Science - USA (2022-01-21)

of the degenerate 2pelectrons participates in
the decay, leading to excited cationic states.
There is a small contribution from the channel
where both 2pelectrons participate, resulting
in a2p^0 ground configuration of the cation ( 14 ).
The circularly polarized laser field maped the
temporal profile of the electron emission on
to the momentum measured at the detector.
When electrons were released from the mole-
cule following interaction with the x-ray pulse,
their trajectory was altered by the presence of
the infrared (IR) laser field, similar to the prin-
ciple of a time-resolving streak camera ( 15 , 16 ).

This interaction altered (or“streaked”) the final

electron momentum, which was measured at

the detector. In a semi-classical approxima-

tion, the final momentum of an ionized elec-

tron is given by

p

→

ðÞ¼t→∞ p

→

0 þeA

→

ðÞt 0 ð 1 Þ

whereA

→

ðÞ¼t 0 ∫t^0 ∞E

→

LðÞt′dt′is the vector po-

tential of the circularly polarized laser field,

ELðÞt, at the time of ionizationt 0 ,eis the charge

of an electron (1 atomic unit), andp

→

0 is the

momentum of the electron in the absence of

the IR laser field. All the quantities are ex-

pressed in atomic units.

In our measurement, the temporal duration

of the circularly polarized“streaking”laser

field (∼100 fs) was much longer than the laser

period (TL¼ 7 :7 fs). This fact implies that

over the time scale of a single laser period, the

vector potential had nearly constant ampli-

tude (A

(^) →
) but a direction that rotated with
constant angular velocity 2p=TL.Thus,Eq.1
describes how the streaking technique encodes
the temporal evolution of the electron emission
rate onto the electron momentum spectrum: An
286 21 JANUARY 2022•VOL 375 ISSUE 6578 science.orgSCIENCE
Fig. 1. Experimental observation of Auger-Meitner emission.(A)NOgasis
ionized by an attosecond x-ray free-electron laser (XFEL) pulse (∼530 to
540 eV, central photon energy) in the presence of a 2.3-mm circularly
polarized streaking field. The resultant photoelectron momentum distribution
is measured by a coaxial velocity map imaging spectrometer (c-VMI) ( 13 ). The
streaking field maps the instantaneous ionization rate onto the measured
photoelectron momentum distribution. (B) Single-color electron momentum
spectrum projected along the axis of the c-VMI in the absence of the
streaking field. Atomic units are denoted here and throughout as“a.u.”We
definepxto lie along the x-ray polarization axis. (C) Applying an inverse Abel
transform to this image, we retrieve the electron kinetic energy distribution
(“arb.”denotes arbitrary units). (D) Change in the projected momentum
distribution as the direction of the streaking laser vector potential (A
→
0 ,
light-gray line pointing along 0 fs of the stopwatch face) is varied. The projected
momentum distribution is presented as a difference image where the
electron momentum shown in (B) is used as a background. To observe the
temporal evolution of AM emission, we monitor the AM yield in a small (15°)
region of the detector [black box shown in the panels of (D); energetic
position also shown in pale red in (C)]. The time dependence of this yield
is shown in black dots in (E) (dashed red line shows trace with high-frequency
noise filter applied; see supplementary materials for further details).
The AM yield in (E) is plotted as a function of angle between the streaking
laser vector potential at the time of ionization and the angle of the detection
box, which is shown as a gray shaded area in (D).E
→
shows the direction of
rotation of the electric field. The red error bars have a total length of four
times the SEM of the measured electron yield,T 2 sx.
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