Figure 1: Coherent momentum transfer and heating in an optical grating.

(a) Photoabsorption in the singlet ladder of C₇₀ is followed by a rapid inter-system crossing (ISC) to the lowest triplet state^29,44 whose lifetime exceeds s, even at high temperatures^45,46. The singlet-state lifetime is on the order of τ_s≃1 ns. The absorbed photon energy is therefore predominantly dissipated non-radiatively into the vibrational degrees of freedom, increasing the internal temperature in steps of ΔT per photon. (b) A molecular ensemble characterized by the transverse momentum state |p〉 and internal, microcanonical temperature T₀ diffracts at a light grating. The interaction of each molecule with the light grating, described by the operator , correlates the internal temperature increase ΔT with a coherent momentum change Δp=±ħk. An additional amplitude and phase modulation of the molecular wave function results in a superposition of diffraction orders, separated by two photon momenta for each temperature class. The temperature of each sub-ensemble is related to the number of absorbed photons n through T_n=T₀+nΔT, where T₀ is the internal temperature of the molecules when they reach the light grating.