Knowledge Bank

A splitting of the ground state K $=3$ levels has been observed in the $\nu 3$ infrared band of $AsH3$ at high J. The splitting follows the expected equation: $$\Delta_{3}=A[J^{3}(J+1)^{3}-8J^{2}(J+1)^{2}+12 J(J+1)]$$ with $A=115$ Hz. This splitting constant is somewhat larger than the 86 Hz observed for $PH3.1$ The magnitudes of the observed splitting constants in $PH3$ and $AsH3$ can be explained by consideration of the effect of a term in the centrifugal distortion energy which is non-diagonal in the K quantum number. The centrifugal distortion contribution to the splitting constant is $2(\tau xxxz)2/(C-B)$. From a normal coordinate treatment of $PH3$ and $AsH3$ this contribution to the splitting constant is calculated to be 110 Hz and 91 Hz for $PH3$ and $AsH3$, respectively, and the $A+$ levels ($A1$ for J even and $A2$ for J odd) should lie below the $A-$ levels ($A2$ for J even and $A1$ for J odd), as is observed for both molecules. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$P.B. Davies, R. M. Neumann, S. C. Wofsy, and W. Klemperer, J. Chem. Phys. 55, 3564 (1971).