  The octet-singlet $\eta -\eta^{\prime}$ mixing mass term could have a
derivative $O(p^{2})$ term as found in recent analysis of the $\eta
-\eta^{\prime}$ system. This term gives rise to an additional
momentum-dependent pole contribution which is suppressed by a factor
$m_{\eta}^{2}/m_{\eta^{\prime}}^{2}$ for $\eta$ relative to the $\eta^{\prime}$
amplitude. The processes with $\eta$ meson can then be described, to a good
approximation, by the momentum-independent mixing mass term which gives rise to
a new $\eta -\eta^{\prime}$ mixing angle $\theta_{P}$, like the old $\eta
-\eta^{\prime}$ mixing angle used in the past, but a momentum-dependent mixing
term $d$, like $\sin(\theta_{0}-\theta_{8})$ in the two-angle mixing scheme
used in the parametrization of the pseudo-scalar meson decay constants in the
current literature, is needed to describe the amplitudes with $\eta^{\prime}$.
In this paper, we obtain sum rules relating $\theta_{P}$ and $d$ to the
physical vector meson radiative decays with $\eta$ and $\eta^{\prime}$, as done
in our previous work for $\eta$ meson two-photon decay, and with nonet symmetry
for the $\eta^{\prime}$ amplitude, we obtain a mixing angle
$\theta_{P}=-(18.76\pm 3.4)^{\circ}$, $d=0.10\pm 0.03$ from $\rho\to\eta\gamma$
and $\eta^{\prime}\to\rho\gamma$ decays, for $\omega$, $\theta_{P}=-(15.81\pm
3.1)^{\circ}$, $d=0.02\pm 0.03$, and for $\phi$, $\theta_{P}=-(13.83\pm
2.1)^{\circ}$, $d=0.08\pm 0.03$. A larger value of $0.06\pm 0.02$ for $d$ is
obtained directly from the nonet symmetry expression for the
$\eta^{\prime}\to\omega\gamma$ amplitude. This indicates that more precise
vector meson radiative decay measured branching ratios and higher order SU(3)
breaking effects could bring these values for $\theta_{P}$ closer and allows a
better determination of $d$.
