SPECTROSCOPY
Photon-recoil imaging: Expanding the view of
nonlinear x-ray physics
U. Eichmann^1 *, H. Rottke^1 , S. Meise^1 , J.-E. Rubensson^2 , J. Söderström^2 , M. Agåker2,3, C. Såthe^3 ,
M. Meyer^4 , T. M. Baumann^4 , R. Boll^4 , A. De Fanis^4 , P. Grychtol^4 , M. Ilchen4,5, T. Mazza^4 , J. Montano^4 ,
V. Music4,5, Y. Ovcharenko^4 , D. E. Rivas^4 , S. Serkez^4 , R. Wagner^4 , S. Eisebitt1,6
Addressing the ultrafast coherent evolution of electronic wave functions has long been a goal of nonlinear
x-ray physics. A first step toward this goal is the investigation of stimulated x-ray Raman scattering
(SXRS) using intense pulses from an x-ray free-electron laser. Earlier SXRS experiments relied on signal
amplification during pulse propagation through dense resonant media. By contrast, our method reveals the
fundamental process in which photons from the primary radiation source directly interact with a single
atom. We introduce an experimental protocol in which scattered neutral atoms rather than scattered
photons are detected. We present SXRS measurements at the neon K edge and a quantitative theoretical
analysis. The method should become a powerful tool in the exploration of nonlinear x-ray physics.
O
ur awareness of nonlinear processes has
changed the way we think about the
interaction between light and matter,
and has inspired the development of
numerous optical techniques ( 1 – 3 ).
Among the most fundamental and practically
important applications of these processes is
the control and manipulation of quantum dy-
namics in atoms and molecules by short and
intense laser pulses. Typically, such experi-
ments involve a combination of light pulses
that couple to the coherent wave function
dynamics of the investigated system (e.g., in
the stimulated Raman adiabatic passage
technique) ( 4 ). This type of experiment has
been crucial in the progress of quantum
engineering of chemistry and quantum mat-
ter ( 5 ). It has been a long-standing vision
to extend the concepts from the optical into
the x-ray regime; combining the merits of
laser and x-ray spectroscopy would enable
entry into the world of nonlinear x-ray phys-
ics ( 6 ), where local quasi-atomic core levels
give access to interactions on atomic length
and time scales. This goal has been one of
the motivations for developing x-ray free-
electron lasers (XFELs), and indeed these
facilities now provide pulse intensities suf-
ficient for investigation of nonlinear x-ray
physics ( 6 – 11 ).Stimulated x-ray emission has been ob-
served in dense gases ( 12 , 13 ), in liquids ( 14 ),
and in solids ( 15 , 16 ), and considerable effort
has gone into the development of stimulated
x-ray Raman scattering (SXRS), which is the
basic building block of nonlinear x-ray spec-
troscopy ( 13 , 17 ). SXRS was first observed ( 18 )
by direct analysis of the scattered photons in
an x-ray spectrometer, exploiting the amplifi-
cation of the XFEL pulses in a dense medium.
Although such x-ray lasing effects highlight
pulse propagation in media and are of great
importance in nonlinear x-ray optics, their
modeling introduces uncertainties that im-
pede investigation of the quantum dynam-
ics in single atoms or molecules.
For dilute samples, direct measurement of
SXRS is hampered by a background attribut-
able to the primary photons, which almost coin-
cide with the scattered photons in momentum
space. We introduce photon-recoil imaging
(PRI), which monitors the momentum trans-
fer from the photons to the atoms by the
deflection of their path in a supersonic beam.
The low momentum transfer in SXRS is used
to discriminate against spontaneous x-ray
Raman scattering, in which the momentum
transfer is much larger. PRI also discriminates1630 25 SEPTEMBER 2020•VOL 369 ISSUE 6511 sciencemag.org SCIENCE
(^1) Max Born Institute, 12489 Berlin, Germany. (^2) Department
of Physics and Astronomy, Uppsala University, SE-751 20
Uppsala, Sweden.^3 MAX IV Laboratory, Lund University,
SE-221 00 Lund, Sweden.^4 European XFEL, 22869 Schenefeld,
Germany.^5 Institut für Physik, University of Kassel, 34132
Kassel, Germany.^6 Institut für Optik und Atomare Physik,
Technische Universität Berlin, 10623 Berlin, Germany.
Corresponding author. Email: [email protected]
Fig. 1. Scheme and experimental setup of the
Ne recoil measurement to detect stimulated
x-ray Raman scattering.(A) Excitation and
decay pathways in x-ray scattering in the vicinity
of the inner-shell 1s→3p transition. Ne[2p–^1 3p]
is populated by spontaneous Raman scattering
(solid red arrow plus blue wiggly line) and
by stimulated Raman scattering via two photons
from the XFEL with photon energiesħwLandħw′L
(solid and dashed red arrows). The transiently
excited resonance decays predominantly nonra-
diatively by emitting an Auger electrone–Aug(solid
blue arrow). Detectable long-lived metastable
Ne[2p–^1 3s] atoms are formed by spontaneous
decay of the Ne[2p–^1 3p] state on a nanosecond
time scale (green arrow). (B) Experimental setup
to detect metastable Ne atoms and their
deflection with a position-sensitive MCP detector.
(C) Possible projections of deflected atoms from
the interaction volume on the detector. Left:
Projection of the mere interaction volume (black
line). Center: Ne pattern after spontaneous
Raman scattering (right side). Scheme of Ne
deflection: Absorption of an XFEL photon causes
photon momentum in the forward direction
(red arrows), followed by isotropic momentum transfer after spontaneous x-ray photon emission (blue arrows). Right: Ne* pattern after stimulated
Raman scattering is a mere projection of the interaction volume (red line).
RESEARCH | REPORTS