Nuclear Quadrupole Interaction in CsF

Abstract

The electric resonance method of molecular beam spectroscopy has been used under high resolution conditions to study the rotational Stark spectrum of CsF for a single rotational state of the molecule, $J=1\mathrm{}$. In this method polar molecules in a single rotational state and a particular state of space quantization are selected from a molecular beam by means of inhomogeneous electric fields which give the desired molecules a unique, sigmoid path in the apparatus. Changes in the beam intensity are observed when a change in the space quantization of the molecule is produced by an oscillating electric field transverse to a homogeneous, steady electric field. For weak electric fields the observed line widths agree well with the estimated uncertainty width of 10 kc/sec. At stronger fields inhomogeneities in the field cause a broadening of the lines. At sufficiently strong fields the spectrum for CsF contains several broad lines, each of which is due to transitions of molecules in a particular vibrational state. As the field strength is decreased the resolution improves and these lines reveal a complex fine structure, the principal features of which can be explained by the interaction of the electric quadrupole moment of the Cs nucleus, spin 7/2, with the molecular electrons and the F nucleus. The F nucleus, spin 1/2, has no quadrupole moment. A complete, quantitative explanation of the spectra requires the existence of a cosine type coupling between the nuclear spins and the molecular spin of the form cI·J, and a correction for the spin-spin interaction of the two nuclei. At weak fields a different type of spectrum appears, permitting an independent evaluation of the nuclear-molecular interactions. The data allows a determination of both the magnitude and sign of the interaction constants. The quadrupole interaction, defined by ($\frac{e^2{q}^{\prime }Q}{2h}$), is (+0.310±0.002) mc/sec. The constants, $\frac{c}{h}$, for the I·J interactions for F and Cs are, respectively, (+16±2) kc/sec and (0±1) kc/sec. The difference in the quadrupole interaction for the first two vibrational states is less than the experimental error; i.e., less than one percent. Application of the method to the measurement of various molecular constants is discussed briefly at the end of the paper.