Spectroscopic anharmonicity constant ($x_e$)

In the quadratic vibrational term expression $E_v/(hc)={\tilde{\omega }}_e(v+1/2)-{\tilde{\omega }}_e{x}_e(v+1/2{)}^2$, the constant $x_e$ is dimensionless, while ${\tilde{\omega }}_e{x}_e$ has units of spectroscopic wavenumber. A positive value makes adjacent level spacings decrease. For an exact Morse oscillator, $D_e/(hc)={\tilde{\omega }}_e/(4x_e)$. 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.