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In this paper we study the possibility of generalizing the classicalphotoabsorption ($\gamma a \to b c$) sum rules, to processes $b c \to \gamma a$and crossed helicity amplitudes. In the first case, using detailed balance, thesum rule is written as $\int_{\nu_{th}}^\infty {\frac{{d\nu}}{\nu}} K\Delta\sigma_{Born} (\nu)=0$ where $K$ is a kinematical constant which depends onlyon the mass of the particles and the center of mass energy. For other crossedhelicity amplitudes, we show that there is a range of values of $s$ and $t$ forwhich the differential cross section for the process $\gamma b \to a c$ or $a c\to \gamma b$ in which the helicities of the photon and particle $a$ havespecific values, is equal to the differential cross section for the process inwhich one of these two helicities is reversed (parallel-antiparallel spincorrelation).Comment: 9 pages, 2 figure

Classical sum rules and spin correlations in photoabsorption and photoproduction processes