= Spectroscopic anharmonicity constant
{title2=$x_e$}
In the quadratic vibrational term expression $E_v/(hc)=\widetilde\omega_e(v+1/2)-\widetilde\omega_ex_e(v+1/2)^2$, the constant $x_e$ is dimensionless, while $\widetilde\omega_ex_e$ has units of <spectroscopic wavenumber>. A positive value makes adjacent level spacings decrease. For an exact <Morse oscillator>, $D_e/(hc)=\widetilde\omega_e/(4x_e)$. Using only two low-lying transitions to extrapolate this identity to a real molecule assumes the higher-order anharmonic terms are negligible all the way to dissociation.
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