Atomic and Molecular Spectra Using Polaroid Films A Physical Chemistry Experiment Manil ~appon'and John M. Greer2 Lakehead University, Thunder Bay, ON P7B 5E1, Canada The concent of auantization of enerm in quantum chemistry is, to some &dents, a difficult &ept to grasp. Perhans one of the hestwass toenhance a student's understanding of this concept is by conducting an experiment in spectroscopy. In many teaching laboratories, facilities for conducting emission spectroscopy experiment do exist, the experiment is relatively simple to set up, and i t is often appreciated by students. A spectrograph, a few light sources with a power supply, and a spectral capturing device are all that is reauired for this exoeriment. Construction of a low-cost specfroprnph with moderately hieh resolution has been described in detail in thic Journal (lj. Normally, photographic plates and films are used to capture the spectra (2). While the plates and films may provide good spectra and flexibility (e.g., spectra may be enlarged if required), they require long processing times and often restrict the extent of the spectroscopic investigation during one laboratory period. The use of the Polaroid films has made it nossible for students to conduct atomic and molecular spectroscopic experiments within one normal laboratorv neriod. Since the loneest nrocessine time reauired bv the ~ o l a r o i dfilm is on theUorder of oneuminute,~therefs nlentv of time for students t o exolore the effects of the slit widthand the exposure time on the spectral lines or molecular bauds. Bv svstematicallv varvina the slit width and the exposure time, students areahle t o find the optimum conditions under which a spectrum is to be taken. The use of the Polaroid color films has been found to enhance students' interest in the experiment. This paper describes the following: I ) Detailed modifiratmn of a Polan&i camera to replare the phowgraphir plate ht>lderof a ~ p e c t r ~ p n p h . (2) This setup has been tested by investigating the atomic spectra of hydrogen and helium atoms. For molecular spectroscopy, the Bunsen flame has been used as a source for free radicals of C? and CH. Helium atomic lines are used as reference to identify the atomic lines and molecular bands. Selective applications of
fast black-and-whiteand slow color films are given. (3) Suggestions for further experiments. l
Author to whom correspondence should be addressed. formerly
M. Rujimethabhas.
Summer Assistant.
Modlllcatlon of a Polarold Camera In order to use the Polaroid films i t is necessary to modify a commercially available Polaroid camera to fit the photographic plate holder of a spectrograph. For the present setun the Polaroid camera "Bie swinger 3000" was adapted & fit the Hiker and Watts model ~ 1 8 7 , ' ~ hcamera e was sawed in half along the line indicated by the arrows in Figure 1. The half used to hold the film pack was stuck with black epoxy resin to a brass plate with a hole cut to the same size as the film. Another identical brass plate was used to sandwich shutter B between both plates. Shutter B was made of black "Arborite" (phenolic laminate), and the side facing the inner brass plate was covered with black velvet t o ensure that the ambient light cannot enter the camera. The outer brass d a t e wasused to hold the camera to Figure 1. Diagram of the mcdified Polaroid the snectroeraoh bv slidine camera. the brass pl&e;nto the plat: holder euide rails of the soectrogra&. A cord was tied between the shutter and position C such that when the shutter was fully raised to just above the opening in the brass plates (in preparation for the exposure), the cord was fully extended, thereby preventing the shutter from being inadvertently pulled out completely. Aviewing screen for visual inspection of spectra was found to be helpful, and it was constructed from a piece of Styrofoam 1.3 cm thick and cut to fit in between the guide rails where the photographic plate was normally secured. A rec-
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tangular opening on this piece of Styrofoam was made and the dimension of the opening was slightly larger than the opening on the spectrograph where the light is allowed to expose the film. One side of the opening was covered with a sheet of translucent material such that a strong atomic line or molecular band falling on the screen may be viewed a t the hack of it. These components are also shown in Figure 2.
Table 1.
Atomic line
Position (cm)
A B
0.00 1.00 2.80 3.10 3.75 4.75 5.15 6.38 6.70 7.40 7.90
C
D E F
G H I J K L
Figure 2. Photographof the modifiedPolaroid camera: A, outer brass plate; B. shuner: C, plate holder guide rails: D. viewing screen; E, back view of the camera.
I I B
A
I l l
II
II I I I
C D E F G H I J K L
Figure 3. Hydrogen and helium spectra on black-and-white Polaroid film.
D
E
8.60
Wavelength (A) (ref 4
6678.15 5875.62
5015.66 4921.93 4713.14 4471.48
4367.93 4143.76 4120.82 4026.19 3964.73 3866.65
Color
Red Yellow
Green
Biue-Green Blue Blue Violet
Experimental P r o c e d u r e With the viewing screen held in the position of the plate holder and a helium lamp as the light source, the shutter of the spectrograph is opened. The slit opening is adjusted such that the intense helium atomic lines may be observed on the screen. The intensity may be enhanced with a focusing lens. Students should adjust the slit width and observe how the slit width affects the sharpness and the intensity of a spectral line. The slit width should be left in the position where the lines are sharp and reasonably intense. Next, the wavelength drum is rotated, and its effect on the positions of spectral lines is observed. Rotating the wavelength drum until the red line (line A) and the violet line (line L) of Figure 3 may he observed on the screen. The position of the wavelength drum should he noted since the He lines will he used as reference later. Rotate the knurled knob to adjust the length of the slit. The vertical position of the screen may he racked up or down with the knob. This preliminary operation nrovides students with some of the basic knowledge of experimental spectroscopy. The modified Polaroid camera, loaded with black-andwhite Polaroid film (Type 107, ASA 3000), is secured to the plate holder, and the spectra of helium and hydrogen are taken at various exposure times, racking the plate after each exposure. The relarive vertical position of the plate may be ol)served on the scale located on one side of the plate holder. The relative positions giving the partial overlapping of the helium and hydrogen spectra, as shown in Figure 3, should he noted. We have also performed this experiment using Polaroid color film (Type 108 Polacolor. ASA 7 5 ) .The spectrum shown in Figure 3 was taken on black-and-white film with a slit-width setting of approximately 0.25 mm and with the approximate exposure times indicated in parentheses a9 follows: H suectrum 120s). He spectrum (5s). For color film, at the samesetting, the following exposure times are recommended: H spectrum (35 min), He spectrum (5 min). The power supply (Model S p 100, Electro-Technic Products, Chicago) was used to discharge the gases in the spectrum tubes. For the case of diatomic molecular spectra, the Cz Swan Bands were chosen. This soectrum was obtained by exposing the black-and-white film for 1Yzh to the inner ionebf the Bunsen flame. The He spectrum was again taken as a reference source as shown in Figure 4. The emission from the Swan Bands is too weak to be investigated by the less sensitive color film within one normal laboratory period. ~~
A B C
Poslllons and Wavelengths ot H e Spectrum In Flgure 3
~~~
Results
Figure 4. Spectra of an inner cone of a Bunsen flame showing the Swan Bands (8-D), the CH band (E) and the helium atomic lines, on black-and-white Polaroid film.
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Journal of Chemical Education
The intense He lines labeled A to L (see Fig. 3) with the relative nositions measured from an arhitrarilv chosen zero line, A, h i e been tabulated in Table 1, where the known waveleneths shown are obtained from reference 3 and MIT ~ a v e l e & h Tables. The last column of the table shows the
ldentllication01 Hvdroaen Atomic Lines in Flsure 3
Table 2.
Position from He line A (cm)
A (expt.)
A (!It.)
NO.
(A)
(A)
% Enor
1 2 3 4 5 6
0.12 3.28 5.40 6.80 7.60 8.60
6500 4620 4330 4100 3960 3890
6562 4861 4340 4101 3970 3669
0.9 0.8 0.2 0.0 0.3 0.0
Atomic line
Table 3.
Wavelenglhs 01 Hydrogen Atomic Lines with the Corresponding ir, n,, and l l ( t ~ ~ ) ~
A (A)
F (cm-')
nl
lh)?
6500 4820 4330 4100 3960 3690
15.000 20.700 23.100 24.400 25.200 25.700
3 4 5 6 7
0.111 0.063 0.040 0.028 0.020
8
OOlfi
Table 4.
DIscu88ion The value of R obtained from exoeriment is 112.000 cm-1. which may he compared with the dnown experimental va~ud of 109,677 cm-I (5).This gives the error in R of approximately 2%, which is within our experimental error. I t is instructive for students to calculate the waveleneths of the Lyman, Ritz-Paschen, and Brackett series cokesponding to nz = 1 , 3 , and 4, respectively, by using eq 1and the known Rydherg constant. From these results students should then he able to offer reasons for not ohservina these series with the present films. In addition, students should also attempt to explain why the intensity of the Balmer lines becomes less intense toward shorter wavelengths. The latter phenomenon may he observed clearly on the color film. However. on the black-and-white film (Fie. 3. line 1).the red line ( X = 6500.4) appears t o he weaker th& line 2at = 4820 A due to the fact that the sensitivitv ofthe film is noor in the red wavelength region. I t should he noted that the investigation of atomic hydrogen spectra by the use of a spectrophotometer has also been reported in this Journal (6, 7). For the molecular spectra of the C2 Swan Bands, those measured from A37r, X37ry, as labeled B, C, and D in Figure 4, compared well with the literature values (4) as shown in Tahle 4. These bands are degraded to the violet. Figure 4 also shows the presence of a very strong CH hand
-
Assignment 01 C, Swan Bands (6-D) and CH band (E) of Figure 4
h (expt.)
h (!It.)
Label
Position from yeliow line of He (cm)
(A)
(A)
% Enor
B C D E
0.42 1.40 2.66 4.51
5800 5160 4720 4310
5635 5165 4737
0.6 0.3 0.4
491s
ni
colors of the atomic lines that mav he observed view~ on ~ the ~ ~ ~ ing screen and captured with the color film. Such information helps students identify these atomic lines. These lines are used to prepare a dispersion curve for each setting of the wavelength drum. The curve is used to identify unknown atomic lines and molecular hands. A typical dispersion curve is shown in Figure 5 where the wavelength in angstroms is plotted against the position in centimeters measured from the reference line. Thus an unknown line or a hand mav he assigned a wavelength once its distance from the same reference line is measured. The hydrogen atomic lines of Figure 3 are identified and summarized in Table 2. The known wavelengths for these lines (3) are also provided for comparison and the percentage errors are given in the last column. I t should he noted that the measured wavelengths are in air. In an attempt to find the Rydherg constant (R)from the hydrogen spectrum, eq 1is used in the analysis, ~
i = R ( I / I L; 11$)
~
~
-
-
-
3000 0
1
2
3
4
5
6
7
8
POSITION (cm Figure 5. A typical dispe~sloncurve derived from a helium spectrum showing the plat of wavelenglh (A) versus position (cm).
(1)
where 5 is the wave number, (5 = 1IX) in cm-I, nz is the quantum number of the lower state and is eaual to 2 for the ~ a l m e series r as studied in this experiment,'nl is the quantum numher of the upper state from which the transition t o the nn takes place and it may assume the followingvalues: nl = 3, 4, 5, 6, . . . . Tahle 3 shows the wavelengths and the corresponding wave numbers of atomic hydrogen. The assignment of nl is shown in the third column, and the last column shows the values of ll(n1)2. The plot of T versus l/(n1)2 is shown in Figure 6 and, from the slope of the line. R is found to he 112.000 cm-1 with the a ~ s o r i ~ t eerror d of approximately 2%. The identification o f t h e G S w a n Bandsand the CH hand is summarized in Tahle 4. he known values from the literature (4) are also included for com~arisonwith the Dercentaee errors given in the last column.
k; Figure 6. A plot olF(cm-') against l/(n,)?.
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labeled as E. This is the (0,O) band of the A2A X 2 r and is degraded to the violet. This band is commonly observed in the Bunsen flame. Also appearing in Figure 4 is the atomic line of Na and is labeled A. The measured his 5900 A,which represents the apparent Na doublets (8)a t X's of 5895.9 and 5890.0 A. These lines are unresolved by the low-resolution spectrograph employed in this work. The intensity of the line was intensified by dropping some NaCl on the Bunsen flame. I t should be pointed out that while He lines are used in the present work, Hg and Fe lines are also in common use. Adequate protection against uv radiation from the lamp is recommended. This may be achieved by filtering the emitted light with Pyrex glass. The major source of error in this experiment comes from the measurements of the positions of spectral lines or band heads from a chosen reference line. I t is suggested that Polaroid films may also be used to investigate emission from some other sources, for example, CN red and violet systems generated from the reactions of active nitrogen with carbon containine comoounds (9) r('?F4, for example): C , hands emitted from o;yacerslene ila~neIIO);(:? and (3hands from elwtric arc in air r. I I )..: NY . spectra from electrical discharge (5)and quantitative atomic emission spectroscopy (12). In conclusion, the modification of a Polaroid camera for spectroscopic experiments is given. This method has been tested and shown to provide satisfactory results for teaching
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Journal of Chemical Education
purposes. The short processing times required and the selective applications of the films have made it possible for students to learn some basic principles and techniques of spectroscopy within one normal laboratory period. From our experience, students are found to be more appreciative of and more receptive to the concept of quantization of energy after this experiment has been performed. Acknowledgment Financial assistance in part from the Lakehead University Senate Research Committee is greatly appreciated. We thank L. D. Hawton for reading the manuscript. D. A. Jones and B. K. Morgan are thanked for their technical assistance. Llterature Cited 1. Schoenheck,R.;Tabbutt,F.D. J.Chem.Educ. 1963,10,452. Prentiee-Hall: 2. Harrison. G. R.;Lord, L. C.; Laofbourov. J. R.P~ocrirolS~emra8copy; Englewood Cliffs. NJ. 1948. 3. Weast, R. C.. Ed CRCHondbookafChemisfryondPhysica;6Lsted.; CRCPres: Born Raton. FL. 198&1961. 4. Pearse, R. W. B.: Gaydon, A. G. Tho ldenfificolion of Molrculor Spectra. 3rd ed.: Chapman &Hall: London, 1965; plate 9. 5. Shoemaker. D. P.; Garland. C. W.; Steinfeld, J. I.; Nibler, J. W. Experiments in Physical Ch~mbtry.4th sd.: MeGrsw-Hill: NewYork. 1981: p 412. 6. Hol1enberg.J. L. J . Cham.Educ. 1966.43, 216. 7. Stafford, F.E.; Wartman. J. H. J. ChemEdur. 1962.39.630. 8. Herzher~.G. Atomic Spectra and Atomic Structure: Dover: New York. L9W p 74. 9. Rujimethabhss.M.; Jones, W. E. Con.J. Chem. 1912.60.346. 10. Steinfeld, J. I. J. Chem.Educ. 1965,42,85. 11. Herzherg. G. The Spectra and Structures of Simple Flee Rodirois: Cornell Univ.: Ithaes. 1971:p4. 12. Blyan, F. a.J. Chem.Educ. 1960.37.471.
