Why does ψ'' have a lower leptonic branching ratio compared to ψ' and J/ψ?

[Ichimaru]

I'm looking at charmonium and its decays. Given a list of data on the charmonium states I'm asked to say why ψ'' has a leptonic branching ratio a thousand times small than ψ' and J/ψ.

From my understanding this is due to OZI suppression. But I'm having a hard time understanding it intuitively. Why can't any charmonium state annihilate via the strong force to another quark antiquark pair? What is the decay available to the ψ'' that isn't available to ψ' and J/ψ?

Thanks!

 

[kjhskj75]

Isn't it just that the heavier $\psi(3770)$ is heavy enough it can decay rapidly into D mesons that carry away the charm, the lighter ones can't.

See http://pdg8.lbl.gov/rpp2013v2/pdgLive/Particle.action?node=M053

 

[Einj]

Yes, the right answer is that one. The ψ'' is the first charmonium above the open charm threshold and therefore it's much more convenient for it to decay into two D mesons, basically just splitting the two pre-existing charm quarks.