In one of the first user experiments at the SQS instrument at the European XFEL in Hamburg the German-Swedish collaboration has established the photon-recoil imaging technique to distinguish spontaneous from nonlinear stimulated x-ray Raman scattering. To understand the new technique it is helpful to recall that a photon, i.e., the quantized energy portion of electromagnetic radiation, also carries momentum; a fact that is well-known since the early days of quantum physics at the times of Einstein. As a consequence, absorption of a photon inevitably pushes the atom very much like a billiard ball gets a push when hit by another one. In the spontaneous Raman scattering process, transient absorption of a photon is rapidly followed by spontaneous emission of a photon with less energy. The photons’ energy difference remains in the atom lifting an electron to a bound excited state. While the absorption pushes the atom in the direction of the incoming photon, the spontaneous emission of a photon, which happens into a random direction, scatters the atom accordingly to a corresponding opposite random direction.
To detect the scattered excited atoms in the experiment the scientists used a collimated supersonic beam of neon atoms, which travel towards a position-sensitive detector. The detector is set to be sensitive to impinging excited atoms only. Well in front of the detector the atomic beam is crossed perpendicularly by the XFEL beam defining a sharp elongated interaction volume. If the x-ray photon energy is tuned close to an inner-shell resonance of the Ne atom, about 2 % of the transiently excited atoms undergo spontaneous x-ray Raman scattering leaving the atom intact in finally a long lived bound excited state (the overwhelming portion of transiently excited atoms ionizes through a fast Auger process since the transient excitation energy exceeds the first ionization threshold by a factor of about 40 ). The photon momentum transfer slightly deflects the excited atoms from the collimated atomic beam. Accumulating the resulting detector signal over many XFEL pulses a characteristic pattern on the detector is produced, as shown in Fig. 1a. The pattern has an extended elliptical shape due to the deflections in random directions experienced by the atoms in the interaction volume during spontaneous Raman scattering.