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Resonance-enhanced three-photon single ionization of Ne by ultrashort extreme-ultraviolet
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2013 J. Phys. B: At. Mol. Opt. Phys. 46 164020(http://iopscience.iop.org/0953-4075/46/16/164020)
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OPEN ACCESSIOP PUBLISHING

JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS

doi:10.1088/0953-4075/46/16/164020

J. Phys. B: At. Mol. Opt. Phys. 46 (2013) 164020 (5pp)

Resonance-enhanced three-photon single
ionization of Ne by ultrashortextreme-ultraviolet pulses
Eiji Shigemasa 1,2,3 , Mitsuru Nagasono 3 , Hiroshi Iwayama 1,2,3 ,James R Harries 3,4 and Lisa Ishikawa (Okihara) 1,5
1

UVSOR Facility, Institute for Molecular Science, Nishigonaka 38, Myodaiji-cho, Okazaki 444-8585,
Japan
2
SOKENDAI, Nishigonaka 38, Myodaiji-cho, Okazaki 444-8585, Japan
3
RIKEN, SPring-8, Kouto 1-1-1, Sayo-cho, Hyogo 679-5148, Japan
4
JAEA/SPring-8, Kouto 1-1-1, Sayo-cho, Hyogo 679-5148, Japan
E-mail: sigemasa@ims.ac.jp

Received 28 February 2013, in final form 1 April 2013
Published 13 August 2013
Online at stacks.iop.org/JPhysB/46/164020
Abstract
Time-resolved extreme-ultraviolet (EUV) fluorescence spectroscopy has been applied to study
the multi-photon, single ionization of Ne irradiated by intense EUV-free-electron laser (FEL)
pulses at a wavelength of 51 nm. A broad, intense peak at a wavelength of around 46 nm isobserved, which is shorter than the incident FEL wavelength. The time dependence of thefluorescence reveals that the peak has two unresolved components, which we attribute to the
decay of the excited ion states Ne+ 2s−1(2S) and 2p−2(3Pe)3s(2Pe) at 46.0 and 44.6 nm. From
the observed intensity ratios and fluorescence lifetimes we conclude that theNe+ 2p−2(3Pe)3s(2Pe) state is populated by two-photon resonance enhancement, via a
2p43p2 doubly excited state of Ne.
(Some figures may appear in colour only in the online journal)

1. Introduction

the physics behind the processes. The majority of the early
work performed at FEL facilities made use of ion mass
spectroscopy. Although the observed ion charge distributions
provide general information on multi-photon processes, thedetailed ionization pathways remain inaccessible. Whileelectron spectroscopy can in principle be used to obtain moredetail, only a few successful studies have been reported to date
[7–9], due to the experimental difficulties involved. Reports todate include those on multi-photon ionization processes in Ar[7] and He [9], manifesting the power of the methodology.
Fluorescence spectroscopy can provide information on theenergy levels of the excited neutral and ionic states involved
in de-excitation processes following multi-photon ionization,
which can be complementary to that obtained by photoelectron
spectroscopy. The most remarkable advantage for fluorescence
spectroscopy is that there are no space charge effects [10],which often cause serious problems for both photoelectron
and ion mass spectroscopy. In addition, the recent successful
observations of super-fluorescence from He atoms [11]With the advent of free-electron laser (FEL) facilities operating
at wavelengths ranging from the extreme ultraviolet (EUV)
to x-rays, we are undoubtedly entering a new age of photoscience. In the research field of atomic and molecularphysics, nonlinear processes such as multi-photon absorption
have become a major target to be investigated in the
short wavelength region [1–9]. The modelling of nonlinear
interactions of short-wavelength and short-pulse radiation withmatter is essential for a wide range of basic and appliedresearch, and thus there is a strong demand for understanding
5 Present address: Nagoya Institute of Technology, Gokiso-cho, Showa-ku,
Nagoya 466-8555, Japan.

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J. Phys. B: At. Mol. Opt. Phys. 46 (2013) 164020

E Shigemasa et al

and up-conversion fluorescence from Ar clusters [12] inducedby the irradiation of EUV-FEL pulses clearly suggest thepossibilities for controlling the chaotic nature of self-amplified
spontaneous emission (SASE) FEL pulses by pumping
gaseous, ionic or solid systems using pulses from one of the
FEL sources. In the x-ray wavelength region, lasing has been
observed on an inner-shell transition in Ne atoms [13], andsuggested in molecules [14, 15].
In this work, we have studied EUV fluorescence in Ne
following excitation with FEL pulses at a wavelength of 51 nm.
Previous studies of the photoionization of neon using freeelectron laser radiation have concentrated on much higher
energy regions, where resonant processes in Ne+ can occur
following single-photon ionization [16]. A wavelength of
51 nm corresponds to excitation just above the Ne singlephoton ionization threshold, and there are no excited states inthis region. With intense pulses, however, it can be expectedthat multi-photon processes can occur, and we observed
fluorescence at wavelengths near 46 nm. Fluorescence at thiswavelength can be expected from the 2s12p5 excited ionic state
following either multi-photon absorption of the fundamental
FEL radiation (51 nm) or single-photon absorption from the
FEL harmonics (25.5 nm, 17 nm), but the details of the spectra
cannot be explained by this process alone. By recording timeresolved spectra, we find evidence for resonance-enhanced(2+1) multi-photon ionization via a doubly excited state.

onto the detection plane. Time-resolved spectra were recorded
using microchannel plates with a delay line anode (RoentDek,
DLD-40). The resolving power λ/λ of the present setup was
estimated to be ∼300, and the time resolution to be less than
0.2 ns. The wavelength scale of the spectrometer was calibrated
by using monochromatized synchrotron radiation at BL5B of
the UVSOR facility, Japan, in a separate measurement.
The contribution of higher harmonics to the total FEL
intensity has previously been determined to be 1.1% at afundamental wavelength of 51 nm [19]. In order to estimate
the contribution in our spectra of the higher harmonics,
we recorded spectra with the first-order (51 nm) intensity
suppressed to 3% using the Ar gas attenuator. Since the photoabsorption cross sections of Ar are 36.8 × 10−18 cm2 for thefundamental (λ = 51 nm), 2.9 × 10−18 cm2 for the second
harmonic (λ = 25.5 nm) and 1.2 × 10−18 cm2 for the third
harmonic (λ = 17 nm) [19–21], and it can be expected thatany multi-photon processes will be severely suppressed bysuch a reduction in intensity, the spectra recorded under these
conditions can be regarded as being purely due to singlephoton absorption of the second and third harmonics. By
subtracting these spectra from the spectra recorded with noattenuation, we obtain spectra only reflecting multi-photon
absorption at 51 nm.

2. Experiment

Figures 1 and 2 show two-dimensional representationsof fluorescence spectra recorded following single-photon
absorption of the higher harmonics (figure 1) and multi-photon
absorption of the fundamental at 51 nm (figure 2). The timeaxes correspond to the time differences between the incident
FEL pulses and fluorescence detection. The simple EUV
fluorescence spectra and time evolution of the fluorescenceintensity are shown as (b) and (c), respectively, in each figure.
The time 0 was set to the position of maximum fluorescenceintensity. In the spectral region of interest in the present
experiment (30–50 nm), the observed fluorescence is mainly
due to the de-excitation processes of excited Ne+ states to theground ionic state Ne+ 2p−1(2Po). In figure 1(b), a single broad
structure at a wavelength of ∼46 nm is observed, which canbe attributed to unresolved peaks due to the Ne+ 2s−1(2S)→ Ne+ 2p−1(2Po) and 2p−2(3P)3s(2P) → Ne+ 2p−1(2Po)transitions. Weak peaks at wavelengths of around 33 nm,
36 nm and 41 nm can also be seen. These features can only be
populated by single-photon absorption of the third harmonic at
a wavelength of 17 nm, and can be associated with correlation
satellites converging to double ionization thresholds. Kikas
et al have identified the corresponding correlation satellites
with 2p−2nl configurations in their photoelectron spectrum,
which was recorded at a photon energy hν = 96 eV (13 nm)[22]. The peak positions estimated from the satellites observedin [22] are shown as vertical lines with notations in figures 1(b)and 2(b), and are in good agreement with the observed
structures in figure 1(b).
The peaks observed near a wavelength of 46 nm arevery different for multi-photon and single-photon absorption.
Only one island-like structure centred at 46.0 nm is seen in

3. Results

The experiments were carried out at the EUV-FEL beamline
at the SCSS test accelerator, which provided FEL pulses witha pulse energy of ∼30 μJ and a pulse duration of ∼30 fs [17].
The FEL radiation was linearly polarized in the horizontal
direction, and the repetition rate was 30 Hz. A schematic
diagram of the beamline can be found in [12, 18]. The EUVFEL pulses are transported using a pair of flat mirrors in theaccelerator tunnel to the experimental hall and then focusedwith a pair of elliptical and cylindrical mirrors [18]. The
overall transmission of the beamline optics is estimated to bearound 35% for the fundamental radiation. The pulse energy
was monitored for each pulse using a photoion yield detector
installed upstream of the focusing system. The focus size wasabout 12 μm in diameter. The pulse energy was controlledusing an Ar gas attenuator upstream of the photoion yielddetector. The intensity of the EUV pulses at the sample position
was I ∼ 3 × 1013 W cm−2.
The focused FEL beam was introduced into a
differentially pumped vacuum chamber. The gas nozzle had
an aperture diameter of 500 μm. The effusive beam of Ne
atoms flowed perpendicularly to the EUV-FEL radiation axis.
EUV fluorescence emitted from the interaction region where
the Ne atom beam crossed the FEL radiation was imaged
through the entrance slit (width ∼ 1 mm) of a flat-field grazingincidence spectrometer (manufactured by Shinkukogaku Co.,
Ltd, Japan), which was set perpendicular to the EUV-FEL axis
and parallel to the polarization vector of the FEL. The EUV
fluorescence was dispersed by an aberration-corrected concave
grating with a groove density of 400 l mm−1 and focused2

J. Phys. B: At. Mol. Opt. Phys. 46 (2013) 164020

E Shigemasa et al

(b)

(c)

(a)

Figure 1. Time evolution of EUV fluorescence following
single-photon absorption of harmonic FEL radiation (25.5 nm or
17 nm). The first-order photon intensity was suppressed to 3% using
an Ar gas attenuator. (b) EUV fluorescence spectrum obtained byprojecting the fluorescence intensity in the 2D map onto thehorizontal axis. Some excited states of Ne+ are shown. Note that
only the 2s−1 state can be reached at 25.5 nm. (c) Time evolution of
the fluorescence intensity obtained by projecting the fluorescenceintensity in the 2D map onto the vertical axis. The red line shows atwo-exponential fit to the data over the time range 0–2.8 ns.

Figure 2. (a) Time evolution of EUV fluorescence following
multi-photon absorption at the fundamental FEL wavelength of
51 nm. The contribution of the higher harmonics (figure 1) has been
subtracted. (b) EUV fluorescence spectrum obtained by projecting
the fluorescence intensity in the 2D map onto the horizontal axis.
(c) Time evolution of the fluorescence intensity obtained byprojecting the fluorescence intensity in the 2D map onto the verticalaxis. The red line shows a two-exponential fit to the data over thetime range 0–2.8 ns.

figure 1(a), while a vertically long shape at a wavelength
slightly shorter than 45 nm is also observed in figure 2(a).
This suggests that there are additional processes contributing
to the fluorescence observed for multi-photon absorption.
Figures 1(c) and 2(c) show the time dependence ofthe observed fluorescence. To determine the fluorescence
lifetimes, the spectra have been fitted to an expression of theform

and single-photon absorption are due to fluorescence from the2s−1(2S) state. The lifetime of this state has been determined
to be around 0.15 ns [23], and published [24] and calculated[25, 26] theoretical results give similar values. The wavelengthof the long-lifetime fluorescence (∼44.6 nm) is consistent
with the decay of the 2p−2(3P)3s(2P) excited state to the ionicground state [27, 28]. This state cannot be reached with twophoton absorption at 51 nm, even considering the bandwidthof the SASE pulses. To our knowledge, there have been no
experimental or theoretical reports of the lifetime of this state,
but atomic structure calculations suggest a lifetime of around
0.3 ns [25, 26]. We observe a much longer lifetime than this,suggesting that this transition is part of a cascade process, withthe 2p43s state being populated through slower transitions at
wavelengths beyond the sensitivity of our spectrometer.
The fits suggest that the ratios of the two decay routes
2s−1(2S): 2p−2(3P)3s(2P) are around 1:1 for multi-photon
excitation (figure 2(c)), and around 1:0 for single-photon
excitation (figure 1(c)). This again is consistent with the resultsof [22], where a ratio of 1:0.003 was found for single-photon
excitation at a wavelength of 13 nm. This result is striking,since while the absorbed energy is the same, the fluorescenceintensity observed due to the decay of the 2p−2(3P)3s(2P) stateis at least 370 times as high for three-photon absorption than forln(I(t )) = ln (a1 × exp (−t/τ1 ) + a2 × exp(−t/τ2 )) (1)where I(t) is the fluorescence intensity, t is the detection time,
ai are the fluorescence intensities at t = 0, and τ i are thefluorescence lifetimes. The fits were carried out on the time
range of 0 to 2.8 ns, and we assume that the effects of thedetector instrument function (responsible for the asymmetricprofile for t < 0) can be ignored. The results of the fit are shown
as solid red lines in the figures. The fit to the multi-photon
data (figure 2(c)) revealed two components, with lifetimes of(1.5 ± 0.3) ns and (0.13 ± 0.03) ns. The fit to the singlephoton data (figure 1(c)) revealed one component with alifetime of (0.15 ± 0.02) ns, and a component with a lifetimeof >7 ns, equivalent to a constant background for this dataset.
From a comparison with the photo-electron spectroscopy
results of [22], we can conclude that the short-lifetime
components at a wavelength of 46 nm for both multi-photon
3J. Phys. B: At. Mol. Opt. Phys. 46 (2013) 164020

E Shigemasa et al

single-photon absorption (assuming a 1:0.003 ratio for singlephoton absorption). Further, this enhancement is only seen
for the 2p−2(3P)3s(2P) state, and not, for example, for other
2p−2(3P)ns(2P) states, which could be expected to have similartransition matrix elements. A similar result has previously been
observed in He [9], where the enhancement of a particular
excitation route was attributed to resonance enhancement of
the two-photon absorption process. In neon, however, there
are no states which can be reached with the absorption of asingle photon of wavelength 51 nm (24.3 eV) [29]. The only
possibility remaining is that the process is enhanced by a twophoton resonance, at a wavelength near 25.5 nm (48.6 eV).
While the single-photon excitation of doubly excited statesin this energy region has been studied with synchrotron
radiation [30, 31], to our knowledge there have been noprevious experimental studies of two-photon absorption tothese states. Experimental and theoretical work on EUV orsoft x-ray multi-photon absorption to date has concentrated on
wavelength regions where excitation processes of Ne+ can bestudied [16, 32–34]. Some theoretical calculations also exist
at wavelengths near the threshold for two-photon ionization of
Ne [35, 36], but do not cover the current wavelength region
or the possibility of two-photon doubly excited state resonant
excitation. Considering the 1Se symmetry of the neon groundstate and the selection rules for two-photon absorption, the2p−2(3P)3p2(3P)(1Se) state appears to be a likely candidate.
Langlois and Sichel [37, 38] give an energy of 48.6 eV for
this state (with a claimed accuracy of 0.1 eV), which is wellwithin the bandwidth of the SASE FEL pulses. The absorptionof a third photon can remove one of the 3p electrons, toleave the excited ion state 2p−2(3Pe)3p(2Po). This state is
known to decay to the ionic ground state through a cascade
via the 2p−2(3Pe)3s(2Pe) state, emitting UV fluorescence at awavelength of around 330 nm and with a lifetime which has
been reported experimentally to be 5.7 ns [39]. Theoretical
calculations give values including 5.2 ns [40] and 4.4 ns [41].
The subsequent decay to the ground state is at a wavelengthof 44.6 nm [27, 28]. It is this fluorescence which we detect,but the observed lifetime is dominated by the upper transition.
While 5.7 ns is considerably longer than the 1.5 ns we estimate
from our data, the estimated errors on both the theoretical andexperimental values for the lifetime are quite large [41], andthe count rate in our experiment quite low. While the count rate
at short times is high enough to determine the short lifetime(0.15 ns) to a fair degree of confidence, longer lifetimes
require increasingly higher cumulative counts. Further, wenote that the doubly excited state we suggest for the twophoton resonance has never been observed experimentally (toour knowledge), and there may be other states in the vicinity
within the bandwidth of the SASE pulses. Depending on
which excited ion states are populated, the fluorescence weobserve could be due to multiple cascade processes—again,a larger number of cumulated counts would be required todetermine this. We can however conclude that our results
qualitatively agree with the resonance enhancement of twophoton absorption, but that more detailed experiments are
required to confirm the exact excitation and decay routes.
The process we propose is summarized in figure 3. Note

Figure 3. Schematic of the proposed two-photon resonant,three-photon ionization process. Two-photon absorption at 51 nm(red arrow) is resonantly enhanced by the Ne∗∗ (2p−2(3Pe)3p2(1Se))
doubly excited state (horizontal red line). The absorption of anadditional photon leads to ionization to the singly charged
Ne+∗ (2p−2(3Pe)3p(2Po)) state with the emission of a photoelectron(blue arrow). This state decays to the Ne+∗ (2p−2(3Pe)3s(2Pe)) stateby emitting 331 nm fluorescence (short yellow arrow). Finally, EUV
fluorescence at 44.6 nm is emitted, corresponding to the transition
from the Ne+∗ (2p−2(3Pe)3s(2Pe)) state to the Ne+ (2p−1(2Po)) groundstate (long yellow arrow).

that the occurrence of this process relies on the overlap withthe ‘spiky’ profile of the SASE pulses overlapping strongly
with the resonant state. This varies shot-by-shot, but the nonresonant parts of the spectrum still lead to non-resonantly
enhanced multi-photon ionization leaving the ion in the excited
2s12p6 state.

4. Summary
In summary, we have studied EUV fluorescence produced
following multi-photon, single-ionization of neon, at awavelength slightly above the threshold for ionization, but
far from any single-photon excitation resonances in either Ne
or Ne+. Strong fluorescence was observed at around 46 nm
which can only be attributed to multi-photon ionization to
excited Ne+ states. By studying the time dependence of thefluorescence, two separate processes were revealed, one ofwhich can only be explained by a (2+1) three-photon resonant
process involving the excitation of a neutral doubly excitedstate followed by the absorption of a further photon.

Acknowledgments
This paper is dedicated to the memory of Mitsuru Nagasono,
who tragically died in 2012. Much of the work carried out
at SCSS would have been impossible without his efforts. The
RIKEN SCSS test accelerator engineering team is gratefully
acknowledged for experimental support and stable operationof the accelerator.
4

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