Particle creation in the Bell - Szekeres spacetime

The quantization of a real massless scalar field in a spacetime produced in a collision of two electromagnetic plane waves with constant wavefronts is considered. The background geometry in the interaction region, the Bell - Szekeres solution, is locally isometric to the conformally flat Bertotti - Robinson universe filled with a uniform electric field. It is shown that before the waves interact the Bogoliubov coefficients relating different observers are trivial and no vacuum polarization takes place. In the non-singular interaction region neutral scalar particles are produced with the number of created particles and spectrum typical of gravitational wave collision.