B. Roberts
The HCI Vibrational
University o f Saskatchewan Saskatoon, Canada
Rotational Spectrum
In adapting the experiment on the HCI vibrational rotation spectrum a t 1 . 7 ~previously described by Stafford, Holt, and Paulson,' w~ havefound that if the Cary is run a t its slowest speed (0.5 A/sec) the accuracy of the wavelengths is increased from *10 A to i l .k. At this accuracy, the correction for the refractive index of air becomes appreciably larger than the experimental error and must be taken into account ,X(, - ,X,. 4.8 A). Further, if the rotational constant B. is then computed as suggested by Stafford, et al.,' a distinct bias with increasing rotational quantum number is found as shown in Table 1 for B,=o. The final values for B,, B,_o, Bod are also somewhat lower than one would expect (e.g., typically B, = 10.562; literature value 10.5832). We have found that this bias can be completely removed if the second rotational constant D is included in the calculation. The values obtained are typically B. = 10.597, D = 5.0 X (literature L S ~ . 4F.~ E. ~, ~HOLT, ~ ~ C. , W., CHEM.Eouc,,40,245 (1963b
AND
PAULSON, G. L., J.
value for D = 5.21 X lO-4).2 I n order to avoid unduly tedious calculations, we have had our students make a simple hand calculation omitting D to illustrate the principles involved. Final discussions have, however, been based on a least squares computer evaluation of B, and D so that the full significance of the results is apparent. Table 1.
lo 2
5Ri5.13
Evaluation of Rotational Constants by the Difference Method'
5406.58
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RANK,D. H., el al., J . Opl. SOC.Am.,50, 1275 (1960).
Volume 43, Number 7, July 1966
/ 357
