  We propose a new method to measure the absolute photon absorption cross
section of neutral molecules in a molecular beam. It is independent of our
knowledge of the particle beam density, nor does it rely on photo-induced
fragmentation or ionization. The method is based on resolving the recoil
resulting from photon absorption by means of near-field matter-wave
interference, and it thus applies even to very dilute beams with low optical
densities. Our discussion includes the possibility of internal state conversion
as well as fluorescence. We assess the influence of various experimental
uncertainties and show that the measurement of absolute absorption cross
sections is conceivable with high precision and using existing technologies.

Quantum Physics