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39 2 2 Here B = hJ8:!t Ib and A = hJ8:!t Ia where Ia and Ib are the moments of inertia parallel and perpendicular respectively to the symmetry axis. ~ V I(J + 1) is 2n the total angular momentum of the molecule and K ~ its projection on the 2n symmetry axis. If the molecule possesses a permanent electric dipole moment it can absorb a quantum of microwave energy hv and make a transition to a higher rotational' state governed by the selection rules L1 I = + 1, L1 K = O. The pure rotational absorption frequency is then given by v=2BI (1. 2) where I is the angular momentum quantum number for the upper state of the transition. If an isotopic substitution is made for an atom lying along the sym metry axis of the molecule, thus changing its moment of inertia and hence its B value, the rotational absorption frequency will shift. The program of mass determination from microwave spectra is to correlate this frequency shift with the change in mass of the substituted isotope. These shifts can range from a few Mc to several thousands of Mc for a change of one mass unit, being larger generally for the lighter atoms. Typical isotopic shifts in the middle mass region (A =60 to 130) are 100 Mc per mass unit. As mentioned above, frequency shifts in the 24000 Mc region can be measured to at least an accuracy of 0.
Table des matières
Atomic Masses of Nuclides.- A. Reasons and methods for measuring masses.- B. Atomic masses of nuclides.- C. Some trends in atomic masses of nuclides.- General references.- Determination of atomic Masses by Microwave Methods.- 1. Introduction.- 2. Theory of mass determination from rotational spectra of diatomic molecules.- 3. Approximations contributing to error in mass ratio determinations from spectra of diatomic molecules.- 4. Polyatomic linear and symmetric top molecules.- 5. Experimental techniques for measuring microwave absorption lines.- 6. Experimental results.- 7. Conclusion.- Acknowledgment.- Determination of Nuclear Spins and Magnetic Moments by Spectroscopic Methods.- A. Atomic spectra.- B. Atomic beams.- C. Molecular spectra.- General references.- Isotope Shifts.- A. Nuclear mass effects.- B. Field effects for spherical, incompressible nuclei.- C. Nuclear deformation effect.- D. Nuclear compressibility and surface phenomena.- E. Atomic considerations.- F. Comparison with experiment.- G. Conclusion.- Kernmagnetische Hochfrequenz-Spektroskopie.- Einführung.- A. Physikalische Grundlagen.- B. Experimentelle Technik.- C. Ergebnisse.- Zusammenfassende Berichte über das Gebiet der kernmagnetischen Hochfrequenz-Spektroskopie.- Determination of Nuclear Quadrupole Moments.- A. Determination of nuclear quadrupole moments from atoms or molecules.- B. Determination of nuclear quadrupole moments from nuclear transitions.- C. Mesonic quadrupole effects.- D. Results.- Acknowledgment.- References.- Sachverzeichnis (Deutsch/Englisch).- Subject Index (English/German).
