The electron-nuclear double resonance (ENDOR) spectra of dilute solutions of trivalent ${\mathrm{Nd}}^{143}$ and ${\mathrm{Nd}}^{145}$ ions occurring at the ${\mathrm{La}}^{+3}$ ion sites in the axially symmetric La${\mathrm{Cl}}_{3}$ structure have been measured at the temperature of boiling He. For laboratory fields of \ensuremath{\approx} ${10}^{3}$ G the frequencies ${\ensuremath{\nu}}_{n}$ of the ENDOR transitions are in the range $10\ensuremath{\le}{\ensuremath{\nu}}_{n}\ensuremath{\le}1000$ Mc ${\mathrm{sec}}^{\ensuremath{-}1}$. The experimental results for each nuclide have been summarized by giving values of the parameters in the spin Hamiltonian, ${\mathcal{H}}_{s}=|\ensuremath{\beta}|\mathrm{H}\ifmmode\cdot\else\textperiodcentered\fi{}g\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{S}+\mathrm{S}\ifmmode\cdot\else\textperiodcentered\fi{}{T}^{\ensuremath{'}}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{I}+{P}^{\ensuremath{'}}[{{I}_{z}}^{2}\ensuremath{-}\frac{1}{3}I(I+1)]\ensuremath{-}{\ensuremath{\beta}}_{n}\mathrm{H}\ifmmode\cdot\else\textperiodcentered\fi{}{{g}_{n}}^{\ensuremath{'}}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{I}$, which produce a rigorous least-squares fit to the data for the two experimental conditions H \ensuremath{\perp} c and H\ensuremath{\parallel}c, where c is a vector parallel to the hexagonal axis of symmetry of the La${\mathrm{Cl}}_{3}$ crystal. The frequencies were fitted with rms deviations of $0.08\ensuremath{\delta}$ for H\ensuremath{\parallel}c and $0.3\ensuremath{\delta}$ for H \ensuremath{\perp} c, where $\ensuremath{\delta}$ is the average ENDOR line width, 3\ifmmode\times\else\texttimes\fi{}${10}^{5}$ cps. The ${{g}_{n}}^{\ensuremath{'}}$ factor in the spin Hamiltonian was found to be anisotropic, with $|\frac{{{g}_{n\mathrm{II}}}^{\ensuremath{'}}}{{{g}_{n\ensuremath{\perp}}}^{\ensuremath{'}}}|=0.62$. A comprehensive theoretical interpretation of the spin Hamiltonian parameters using eigenvectors precisely calculated from the best available crystal field interaction parameters yielded $\frac{\ensuremath{\mu}({\mathrm{Nd}}^{143})}{\ensuremath{\mu}({\mathrm{Nd}}^{145})}=+1.60883\ifmmode\pm\else\textpm\fi{}0.00004$, $\ensuremath{\mu}({\mathrm{Nd}}^{143})=\ensuremath{-}1.079\ifmmode\pm\else\textpm\fi{}0.06$ nuclear magneton, $\ensuremath{\mu}({\mathrm{Nd}}^{145})=\ensuremath{-}0.671\ifmmode\pm\else\textpm\fi{}0.04$ nuclear magneton, $\frac{Q({\mathrm{Nd}}^{143})}{Q({\mathrm{Nd}}^{145})}=+1.96\ifmmode\pm\else\textpm\fi{}0.2$, $Q({\mathrm{Nd}}^{143})=(+0.0206\ifmmode\pm\else\textpm\fi{}0.003)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$, $Q({\mathrm{Nd}}^{145})=(+0.0105\ifmmode\pm\else\textpm\fi{}0.002)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$, $〈{r}^{\ensuremath{-}3}({\mathrm{Nd}}^{+3}, 4{f}^{3})〉=(36.9\ifmmode\pm\else\textpm\fi{}4.5)\ifmmode\times\else\texttimes\fi{}{10}^{24}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. An upper limit of one part in two thousand was established for the contribution, if any, of a contact term to the hyperfine interaction. The errors quoted are estimated standard deviations of the mean based upon internal consistency. A discussion of the errors and their sources is given. In particular, the precision of the determination of the nuclear moments $\ensuremath{\mu}$ and $Q$, of $〈{r}^{\ensuremath{-}3}〉$, and of the contact term was limited mainly by the inaccuracies of the best available values of the crystal field parameters ${{A}_{n}}^{m}〈{r}^{n}〉$. Numerical values of the spin Hamiltonian parameters, crystal field eigenvectors, and relevant interaction multiplicative factors are tabulated.

The method of electron nuclear double resonance has recently been employed by Feher for investigation of some solid-state problems. In this letter the application of this method for an investigation of Nd/sup 3+/ in LaCl/sub 3/ crystals is discussed. A tabulation is made of the frequencies so far observed at which the second resonance for Nd/sup 145/ occurs when the c axis is parallel to H. The remainder of the measurements are in progress. (A.C.)