An ensemble of excited molecules collectively emits multiple-frequency real and virtual photons

The interplay of molecules gives rise to collective phenomena absent in a single molecule. Many examples of collective phenomena have been reported as their knowledge is essential for understanding the behavior of matter. Here, we consider molecules sufficiently separated from each other to not form chemical bonds. If these molecules are excited, e.g., by a weak laser, can they concertedly relax by emitting a single high-energy photon possessing the total energy of all the relaxing molecules? We show that this concerted emission process is indeed possible. We estimate its probability and analyze its dependence on molecular properties, intermolecular distances, and relative orientations of the molecules. A numerical example on two pyridine molecules is given. The concerted emission found is a fundamental process expected to be operative in gas phase and clusters. Its true relevance lies in its intimate relationship to concerted emission of virtual photons and thus to collective energy transfer ionizing neighboring systems. The estimated rates and examples discussed of this collective intermolecular Coulombic decay shed much light on recent puzzling experiments.