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We consider two coupled problems.  We study the dependence on photon virtuality $Q^2$ for the semihardquasi--elastic photoproduction of neutral vector mesons on a quark, gluon orreal photon (at $s\gg p_{\bot}^2,\;Q^2; \; p_{\bot}^2\gg \mu^2 \approx (0.3$GeV)$^2$). To this end we calculate the corresponding amplitudes (in ananalytical form) in the lowest nontrivial approximation of perturbative QCD. Itis shown that the amplitude for the production of light meson varies veryrapidly with the photon virtuality near $Q^2=0$.  We estimate the bound of the pQCD validity region for such processes. For thereal incident photon the obtained bound for the $\rho$ meson production is veryhigh. This bound decreases fast with the increase of $Q^2$, and we expect thatthe virtual photoproduction at HERA gives opportunity to test the pQCD results.The signature of this region is discussed. For the hard Compton effect the pQCDshould work good at not too high $p_{\bot}$, and this effect seems measurableat HERA.Comment: ReVTeX, 36 pages, 5 Postscript figures, uses epsf.st

physical review. d. particles, fields, gravitation, and cosmology/physical review. d. particles and fields

<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>Q</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>dependence of the hard diffractive photoproduction of a vector meson or photon and the range of PQCD validity

Q2dependence of the hard diffractive photoproduction of a vector meson or photon and the range of PQCD validity