Photon Factory Activity Report 2022 #40 (2023)

BL-2B/2016G610, 2018S2-003

Auger Decays from Highly-Excited Vibrational Sublevels in Core-Ionized N2Molecules
Tatsuo KANEASU1,*, Takeshi ODAGIRI2, Hirokazu TANAKA3, Jun-ichi ADACHI3,4, andYasumasa HIKOSAKA5
1
SAGA Light Source, Tosu 841-0005, Japan2
Department of Materials and Life Sciences, Sophia University, Tokyo 102-8554, Japan3
Photon Factory, Institute of Materials Structure Science, High Energy Accelerator
Research Organization, Tsukuba 305-0801, Japan
4
School of High Energy Accelerator Science, Graduate University for Advanced Studies(SOKENDAI), Tsukuba 305-0801, Japan
5
Institute of Liberal Arts and Sciences, University of Toyama, Toyama 930-0194, Japan
1 Introduction
Photoelectron–Auger electron spectroscopy is a
powerful method for disentangling complicated Auger
decay paths following core photoionization of matter.
With this method, the photoelectron and Auger
electron emitted in each ionization event are detectedin coincidence and their kinetic energies are recorded,
which enables identification of the Auger decay paths
starting from a specific core-hole state. In this report[1], we present an experimental study on Auger
decays from highly excited vibrational sublevels (v >5) of a core hole state by multielectron coincidence
spectroscopy using a magnetic bottle time-of-flight(TOF) spectrometer. The aspects of Auger decay
starting outside the Franck–Condon region of direct
photoionization are studied.

recorded at a photon energy of 415.7 eV, where thecore-valence doubly excited states lie.
Figure 1 shows the kinetic energy correlations
between the N 1s photoelectrons and the Augerelectrons. The top panel shows a photoelectron
spectrum obtained by projecting the coincidence
counts onto the horizontal axis. The photoelectron
peak shows a long tail on the lower-kinetic-energyside, which corresponds to vibrational progression.

2 Experiment
The experiment was performed at the BL-2B soft
X-ray beamline. The combination of the hybrid-fill
operation of the storage ring and the pulse selector
apparatus [2] provided pulsed light at a repetition rateof 146 kHz. Multielectron coincidence spectroscopy
was performed using the magnetic-bottle electron
spectrometer [3]. The spectrometer was equippedwith a strong permanent magnet (~0.7 T) and a 2.5
m-long flight tube, which enabled full solid angle
detection and TOF measurements of the electrons
emitted following the photoionization of N2 molecules.
3 Results and Discussion
The double-excitation resonance lying above theK-shell ionization threshold was used to prepare
highly excited vibrational sublevels of the core-holestate of the N2 molecules. The autoionization decays
from the doubly excited states lead to the productionof vibrationally excited core-hole states [4] becauseof nuclear motion on the repulsive potential energy
curves. These highly excited vibrational sublevels
were hardly populated by direct photoionization of N
1s electrons because of the weak overlap between
the vibrational wave functions in the ground and corehole states. The multicoincidence dataset wasFig. 1: Two-dimensional map of the kinetic energy
correlation between N 1s photoelectrons and Auger
electrons, measured at a photon energy of 415.7 eV.
We depict in Fig. 2 the coincidence Auger spectra
derived from the two-dimensional map by selecting
the photoelectron energy regions with binding-energy
intervals of 409.6–411.0, 411.0–412.0, 412.0–413.0,
and 413.0–414.0 eV, which correspond mainly to thevibrational sublevels 0 ≤ v ≤ 3, 4 ≤ v ≤ 7, 8 ≤ v ≤ 11,and v > 12, respectively. The three band structures

Photon Factory Activity Report 2022 #40 (2023)

in the spectra are assigned to the formation of doubly
charged states with two holes in the valence orbitals.
A change in spectral shape is clearly observed in
the coincidence Auger spectra in Fig. 2. The threeband structures appear to become broader and lessresolved as the binding energy of the core-hole stateincreases. These features can be essentiallyinterpreted in terms of Franck–Condon overlap
between the core-hole states and Auger final states.
Most of the Auger final states have repulsive potential
energy curves; therefore, the probability distribution
for the vibrational wave function of the core-hole stateis reflected in the energy space because of thereflection principle. The vibrational wave function fora higher vibrational level has a wider internuclear
distance distribution. The spectral profile for theAuger decay is therefore expected to become
broader for such higher vibrational levels of the corehole state.

Fig. 2: Coincidence spectra for single Auger decays
associated with binding-energy intervals of (a) 409.6–
411.0 eV, (b) 411.0–412.0 eV, (c) 412.0–413.0 eV,and (d) 413.0–414.0 eV, respectively. The spectra
are plotted as a function of the N22+ binding energy.
References[1] T. Kaneyasu et al., J. Electron Spec. Relat.
Phoneom. 259, 147244 (2022).
[2] J. Adachi et al., J. Phys.: Conf. Ser. 1412, 152092(2020).
[3] K. Ito et al., Rev. Sci. Instrum. 80, 123101 (2009).
[4] Y. Hikosaka et al., Phys. Rev. A 75, 042708 (2007).
* kaneyasu@saga-ls.jp