Unveiling and Driving Hidden Resonances with High-Fluence, High-Intensity X-Ray Pulses

Abstract

We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s↔2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.

Full text

Unveiling and Driving Hidden Resonances with High-Fluence, High-Intensity X-Ray Pulses
E. P. Kanter,
1,
*B. Kra
¨ssig,
1
Y. Li,
1
A. M. March,
1
P. Ho,
1
N. Rohringer,
2,3,4,5,†
R. Santra,
1,6,7,5,‡
S. H. Southworth,
1
L. F. DiMauro,
8
G. Doumy,
8,§
C. A. Roedig,
8
N. Berrah,
9
L. Fang,
9
M. Hoener,
9
P. H. Bucksbaum,
10
S. Ghimire,
10
D. A. Reis,
10
J. D. Bozek,
11
C. Bostedt,
11
M. Messerschmidt,
11
and L. Young
1,k
1
Argonne National Laboratory, Argonne, Illinois 60439, USA
2
Lawrence Livermore National Laboratory, Livermore, California 94551, USA
3
Max Planck Institute for the Physics of Complex Systems, 01187 Dresden, Germany
4
Max Planck Advanced Study Group, Center for Free-Electron Laser Science, DESY, Notkestraße 85, 22607 Hamburg, Germany
5
Kavli Institute for Theoretical Physics, University of California, Santa Barbara, California 93106, USA
6
Center for Free-Electron Laser Science, DESY, Notkestraße 85, 22607 Hamburg, Germany
7
Department of Physics, University of Hamburg, Jungiusstraße 9, 20355 Hamburg, Germany
8
Ohio State University, Columbus, Ohio 43210, USA
9
Western Michigan University, Kalamazoo, Michigan 49008, USA
10
PULSE Center, SLAC, Menlo Park, California 94025, USA
11
Linac Coherent Light Source, SLAC, Menlo Park, California 94025, USA
(Received 15 June 2011; published 30 November 2011)
We show that high fluence, high-intensity x-ray pulses from the world’s first hard x-ray free-electron
laser produce nonlinear phenomena that differ dramatically from the linear x-ray–matter interaction
processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs
duration to first reveal and subsequently drive the 1s$2presonance in singly ionized neon. This photon-
driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape
modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the
target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically
inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at
unexpectedly low photon energies.
DOI: 10.1103/PhysRevLett.107.233001 PACS numbers: 32.80.Aa, 32.70.Jz, 32.80.Hd, 32.80.Wr
Ultraintense, tunable x-ray pulses recently available
from x-ray free-electron lasers (XFELs) [1] increase the
intensity and fluence available in a single x-ray pulse up to
a billion-fold over that typically available at synchrotron
facilities. As a result, XFELs provide a unique opportunity
to investigate nonlinear phenomena at short wavelengths.
Initial experiments with XFELs demonstrated the most
basic nonlinear x-ray process, multiphoton absorption, us-
ing photon energies far removed from absorption reso-
nances, first in the extreme ultraviolet [2] and later in the
soft x-ray regimes, in atoms [3,4] and molecules [5–7]. The
first experiment at the LCLS (Linac Coherent Light
Source), the world’s first hard x-ray free-electron laser,
revealed the ability of a single !100-fs x-ray pulse, at a
fluence of !1020 !=cm2to strip a neon atom of all its
electrons thereby irrevocably altering the target [3]. Thus,
the use of high-fluence, high-intensity x-ray radiation as a
controlled probe of atomic, molecular and material prop-
erties poses a unique challenge for experimentalists, one
where characterization of interaction mechanisms at a
fundamental level will play an important role.
These early experiments at LCLS [3–7] all studied
photon-matter interactions in a continuum, in principle,
far removed from resonances. In this study, we focus on
resonant interactions. (Earlier work in the EUV using the
FLASH FEL to study laser interactions in rare gases [8,9] at
intensities and fluences up to 1016 W=cm2and 1018 !=cm2,
invoked resonances to explain enhanced ion yields, but did
not observe multiphoton resonance behavior directly, in
contrast to the present study.) Resonances provide interac-
tion strengths that are more than 1000-fold larger than those
in the continuum and the ability to selectively address
quantum states. Specifically, at extreme intensities ap-
proaching 1018 W=cm2, Rabi cycling [10,11] can effec-
tively compete with Auger decay [12] and directly modify
the branching between decay channels. Here, starting with a
neutral neon target, we used ultraintense, high-fluence x-ray
pulses from the LCLS to first reveal and subsequently drive
the ‘‘hidden’’ 1s!2presonance in singly ionized neon
and thus demonstrate the ability to modify femtosecond
Auger decay. Our work illustrates the complexities associ-
ated with using ultraintense, high-fluence x-ray pulses as a
controlled probe of matter and is a first step toward photonic
control of inner-shell electrons.
While considerable effort has been devoted to the con-
trol of atomic and molecular processes using ultrafast laser
technology to manipulate valence electrons [13–15], active
control of inner-shell electron processes is unexplored.
There is potential for wide-ranging applications, e.g., in-
hibition of Auger decay could suppress x-ray radiation
damage [16] and modification of inner-shell electronic
structure can alter nuclear lifetimes dominated by internal
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0031-9007=11=107(23)=233001(5) 233001-1 !2011 American Physical Society

conversion [17] or electron capture decay [18]. The ab-
sence of research is due to the ultrafast nature of inner-shell
decay and the lack of a suitably intense radiation source to
selectively address inner-shell electron motion on the rele-
vant time scale. With the realization of the LCLS [1] and
impending arrival [19–21] of several other x-ray free-
electron lasers (XFELs), this longstanding deficiency has
been, to some extent, alleviated. However, the properties of
present-day XFELs, based on the self amplified spontane-
ous emission (SASE) mechanism, [22] are not ideal for
quantum control experiments. The lack of longitudinal
(temporal) coherence prevents the direct observation of
Rabi cycling, even in an isolated two-level system. The
current situation is not unlike early research with intense
optical lasers, where the effects of a strong stochastic field
on atomic transitions were considered more than 30 years
ago [23,24].
In anticipation of XFELs theoreticians have considered
the effects of strong-field excitation of inner-shell reso-
nances [12,25]. The incoherent nature of the SASE exci-
tation pulse of current XFELs causes the mean times
between excitation and stimulated emission to fluctuate.
Rohringer and Santra (RS) considered the specific question
of how such XFEL fields would affect resonant Auger
transitions in the x-ray regime [12]. RS showed that
although it was impractical to observe the fs-scale Rabi
cycling directly, the effect of the excited state population
time being shortened by stimulated emission could be
observed by the resultant broadening of the Auger emis-
sion lines. It is that effect we investigate here.
We chose to study the Neþ1s!2ptransition, although
the quasi-two-level system originally treated [12] was the
prominent 1s!3presonance in Ne. This choice facili-
tates both theory and experiment because the 1s"2p
resonance is better isolated (more than 70 natural line-
widths separated from the next Rydberg excitation,
1s"3p), allowing freedom from lineshape distortion
[26] and is stronger by 30#than that Rydberg transition,
decreasing the intensity requirements for Rabi cycling. The
only experimental drawback is the lack of a 2phole in the
ground state of neon, thus normally ‘‘hiding’’ the reso-
nance. We overcome this by using a single SASE FEL
pulse both to prepare the desired state, singly ionized neon
containing a 2phole, and to drive the 1s!2ptransition
resonantly, as shown in Fig. 1. By using a relatively short
FEL pulse we accrue the additional advantage of relatively
clean electron spectra in the region of interest. (We pre-
viously demonstrated [3] that longer pulses produce photo-
and Auger electrons from higher ionization stages
significantly complicating the spectra.) Since the energy
of the 1s"2presonance, 848 eV, lies well below the
binding energy of a 1selectron in neutral neon, 870 eV, a
single-photon excitation at 848 eV cannot produce a 1s
hole without a 2pvacancy and the appearance of Auger
electrons is a clear signature of the 2presonance. A
comparison of the Auger line profiles for resonant
(848 eV) and nonresonant (930 eV) excitation with theory
then constitutes the evidence for the modification of the
Auger decay through Rabi cycling of inner-shell electrons.
We stepped the x-ray energy through the region of
interest (840–860 eV), by tuning the electron beam energy
of the LCLS, and recorded electron emission spectra at
each step. The x-ray pulses (of nominal energy, duration,
and focus 0.3 mJ, 8.5 fs, and 1#2!m2would yield
intensities approaching 1018 W=cm2, assuming a beam
line transmission of $20%) intersected a Ne gas jet in
the High Field Physics chamber. These pulse parameters
were confirmed through comparisons of measured charge
state distributions with theoretical simulations. [3]
The experimental setup and protocol have been detailed
in [3] and references therein. The only significant change
was to record the LCLS electron beam energy shot-by-shot
in the data stream [1]. This allowed us to characterize the
x-ray radiation bandwidth and jitter. Because the x-ray
energy, Exin eV, is related to the electron energy, Eein
GeV, by Ex¼44:25E2
e, we could (1) determine the photon
energy of individual x-ray pulses to &0:1 eV, and (2)
decompose the observed x-ray photon energy spread
($0:7%) into components due to jitter ($0:5%) and in-
trinsic bandwidth ($0:5%)
Electron yields versus incident x-ray energy, corrected
on a shot-by-shot basis, are mapped in Fig. 2(a). Only
electrons emitted perpendicular to the x-ray polarization
FIG. 1 (color online). Revealing and driving a hidden reso-
nance within a single SASE pulse. An x-ray pulse at 848 eV first
strips a 2pelectron from Ne to reveal and then excite the
Neþ1s!2presonance. Stimulated emission competes with
Auger decay to refill the 1shole. Cycling is terminated by
Auger decay which changes the resonance energy.
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axis are shown, thus suppressing 2sphotoelectrons. The 2p
photoelectrons disperse linearly with the incident photon
energy, forming a prominent diagonal line, whereas the
Auger electrons are independent of Ex, forming vertical
lines. Additional higher-lying vertical lines represent
Auger decays from higher charge state Neþqions.
Unlabeled diagonal features are photoelectron correlation
satellites. The appearance of Auger lines near Ex¼848 eV
is a clear signature of the 1s#2presonance in Ne1þ
resulting from the valence ionization, resonant excitation
sequence shown in Fig. 1.
This resonance is shown in Fig. 2(b), where the 1D
Auger yield is projected onto the x-ray photon energy
axis, Ex. The signature Auger electrons appear only within
a few eV of the expected 1s!2presonance of singly-
ionized neon at 848 eV. The small resonance $8:5 eV
higher in energy is attributed to the 1s#2pexcitation in
doubly-ionized neon (2p#2!1s#12p#1). The data are fit
using Voigt profiles with a fixed Lorentzian width of
0.27 eV (corresponding to a lifetime of 2.4 fs) [27,28]
and Gaussian FWHM of 5.6 eV. These resonances are
not present in the original target and are only revealed as
a consequence of sequential valence photoionization at
photon energies below the 1s-binding energy of neutral
neon, 870 eV. Figure 2(c) shows, for the first three
ionization stages the strength and location of hidden ab-
sorption resonances that are induced by the high-fluence
LCLS pulses. The 1s!2pabsorption resonances are
enormous (10’s of Mb), roughly 3 orders of magnitude
larger than neutral neon absorption cross sections in this
vicinity ($10 kb) arising from valence photoabsorption.
(Our simple Hartree-Fock-Slater calculations provide
inner-shell transition energies accurate to $10 eV;
energetics in extended molecular systems would be more
difficult to predict.)
Next, in Fig. 3, we show the Auger line profile off- and
on-resonance, as evidence of Rabi cycling. The off-
resonant Auger line profile, shown in (a), is used to
determine the instrumental function of the electron spec-
trometer (Gaussian FWHM 0.56 eV). The on-resonance
Auger line profile (b), is obtained by projecting electron
kinetic energies for incident photon energies within %1 eV
of the 1s#2presonance energy. The theoretical simula-
tion (an extension of [12] to be published elsewhere) for
the resonant Auger line shape, using beam parameters
comparable to the experiment, is overlaid. The Auger
line profile was averaged over a large ensemble of chaotic
x-ray pulses [12] and integrated over the laser spatial
profile, (transversely Gaussian with a Rayleigh range of
1.5 mm), and weighted by the spectrometer efficiency and
gas density distribution (Gaussian FWHM of 1.6 mm). The
agreement is excellent. Panel (c) shows the theoretical
simulations for off- and on-resonance excitation prior to
convolution with the instrument function, and more clearly
demonstrates the effect of the resonant strong-field
excitation.
Intensity averaging over the focal volume—a phenome-
non common to all single-beam experiments—leads to the
FIG. 2 (color online). (a) Electron emission from Ne vs x-ray energy, observed at 90&to the x-ray polarization axis. Photoelectron
lines disperse linearly with photon energy while Auger lines are independent of photon energy [31]. The data are normalized to
represent electron yields from x-ray irradiation of 30 Joules=eV. (b) 1DAuger electron yield as a function of x-ray photon energy.
(c) Absorption resonances for higher charge states that can be produced by high-fluence x-ray pulses calculated by the Hartree-Fock-
Slater (HFS) method. Neutral Ne resonances, with a maximum cross section of 1.5 Mb, are barely visible, compared to the many ionic
resonances.
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rather modest modification of the Auger line profile. The
calculated occupation probabilities of the relevant configu-
rations, Fig. 4(a), show that only a small fraction (<2%) of
the sample contributes to the Rabi cycling. Note that as a
consequence of the valence ionization step (Fig. 1), the
coherence, defined in this case of a symmetric 2!2
density matrix, as the ratio of the magnitude of the off-
diagonal matrix element to the square root of the product of
the diagonal elements [29], differs from unity even for a
single SASE pulse, as shown in Fig. 4(b). With a longi-
tudinally coherent pulse, as should soon be available
through self-seeding schemes [30], the situation improves
dramatically, as shown in Fig. 4(c), where resonant Auger
line profiles are compared for an ensemble of SASE and
Gaussian pulses of equal fluence and pulse duration
(FWHM). Gaussian pulses drive the resonance more effec-
tively than SASE pulses because the incoherent spikes in
the SASE ensemble have a larger effective bandwidth.
While the excitation of the hidden 1s!2presonance is
advantageous here, it is a potential liability for other XFEL
experiments. A recent example was the investigation of
multiphoton x-ray ionization in which a significant back-
ground contribution was attributed to a hidden single-
photon resonance in Ne7þthat allowed sequential valence
ionization to compete with direct multiphoton ionization of
ground state Ne8þto produce Ne9þ[4].
In summary, this work illustrates the nuances associated
with using high-fluence, high-intensity femtosecond x-ray
pulses for controlled investigations of material properties.
We demonstrate that high-fluence x-ray pulses reveal oth-
erwise hidden resonances through sequential valence
ionization. Photoexcitation of these resonances break
open inner shells at unexpectedly low photon energies,
i.e., below the 1sthreshold, and thereby unleash damaging
Auger electron cascades. These phenomena must be
considered in the design of all future XFEL experiments.
We further demonstrate that a strong, incoherent
SASE pulse can induce Rabi cycling on a deep inner-shell
transition and thus modify Auger decay. Control of
inner-shell electron dynamics should be markedly en-
hanced with soon-to-be-available longitudinally coherent
x-ray pulses.
This work was supported by the Chemical Sciences,
Geosciences, and Biosciences Division of the Office of
Basic Energy Sciences, Office of Science, U.S.
Department of Energy (DE-AC02-06CH11357, DE-
FG02-04ER15614, DE-FG02-92ER14299). N. R. was sup-
ported by the U.S. Department of Energy by Lawrence
Livermore National Laboratory (DE-AC52-07NA27344).
N. R. and R. S. were supported in part by the National
Science Foundation under Grant No. NSF PHY05-51164.
M. H. thanks the Alexander von Humboldt Foundation for
a Feodor Lynen fellowship. P. H. B., S. G., and D. A. R.
were supported through the PULSE Institute, which is
jointly funded by the Department of Energy, Basic
Energy Sciences, Chemical Sciences, Geosciences and
Biosciences Division and Division of Materials Science
and Engineering. LCLS is funded by the U.S. Department
of Energy’s Office of Basic Energy Sciences.
0 5 10 15 20
Time(fs)
0
0.01
0.02
a
[1s] occupation
[2p0] occupation
0.2
0.4
0.6
0.8
b
SASE x-ray pulse
coherence
-5 -4 -3 -2 -1 0 1 2 3 4 5
Relative electron energy (eV)
0
0.2
0.4
0.6
0.8
Auger Yield (arb.)
c
Gaussian
SASE
FIG. 4 (color online). Theoretical simulations for resonant
1s!2pexcitation of neon with FEL pulses of intensity
3:5!1017 W=cm2. (a) Occupation probabilities for the [1s]
and [2p0] vacancy states of the Neþion as a function of time
for irradiation by the single SASE pulse shown in (b). (b) The
degree of coherence between the [1s] and [2p0] vacancy states
and SASE pulse profile used in (a) and (b). (c) The resonant
Auger line shape generated by an ensemble of SASE pulses
(averaged Gaussian temporal profile of 8.5 fs FWHM, 6 eV
bandwidth) and a longitudinally coherent Gaussian pulse (8.5 fs
FWHM, transform-limited).
FIG. 3 (color online). Electron kinetic energy spectra of the 1D
Auger line. (a) Nonresonant Auger, Ex¼930 eV. (b) Resonant
Auger, Ex¼848 $1 eV. Solid lines are the simulations for the
experimental conditions: resonant (blue), nonresonant (red).
(c) Simulations of Auger line shape before convolution with
the instrumental function. All curves in this figure are normal-
ized to the integrals over the displayed energy region.
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*kanter@anl.gov
†
Present Address: Max Planck Advanced Study Group,
Center for Free-Electron Laser Science, Hamburg
22607, Germany.
‡
Present Address: Center for Free-Electron Laser Science,
Hamburg 22607, Germany.
§
Present Address: Argonne National Laboratory, Argonne,
IL 60439, USA.
k
young@anl.gov
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