Richard A. Ashby The N.S.W. lnstitlrte of ~echnology P.O. Box 123
I I
Apparatus to obtain the absorption spectrum of Na vapor was described by the author in a recent article ( I ) in this Journal: The novel portion of the apparatus was a 25 cm long cell in which to contain alkali metal vapors a t temperatures ranging up to -500°C. This temperature range is adequate to produce a sufficient quantity of the vapors of Na, K, Rh, and Cs for soectral observations.' However. Li has a lower than the other alkali metals (for a given temvapor perature) so to obtain the corresponding spectrum of Li vapor the cell temperature must he raised. This can he done by using a higher outout transformer than oreviouslv described ( I ) to drive the ceil to -660°C. Another factor must he considered when working with Li. Nitrogen, the unreactive gas recommended to accompany Na and K vapors (I), cannot he used with Li, as a t elevated temperatures they comhine to form LinN (3).Therefore, argon (less exoensive) or helium (more exoensive) must he used. In this article the results of measur&nents on the absorption spectra of Li, Na, and K vapors are reported. The presentation of the data (Figs. 1and 2) shows the ease with which ionization potentials can he measured and how they vary within Group 1.As well, the variation of the quantum defect, a,, is clear. Data Evaluation The ahsorotion soectrum of each alkali metal vapor shows the principal series of the appnrprintr atom. The spectra are shown in Figure I . llnder the experimmml cmditions referred
'Since the vapor pressures of Rh and Cs are greater than that of K
(a), the measurement of the absorption spectra of their vapors could
be accomplished using the apparatus described herein. However, it is recommended that they not be used in an undergraduate laboratory experiment. Rh and Cs are difficult to "handle" and are more dangerous than the other members of Group 1.For example, they "react explosively with water" (3).Even with K, care should he taken when removing it from the cell. To dispose of it a mixture of dry t-butanollisoamyl alcohol is recommended.
500 / Journal of Chemical Education
The Absorption Spectra of Alkali Metal Vapors to on the figure, 25-30 members of the principal series were observed for each of Na and K and 10-12 for Li. A casual observation of Figure 1 shows that the principal series of the Li atom converges to a shorter wavelength than that of the Na atom and thev both converee a t a shorter Since tGe point of conwavelength than that of the vereence is a measure of the first ionization ooteutial(1.P.) it iiohvious that the 1.P.k of the alkali atoms i r e in the order Li > Na > K. Values tor these l.l'.'s w r e extracted from measurements ofthe wavelengthi of thv nt,a,rption lines I,\, n~memhvrinrr l ) that the principal series of eachalkali atom can he represeited by an equation of the form
atom.
Z = [R,l(n~- a,,)2- R,l(n - a,,)2]em-'
(1)
where; i is the frequency of a line in cm-1 units; nl = 2 for Li,
-
Figure 1. The absorption spectra of Li. Na, and K vapors (schematic). The principal series for each atom arises from the transitions n2P-,, *,, . ZZS.,, .. .for 3'51 .(torNa)and r?P2 4?SI ,(forKl Theval~erol L P 2.9 napprapriate to each sere3 are shown on me d agram Tne broken line (n -1 at the high frequencr eKI of eacn spectrm md cates me mvergence omit (i.e, ionization potential) of the principal series appropriate to each atom. The first member of the principal series of K (the broken line daubletat the low frequency end) is not recorded on our film. The density of cross hatching in the Na and K Spectra indicates the intensity of absorption by Nagand K2 molecules. respectively. The temperature of the absorption cell ( 1 ) used to record the spectra represented in this diagram was - 6 6 0 ' C for Li. -550°C for Na, and -500'C for K.
,., ,-
-
ol,, and ap,. That a,, increases from Li to K is seen in the curvature of the G versus l/n2 plots (solid lines on Fig. 2). To emphasize the curvature, plots for each atom of ;ver&s (LP. - R,/n2) are shown as broken lines on Figure 2. These lines represent the situation that would apply if a,, was equal t o zero. The difference, Ax, between an experimental curve and the appropriate broken line is given by the equation
sre The o,,.'s weredeterminrd by s~thstirutingmeasured (I,, Fig. 21and known ( K , and n j facturs ineun. 12).R, varies from atom to atom and can he calculated from universal constants (9).T o the first decimal place R, = 109728.8, 109734.9, and 109735.8 cm-I, respectively, for Li, and Na, and K. The a,,'s were calculated from the measured LP.'s by noting that the LP. is equal to the first term on the right-hand side of eqn. (1). Figure 2. Absorption line position (cm-') versus l / n 2 for Me principal series ol the Li. Na, and K atoms (solid lines).The braken llnes represent plots of F ve(I.P.- R.lfi i.e. lhe curves which MUM resun il a, = 0. The annotations are explained in the text. lonlzatlon Potentials (I.P.) and Quantum Delects of Alkali Atoms
Alkali Atom
I.P. (eV) (experimental)
+
Li Na K
5.391 0.003 5.139 i 0.003 4.341 1 0 . 0 0 3
I.P. (eV) (literatured) 5.3gb 5.138r 4.339
L*s
(experimental) 0.41b 1.37 2.23
0.056 0.86 1.73
-see nef. (4. %ee Ref. (5). %ee Ref. (61.
Figure 2.
and 3 for Na, and 4 for K; n = 2,3,4. . . for Li, 3,4,5. . .for Na, and 4,5,6.. . for K; a,, and ap, are the quantum defects for the s a n d p states appropriate to each alkali atom,^; R, is the appropriate Rydberg constant. The plot, for each series of lines, of F versus l/nz (Fig. 2) on extrapolation to lln2 0 (i.e. -) gives an accurate value for the appropriate I.P. n The results obtained for such extrapolations (numerical), using data from our equipment ( I ) and after making corrections from air to vacuum wavelengths (7), are given in the table. Also given in the table are estimates of the quantum defects,
-
-
Conclusion
The absorption technique results in spectra relatively free from i m.~ u r i.t vinterferences. a t least comoared to the corresponding emission technique (5). The mnjm interferenre is from Na in the 1.i and K bur the one or two N\'Rlines in evi. dence can he easily identified. The method of presentation of the data (Fig. 2) clearly shows the variation of I.P. and a, amongst the alkali atoms which leads to discussions of the effect of atomic size, nuclear charge, and radial probability diagrams on those parameters. Acknowledgment
I would like to thank two final year undergraduate students, Mr. R. Schade and Mr. A. Vassalo, for their assistance with the experimental work, and Mr. H.W. Gotthard for preparing the diagrams for this article. Llterature Cited (1) Aahhy,R.A.,and Gottherd,H.W.,J. CHEM. EDUC.,51.408 (1974). (2) Ret il)p.411. (3) Cotton. F A , and Wilkinaan. G.. "Basic Inorganic Chemistry: John Wiiey and Sons, Ine.. New York. 1976. p. 218. 14) Weast, R.G.(Editor), "Handhookof Chemistry and Phynia: 5SthEd.. The Chemical Rubber Co.. Ohio, 1974-1975, p. E68. (5) Miiler,K.J.,J. CHEM. EDUC.,51,805 (1974). (6)Sfafford,F.E.,and Wortman, J.H.,J.CHEM. EDUC.,39.630 (1962). (7)P c m , R.W.B.,and Gaydon. A.G. '"TheIdentification ofMol~ularSpfftrs:Chapman and Hall, London, 1965. p. 337. 18) ReL (4Ip.0190. ( 9 ) ReL (41 p. F222.
Volume 55, Number 8. August 1978 / 501
