and NAIPs (21), at the interface with microbial
ligands (36). Thus, we propose that multisurface
recognition is one strategy in the arsenal deployed
by hosts to counteract the intrinsic advantage
held by large populations of rapidly evolving pathogens in their “arms race” with eukaryotic immune systems.
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We thank T. Houweling and A. Chintangal for computer support.
The EM data was collected in the EM facility of the Howard
Hughes Medical Institute Janelia Research Campus. We are
thankful to C. Hong and Z. Yu for expert EM assistance; B. Greber,
R. Louder, A. Patel, and A. Sandstrom for discussion; P. Dietzen
for technical support; K. Namba for the coordinates of the
R-type flagellar filament (26); and A. Truxal for generating the
illustration in Fig. 3C. We acknowledge the use of the LAWRENCIUM
computing cluster at Lawrence Berkeley National Laboratory and
the resources of the National Energy Research Scientific Computing
Center, a U.S. Department of Energy (DOE) Office of Science user
facility supported by the Office of Science of the DOE under contract
no. DE-AC02-05CH11231. We thank K. Smith for the contribution of
a CHO cell line stably expressing Hs TLR5 and NFkB luciferase
reporters, G. Barton for S. typhimurium strains, S. Brubaker and
D. Monack for P22 transducing phage, and H. Darwin for advice on
analysis of flagellin expression. This work was funded by the
Spanish Ministry of Economy, Industry and Competitiveness grant
BFU2016-76220-P (P.C.) and NIH grants AI075039 and AI063302
(R.E.V.). N.H. and J.L. T. were supported by the NSF Graduate
Research Fellowship Program. R.E.V. and E.N. are Howard Hughes
Medical Institute Investigators. The cryo-EM map has been
deposited in the Electron Microscopy Databank with accession code
EMD-7055. The atomic coordinate model has been deposited in the
Protein Data Bank (PDB) with accession code 6B5B.
Materials and Methods
Figs. S1 to S17
15 June 2017; accepted 4 October 2017
Photoionization in the time and
M. Isinger,1 R. J. Squibb,2 D. Busto,1 S. Zhong,1 A. Harth,1 D. Kroon,1 S. Nandi,1
C. L. Arnold,1 M. Miranda,1 J. M. Dahlström,1,3 E. Lindroth,3 R. Feifel,2
M. Gisselbrecht,1 A. L’Huillier1
Ultrafast processes in matter, such as the electron emission after light absorption, can now
be studied using ultrashort light pulses of attosecond duration (10−18 seconds) in the
extreme ultraviolet spectral range. The lack of spectral resolution due to the use of short
light pulses has raised issues in the interpretation of the experimental results and the
comparison with theoretical calculations. We determine photoionization time delays in
neon atoms over a 40–electron volt energy range with an interferometric technique
combining high temporal and spectral resolution. We spectrally disentangle direct
ionization from ionization with shake-up, in which a second electron is left in an excited
state, and obtain excellent agreement with theoretical calculations, thereby solving a
puzzle raised by 7-year-old measurements.
Although femtosecond lasers allow for the study and control of the motion of nuclei n molecules, attosecond light pulses give access to even faster dynamics, such as electron motion induced by light-matter
interactions (1). During the last decade, seminal
experiments with subfemtosecond temporal resolution have allowed the observation of the electron valence motion (2), monitoring of the birth
of an autoionizing resonance (3, 4) or the decay
of a core vacancy (5). Fast electron motion occurs
even when electrons are directly emitted from
materials upon absorption of sufficiently energetic
radiation (the photoelectric effect). The time for
the photoelectron emission (6), called photoionization time delay (7, 8), is typically on the order
of tens of attoseconds, depending on the excitation
energy and the underlying ionic core structure.
Photoemission has traditionally been studied
in the frequency domain, using high-resolution
photoelectron spectroscopy with x-ray synchro-
tron radiation sources, and such methods have
provided a detailed understanding of the elec-
tronic structure of matter (9, 10). Absorption of
light in the 60 to 100 eV range by neon atoms, for
example, leads to direct ionization in the 2s or 2p
shells and to processes mediated by electron-
electron interaction, leaving the residual ion in
an excited state (often called shake-up) or doubly
It may be argued that the high temporal resolution achieved in attosecond experiments prevents
any spectral accuracy and thus may affect the
interpretation of experimental results. This is especially true when different processes can be induced simultaneously and lead to photoelectrons
with kinetic energies within the bandwidth of
the excitation pulse. In fact, the natural trade-off
between temporal and spectral resolution may
be circumvented, as beautifully shown in the visible spectrum using high-resolution frequency
combs based on phase-stable femtosecond pulse
Here, we bridge the gap between high-resolution
photoelectron spectroscopy and attosecond dynamics, making use of the high-order harmonic
spectrum obtained by phase-stable interferences
between attosecond pulses in a train. Photoionization time delays of the 2s and 2p shells are
measured in neon over a broad energy range from
65 to 100 eV, with high temporal (20 as) and spectral (200 meV) accuracy. Our spectral resolution
comes from the sharpness of the harmonic comb
teeth (harmonic bandwidth), which is larger
than the spectral resolution of our spectrometer.
Time delays are obtained by measuring spectral
phase derivatives with an interferometric technique
originally introduced for characterizing attosecond
pulses in a train, called RABBIT (reconstruction
of attosecond beating by interference of two-photon transition) (15, 16). Temporal accuracy
SCIENCE sciencemag.org 17 NOVEMBER 2017 • VOL 358 ISSUE 6365 893
1Department of Physics, Lund University, P.O. Box 118, SE-22
100 Lund, Sweden. 2Department of Physics, University of
Gothenburg, Origovägen 6B, SE-41 296 Göteborg, Sweden.
3Department of Physics, Stockholm University, SE-106 91
*Corresponding author. Email: firstname.lastname@example.org