  We study the hard color dipole-nucleon cross section within perturbative QCD
and discuss its relation to observables in diffractive leptoproduction of heavy
quarkonium. The dipole cross section calculated with the unintegrated gluon
density of the nucleon substantially differs from the well-known perturbative
form $\sigma_{dip} \sim b^2$ for $b > 0.3$fm, where $b$ is the transverse
separation of the dipole. We show the measured ratio of $\psi '$ to $J / \psi$
photoproduction cross sections constrains the dipole cross section at
intermediate $b$, and in fact excludes the simple $\sigma_{dip} \sim b^2$
behavior. We also calculate the $t$-slopes of the diffractive $J / \psi, \psi
'$ productions. We emphasize the difference of $t$-slopes, $B_{J/\psi} -
B_{\psi '}$, is dominated by the dipole-nucleon dynamics. This difference is
found to be about $0.3 {GeV}^{-2}$ with our dipole cross section.
