Climbing the N -shell resonance ladder of xenon Steffen Palutke1 Michael Martins2 Stephan Klumpp1 Karolin Baev12 Mathias Richter3 Tobias Wagner2 Marion Kuhlmann1 Mabel Ruiz -Lopez1 Michael Meyer4 and Kai Tiedtke1

2025-04-29 0 0 835.8KB 12 页 10玖币

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Climbing the N-shell resonance ladder of xenon

Steffen Palutke1, Michael Martins2, Stephan Klumpp1, Karolin Baev1,2, Mathias Richter3, Tobias

Wagner2, Marion Kuhlmann1, Mabel Ruiz-Lopez1, Michael Meyer4 and Kai Tiedtke1

1Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany

2Department of Physics, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany

3Physikalisch-Technische Bundesanstalt, Abbestr. 2-12, 10587 Berlin, Germany

4European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany

The dependency on the excitation energy of ultrafast multi-photon ionization of xenon by intense,

short extreme ultraviolet pulses (XUV) was investigated in the vicinity of the 4d ‘giant’ resonance

using ion time-of-flight spectroscopy. The yields of the high charge states of xenon show strong

variations with the excitation energy. With reference to simulated absorption spectra, we can link

the photon energy dependency to resonance structures of single-electron excitations mainly in

the xenon N-shell and purely sequential multi-photon absorption.

I. Introduction

Since the early days of spectroscopy, rare

gases are widely used, either as

commissioning and calibration sample of

spectrometers or as part of pioneering

experiments whenever new light sources

become available, such as free-electron

lasers (FELs).

Free-electron lasers allow the generation of

intense short light pulses from the XUV to the

hard x-ray regime. With up to 1013 photons

per pulse with pulse durations from a few fs

to a few 100 fs, FELs generate peak

brilliances which are up to ten orders of

magnitude higher than those of modern

third-generation radiation sources. FELs

enable the investigation of the interaction of

matter with extreme laser fields and atomic

and electronic dynamics on fs time scales.

Pioneering experiments have been

performed in the XUV and x-ray regime

mostly on the aforementioned rare-gas

atoms, but also on molecules and clusters [1-

7].

Despite being seemingly simple systems,

rare gases, especially xenon, generate up to

now new insides into multi-photon

ionization and correlated processes induced

by the short XUV and x-ray pulses, see for

example [3, 5, 8-13].

The electron-rich xenon atom acts as a model

system to pave the way to understand the

ionization and relaxation processes in more

complex electronic systems such as

molecules and proteins which contain heavy

atoms. An important specific example is

iodine, which is a trace element with a vital

impact on atmospheric chemistry [14] and

biological processes in living organisms [15].

One of the pioneering experiments was

performed by Sorokin et al. [3] on xenon

using FEL pulses in the XUV region. They

found Xe21+ ions after the irradiation with

highly intense XUV pulses in the vicinity of

the xenon 4d ‘giant’ resonance [16, 17] at a

photon energy of about 93 eV (13.3 nm).

According to this work, at least 57 photons

have to be absorbed in total by a xenon atom

to be ionized 21-fold. Seven XUV photons

alone are required at least to overcome the

Xe20+ ionization threshold from its ground

state.

Under so far considered experimental

conditions, it is established that up to a

charge state of Xe5+ the ionization can be

explained by sequential one-photon

absorption steps followed by Auger-Meitner

processes or direct photoelectron emission.

Starting from Xe5+ multi-photon processes

play a role. [18] However, even at fluences

typically realized at FELs, the simultaneous

absorption of up to seven XUV photons is

very unlikely. Therefore, the emergence of

Xe21+ at 93.2 eV photon energy was discussed

intensively and controversially [19-21], but

the underlying mechanisms are still not

conclusively clarified.

In order to shed light on this question,

efficient photon energy tunability and high

levels of irradiance are required in

combination with instrumentation that

allows for collecting all created ions with

high accuracy and mass-to-charge

resolution.

For this, we performed excitation energy-

dependent ion time-of-flight (iToF)

spectroscopy of xenon ions interacting with

short XUV FEL pulses. We found a very

strong dependency of the yield of highly

charged ions on the excitation energy.

To explain this, we present a possible

mechanism to achieve very high charge

states in xenon by multiple resonant

sequential single-photon absorption

processes via intermediate resonances. This

mechanism mainly involves single-electron

excitations within the xenon N-shell.

II. Experiment

The experiment has been performed at the

FL24 beamline of the free-electron laser

facility FLASH2 at DESY in Hamburg,

Germany, which fulfills the aforementioned

requirements. FLASH2 is equipped with

variable gap undulators, which enable

changing the FEL photon energy within

minutes [22]. In addition, the FL24 beamline

is equipped with a set of bendable

Kirkpatrick–Baez mirror optics to adjust the

focus during the experiment [23]. To excite

the neutral xenon, 13 different photon

energies ranging from 82.6 eV up to 126.5 eV

have been chosen in the region of the xenon

4d ‘giant’ resonance [16, 17]. The photon

energy bandwidth of a self-amplified

spontaneous emission FEL [24], such as

FLASH, is typically about 1%.

Ion time-of-flight mass spectra of the excited

xenon have been recorded with a shot-to-

shot scheme operating FLASH2 in single-

bunch mode. The interaction volume was

located between a pusher plate set to

negative voltage and an iToF spectrometer

working in a Wiley-McLaren configuration

[25]. The pulse intensity of the delivered XUV

pulses was determined by a gas monitor

detector (GMD) [26] in front of the beamline

as part of the beamline infrastructure and

behind the spectroscopic chamber with a

second GMD. With thin metal filters, the

average pulse energy was varied between

3 µJ and 25 µJ. The vertical and horizontal

diameter of the FEL beam focus was

determined by wavefront measurements

using a Hartmann-type setup [27] for each

individual FEL photon energy and ranged

from 3.5 µm to 7.0 µm full width at half

maximum (FWHM). From the pulse energy

and spot size, the fluence of the XUV pulses in

the focal volume of the iToF spectrometer

was calculated in J/cm2. All measured

spectra have been sorted according to the

measured pulse fluence value and binned

with a bin size of 2 J/cm2 per bin in the

fluence range up to 100 J/cm2. The FWHM

pulse duration was estimated from electron

bunch parameters to be about 150±50 fs. The

FEL photon energy has been measured via an

online electron spectroscopy-based

diagnostics setup [28].

III. Results

Figure 1: Excitation energy dependent

photoionization spectra of xenon at a pulse

fluence of 53 J/cm2 normalized by the

respective number of single-shot spectra.

Figure 1 shows the ion yield spectra of xenon

at different excitation energies from 82.6 eV

to 126.5 eV (15.0 nm to 9.8 nm) at 53 J/cm2

(~3.5×1018 photons/cm2). Charged states of

up to Xe19+ are created for photon energies

close to the maximum of the giant resonance

at around 112 eV photon energy. Upon

excitation the wings of the giant resonance,

the highest charge state observed is around

Xe10+. All in all, the spectra show strong

changes in the ion yield production by XUV

pulses for the chosen different photon

energies. Every spectrum was normalized by

the number of single-shot spectra measured

for the respective beam parameters.

By fitting the observed charge state peaks in

Figure 1 with multi-Gaussian profiles and

plotting the integral peak area over the

photon energy, Figure 2 shows the

resonance spectrum of the respective xenon

charge state Xeq+.

For the lower xenon charge states q=2–6 in

Figure 2(a), a broad distribution can be

observed, which follows in general the shape

of the 4d ‘giant’ resonance of neutral xenon

[16]. Reaching higher charge states

(q=8,10,15) in Figure 2(b), the width of the

charge distribution shrinks and shifts to

higher excitation energy for higher charge

states.

Figure 2: Yields of the xenon charge states (a)

q=2–6 and (b) q=8, 10 and 15 as a function of

the FEL photon energy for a fluence of

53 J/cm2. In (a) cross section of the ‘giant’

resonance of neutral xenon (grey dashed line)

is plotted for comparison [16]. The connecting

lines serve for a better overview.

For selected fluences (13 J/cm2, 53 J/cm2 and

93 J/cm2) and excitation energies (92.5 eV,

104.2 eV, 112.8 eV and 118.0 eV) the matrix

in Figure 3 shows the evolution of the ion

spectra with fluences compared to changes

with the photon energy. The spectra are

normalized to Xe2+ as Xe2+ is predominantly

produced by a one-photon process at this

photon energy. They show the expected shift

towards higher charge states with increasing

fluence, but the changes of the ion yields with

photon energy are much more pronounced.

The yields of the highest charge states are

strongly enhanced at 112.8 eV compared to

the other excitation energies. Most striking is

the difference to the spectra taken at a

photon energy of 92.5 eV, which is close to

the excitation energy used by Sorokin et al.

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ClimbingtheN-shellresonanceladderofxenonSteffenPalutke1,MichaelMartins2,StephanKlumpp1,KarolinBaev1,2,MathiasRichter3,TobiasWagner2,MarionKuhlmann1,MabelRuiz-Lopez1,MichaelMeyer4andKaiTiedtke11DeutschesElektronen-SynchrotronDESY,Notkestr.85,22607Hamburg,Germany2DepartmentofPhysics,UniversityofHambur...

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Climbing the N -shell resonance ladder of xenon Steffen Palutke1 Michael Martins2 Stephan Klumpp1 Karolin Baev12 Mathias Richter3 Tobias Wagner2 Marion Kuhlmann1 Mabel Ruiz -Lopez1 Michael Meyer4 and Kai Tiedtke1

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