Inner-shell photoionization dynamics probed by momentum vector correlation

The main justifications for photoionization dynamics studies of isolated atoms and molecules in this energy range are the following: (i) the core-hole lifetimes in this energy range are in the order of one femtosecond or less. This very short lifetime can be used as an internal clock allowing ultrafast nuclear motion to be studied. (ii) Because of the very short lifetimes, and high kinetic energies of Auger electrons, post collision interaction leads to large effects that can be studied in details. (iii) Because of the large lifetime broadenings, electronic state interferences can be studied. (iv) Finally, multiple cascade Auger decays lead to highly charged molecular ions which dissociation dynamics are of particular interest to study.

We developed a momentum spectrometer, CELIMENE, dedicated to the study of vector correlations, and the measurement of molecular frame photoelectron angular distributions (MFPADs), in the context of dissociative deep core-shell photoionization in the tender x-ray region (1 to 10 keV).

Experimental setup : CELIMENE

Fig. 1: the experimental setup installed on beamline LUCIA at Synchrotron SOLEIL.

CELIMENE combines time-of-flight and imaging techniques for momentum vector measurements of several charged particles detected in coincidence.

The experimental setup is composed of two vacuum chambers: the expansion chamber (A in Fig. 2) and the interaction chamber (B in Fig. 2). The expansion chamber houses a molecular beam source. A continuous molecular beam is produced by adiabatic expansion of the sample gas at a backing pressure of 0.25 to 1 bar through a nozzle (C in Fig. 2). The expansion chamber is pumped with two 1600 l/s turbo molecular pumps and pressure in the chamber is typically 10−4 mbar. To select the cold molecules in the beam, a skimmer (D in Fig. 2) is placed in the region of overexpansion in the free jet zone of silence. In our setup, a 100 μm conical nozzle is mounted on an XYZ-manipulator (E in Fig. 2) for precise alignment with a 700 μm aperture skimmer. The skimmer is mounted on a custom conical flange that isolates the two chambers and penetrates into the interaction chamber allowing the skimmer to be placed at 90 mm from the interaction region. The interaction chamber is pumped by a set of 1000 l/s and two additional 70 l/s turbo molecular pumps are mounted next to the detectors. The base pressure is 1.10−8 mbar and rises to 1.10−6 mbar when the molecular beam is flowing.

The spectrometer is composed of two strictly identical time-of-flight (TOF) spectrometers, symmetrically mounted in the interaction chamber. The geometry of the TOF spectrometers was chosen to satisfy the Wiley-McLarren time focusing criteria. Electrons and ions are accelerated in opposite direction by a uniform electrostatic field by two sets of electrostatic lenses polarized with opposite voltages (H in Fig. 2). The impact position of each particle is measured using commercial delay lines as part of the detector assembly. The impact position coordinates x and y measured on the detector and the time of flight (TOF) T of the particle through the spectrometer are recorded, to determine the three components (Px, Py, Pz) of the initial momentum vector P.

FIG. 2: Overview of the experimental setup. A: expansion chamber. B: interaction chamber. C: nozzle. D: skimmer. E: XYZ-manipulator. F: linear manipulator. G: μ-metal shield. H: extraction lens. J: refocusing lens. K: drift tube. L: delay line detector

Reference: C. Bomme, R. Guillemin, T. Marin, L. journel, T. Marchenko, D. Dowek, N. Trcera, B. PIlette, A. Avila, H. Ringuenet, R.K. Kushawaha, and M. Simon, Review of Scientific Instruments 84, 103104 (2013).

Ion recoil momentum spectroscopy

The decay processes following Ar 1s photoionization involves both photon emission (radiative decay) and electron emission (nonradiative decay) which happen in competition. In turn, the radiative and nonradiative pathways branch into several different subchannels. As a result, complex decay channels lead to the formation of highly charged ions, up to 7+. We used ion-recoil momentum spectroscopy to disentangle the multiple decay patterns. For momentum conservation, the departing electron induces recoil on the ion which is directly proportional to the ion mass and to the electron velocity. From the ion-recoil momentum vector, it is possible to correlate the creation of a specific charged species Arn+, with n = 1–7, to a radiative, nonradiative, or mixed multistep process following Ar 1s photoionization or resonant excitation.

Fig. 3: (a) Kinetic-energy correlation diagram: photoelectron energy as a function of ion-recoil energy, measured at 3208.3-eV photon energy, 2 eV above threshold. (b) Auger electron momentum derived from the ion-recoil by momentum conservation.

In this study, we have shown on a simple atomic system that ion-recoil momentum spectroscopy can be used to probe intricate decay processes that follow core ionization or excitation. The method is also unique in providing direct information on the branching between radiative and nonradiative decay.

Reference: R. Guillemin, C. Bomme, T. Marin, L. Journel, T. Marchenko, R.K. Kushawaha, N. Trcera, M.N. Piancastelli, and M. Simon, Physical Review A 84, 063425 (2011).

Post Collision interaction and core-hole lifetime

The natural lifetime of excited electronic states determines the time scale on which processes following the photoexcitation of a system, an atom, or a molecule will take place. Core-level photoexcitation brings this time scale down to the femtosecond, as core-hole lifetimes are typically a few femtoseconds for shallow core levels in the soft x-ray region and 1 fs or less for deep core levels, and thus provides an internal clock that allows the study of ultrafast phenomena. One possible way to look at the dynamics of photoemission is to study the effects of postcollision interaction (PCI) on the line shape of the photoelectron peak. In the cascade process, a series of subsequent nonradiative relaxations leaves the atom in different core excited states, each with a different lifetime, depending on the decay pathways. The nascent photoelectron thus ‘‘feels’’ an ionic charge that changes with time as the cascade unfolds. In general, PCI takes into account the response of the photoelectron to the variation of the ionic field during the Auger decay, and the interaction between the photoelectron and Auger electrons. Therefore, the dynamical change in the Coulomb potential felt by the photoelectron on a femtosecond scale can be revealed by the analysis of the photoelectron line shapes in terms of PCI.

Fig. 4: Experimental (circles) and theoretical (solid) partial photoelectron spectra measured in coincidence with Arn+ ions (n=1–5) for excess energy 2 eV above IP. The top curve shows the noncoincident photoelectron spectrum.

Ar 1s photoelectron spectra associated with the different ionic states created after core excitation were investigated by electron-ion coincidences. Both measurements and calculation reveal a strong PCI distortion of the photoelectron line. Theoretical analysis allows us to clarify the complicated dynamics of the deep vacancy Auger decay and to estimate the average lifetime of the intermediate states involved in the cascade Auger decay.

Reference: R. Guillemin, S. Sheinerman, C. Bomme, L. journel, T. marin, T. Marchenko, R.K. Kushawaha, N. Trcera, M.N. Piancastelli, and M. Simon, Physical Review Letters 109, 013001 (2012).

The same procedure was successfully used on a molecular system, namely carbonyl sulfide (OCS), demonstrating that this technique is general and can be used to study more complex systems.

Reference: C. Bomme, R. Guillemin, S. Sheinerman, T. marin, L. Journel, T. Marchenko, R.K. Kushawaha, N. Trcera, M.N. Piancastelli, and M. Simon, J. Phys. B: At. Mol. Opt. Phys. 46, 215101 (2013).

Molecular frame Photoelectron Angular distributions

The study of fully energy- and angle-resolved photoelectron emission has been the subject of great interest. Theoretical calculations and experimental methods have been well developed over the years for shallow core holes (in the soft x-ray regime) and for valence electrons (VUV). A powerful technique to reveal the orientation of a molecule at the time of photoemission is to detect in coincidence the ionic fragment originating from the fast decay and dissociation of the primary molecular ion. We have applied these theoretical and experimental tools to a linear triatomic molecule, namely carbonyl sulphide, OCS, photoionized by high-energy photons at the S 1s threshold (~2400eV).

Fig. 5: experimental and theoretical 3D MFPADs at 10 eV above threshold, for χ = 0◦, χ = 54.7◦ and χ = 90◦./span>

The molecular-frame photoelectron angular distribution arising from S 1s ionization in OCS were studied both experimentally and theoretically, and the good agreement obtained demonstrates the feasibility of such studies even for deep core levels, which are relevant in the hard x-ray domain of wide current interest, and may offer advantages over the intermediate core holes generally studied because of the simplicity of the 1s holes and the more extensive fragmentation pattern that is obtained.

Reference: C. Bomme, R. Guillemin, T. Marin, L. Journel, T. Marchenko, N. Trcera, R.K. Kushawaha, M.N. Piancastellli, M. Simon, M. Stener, and P. Decleva, J. Phys. B: At. Mol. Opt. Phys. 45, 194005 (2012).

Dernière modification : January 12 2015 17:27:04.