limit of this region is∼6.4 atomic units, and
the upper limit extends to include all electrons
at higher momenta. Equation 2 can be used to
convert the streak angle into a time delay, and
this value is used to label the clock face in Fig.
1D and the lower horizontal axis in Fig. 1E.
At the Linac Coherent Light Source (LCLS),
the synchronization of the streaking laser and
x-ray pulse has a jitter of roughly∼500 fs ( 20 ),
which is orders of magnitude below the re-
quired precision for directly timing the AM
process. Thus, to produce the images shown in
Fig. 1D, we must use a single-shot diagnostic of
the relative arrival time between the x-rays
and laser pulse. As described above, in addi-
tion to driving resonant excitation near the
oxygen K-edge, the attosecond x-ray pulse
ionized electrons from the nitrogen K-shell
oftheNOmolecule(seeFig.1,BandC).This
direct photoionization process produced high
energy (∼120 eV) electrons. The photoionization
delay between the arrival of the x-ray pulse and
the appearance of these fast photoelectrons in
the continuum was negligibly small (≲5 as)
compared to the streaking laser periodTLof
7.7 fs ( 21 – 23 ). Therefore, the momentum shift
observed for the nitrogen K-shell photoemission
feature provided an accurate, single-shot mea-
surement of the direction of the streaking laser
vector potentialA→
0 at the time of arrival of
the x-ray pulse.
We monitor the AM yield in a small angular
region of the detector to avoid introducing
artifacts in the extracted time-dependent trace
due to angular anisotropy in the AM emission
( 24 ). The period of the streaking field was
chosen to be longer than the dominant time
scale of the AM process. This fact simplifies
interpretation of the streaking measurement
by limiting the effect of“wrapping,”where
electrons released into the continuum at time
tandtþTLexperience a similar momentum
kick from the streaking field.
The time-dependent electron yield shown
in Fig. 1E shows a maximum att=0,when
A→
0 was directed along the detection direction
and the the core-excited population (and AM
emission rate) was at a maximum. In additionto an exponentially decaying electron emis-
sion rate, we observed a revival in the time-
dependent emission rate att= 3.5 fs.Model
We modeled our measurement according to
the theory of attosecond streaking of multiple
Fano resonances described by Wickenhauseret al.
( 25 , 26 ). Our model, illustrated in Fig. 2A, in-
cluded a ground state that is doubly degenerate
and was resonantly coupled to four bound
states, one of which (^2 D) is also doubly de-
generate, and thus is labeled as a single state
in the figure. These bound states were also
coupled to a single, structure-less continuum,
which was dressed by the circularly polarized,
2 : 3 mm streaking laser field. The coupling
between the bound and continuum states was
the result of electron correlation interactions
and drove the AM decay process. The bound
states had excitation energies of 531:5 eV (^2 S ),
532 :6 eV (^2 D), and 533:5 eV (^2 Sþ), which re-
presented the core-excitation spectrum of nitric
oxide ( 27 ). The continuum coupling constant288 21 JANUARY 2022•VOL 375 ISSUE 6578 science.orgSCIENCE
Fig. 3. Comparison between model and experimental results for resonant
Auger-Meitner emission.(A) Measurement of time-resolved Auger-Meitner
emission from core-excited NO. The left panel shows the experimentally
measured time-dependent AM yield for the various central XFEL photon energies
(black dots). Colored error bars have a total length of four times the SEM
of the measured electron yield,T 2 sx. This measurement is compared with the
results of the model shown in Fig. 2 (solid colored lines). The right panel shows
total electron yield, which decreases as the central photon energy moves away
from the center of the 1 s→presonance (bars). The time-dependent yields
change by a factor of 2 between the minimum (normalized to 0) and maximum
(normalized to 1) values. The coherent bandwidth of the attosecond XFEL pulse
spans∼5 eV, as illustrated by a Gaussian curve of equivalent full width at half
maximum at each central photon energy. The black line shows the O1s→p
feature reported in ( 27 ), comprising the^2 S ,^2 D, and^2 Sþelectronic states. The
revival att∼3.5 fs, marked by the black vertical arrow, is due to the rephasing
(constructive interference) of the AM emission from the core-excited states
(^2 Sand^2 D) populated by the x-ray pulse. The coherent revival is suppressed as
the photon energy moves above the 1 s→presonance and the contribution
from the direct photoionization channel increases. The photon energy–
dependence of the quantum beat is shown in the magnified image in (B), for
experiment (left) and simulation (right). The shaded area represents the streak-
angle–dependent yield with corresponding error barT 2 sx, and the solid line
shows this electron yield after application of a high-frequency filter along the
time axis (see supplementary materials for further details). The color of the
curves corresponds to the central photon energies shown on the left side of (A).
(C) Comparison between two different models where core-excited states are
populated coherently (deep red) and incoherently (pale red) at 533-eV central
photon energy. The experimental measurement is shown in black dots with error
barsT 2 sx. Coherent interaction between the core-excited states is required to
account for the measured data.RESEARCH | RESEARCH ARTICLES