Gas phase Photoemission studies in the hard X-ray domain

. Recent results obtained in gas phase photoemission studies are presented in this article with particular emphasis on recoil, Double Core Hole and Post Collision Interaction. These three important effects are not specific to the gas phase and could have more general applications in condensed matter studies.


Introduction
Light-Matter interaction studied by photoemission on gas phase samples has proven to furnish a wealth of information.This article deals with Hard X-ray Photoelectron Spectroscopy (HAXPES) with a focus on the tender x-ray domain (2-12 keV).One of the interests of this photon energy range is the short (femtosecond or less) corehole lifetime of the reached electronic states by photoabsorption.Among the different studies performed by our group, I will select three important fundamental processes, which could attract the condensed phase community participating in the JNSPE: high recoil effects due to the high kinetic energy of the emitted photoelectron, Double Core Hole Spectroscopy and Post Collision Interaction.These studies have important applications like radiationdamage in the hard x-ray domain.The measurements were performed at SOLEIL, the French Synchrotron facility, located close to Paris.The perspectives offered by these results are discussed.We have published several review articles in the past where the reader could find more details [1][2][3].

Experimental setup
The HAXPES experimental chamber is a permanent endstation of the SOLEIL GALAXIES beamline.

The SOLEIL GALAXIES beamline
The GALAXIES beamline is an undulator beamline, optimized to operate in the 2-12 keV energy range [4].The undulator beam is monochromatized by a double-crystal monochromator providing a photon flux of few 10 12 photons per second on the sample.A 4-bounce, high-resolution monochromator (HRM) has been recently commissioned allowing to obtain a photon bandwidth of 100 meV between 4 and 12 keV.

The HAXPES endstation
HAXPES is a permanent endstation of the GALAXIES beamline [5].A high-resolution hemispherical electron analyzer allows to measure electron spectra with a kinetic energy resolution of 35 meV for electron kinetic energies up to 12 keV.For gas phase studies, a gas cell is installed into the photon beam thanks to a motorized 4-axes manipulator.

Recoil effects
In the hard x-ray domain, the momentum of the departing photoelectron is far to be negligeable as it is the case in the soft x-ray domain.This leads to translational recoil for isolated atoms and molecules.Additionally, depending on the direction in which the photoelectron is ejected, vibrational and rotational recoil have been observed for molecules.

Translational recoil
Core ionization of neon has been studied as a demonstration case [6].Ne1s ionization has been performed at high photon energy and the Auger electrons have been measured for different photon energies.The recoil momentum of the Ne + ion is so high that Atomic Auger Doppler effects have been observed, as illustrated in figure 2.

Vibrational recoil
In the case of carbon 1s photoionization of the CF4 molecule, the escaping C1s photoelectron cannot induce rotational recoil of the CF4 + ion because the carbon atom is located at the center of gravity of the tetrahedral molecule.In addition to the translational recoil, only vibrational recoil is then induced by the fast C1s photoelectron emission, as illustrated in figure 3   vibrational level of the CF4 + ion is populated.When the photon energy is increased, a wider vibrational envelope is observed as a signature of the vibrational recoil.
4 molecule measured at different photon energies.

Rotational recoil
In case of the CO molecule, C1s photoionization can lead to vibrational recoil if the photoelectron is emitted parallel to the molecular axis and to rotational recoil if the photoelectron is emitted perpendicular to the molecular axis.Rotational levels cannot be resolved at such high kinetic energies.Rotational recoil effects are observed as a shift of the centroid of the photoelectron peak toward lower kinetic energy, the difference in energy being the rotational energy of the molecular ion.This rotational recoil has been precisely determined by a mixture of CF4 and CO molecules as illustrated in figure 4.

Recoil induced ultrafast molecular rotation of the CO molecule probed by dynamical rotational Doppler effect
Photoionization at high photon energy induces ultra-fast rotation up to an effective rotational temperature of about 10,000 K due to the recoil caused by emission of a fast photoelectron.In order to monitor the recoilinduced rotational motion of the CO molecule, high-resolution Auger spectra of the C1s core-ionized molecular ion have been recorded at various photon energies [8].Given the C1s core-hole lifetime, the molecular ion has a time of 5 femtoseconds to rotate before the Auger electron emission.This is illustrated in figure 5.

Double Core Hole Spectroscopy
Double Core Hole (DCH) studies are attract considerable interest mainly because of the high chemical sensitivity of the DCH binding energy compared to the traditional ESCA (Electron Spectroscopy for Chemical Analysis) technique [9].In case of ESCA, a chemical shift of 700 meV has been recently measured between the CH3I and CF3I molecules at the iodine 1s ionization threshold around 33 keV binding energy [10].These results have been obtained at the Spring8 synchrotron radiation facility where an HAXPES endstation has been successfully commissioned in the hard x-ray domain [11].The high sensitivity of DCH states has been theoretically predicted during the 1980's and but only experimentally confirmed very recently [12].Most of the DCH studies have used sophisticated magnetic bottles in order to measure both core-ionized photoelectrons in a coincidence regime.An interesting way to study DCH states has been recently founded [13]: one of the core electrons is ejected into the continuum whereas a second core electron is promoted to an empty molecular orbital.

Spectroscopy of DCH states
The high-resolution of the HAXPES setup is particularly well adapted to perform DCH studies with one core electron ionized and the other promoted into an unoccupied level.A proof of principle has been performed with the argon 1s -1 2p -1 shake-up photoelectron spectrum shown figure 6 [14].The Rydberg series of the DCH states converging towards the 3 P2, 3 P1, 3 P0 and 1 P1 DCH thresholds are clearly visible with a good signal/noise ratio.Spectral bandwidth has been measured allowing to determine the lifetime of the DCH states.This technique has been applied to the neon atom [15], the H2O molecule [16], the CS2 and SF6 molecules [17], the HCl molecule [18], the O K -2 CO2 molecule [19].

Electronic relaxation of DCH states
The Auger relaxation of these DCH states mainly occurs via a 2 step Auger decay: a first Auger decay filling one core hole of the DCH state, called hypersatellite Auger decay, occurs followed by a second Auger decay filling the remaining core hole.Even if the relative ratio of DCH/SCH states is in the order of 10 -3 , good signal/noise ratio spectra have been obtained on neon [15,20] as illustrated in figure 7.
Hypersatellite Auger spectrum of neon.
The relaxation of these DCH states mainly occurs via a 2 step Auger decay: a first Auger decay filling one core hole of the DCH state, called hypersatellite Auger decay, occurs followed by a second Auger decay filling the remaining core hole.Even if the relative cross section of DCH/SCH states is in the order of 10 -3 , nice signal/noise ratio spectra have been obtained.
The hypersatellite Auger spectrum of the H2O molecule has revealed [21] ultra-fast nuclear motion occurring within the lifetime of the DCH state which is equal to 1.5 femtosecond.

Post Collision Interaction (PCI)
Post Collision Interaction (PCI) effect takes place in presence of a slow photoelectron and a fast Auger electron.Close to the ionization threshold, the slow photoelectron starts to leave a singly charged ion, but then the faster Auger electron overtakes it.Then, instead of "feeling" the potential of a singly charged ion, the photoelectron suddenly 'feels' a doubly charged ion.Due to PCI, the photoelectron loses kinetic energy and the Auger electron simultaneously gains kinetic energy [25][26][27].Just above threshold, the photoelectron can be recaptured into a unoccupied Rydberg orbital.Momentum transfer between the slow photoelectron and the fast Auger electron can also takes place [28].
PCI is a quite general effect.Several groups are working for the moment on PCI on liquids and solid states compounds.

Conclusion
All the presented effects presented in this overview have been observed in the gas phase during the last ten years.Recoil, Double Core Hole and PCI are easier to observe with diluted species but can be studied in any phases.

4 EPJFig. 5 .
Fig. 5. Classical picture of the dynamical rotational Doppler effect of the C1s core-ionized CO molecule