PI, D, and S are passed through the absorption cell only PI will be absorbed to any great extent as there are few atoms in the 3ZP3i2,1/2 states to absorb D and S. This aspect of the experiment proves that the red (S) and green (D) transitions of the sodium atom do not involve the ground state whereas the yellow (PI)transitions do. As well a s the students making ohsewations by eye, we get them to photograph the spectra over a range of temperatures of the absorption cell and a schematic diagram of the results they ohtain is shown in Figure 4. These spectra clearly show up the concept of resonance lines and their application in atomic absorption spectrometry (AAS) (i.e., of the visible emission lines of a sodium vapor lamp only the yellow lines could he used to monitor sodi'Other nonrearrwe gases, helium and argon, can and have been used but these are mvre exprnaiw Howewr. for equal prrsrurrs of fomipn gas, helium has t h r leasr pressure broadening elfrct on the spectral lines. Too low a pressure of foreign gas leads to a quicker huild-up of sodium on the windows.
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GROUND STATE
green
yellow
red
Figure 4. The emission spectrum of a sodium vapor lamp as seen through the Sodium absorption cell. Note the attenuation of the yellow lines and the unaffected red and green lines. The temperatures refer to the temperature of the absorptioncdl.
7
Figure 5. The molecular energy level diagram of the Na. molecule. The small groups of horizontal bars indicate vibrational fine structure of the electronic states. Only the wavelength for a pure electronic transition is indicated. The location of transitions involving both vibrational and electronic energy changes can be found in 16). The notation of the electronic slates is according to Herzberg (7).
Electron Configuration of Some Electronic States of the Na. Molecule
um by A M ; the other strong lines would have no useful sensitivity). Arguments based on these spectra also allow one to exnlain whv some emission lines of a hieh nressure source show the phenomenon of self-absorpt& -or linereversal whereas others do not (only resonance lines show this property to any great extent).-~urther,a clear demonstration of the effect of temperature on line widths is shown. The effective temperature of the sodium atoms in the vapor lamp is of the order of 103"C and hence the emission line width is broader than the absorption line width of the sodium atoms in the absorption cell where the temperature is of the order of 10Z"C. This effect is clearly shown in the spectra taken a t 340 and 360°C (Fig. 4) where absorption at the wavelengths of peak yellow emission is almost complete. One can note here that a broader emission line width than absorption line width gives rise. to nonlinear calibration curves and, in part, limits the ultimate sensitivity for the analysis of an element using AAS (8). A link with astrophysics and astrochemistry can be achieved by relating the absorption spectra observation herein to the spectrum of the sun. We have photographed the solar spectrum using the same spectrograph and the students can clearly see in the yellow, absorption in the chromosphere by sodium atoms (also there is a noticeable Donoler shift). & accurate estimate of the first ionization potential of sodium can be obtained bv fittine the 20 or so absorntion lines (visible and ultraviolei) to th~expression(3) Y
+ a,J2 cm-'
+
= R N , / ( ~ a,)2 - R N a l ( n
where RNa = Rydberg constant for sodium (==R,), a, = quantum defect for s states, a, = quantum defect for p states, and n = 3 , 4 , 5 . . .20,21. . . . Plotting i versus lln2 and extrapolating (either graphical0 gives the value of the first ly or numerically) to l / n z ionization potential, R N , / ( ~ + a#. A typical value obtained with our equipment is 5.139 f 0.001 eV which is to be compared with the reported values of 5.138 (5) and
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Electronic state
Ohserved
symmetry
X A B C
n
I&+
%+
TI,+
,xu+ 7
Electron configuration KKLL KKLL KKLL KKLL
(~7~3s)~ (a,") (nu%) inas) ( ~ , 3 p )
( ~ ~ 3(ru3p) s) ?
5.139 eV (2). We can measure line positions to *0.02 nm and correction from air to vacuum wavelengths has to be made. Molecular Absorption
A bonus is obtained from this experiment in that the absorption spectrum of Naz molecules can be observed when the cell is operating between 400600°C (6). Discrete absorption is noted in the blue (strong), red (medium), and ultraviolet (medium and weak) (cf. Fig. 3). The observed pure electronic energy level transitions appropriate to these absorption bands are shown on Figure 5 where the electronic states are labelled according to Herzberg's notation (7). Unfortunately, the spectral bands are complex and it is not easy to assign the vibrational structure so the measurement of ,chemically interesting constants (e.g., dissociation energies) is impossible in the context of this experiment. Nevertheless, the observation of four different discrete absoration bands. simifvina the existence of a t least five stable electronic states for this simple molecule, serves to indicate the need for a comprehensive bonding theory which includes excited states as well as ground states of molecules. Therefore, this aspect of the experiment gives a nice lead into descriptive molecular orbital theory. The table shows the observed symmetries (7) and electron configurations, according to molecular orbital theory, for some of the electronic states of the Naz molecule (compare Figure 5 and the table).
Possible Extensions to this Experiment The experiment can be extended to include the absorption spectra of the vapors of K, Rb, and Cs. Li has t w low a vapor pressure a t 500°C (4); so to obtain its vapor absorption spectrum one would have to construct a longer or higher temperature absorption cell. Conclusions We feel that an experiment on the absorption spectra of atoms is a necessary adjunct to the more common experiments on atomic emission spectra. As well as being able to build up a very complete picture of the behavior of an atmi by combining the absorption data obtained as in this experiment with pertinent emission data, this experiment clearly demonstrates the fundamental principles on which rests the powerful analytical technique of atomic absorption spectrometry. While the experiment is geared primarily to the obser-
vation of atomic spectra, the observation of spectra due to the Naz molecule can serve as a starting point for simple descriptive molecular orbital theory. Finally, the experiment as described can be completed in one three hour laboratory period even to the extent of measuring up the photographic plates. Calculations and writing up are done away from the laboratory. Literature Cited (11 Hallonbrg. J.L.,J.CHEM. EDUC.,43,216(1966). (21 Stafford, F.E.. and Wortman. J. H., J . CHEM. EDUC., 39,630 119621. (31 Henberg, G.. "Atomic SpecVe and Atomic Structure/ Dover, New York. 1944. pp. 55-56. 141 Weast, R. G. (Editor), "Handbook of Chemistry and Physics." 83th FA.. The chemical Rubber Co., Ohio, 1969.p. D170. (51 Reference(4J p. E74. 161 Peame, R. W. B. and Gaydon. A. G.. "The identification of Molecular Spcelm." Chapman end Hall, London, 196s. pp. 232-233. 171 Herzbeq, G., "Tho Swefra of Diatomic Moleeulss." 2nd. Ed.. D. Van Natrand Reinhold Co., Princeton, 1950, p. 36d. IS1 Price. W. J., "Anslytied Atomic Absorption Spctmmetry." Heyden. Landon. 1972, p. 79.
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