A. I. Baise' University College of Wales Aberystwyth, United Kingdom
II
A Novel Method for Measuring Molecular Emission Spectra
In the course of developing an undergraduate experiment illustrating the electronic spectra of diatomic molecules, a novel method for measuring molecular emission spectra using an atomic absorption spectrophotometer has been used. The method has been applied to record the spectrum of the C2 molecule (Swan hands), and students have successfully obtained results which show good agreement with literature values. Atomic absorption spectrophotometers are normally used (in both absorption and emission modes) for the quantitative determination of elements in solution. By means of minor adjustments, however, it is possible to use this relatively inexpensive instrument for recording flame emission spectra. This is conveniently provided by the burner system of the instrument, operating in the emission mode. The spectrophotnmeter used2 has hurners for various gas mixtures, and the airlpropane burner was chosen for studying the Swan hands. An intense emission is observed in the C2 spectrum a t 517 nm (see the figure), and optimum operating conditions were found by observing this band head and adiusting the burner height and airlpropane ratio to give a maximum signal. The monochromator associated with the instrument ii generally set at a single wavelength for elemental analysis, hut the essential feature of the method described here is to remove the wavelength control knob and attach a synchronous motor to the control shaft. In this way one can readily scan through the wavelengths of interest, and the motor can be removed easily and the instrument restored to its normal operating condition. The detector signal was displayed on a pen recorder, and the spectrum was scanned a t approximately 10 nmlmin, so that the time to scan from 400-600 nm was thus about 20 min. The spectral bandpass used was the narrowest obtainable with the instrument (0.15 nm). The Swan bands form four sequences in the region under observation (1, 3). These are given in parentheses in the figure, and the individual peaks in each sequence are indicated by vertical lines. In order to calibrate the record, a solution containing Ca2+,Sr2+,TI+, and Na+ was aspirated into the flame at appropriate points during the scan. These atomic emission lines with their wavelengths in nm (2) are also shown in the figure. This provides an internal calihration during the run, and accurate interpolation (to 0.1 nm) of the band heads. The estimated maximum error in each measurement was +0.2 nm, which leads to an estimated error o f f 10 cm-1 in the wavenumbers given in Table 1. There is a ~ r o m i n e n tband head a t 431 nm due to (CH I , u,ith its associated rotutional tine structuri~,the full detail of which is quite c u m ~ l e x(41. .4 slow scan thrw~ghthis region provided-a regular-sequence of maxima: if the mean "line" separation observed is assumed to be 2B, where B is the rotational constant in cm-I, then a value of 17.6 cm-' is obtained for B. The completely resolved structure leads to two values (5): 14.5 cm-' (ground state) and 14.9 cm-I (excited state). 'Present address: Chemistry Department, Temple University, Philadelphia, Pennsylvania 19122. 2Model A3000. suoolied bv Shandon Southern Instruments Ltd., Camberley, ~ngia.nd 58 I Journal of Chemical Education
I
Reproduction d an actual recording of the
C2 Swan
bands emission spec-
trum. The vertical lines indicate the band heads. The wavelengths of the calibrating lines are given in nm
The results ohtained in a typical run are given in the Deslandres tahle (Table 11, where our values are compared with band head wavenumbers given in the literature (3). An earlier article in this Journal ( I ) described the measurement of the Swan bands visually, a lengthy and less accurate procedure than that described here. The notation of that article is used here to facilitate comparison. Thus the expressions for the vibrational energy levels in the excited (E')and ground states (E) in wavenumbers are ( I )
E' E
= =
Et0 E,
+ w',(n' + ll,) - o',x:(n' + %)2 + wdn + %) - wsxp(n + % Y
The w's represent the vibrational wavenumhers for zero amplitudes, the x's are the anharmonicity constants, and n and n' are the vibrational quantum numbers in the ground and excited states.. res~ectivelv. From these eauations it . follows that the wavenumbers for transitions from a given level n' to various ground state levels n are given by Table 1.
Deslandres Tabla of Swan Bands (cm-'I
where a is a constant. These wavenumhers correspond to those in any row of the Deslandres tahle. Thus for any row containing three or more entries, one can solve the three simultaneous equations obtained from eqn. (1) to give the values of m, and w$, (and a). A similar procedure is followed for any column containing three or more entries, in which case the relevant equation is
where b is a constant. The results of these calculations are collected in Table 2. In general, the wavenumbers ohtained (Table 1) agree well with literature values (< 0.1% error), although even these small differences lead to relatively large errors in the values of the anharmonicity constants. A possible source of error is non-uniform movement of the synchronous motor used or of the pen recorder. I t should also he noted that the results obtained from these measurements will necessarily deviate from the "best" literature values: the latter take account of the displacement between the band heads (measured here), in which the rotational structure is not resolved, and the hand origins, to which the molecular parameters are correctly related (3). Literature CHed
Q
valuer
initalicrar from
reference
(1) Dauioa. Mannel. J. CHEM.EDUC..Z8.474 11951l. 121 "Handhook or Chemistry and Phyricr," 50th Ed., The Chemiesl Ruhber Co.. Cleve1and.Ohio. 1969.p. E2l6. 13) Walker. S.. and Straw, H.:'Spectronmpy," Chapman and Hall. London. 1961. Val. 2. pp. 194-201. 141 Mcxm.C. E..and Broida. H. P.. J. ROS.Not. Rur. Stand., A63, 19. (1969). l i l Hernher., G.."Spectra o l Diatomic Mdeeules." D. van Nostrend Co.. P~inceton. 1950. D.618.
(3) Table 2.
Molecular Constants of C P
G r o u n d State
n
2 3
1. 2. 3 1. 2. 4
1637 1631 -
1661 1624
Mean Accepted value@
1634 -
1643
WI
; v a l u e r -are Reference
is), p.
(cm-')
1641.35
from reference
513.
E x c i t e d State
103x,
n'
7.0 6.6 -
10.2 6.3
6.6
8.2
-
n
n'
1 2 2
1. 2. 3 1.2.3 2.3.4
7.11
(3).
w ~ (cm-' ' I
-
1794
1794 1824 1798 -
1796 1764 1836 1800
1788.22
.
Volume 53,Number 1, January 1976 / 59
