The Radiofrequency Spectrum ofK39F by the Electric Resonance Method

Abstract

The molecular beam electric resonance method has been used to investigate the rotational Stark spectrum of ${\mathrm{K}}^{39}$F. In this method linear polar diatomic molecules in one, or at most two, rotational states and definite states of space quantization are selected by means of inhomogeneous electric fields. The molecular transitions are not of the $\ensuremath{\Delta}J$ type (transition between different rotational states). Rather, the transitions are between states with the same value of $J$ but different values of ${m}_{J}$, the electric quantum number characterizing the state of space quantization. Lines are observed as a change in intensity of the refocused beam when the frequency of an oscillating field, which is perpendicular to an homogeneous field, times Planck's constant is equal to the energy difference between different states of space quantization. The frequency of the rotational Stark line is determined by the value of the homogeneous static field. The Stark line was observed at low and high fields for $J=1$ and low fields for $J=2$. It is found to be split. The fine structure is due to an interaction of the nuclear electric quadrupole moment of the ${\mathrm{K}}^{39}$ nucleus with the rest of the charges of the molecule as well as a variation of ${\ensuremath{\mu}}^{2}A$ with the vibrational state of the molecule. Here $\ensuremath{\mu}$ is the permanent electric dipole moment, and $A$ is the moment of inertia. In addition, the quadrupole interaction constant is found to vary with the vibrational state. It is measured when the molecule is in the rotational state $J=1$ and the vibrational states $v=0,1,2,3 \mathrm{and} 4$; and $J=2$, $v=0,1\mathrm{and}2$.The frequency dependence of the Stark line at strong fields permits the determination of $\ensuremath{\mu}$ and $A$; they were determined only for the zeroth vibrational state.The results are: For $J=1$, $v=0$, $\frac{\mathrm{eqQ}}{h}=(\ensuremath{-}7.938\ifmmode\pm\else\textpm\fi{}0.040)$ Mc/sec. The absolute value of $\frac{\mathrm{eqQ}}{h}$ decreases about 1.3 percent when the vibrational quantum number increases by unity. Within experimental error the quadrupole interaction and its variation with vibrational state for $J=2$ is the same as for $J=1$. A complete tabulation of the results will be found in Table II. For the zeroth vibrational state $\ensuremath{\mu}=(7.33\ifmmode\pm\else\textpm\fi{}0.24)$ Debye, $A=(138.4\ifmmode\pm\else\textpm\fi{}6.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}40}$ g-${\mathrm{cm}}^{2}$. From $A$ we find for the internuclear distance for the zeroth vibrational state $r=(2.55\ifmmode\pm\else\textpm\fi{}0.06)$ angstr\oms. From the variation in intensity with vibrational state for a single transition the vibrational constant ${\ensuremath{\omega}}_{0}=(390\ifmmode\pm\else\textpm\fi{}39)$ ${\mathrm{cm}}^{\ensuremath{-}1}$.