Extreme Ultraviolet Laser Excites Atomic Giant Resonance
M. Richter,
1,
*
M. Ya. Amusia,
2
S. V. Bobashev,
2
T. Feigl,
3
P. N. Juranic
´
,
4
M. Martins,
5
A. A. Sorokin,
1,2
and K. Tiedtke
4
1
Physikalisch-Technische Bundesanstalt, Abbestraße 2-12, 10587 Berlin, Germany
2
Ioffe Physico-Technical Institute, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia
3
Fraunhofer-Institut fu
¨
r Angewandte Optik und Feinmechanik, Albert-Einstein-Straße 7, 07745 Jena, Germany
4
Deutsches Elektronen-Synchrotron, Notkestraße 85, 22603 Hamburg, Germany
5
Universita
¨
t Hamburg, Institut fu
¨
r Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
(Received 15 January 2009; published 24 April 2009)
Exceptional behavior of light-matter interaction in the extreme ultraviolet is demonstrated. The
photoionization of different rare gases was compared at the free-electron laser in Hamburg, FLASH,
by applying ion spectroscopy at the wavelength of 13.7 nm and irradiance levels of thousands of terawatts
per square centimeter. In the case of xenon, the degree of nonlinear photoionization was found to be
significantly higher than for neon, argon, and krypton. This target specific behavior cannot be explained by
the standard theories developed for optical strong-field phenomena. We suspect that the collective giant 4d
resonance of xenon is the driving force behind the effect that arises in this spectral range.
DOI: 10.1103/PhysRevLett.102.163002 PACS numbers: 32.80.Rm, 32.80.Fb, 41.60.Cr, 42.50.Hz
The new Free-Electron Laser (FEL) in Hamburg
FLASH [1] currently produces the highest irradiance of
extreme ultraviolet (EUV) and soft x-ray pulses in the
world. 10
16
Wcm
2
were recently achieved in a photo-
ionization experiment performed at FLASH on xenon
atoms in the EUV wavelength region at about 13 nm [2].
Under these extreme conditions of ultrahigh intensities at
short wavelengths, the mechanisms of light-matter inter-
action are not well understood, as the above xenon study
clearly demonstrates. Here, we show now that the nature of
the EUV laser light on the interaction, i.e., as weak per-
turbation in terms of individual photons or as a strongly
interacting electromagnetic wave, depends significantly on
the electron structure of the target and the excitation of
strong resonances. The quiver energy transferred to free
electrons which defines the degree of perturbation of mat-
ter by powerful lasers in the optical regime [3–5] plays a
minor role in the EUV. This comes out by comparing the
nonlinear photoionization of different rare gases by ion
mass-to-charge spectroscopy. Our experiments were per-
formed at FLASH at the wavelength of 13.7 nm. The work
refers to the fundamental aspects of the photoelectric effect
[6] and is significant for any investigation on nanometer
and femtosecond scales using x-ray lasers [7–11].
The gases were investigated under equivalent conditions
in the microfocus region of a spherical multilayer mirror
developed for EUV lithography [12]. The mirror could be
moved along the FEL beam in order to shift the focus in
back-reflection geometry into and out of the interaction
volume of our ion time-of-flight (TOF) spectrometer
[2,13–15] and to vary the effective FEL beam diameter
from 4 to 200 m for our measurements. As a result, the
peak irradiance of the FEL pulses with pulse durations in
the order of 10 fs could be varied from 10
12
to 2
10
15
Wcm
2
. The respective target gas filled the experi-
mental vacuum chamber homogeneously at the consider-
ably low pressure of about 10
4
Pa to avoid any interaction
between neighboring atoms and ions. Absolute FEL pulse
energy in the microjoule regime was monitored with a
relative standard uncertainty of 15% on a shot-to-shot basis
by means of calibrated gas-monitor detectors [16,17].
Figure 1 shows ion TOF spectra of xenon measured at
the wavelength of 13.7 nm, i.e., at the photon energy of
90.5 eV, at considerably low irradiance of ð2:5 0:7Þ
10
12
Wcm
2
[Fig. 1(a)] and at high irradiances of (1:7
0:5) and ð2:0 0:6Þ10
15
Wcm
2
[Figs. 1(b) and 1(c)].
The spectra confirm our former results obtained at the
wavelength of 13.3 nm, i.e., 93-eV photon energy [2]. At
low irradiance, the photoionization of xenon (ground state
electron configuration Xe: [Kr] 4d
10
5s
2
5p
6
) is dominated
by the so-called giant 4d ! "f continuum resonance
which ranges from 14 to 11 nm, i.e., from about 88 to
113 eV photon energy [18]. Here, it is a one-photon
process and leads via Auger decay mainly to doubly or
triply charged final states with two or three electron vacan-
cies in the outer 5p shell [19]. Direct 5p emission ending
up in a singly charged final state plays a minor role. At
1:7 10
15
Wcm
2
[Fig. 1(b)], on the other hand, charge
states up to Xe
14þ
occur. Here, to achieve this highly
charged state, a single Xe atom must have absorbed 22
EUV photons of 90.5 eV each, or 1.93 keV total [20],
during a FEL pulse with a duration of 10 fs [1]. At 2:0
10
15
Wcm
2
,evenXe
19þ
is observed [Fig. 1(c)] which
requires at least 46 EUV photons to be absorbed. Thus,
the situation is beyond the scope of low-order perturba-
tion theory and a multiphoton scheme, as discussed pre-
viously [2].
Compared to xenon, the high-irradiance spectrum of
krypton, the next heaviest rare gas (Kr: [Ar] 3d
10
4s
2
4p
6
), looks significantly different as shown in Fig. 2(a).
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0031-9007=09=102(16)=163002(4) 163002-1 Ó 2009 The American Physical Society
The highest charge state of Kr at ð1:5 0:5Þ
10
15
Wcm
2
is only Kr
7þ
, which can be simply explained
by a sequence of one- and two-photon ionization processes
in which an ion created in a preceding step represents the
target for a subsequent step. Sequential multiphoton ion-
ization has already been found to be generally dominant at
moderate irradiance levels below 10
14
Wcm
2
[14,15,21].
The first four rows of Table I summarize, for all rare gases
investigated, (a) the highest charge state observable in the
spectra of Figs. 1(b) and 2 obtained at irradiance levels in
the range from 1.5 to 1:8 10
15
Wcm
2
, (b) the ioniza-
tion energy required to reach this state starting from the
atomic ground state [20,22], and (c) the corresponding
minimum number n
min
of EUV photons of 90.5 eV photon
energy which must have been absorbed within a single FEL
pulse by an individual atom to deliver this amount of
energy. As a result, n
min
strongly varies from n
min
¼ 5
for krypton and argon (Ar: [Ne] 3s
2
3p
6
), n
min
¼ 8 for
neon (Ne: 1s
2
2s
2
2p
6
), and n
min
¼ 22 for xenon.
FIG. 2 (color online). Ion time-of-flight (TOF) mass-to-charge
spectra of (a) krypton (Kr), (b) argon (Ar), and (c) neon (Ne),
taken at 90.5 eV photon energy and irradiance levels between 1.5
and 1:8 10
15
Wcm
2
. The C
n
þ
signals (n ¼ 2 to 5) possibly
arise from carbon clusters desorbed from the carbon coated BL2
focusing mirror.
FIG. 1 (color online). Ion time-of-flight (TOF) mass-to-charge
spectra of xenon (Xe) taken at 90.5 eV photon energy and ir-
radiance levels of (a) 2:5 10
12
Wcm
2
, (b) 1:7 10
15
Wcm
2
,
and (c) 2:0 10
15
Wcm
2
. Signals from residual gas are also
indicated.
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With the number of photons absorbed per atom and
pulse, the perturbation of the atom by the electromagnetic
radiation generally increases. More and more high-
perturbation orders have to be taken into account to de-
scribe the excitation process which may be interpreted as a
gradual increase of the wave character of the EUV light on
the interaction. It obviously depends strongly on the indi-
vidual target and its electron structure as demonstrated by
our results in Figs. 1 and 2, and Table I. In the optical
regime, on the other hand, the wave character of light and
the application of nonperturbative theories on the excita-
tion process like the tunneling model and the Keldysh
theory are defined by high ponderomotive energy [3–5]:
U
p
=eV 1:44
E=ð10
13
Wcm
2
Þ
ð@!=eVÞ
2
: (1)
U
p
does not depend on the target gas and its individual
electron structure but on the characteristics of the incident
radiation field only, the irradiance E and photon energy
@!, because it represents the quiver energy transferred
from the oscillating field of an electromagnetic wave to
the almost free electron of a highly excited Rydberg state.
The latter must have been populated by preceding excita-
tion steps. In the EUV, however, photoexcitation into the
continuum is the dominant process. Excitation into
Rydberg states plays generally a minor role. Thus, non-
perturbative theories which are based on ponderomotive
motion of highly excited Rydberg electrons seem not to be
adequate to describe strong-field phenomena in the EUV
which is confirmed by the target dependence of our results.
As a consequence, Eq. (1) is not applicable either to define
in the EUV the nature of light on the excitation process.
Our comparative study of rare-gas photoionization in the
EUV at ultrahigh irradiance levels clearly demonstrates the
particular behavior of Xe compared to Ne, Ar, and Kr. The
highest charge state observable by our experiments at al-
most 2 10
15
Wcm
2
, the minimum number of photons
which must have been almost simultaneously absorbed by
an individual atom to reach this state, and, hence, the
degree of atomic perturbation by the electromagnetic
field are significantly higher for Xe than for the other
gases. This is obviously reflected by the respective one-
photon ionization cross section values for 90.5 eV photons
which are listed in the last row of Table I, known from
low photon intensity experiments [17]. In the case of Xe,
the photoionization cross section is strongly enhanced by
the giant 4d ! "f continuum resonance and amounts to
24 10
18
cm
2
.
In this context, it should be noted that the 4d giant
resonance in Xe and the subsequent elements in the peri-
odic table represent prime examples for strong electron
correlation within an atomic system [23,24]. The correla-
tions may be described by a collective motion of a full
ensemble of quantum particles, where the ten electrons of
the 4d shell, driven by the oscillation field of the electro-
magnetic wave, emit one of their members. The mecha-
nism is in analogy to nuclear giant resonances of protons
and neutrons or plasmon excitation in solids and has been
applied, for many years, to describe atomic giant reso-
nances excited at low irradiances [23,24]. The application
of this idea to high irradiance, on the other hand, can
explain our Xe results: due to the higher amplitudes, the
collective oscillations within the 4d shell may lead to the
emission of more than one 4d electron, up to all ten, more
or less simultaneously and coherently. Subsequent Auger
decay cascades, then, result in the higher charge states.
Such a photoionization scheme represents an extension of
the classical photoelectric effect or strong-field ionization
in the optical regime which are described by a single
photon or an electromagnetic wave interacting just with a
single bound outer electron. The scheme might also ex-
plain the unexpected irradiance dependence of the higher
charge states as discussed in our previous work [2].
The direct multiple ionization of Xe in the inner 4d shell
represents, however, a higher order effect and is expected
to principally occur at the higher irradiance levels.
Moreover, each additional 4d electron simultaneously
emitted with its associates should affect the corresponding
ion spectrum because it is related with a considerable
amount of additional energy transferred to the atom. The
sudden rise of higher charge states from Xe
15þ
to Xe
19þ
when slightly increasing the irradiance just from 1.7 to
2:0 10
15
Wcm
2
as demonstrated by Figs. 1(b) and 1(c)
might indicate such step in the direct multiple 4d
ionization.
In conclusion, our comparative rare-gas study at FLASH
and the particular behavior of xenon show that in the
extreme ultraviolet the interaction of high-power lasers
with matter cannot be described in the same manner as in
the optical regime. Nonperturbative theories based on pon-
deromotive motion of quasifree electrons are not appli-
cable. On the other hand, the inner-shell electron structure,
electron correlation, and resonances play a significant role
in explaining strong-field phenomena on photoionization
TABLE I. Highest charge state q
þ
max
observed at irradiance
levels in the range from 1.5 to 1:8 10
15
Wcm
2
, ionization
energy I required to reach this state starting from the atomic
ground state [20,22] corresponding minimum number n
min
of
EUV photons of 90.5 eV photon energy which must have been
absorbed within a single FEL pulse by an individual atom to
deliver this amount of energy, and one-photon ionization cross
section at 90.5 eV photon energy [17] for the rare gases Ne,
Ar, Kr, and Xe, respectively.
Gas q
þ
max
I=eV n
min
=10
18
cm
2
Ne 7þ 715 8 4.4
Ar 7þ 434 5 1.35
Kr 7þ 383 5 0.55
Xe 14þ 1930 22 24
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in the short-wavelength regime of Einstein’s photoelectric
effect. First attempts to theoretically approach our xenon
results have recently been published [25,26]. However, we
hope to stimulate further theoretical investigations, in par-
ticular, into the role of giant resonances and collective
effects on photoionization in the high-intensity short-
wavelength regime.
We thank the FLASH team for the very successful
operation of the FEL; we also thank J. Costello, P.
Lambropoulos, B. Sonntag, G. Ulm, and P. Zimmermann
for continuous support and many helpful discussions; sup-
port by the Deutsche Forschungsgemeinschaft (DFG) is
gratefully appreciated.
*mathias.richter@ptb.de
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![TABLE I. Highest charge state qþmax observed at irradiance levels in the range from 1.5 to 1:8 1015 Wcm 2, ionization energy I required to reach this state starting from the atomic ground state [20,22] corresponding minimum number nmin of EUV photons of 90.5 eV photon energy which must have been absorbed within a single FEL pulse by an individual atom to deliver this amount of energy, and one-photon ionization cross section at 90.5 eV photon energy [17] for the rare gases Ne, Ar, Kr, and Xe, respectively.](/figures/table-i-highest-charge-state-qthmax-observed-at-irradiance-68zfarcz.webp)
