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Register a new userTheory on polarization-averaged core-level molecular-frame photoelectron angular distributions: II. Extracting the X-ray induced fragmentation dynamics of carbon monoxide dication from forward and backward intensities
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Recent developments of high-reputation-rate X-ray free electron lasers (XFELs) such as European XFEL and LSCS-II, combined with coincidence measurements at the COLTRIMS-Reaction Microscope, is now opening a door to realize a long-standing dream to create molecular movies of photo-induced chemical reactions of gas-phase molecules. In this paper, we theoretically propose a new method to experimentally visualize dissociation of diatomic molecules via time-resolved polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) measurements using the COLTRIMs--Reaction Microscope and two-color XFEL pump-probe set-up. The first and second order scattering theories with the Muffin-tin approximation give us simple EXAFS type formula for the forward and backward scattering peaks in the PA-MFPADs structure. This formula acts as an experimentally applicable bond length ruler by adjusting only three semi-empirical parameters from the time-resolved measurements. The accuracy and applicability of a new ruler equation are numerically examined against the PA-MFPADs of CO<sup>2+</sup> calculated by Full-potential multiple scattering theory as a function of the C-O bond length reported in the preceding work. The bond lengths retrieved from the PA-MFPADs via the EXAFS formula well reproduce the original C-O bond lengths used in the reference <i>ab-initio</i> PA-MFPADs with accuracy of 0.1 {AA}. We expect that time-resolved PA-MFPADs will be a new attractive tool to make molecular movies visualizing intramolecular reactions.
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We present a theoretical study on polarization-averaged molecular-frame photoelectron angular distributions (PA-MFPADs) emitted from $1s$ orbital of oxygen atom of dissociating dicationic carbon monoxide CO$^{2+}$. Due to the polarization-average, contribution of direct wave of photoelectron which has the biggest contribution to MFPADs is removed, so that PA-MFPADs clearly show the detail of scattering image of the photoelectron. As a result, it is necessary to employ well precise theory for the continuum state for the theoretical analysis. In this study, we applied our Full-potential multiple scattering theory, where the space is partitioned by using Voronoi polyhedra and truncated spheres to take into account the electron charge density outside the physical atomic spheres. We did not use spherical harmonic expansion of the cell shape functions to avoid convergence problems.The potentials in scattering cells are prepared employing Multiconfigurational Second-Order Perturbation Theory Restricted Active Space (RASPT2) method in order to take into account the influence of core hole in the electron charge density in the final state to realize realistic relaxation. We showed that the Full-potential treatment plays an important role for the PA-MFPADs at 100 eV of kinetic energy of photoelectron. Instead, the PA-MFPADs are not sensitive to type of major excited state in the Auger final state.We also studied the dynamics of CO$^{2+}$ dissociation. We found that the PA-MFPADs dramatically change its shape as a function of C-O bond length.
We investigate angular emission distributions of the 1s-photoelectrons of N$_2$ ionized by linearly polarized synchrotron radiation at $h u=40$ 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 XFELs operating in the few tens of keV regime, where such nondipole and recoil effects will be essential.
The application of a matrix-based reconstruction protocol for obtaining Molecular Frame (MF) photoelectron angular distributions (MFPADs) from laboratory frame (LF) measurements (LFPADs) is explored. Similarly to other recent works on the topic of MF reconstruction, this protocol makes use of time-resolved LF measurements, in which a rotational wavepacket is prepared and probed via photoionization, followed by a numerical reconstruction routine; however, in contrast to other methodologies, the protocol developed herein does not require determination of photoionization matrix elements, and consequently takes a relatively simple numerical form (matrix transform making use of the Moore-Penrose inverse). Significantly, the simplicity allows application of the method to the successful reconstruction of MFPADs for polyatomic molecules. The scheme is demonstrated numerically for two realistic cases, $N_2$ and $C_2H_4$. The new technique is expected to be generally applicable for a range of MF reconstruction problems involving photoionization of polyatomic molecules.
The fragmentation of carbon monoxide dimers induced by collisions with low energy Ar$^{9+}$ ions is investigated using the COLTRIMS technique. The presence of a neighbor molecule in the dimer serves here as a diagnostic tool to probe the lifetimes of the $rm CO^{2+}$ molecular dications resulting from the collision. The existence of metastable states with lifetimes ranging from 2~ps to 200~ns is clearly evidenced experimentally through a sequential 3-body fragmentation of the dimer, whereas fast dissociation channels are observed in a so-called concerted 3-body fragmentation process. The fast fragmentation process leads to a kinetic energy release distribution also observed in collisions with monomer CO targets. This is found in contradiction with the conclusions of a former study attributing this fast process to the perturbation induced by the neighbor molecular ion.
Using a quantum wave packet simulation including the nuclear and electronic degrees of freedom, we investigate the femtosecond and picosecond energy- and angle-resolved photoelectron spectra of the E($^1Sigma_g^+$) electronic state of Li$_2$. We find that the angular distributions of the emitted photoelectrons depend strongly on the pulse duration in the regime of ultrashort laser pulses. This effect is illustrated by the extraction of a time-dependent asymmetry parameter whose variation with pulse duration can be explained by an incoherent average over different ion rotational quantum numbers. We then derive for the variation of the asymmetry parameter a simple analytical formula, which can be used to extract the asymptotic CW asymmetry parameters of individual transitions from measurements performed with ultra-short pulses.
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