Diffractive photoproduction of vector mesons, \gamma p \rightarrow VY, where Y is the proton-dissociative system, has been measured in e⁺p interactions with the ZEUS detector at HERA using an integrated luminosity of 25 pb^-1. The differential cross section, d\sigma /dt, and a decay-angle analysis for \rho ⁰, \phi and J/\psi mesons are presented for a photon--proton centre-of-mass energy 80<W<120 GeV and -t<12 GeV, where t is the square of the four-momentum transferred to the vector meson. The t distributions are well fit by a power law, d\sigma /dt \propto (-t)ⁿ. The slope of the Pomeron trajectory, measured from the W dependence of the \rho and \phi cross sections in bins of t, is consistent with zero. The ratios d\sigma _{\gamma p \rightarrow \phi Y}/dt to d\sigma _{\gamma p \rightarrow \rho Y}/dt and d\sigma _{\gamma p \rightarrow J/\psi Y}/dt to d\sigma _{\gamma p \rightarrow \rho Y}/dt increase with increasing -t. For the \rho ⁰ and \phi mesons a contribution from single and double helicity flip is observed. The results are compared to expectations of theoretical models.

PACS numbers: 13.60.Le