Atomic Physics and Related Subjects.: Communications to Nature.: Moment of Inertia of Hydrogen from Band Spectra

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THE commonly accepted value of the moment of inertia of the hydrogen molecule, in the normal state, is due to Hori (Zeit. f. Phys., 44, 834; 1927). In deriving this value, Hori expressed the rotational energy as Em = Bvm2 + Dvm4, and calculated empirical values of Dv directly from the data. The term Dvm4 is due to the swelling of the molecule with rotation, and it is now well established that the theoretical values of Dv hold accurately in the case of electronic levels for which there is no resultant electron momentum to complicate matters. This is the situation in the normal level of hydrogen (1Σ). Hori's values of Dv are several times as small as those given by theory and, what is more important, are positive, whereas a swelling of the molecule with rotation requires that they be negative. Hori made note of this discrepancy in sign, but was unable to explain it. We find, however, in agreement with Schaafsma and Dieke (Zeit. f. Phys., 55, 164; 1929) that Hori's data, although relatively inaccurate and meagre, do indicate negative values of Dv, and we are unable to locate the origin of his published values. It should be pointed out, in addition, that Hori's data give probable errors for Bv (his Av) and Dv (his β) ten to one hundred times as large as his published errors for these quantities.

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