In KDTLI, the molecular interference patterns have a period of 266 nm. Such fringe systems are easily shifted when every single molecule absorbs even only a single visible photon. Subsequent decoherence by the emission of photons is often strongly suppressed by internal energy conversion in intersystem crossings.
The incident light arrives as a Poissonian photon distribution and not every molecules receives a momentum kick. This makes the molecular interference pattern self-referencing: the ratio of shifted to unshifted amplitudes is a direct measure for the average number of photons absorbed per molecules. This provides us with absolute absorption cross sections of even very dilute molecular beams.
In Talbot-Lau interferometry with optical gratings, the diffraction pattern also depends on the phase the molecular de Broglie wave accumulates in the standing light wave. This is proportional to both the laser intensity and the optical molecular polarizability at the diffracting laser wave length. A trace of the molecular interference fringe visibility as a function of the diffracting laser power (or molecular velocity) allows us to extract the optical polarizability.