Recoil excitation of vibrational structure in the carbon 1s photoelectron spectrum of CF4

Orientational anisotropy due to molecular field splitting in sulfur 2p photoemission from CS2 and SF6 - theoretical treatment and application to photoelectron recoil

Photoelectron recoil strongly modifies the high kinetic energy photoemission spectra from atoms and molecules as well as from surface structures. In most cases studied so far, photoemission from atomic-like inner-shell or core orbitals has been assumed to be isotropic in the molecular frame of reference. However, in the presence of molecular field splitting of p or d orbitals, this assumption is not justified per se. We present a general theoretical treatment, linking the orientational distribution of gas-phase molecules to the electron emission and detection in a certain direction in the laboratory frame. The approach is then applied to the S 2p photoemission from a linear molecule such as CS2 and we investigate, how the predicted orientational anisotropies due to molecular field splitting affect the photoelectron recoil excitations. Lastly, experimental S 2p high-kinetic-energy photoelectron spectra of SF6 and CS2 are analyzed using the modeled recoil lineshapes representing the anisotropy-affected recoil effects.

Recoil lineshapes in hard X-ray photoelectron spectra of large molecules - free and anchored-on-surface 10-aminodecane-1-thiol

Core-level photoelectron spectroscopy of molecules presents unique opportunities but also challenges in the Hard X-ray Spectroscopy (HAXPES) realm. Here we focus on the manifestation of the photoelectron recoil effects in core-level photoemission spectra, using the independent normal-mode oscillators approach that allows to model and investigate the resulting recoil lineshapes for molecules of large sizes with only a slight computational effort. We model the recoil lineshape for N 1s and C 1s photoemission using the 10-aminodecane-1-thiol molecule as an example. It represents also a class of compounds commonly used in creating self-assembled monolayers (SAMs) on surfaces. Attachment of the -SH head group to the surface is modelled here in a simplified way by anchoring the sulfur atom of a single molecule. The effects of the orientation of photoemission in the molecular frame on the recoil lineshape of such anchored molecules are illustrated and discussed as a possible geometry probe. Time-evolution of the recoil excitations from the initial emission site across the entire molecule is also visualized.

Recoil-Induced Asymmetry of Nondipole Molecular Frame Photoelectron Angular Distributions in the Hard X-ray Regime

We investigate angular emission distributions of the 1s photoelectrons of N_{2} ionized by linearly polarized synchrotron radiation at hν=40  keV. As expected, nondipole contributions cause a very strong forward-backward asymmetry in the measured emission distributions. In addition, we observe an unexpected asymmetry with respect to the polarization direction, which depends on the direction of the molecular fragmentation. In particular, photoelectrons are predominantly emitted in the direction of the forward nitrogen atom. This observation cannot be explained via asymmetries introduced by the initial bound and final continuum electronic states of the oriented molecule. The present simulations assign this asymmetry to a novel nontrivial effect of the recoil imposed to the nuclei by the fast photoelectrons and high-energy photons, which results in a propensity for the ions to break up along the axis of the recoil momentum. The results are of particular importance for the interpretation of future experiments at x-ray free electron lasers operating in the few tens of keV regime, where such nondipole and recoil effects will be essential.

Photon-Momentum-Induced Molecular Dynamics in Photoionization of N_{2} at hν=40 keV

We investigate K-shell ionization of N_{2} at 40 keV photon energy. Using a cold target recoil ion momentum spectroscopy reaction microscope, we determine the vector momenta of the photoelectron, the Auger electron, and both N^{+} fragments. These fully differential data show that the dissociation process of the N_{2}^{2+} ion is significantly modified not only by the recoil momentum of the photoelectron but also by the photon momentum and the momentum of the emitted Auger electron. We find that the recoil energy introduced by the photon and the photoelectron momentum is partitioned with a ratio of approximately 30∶70 between the Auger electron and fragment ion kinetic energies, respectively. We also observe that the photon momentum induces an additional rotation of the molecular ion.

Full-dimensional theoretical description of vibrationally resolved valence-shell photoionization of H2O

We have performed a full-dimensional theoretical study of vibrationally resolved photoelectron emission from the valence shell of the water molecule by using an extension of the static-exchange density functional theory that accounts for ionization as well as for vibrational motion in the symmetric stretching, antisymmetric stretching, and bending modes. At variance with previous studies performed in centrosymmetric molecules, where vibrationally resolved spectra are mostly dominated by the symmetric stretching mode, in the present case, all three modes contribute to the calculated spectra, including intermode couplings. We have found that diffraction of the ejected electron by the various atomic centers is barely visible in the ratios between vibrationally resolved photoelectron spectra corresponding to different vibrational states of the remaining H2O+ cation (the so-called v-ratios), in contrast to the prominent oscillations observed in K-shell ionization of centrosymmetric molecules, including those that only contain hydrogen atoms around the central atoms, e.g., CH4. To validate the conclusions of our work, we have carried out synchrotron radiation experiments at the SOLEIL synchrotron and determined photoelectron spectra and v-ratios for H2O in a wide range of photon energies, from threshold up to 150 eV. The agreement with the theoretical predictions is good.

Energy Transfer into Molecular Vibrations and Rotations by Recoil in Inner-Shell Photoemission

A mixture of CF_{4} and CO gases is used to study photoelectron recoil effects extending into the tender x-ray region. In CF_{4}, the vibrational envelope of the C 1s photoelectron spectrum becomes fully dominated by the recoil-induced excitations, revealing vibrational modes hidden from Franck-Condon excitations. In CO, using CF_{4} as an accurate energy calibrant, we determine the partitioning of the recoil-induced internal excitation energy between rotational and vibrational excitation. The observed rotational recoil energy is 2.88(28) times larger than the observed vibrational recoil energy, well in excess of the ratio of 2 predicted by the basic recoil model. The experiment is, however, in good agreement with the value of 2.68 if energy transfer via Coriolis coupling is included.

Experimental observation of rotational Doppler broadening in a molecular system

The first experimental evidence of rotational Doppler broadening in photoelectron spectra, reported here, show good agreement with recently described theoretical predictions. The dependence of the broadening on temperature and photoelectron kinetic energy is quantitatively predicted by the theory. The experiments verify that the rotational contributions to the linewidth are comparable to those from translational Doppler broadening and must be considered in the analysis of high-resolution photoelectron spectra. A classical model accounting for this newly observed effect is presented.

Electron scattering at high momentum transfer from methane: analysis of line shapes

The measurement of the energy distribution of keV electrons backscattered elastically from molecules reveals one or more peaks. These peaks are at nonzero energy loss and have an intrinsic width. The usual interpretation of these measurements is attractively simple and assumes billiard-ball-type collisions between the electron and a specific atom in the molecule, and the scattering atom is assumed to behave as a free particle. The peak position is then related to the mass of the scattering atom, and its width is a Compton profile of the momentum distribution of this atom in the molecule. Here we explore the limits of the validity of this picture for the case of electrons scattering from methane. The biggest discrepancy is found for electrons scattering from carbon. For electrons scattering from hydrogen the effects are substantial at relatively low incoming energies and appear to decrease with increasing momentum transfer. The discrepancy is analyzed in terms of the force the atom experiences near the equilibrium position.

Exploring nonadiabatic effects by recoil of fast photoelectrons

The recoil momentum imparted on the residual ion by the emission of a fast photoelectron gives rise to internal excitations of the ion. Because of this recoil, the photoionization transition is not a vertical one. By varying the photon energy and/or orientation of the system one can systematically create nuclear wave packets with different initial locations and momenta in nuclear space and thus explore nonadiabatic effects. As an illustrative example we study the dynamics in the vicinity of a conical intersection of a Jahn-Teller system.

Photo- and auger-electron recoil induced dynamics of interatomic Coulombic decay

At photon energies near the Ne K edge it is shown that for 1s ionization the Auger electron, and for 2s ionization the fast photoelectron, launch vibrational wave packets in a Ne dimer. These wave packets then decay by emission of a slow electron via interatomic Coulombic decay (ICD). The measured and computed ICD electron spectra are shown to be significantly modified by the recoil induced nuclear motion.

Recoil effect of photoelectrons in the Fermi edge of simple metals

High energy resolution photoelectron spectroscopy of conduction electrons in the vicinity of the Fermi edge in Al and Au at excitation energies of 880 and 7940 eV was carried out using synchrotron radiation. For the excitation energy of 7940 eV, the observed Fermi energy of Al shows a remarkable shift to higher binding energy as compared with that of Au, with accompanying broadening. This is due to the recoil effect of the emitted photoelectrons. The observed spectra are well reproduced by a simple model of Bloch electrons based on the isotropic Debye model.