Two-photon width of eta_b, eta'_b and eta''_b from Heavy-Quark Spin Symmetry

We predict the two-photon width of the pseudoscalar bottomonia, Gamma(eta_b -> gamma gamma), Gamma(eta'_b -> gamma gamma) and Gamma(eta''_b -> gamma gamma) within a Heavy-Quark Spin-Symmetry setting following the same line as our recent work for the corresponding decays for charmonia. Binding-energy effects are included for excited states and are shown to shift up the eta'_b and eta''_b widths by 10 %. We point out that the essentially model independent ratio Gamma(eta_b -> gamma gamma)/Gamma(Upsilon -> ll) and the branching ratio BR(eta_b -> gamma gamma) obtained could be used to extract the coupling constant alpha_s.