Centrifugal distortion effects in the hyperfine spectrum of KCl

The hyperfine spectrum of KCl has been examined at near-zero electric field and zero magnetic field using a molecular beam electric resonance spectrometer. Rotational as well as vibrational shifts have been observed in both nuclear quadrupole interactions. With $\text{eqQ = Q}{}_{00}\text{+ Q}{}_{10}\text{(v +}\text{1}\text{2}\text{) + Q}{}_{20}\text{(v +}\text{1}\text{2}\text{)}{}^2\text{+ Q}{}_{01}\text{J(J + 1)}$, we find (all in units of kHz) for K in ³⁹K³⁵Cl: Q₀₀ = ?5691.47 ± 0.04, Q₁₀ = 51.32 ± 0.06, Q₂₀ = ?0.205 ± 0.020, Q₀₁ = 0.014 ± 0.007, Q₀₀(K³⁷Cl) ? Q₀₀(K³⁵Cl) = ?0.03 ± 0.07; for Cl in ³⁹K³⁵Cl: Q₀₀ = 137.0 ± 0.3, Q₁₀ = ?163.2 ± 0.5, Q₂₀ = 1.57 ± 0.15, Q₀₁ = 0.07 ± 0.03, $\text{Q(}{}^{35}\text{Cl}\text{)}\text{Q(}{}^{37}\text{Cl}\text{)}\text{]Q}{}_{00}\text{(}\text{K}{}^{37}\text{Cl}\text{) ? Q}{}_{00}\text{(}\text{K}{}^{35}\text{Cl}\text{) = ?0.5 ± 0.6}$; and magnetic constants c_K = 0.154 ± 0.007, c_Cl = 0.435 ± 0.010, c₃ = 0.035 ± 0.012, and c₄ = 0.009 ± 0.006. These have been used to provide a mapping of the field gradients at both nuclear sites to fourth order in $\text{ξ =}\text{(r ? r}{}_{\mathrm{e}}\text{)}\text{r}{}_{\mathrm{e}}$. We find eQq_K(ξ) = (?5692.5 ± 2.5) + (1.7 ± 0.8) × 10⁴ξ + (?2. ± 4.) × 10⁴ξ² + (?8. ± 18.) × 10⁵ξ³ + (8. ± 15.) × 10⁶ξ⁴ and eQq_Cl(ξ) = (120. ± 22.) + (8. ± 4.) × 10⁴ξ + (?5.8 ± 2.0) × 10⁵ξ² + (?1.1 ± 1.6) × 10⁷ξ³ + (1.1 ± 1.3) × 10⁸ξ⁴.