2021 |
Holzmeier, F., Joseph, J., Houver, J. C., Lebech, M., Dowek, D., & Lucchese, R. R. (2021). Influence of shape resonances on the angular dependence of molecular photoionization delays. Nat. Commun., .
Résumé: Characterizing time delays in molecular photoionization as a function of the ejected electron emission direction relative to the orientation of the molecule and the light polarization axis provides unprecedented insights into the attosecond dynamics induced by extreme ultraviolet or X-ray one-photon absorption, including the role of electronic correlation and continuum resonant states. Here, we report completely resolved experimental and computational angular dependence of single-photon ionization delays in NO molecules across a shape resonance, relying on synchrotron radiation and time-independent ab initio calculations. The angle-dependent time delay variations of few hundreds of attoseconds, resulting from the interference of the resonant and non-resonant contributions to the dynamics of the ejected electron, are well described using a multichannel Fano model where the time delay of the resonant component is angle-independent. Comparing these results with the same resonance computed in e-NO+ scattering highlights the connection of photoionization delays with Wigner scattering time delays. It is an interesting topic to find the time it takes for an electron to escape an atom or a molecule after photoionization. Here the authors measure the angular dependence of photoionization time delay in the molecular frame and discuss the role of shape resonances.
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2017 |
Poullain, S. M., Cireasa, R., Cornaggia, C., Simon, M., Marin, T., Guillemin, R., Houver, J. C., Lucchese, R. R., & Dowek, D. (2017). Spectral dependence of photoemission in multiphoton ionization of NO2 by femtosecond pulses in the 375-430 nm range. Physical Chemistry Chemical Physics, 19(33), 21996–22007.
Résumé: We investigate the multiphoton ionization of NO2 using tunable (430-375 nm) femtosecond pulses and photoelectron-photoion coincidence momentum spectroscopy. In order to understand the complex electronic and nuclear photodynamics at play following absorption of three to five photons, we also report extended photoionization calculations using correlated targets and coupled channels. Exploring the multiphoton dissociative ionization (MPDI) and multiphoton ionization (MPI) processes over such a broad energy range enables us to lend further support to our work carried out around 400 nm of a femtosecond laser [S. Marggi Poullain et al., J. Phys. B: At., Mol. Opt. Phys., 2014, 47, 124024]. Two excitation energy regions are identified and discussed in terms of the proposed reaction pathways, highlighting the significant role of Rydberg states, such as the [R*(6a(1))(-1), 3p sigma] Rydberg state, in the NO2 multiphoton excitation and photoionization. These new results support our previous assumption that different bent and linear geometries of the NO2 +(X-1 Sigma(g)) ionic state contribute to the MPDI and MPI, consistent with the reported calculations which reveal an important vibronic coupling characterizing the photoemission. Remarkably, the strong anisotropy of the recoil frame photoelectron angular distribution (RFPAD) previously observed at 400 nm appears as a fingerprint across the whole explored photon energy range.
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2016 |
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Decleva, P., Orr-Ewing, A. J., Kowalewski, M., Kornilov, O., Marangos, J. P., Worner, H. J., Johnson, A. S., Forbes, R., Rolles, D., Townsend, D., Schalk, O., Mai, S., Penfold, T. J., Miller, R. J. D., Centurion, M., Ueda, K., Domcke, W., Weber, P. M., Baeck, K. K., Travnikova, O., Liekhus-Schmaltz, C., Figueira, J. A. P., Neumark, N. D. M., Gessner, O., Stolow, A., Rudenko, A., Mishra, P. K., Kirrander, A., Dowek, D., Martin, F., Vibok, A., Minitti, M. P., Stankus, B., & Burger, C. (2016). Structural dynamics: general discussion. Faraday Discussions, 194, 583–620.
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Milne, C. J., Weber, P. M., Kowalewski, M., Marangos, J. P., Johnson, A. S., Forbes, R., Worner, H. J., Rolles, D., Townsend, D., Schalk, O., Mai, S., Vacher, M., Miller, R. J. D., Centurion, M., Vibok, A., Domcke, W., Cireasa, R., Ueda, K., Bencivenga, F., Neumark, D. M., Stolow, A., Rudenko, A., Kirrander, A., Dowek, D., Martin, F., Ivanov, M., Dahlstrom, J. M., Dudovich, N., Mukamel, S., Sanchez-Gonzalez, A., Minitti, M. P., Austin, D. R., Kimberg, V., & Masin, Z. (2016). Attosecond processes and X-ray spectroscopy: general discussion. Faraday Discussions, 194, 427–462.
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Orr-Ewing, A. J., Verlet, J. R. R., Penfold, T. J., Minns, R. S., Minitti, M. P., Solling, T. I., Schalk, O., Kowalewski, M., Marangos, J. P., Robb, M. A., Johnson, A. S., Worner, H. J., Shalashilin, D. V., Miller, R. J. D., Domcke, W., Ueda, K., Weber, P. M., Cireasa, R., Vacher, M., Roberts, G. M., Decleva, P., Bencivenga, F., Neumark, D. M., Gessner, O., Stolow, A., Mishra, P. K., Polyak, I., Baeck, K. K., Kirrander, A., Dowek, D., Jimenez-Galan, A., Martin, F., Mukamel, S., Sekikawa, T., Gelin, M. F., Townsend, D., Makhov, D. V., & Neville, S. P. (2016). Electronic and non-adiabatic dynamics: general discussion. Faraday Discussions, 194, 209–257.
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Veyrinas, K., Gruson, V., Weber, S. J., Barreau, L., Ruchon, T., Hergott, J. - F., Houver, J. - C., Lucchese, R. R., Salieres, P., & Dowek, D. (2016). Molecular frame photoemission by a comb of elliptical high-order harmonics: a sensitive probe of both photodynamics and harmonic complete polarization state. Faraday Discussions, 194, 161–183.
Résumé: Due to the intimate anisotropic interaction between an XUV light field and a molecule resulting in photoionization (PI), molecular frame photoelectron angular distributions (MFPADs) are most sensitive probes of both electronic/nuclear dynamics and the polarization state of the ionizing light field. Consequently, they encode the complex dipole matrix elements describing the dynamics of the PI transition, as well as the three normalized Stokes parameters s(1), s(2), s(3) characterizing the complete polarization state of the light, operating as molecular polarimetry. The remarkable development of advanced light sources delivering attosecond XUV pulses opens the perspective to visualize the primary steps of photochemical dynamics in time-resolved studies, at the natural attosecond to few femtosecond time-scales of electron dynamics and fast nuclear motion. It is thus timely to investigate the feasibility of measurement of MFPADs when PI is induced e.g., by an attosecond pulse train (APT) corresponding to a comb of discrete high-order harmonics. In the work presented here, we report MFPAD studies based on coincident electron-ion 3D momentum imaging in the context of ultrafast molecular dynamics investigated at the PLFA facility (CEA-SLIC), with two perspectives: (i) using APTs generated in atoms/molecules as a source for MFPAD-resolved PI studies, and (ii) taking advantage of molecular polarimetry to perform a complete polarization analysis of the harmonic emission of molecules, a major challenge of high harmonic spectroscopy. Recent results illustrating both aspects are reported for APTs generated in unaligned SF6 molecules by an elliptically polarized infrared driving field. The observed fingerprints of the elliptically polarized harmonics include the first direct determination of the complete s(1), s(2), s(3) Stokes vector, equivalent to (psi, epsilon, P), the orientation and the signed ellipticity of the polarization ellipse, and the degree of polarization P. They are compared to so far incomplete results of XUV optical polarimetry. We finally discuss the comparison between the outcomes of photoionization and high harmonic spectroscopy for the description of molecular photodynamics.
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2015 |
Veyrinas, K., Gruson, V., Weber, S. J., Barreau, L., Ruchon, T., Houver, J. C., Carre, B., Lucchese, R. R., Salieres, P., & Dowek, D. (2015). Molecular frame photoemission: a sensitive probe of the complete polarization state of high harmonic generation. In J. Phys.: Conf. Ser. (Vol. 635, 112140).
Résumé: Molecular polarimetry, based on the determination of molecular frame photoemission in dissociative photoionization (DPI) studied by electron-ion coincident momentum spectroscopy, allows us to measure the complete state of elliptically polarized light. Here, we demonstrate the first complete characterization of high-order harmonic polarization state for three benchmark generation processes with inherent symmetry breaking based on the polarization of the generating beam (elliptical or counter-rotating fields) or on anisotropic targets e.g. aligned molecules. The ability to disentangle circular and unpolarized components of the XUV pulses is of particular interest to qualify the HHG sources.
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2014 |
Marggi Poullain, S., Elkharrat, C., Li, W. B., Veyrinas, K., Houver, J. C., Cornaggia, C., Rescigno, T. N., Lucchese, R. R., & Dowek, D. (2014). Recoil frame photoemission in multiphoton ionization of small polyatomic molecules: photodynamics of NO2 probed by 400 nm fs pulses. JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 47(12), 124024.
Résumé: We report a general method for the complete analysis of the recoil frame photoelectron angular distribution (RFPAD) in n-photon dissociative ionization of small polyatomic molecules, resulting from (n – 1) bound-to-bound transitions plus one-photon ionization of a neutral excited state of the target. This method relies on the decomposition of the RFPAD in terms of the R-K (chi, theta(e)) recoil frame azimuthal harmonics (RFAHs) which are the components of its Fourier expansion in phi(e), where chi and theta(e) are the polar angles referring to the polarization axis P and the photoelectron momentum k relative to the ion fragment recoil direction, respectively, and fe is the azimuth of k relative to P. The RFAH expansion method is illustrated by a detailed experimental and theoretical study of one-colour multiphoton dissociative and non-dissociative ionization of the NO2 molecule of C-2v symmetry induced by 400 nm fs laser pulses, which involve electronic and nuclear dynamics within the pulse duration of the order of 70 fs. The reaction mechanism proposed to account for five-photon dissociative ionization of NO2 involves the role of [R*(6a(1))(-1)] Rydberg states populated by three-photon absorption, subsequently ionized by a fourth photon into the NO2+ (X-1 Sigma(+)(g), upsilon(1),upsilon(2),upsilon(3)) manifold involving autoionization of [R*(4b(2))(-1)] Rydberg states, and linear versus bent geometry selective dissociation of NO2+ (X-1 Sigma(+)(g), upsilon(1),upsilon(2),upsilon(3)) by a fifth photon. The reported calculations provide a coherent picture of the experimental findings although all features are not yet well reproduced.
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Perez-Torres, J. F., Sanz-Vicario, J. L., Veyrinas, K., Billaud, P., Picard, Y. J., Elkharrat, C., Marggi Poullain, S., Saquet, N., Lebech, M., Houver, J. C., Martin, F., & Dowek, D. (2014). Circular dichroism in molecular-frame photoelectron angular distributions in the dissociative photoionization of H-2 and D-2 molecules. PHYSICAL REVIEW A, 90(4), 043417.
Résumé: The presence of net circular dichroism in the photoionization of nonchiral homonuclear molecules has been put in evidence recently through the measurement of molecular-frame photoelectron angular distributions in dissociative photoionization of H-2 [Dowek et al., Phys. Rev. Lett. 104, 233003 (2010)]. In this work we present a detailed study of circular dichroism in the photoelectron angular distributions of H-2 and D-2 molecules, oriented perpendicularly to the propagation vector of the circularly polarized light, at different photon energies (20, 27, and 32.5 eV). Circular dichroism in the angular distributions at 20 and to a large extent 27 eV exhibits the usual pattern in which inversion symmetry is preserved. In contrast, at 32.5 eV, the inversion symmetry breaks down, which eventually leads to total circular dichroism after integration over the polar emission angle. Time-dependent ab initio calculations support and explain the observed results for H-2 in terms of quantum interferences between direct photoionization and delayed autoionization from the Q(1) and Q(2) doubly excited states into ionic states (1s sigma(g) and 2p sigma(u)) of different inversion symmetry. Nevertheless, for D-2 at 32.5 eV, there is a particular case where theory and experiment disagree in the magnitude of the symmetry breaking: when D+ ions are produced with an energy of around 5 eV. This reflects the subleties associated to such simple molecules when exposed to this fine scrutiny.
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2013 |
Bomme, C., Guillemin, R., Marin, T., Journel, L., Marchenko, T., Dowek, D., Trcera, N., Pilette, B., Avila, A., Ringuenet, H., Kushawaha, R. K., & Simon, M. (2013). Double momentum spectrometer for ion-electron vector correlations in dissociative photoionization. REVIEW OF SCIENTIFIC INSTRUMENTS, 84(10), 103104.
Résumé: We have developed a new momentum spectrometer dedicated to momentum vector correlations in the context of deep core photoionization of atomic and molecular species in the gas phase. In this article, we describe the design and operation of the experimental setup. The capabilities of the apparatus are illustrated with a set of measurements done on the sulphur core 1s photoionization of gas-phase CS2. (C) 2013 AIP Publishing LLC.
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Hoendervanger, A. L., Clement, D., Aspect, A., Westbrook, C. I., Dowek, D., Picard, Y. J., & Boiron, D. (2013). Influence of gold coating and interplate voltage on the performance of chevron micro-channel plates for temporally and spatially resolved single particle detection. REVIEW OF SCIENTIFIC INSTRUMENTS, 84(2), 023307.
Résumé: We present a study of two different sets of Micro-Channel Plates used for time and space resolved single particle detection. We investigate the effects of the gold coating and that of introducing an interplate voltage between the spatially separated plates. We find that the gold coating increases the count rate of the detector and the pulse amplitude as previously reported for non-spatially resolved setups. The interplate voltage also increases count rates. In addition, we find that a non-zero interplate voltage improves the spatial accuracy in determining the arrival position of incoming single particles (by ~20%) while the gold coating has a negative effect (by ~30%).
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Marggi Poullain, S., Veyrinas, K., Billaud, P., Lebech, M., Picard, Y. J., Lucchese, R. R., & Dowek, D. (2013). The role of Rydberg states in photoionization of NO2 and (NO+, O-) ion pair formation induced by one VUV photon. JOURNAL OF CHEMICAL PHYSICS, 139(4), 044311.
Résumé: We report a combined experimental and theoretical study of photoionization (PI) of the NO2 molecule into the NO2+ (X-1 Sigma(+)(g)) ground state and the photodissociation of NO2 into the NO+((1)Sigma(+)) + O-(P-2) ion pair. These processes were induced by 10.9 eV-13 eV synchrotron radiation and the products were detected using electron-ion or O--NO+ coincident momentum spectroscopy. The results demonstrate the strong influence of [R*(4b(2))(-1), nl alpha(i), v'(2)] Rydberg states vibrationally resolved in the v'(2) bending modes for both processes. In particular, we emphasize two regions around 11.5 eV and 12.5 eV that were studied in more detail for their relevance to 400 nm multiphoton ionization induced by femtosecond pulses. The photoelectron energy spectra and asymmetry parameters support the existence of two PI mechanisms, as probed with the help of fixed-nuclei frozen-core Hartree-Fock calculations. We found significant deviations from Franck-Condon ionization predictions which may be assigned to vibronic coupling of NO2* states such as that induced by a conical intersection. The limited agreement between theory and experiment, even for the non-resonant processes, indicates the need for calculations that go beyond the approximations used in the current study. Ion pair formation leads to strong vibrational and rotational excitation of the NO+((1)Sigma(+), v) product, with an ion fragment angular anisotropy depending on both the v'(2) bending quantum number of the excited parent molecule and the v vibrational level of the fragment. (C) 2013 AIP Publishing LLC.
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Veyrinas, K., Elkharrat, C., Marggi Poullain, S., Saquet, N., Dowek, D., Lucchese, R. R., Garcia, G. A., & Nahon, L. (2013). Complete determination of the state of elliptically polarized light by electron-ion vector correlations. PHYSICAL REVIEW A, 88(6), 063411.
Résumé: We propose a method, molecular polarimetry, applicable to an extended VUV-x-ray range, allowing us to determine the complete state of elliptically polarized light, including the challenging disentanglement of the circular and unpolarized components. It relies on the determination of the molecular frame photoelectron angular distributions derived from electron-ion velocity vector correlations in dissociative photoionization of simple molecules. The high accuracy of the full set of Stokes parameters determination has been established by comparison with data obtained in parallel with a VUV optical polarimeter used as a benchmark.
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2012 |
Billaud, P., Geleoc, M., Picard, Y. J., Veyrinas, K., Hergott, J. F., Poullain, S. M., Breger, P., Ruchon, T., Roulliay, M., Delmotte, F., Lepetit, F., Huetz, A., Carre, B., & Dowek, D. (2012). Molecular frame photoemission in dissociative ionization of H-2 and D-2 induced by high harmonic generation femtosecond XUV pulses. JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS, 45(19), 194013.
Résumé: We report the first results of molecular frame photoelectron emission for dissociative photoionization (DPI) of H-2 and D-2 molecules induced by a spectrally filtered single high harmonic of a few femtosecond duration, using coincident electron-ion velocity vector correlation techniques. For the studied photon energies around 32 eV, where the resonant excitation of the Q(1) and Q(2) doubly excited states occurs, autoionization and nuclear dynamics are coupled on a few femtosecond timescale, giving rise to quantum interferences. Molecular frame photoelectron angular distributions (MFPADs), traced as a function of the kinetic energy release of the atomic fragments, provide the most sensitive observables for such complex dynamics. These results compare well with recent spectrally resolved experiments using synchrotron radiation which are also reported. As a novel XUV light source running at multi-kHz repetition rate and synchronized with laser pulses, high-order harmonic generation (HHG) opens new possibilities for extending these investigations to time-resolved studies at the femtosecond scale.
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Lebech, M., Houver, J. C., Raseev, G., dos Santos, A. S., Dowek, D., & Lucchese, R. R. (2012). Valence and inner-valence shell dissociative photoionization of CO in the 26-33 eV range. II. Molecular-frame and recoil-frame photoelectron angular distributions. JOURNAL OF CHEMICAL PHYSICS, 136(9), 094303.
Résumé: Experimental and theoretical results for molecular-frame photoemission are presented for inner-valence shell photoionization of the CO molecule induced by linearly and circularly polarized light. The experimental recoil frame photoelectron angular distributions (RFPADs) obtained from dissociative photoionization measurements where the velocities of the ionic fragment and photoelectron were detected in coincidence, are compared to RFPADs computed using the multichannel Schwinger configuration interaction method. The formalism for including a finite lifetime of the predissociative ion state is presented for the case of general elliptically polarized light, to obtain the RFPAD rather than the molecular frame photoelectron angular distribution (MFPAD), which would be obtained with the assumption of instantaneous dissociation. We have considered photoionization of CO for the photon energies of 26.0 eV, 29.5 eV, and 32.5 eV. A comparison of experimental and theoretical RFPADs allows us to identify the ionic states detected in the experimental studies. In addition to previously identified states, we found evidence for the 2 (2)Delta state with an ionization potential of 25.3 eV and (2)Sigma(+) states with ionization potentials near 32.5 eV. A comparison of the experimental and theoretical RFPADs permits us to estimate predissociative lifetimes of 0.25-1 ps for some of the ion states. Consideration of the MFPADs of a series of (2)Pi ion states indicates the importance of inter-channel coupling at low photoelectron kinetic energy and the limitations of a single-channel analysis based on the corresponding Dyson orbitals. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3681920]
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2011 |
Siu, W., Kelkensberg, F., Gademann, G., Rouzee, A., Johnsson, P., Dowek, D., Lucchini, M., Calegari, F., De Giovannini, U., Rubio, A., Lucchese, R. R., Kono, H., Lepine, F., & Vrakking, M. J. J. (2011). Attosecond control of dissociative ionization of O-2 molecules. PHYSICAL REVIEW A, 84(6), 063412.
Résumé: We demonstrate that dissociative ionization of O-2 can be controlled by the relative delay between an attosecond pulse train (APT) and a copropagating infrared (IR) field. Our experiments reveal a dependence of both the branching ratios between a range of electronic states and the fragment angular distributions on the extreme ultraviolet (XUV) to IR time delay. The observations go beyond adiabatic propagation of dissociative wave packets on IR-induced quasistatic potential energy curves and are understood in terms of an IR-induced coupling between electronic states in the molecular ion.
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2010 |
Dowek, D., Perez-Torres, J. F., Picard, Y. J., Billaud, P., Elkharrat, C., Houver, J. C., Sanz-Vicario, J. L., & Martin, F. (2010). Circular Dichroism in Photoionization of H-2. PHYSICAL REVIEW LETTERS, 104(23), 233003.
Résumé: Circular dichroism is a consequence of chirality. However, nonchiral molecules can also exhibit it when the measurement itself introduces chirality, e. g., when measuring molecular-frame photoelectron angular distributions. The few such experiments performed on homonuclear diatomic molecules show that, as expected, circular dichroism vanishes when the molecular-frame photoelectron angular distributions are integrated over the polar electron emission angle. Here we show that this is not the case in resonant dissociative ionization of H-2 for photons of 30-35 eV, which is the consequence of the delayed ionization from molecular doubly excited states into ionic states of different inversion symmetry.
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Elkharrat, C., Picard, Y. J., Billaud, P., Cornaggia, C., Garzella, D., Perdrix, M., Houver, J. C., Lucchese, R. R., & Dowek, D. (2010). Ion Pair Formation in Multiphoton Excitation of NO2 Using Linearly and Circularly Polarized Femtosecond Light Pulses: Kinetic Energy Distribution and Fragment Recoil Anisotropy (vol 114A, pg 9902, 2010). JOURNAL OF PHYSICAL CHEMISTRY A, 114(50), 13288.
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2009 |
Dowek, D., Haouas, A., Guillemin, R., Elkharrat, C., Houver, J. - C., Li, W. B., Catoire, F., Journel, L., Simon, M., & Lucchese, R. R. (2009). Recoil frame photoemission in inner-shell photoionization of small polyatomic molecules. EUROPEAN PHYSICAL JOURNAL-SPECIAL TOPICS, 169, 85–93.
Résumé: Going from a diatomic to a polyatomic linear or non-linear molecule, the description of molecular frame photoemission in photoionization of molecules involves a function of increasing complexity. Using the electron-ion Vector Correlation method, which provides the emission velocity vectors of the emitted charged particles in a dissociative photoionization reaction, generally gives access to Recoil Frame Photoelectron Angular Distributions (RFPADs). We report results for site selected inner-shell photoionization of the N(2)O molecule induced by elliptically polarized light, where the primary ionization reaction leads to the production of several fragmentation channels. Ion fragment kinetic energy release distributions as well as RFPADs are reported for selected two ion-channels containing the complete parent molecule. Experimental results are compared with recent multi-channel Schwinger configuration interaction (MCSCI) ab initio calculations at the level of the RFPADs. The link with the original computed molecular frame photoelectron angular distributions (MFPADs) is discussed.
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Dowek, D., Picard, Y. J., Billaud, P., Elkharrat, C., & Houver, J. C. (2009). Molecular Frame Photoemission: Probe of the Photoionization Dynamics for Molecules in the Gas Phase. CHINESE JOURNAL OF CHEMICAL PHYSICS, 22(2), 178–186.
Résumé: Molecular frame photoemission is a very sensitive probe of the photoionization (PI) dynamics of molecules. This paper reports a comparative study of non-resonant and resonant photoionization of D(2) induced by VUV circularly polarized synchrotron radiation at SOLEIL at the level of the molecular frame photoelectron angular distributions (MFPADs). We use the vector correlation method which combines imaging and time-of-flight resolved electron-ion coincidence techniques, and a generalized formalism for the expression of the I(chi, theta(e), phi(e)) MFPADs, where chi is the orientation of the molecular axis with respect to the light quantization axis and (theta(e), phi(e)) the electron emission direction in the molecular frame. Selected MFPADs for a molecule aligned parallel or perpendicular to linearly polarized light, or perpendicular to the propagation axis of circularly polarized light, are presented for dissociative photoionization (DPI) of D(2) at two photon excitation energies, hv=19 eV, where direct PI is the only channel opened, and hv=32.5 eV, i.e. in the region involving resonant excitation of Q(1) and Q(2) doubly excited state series. We discuss in particular the properties of the circular dichroism characterizing photoemission in the molecular frame for direct and resonant PI. In the latter case, a remarkable behavior is observed which may be attributed to the interference occurring between undistinguishable autoionization decay channels.
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Lebech, M., Houver, J. C., & Dowek, D. (2009). Valence and inner-valence shell dissociative photoionization of CO in the 26-33 eV range. I. Ion-electron kinetic energy correlation and laboratory frame photoemission. JOURNAL OF CHEMICAL PHYSICS, 130(19), 194307.
Résumé: The (V(A+), V(e), e) vector correlation method, combining imaging and time-of-flight resolved electron-ion coincidence techniques, is used to probe dissociative photoionization (DPI) of CO induced by vacuum ultra violet linearly or circularly polarized synchrotron radiation in the 26-33 eV photon excitation energy range. It provides original information about both the photoionization dynamics of the CO molecule and the dissociation dynamics of the CO(+) molecular ions. The explored region corresponds to valence and inner-valence CO(+) ionic states, which involve doubly or multiply excited electronic configurations. In this paper I we identify up to 17 DPI reaction pathways by the position of the intermediate CO(+) molecular states in the Franck-Condon region and the (C(+) + O) or (O(+) + C) dissociation limits to which they correlate. For these processes we report the laboratory frame beta(C+/O+) and beta(e) asymmetry parameters as well as the relative branching ratios in selected binding energy bands. The I(chi,theta(e),phi(e)) molecular frame photoelectron angular distributions for selected PI processes will be reported in a companion paper II and compared with multichannel Schwinger configuration interaction ab initio calculations of these observables.
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2008 |
Elkharrat, C., Picard, Y. J., Billaud, P., Houver, J. C., & Dowek, D. (2008). Electron-ion vector correlations for the study of photoionization of molecules in the UVX range: From synchrotron radiation to short light pulses. In UVX 2008: 9E COLLOQUE SUR LES SOURCES COHERENTES ET INCOHERENTES UV (pp. 35–43).
Résumé: Molecular frame photoemission is a very sensitive probe of the photoionization (PI) dynamics of molecules. The electron-ion vector correlation (VC) method takes advantage of dissociative photoionization (DPI) reactions of small molecules to measure such observables. It relies on the coincident detection of the photoelectron and the ion fragment emitted from the same DPI event and the determination of their velocity vectors. Most of the VC studies so far have been performed using synchrotron radiation (SR) light sources which benefit from a high repetition rate (1-10 MHz) favorable for coincidence experiments. In this paper we discuss the extension of this method to the study of PI processes induced by ultra-short VUV light sources, which provide the capability for investigating processes characterized by femtosecond or subfemtosecond dynamics. We first illustrate the VC method by the report of recent results of a comparative study of resonant photoionization of the H(2) and D(2) molecules induced by VUV circularly polarized synchrotron radiation at SOLEIL in the region involving resonant excitation of Q(1) and Q(2) doubly excited state series. This problem is of particular interest since autoionization and dissociation of selected Q, or Q(2) states occur on a comparable time scale of few femtoseconds, which implies a coupling between the electronic and nuclear motion. The extension of the VC method using femtosecond laser sources is then demonstrated by results for multiphoton PI of the Xe atom induced by 70 fs pulses centered at 400 nm delivered by the SOFOCKLE and PLFA Sources (SLIC, CEA-Saclay) after frequency doubling of a 1 kHz Ti:sapphire laser.
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Journel, L., Guillemin, R., Haouas, A., Lablanquie, P., Penent, F., Palaudoux, J., Andric, L., Simon, M., Ceolin, D., Kaneyasu, T., Viefhaus, J., Braune, M., Li, W. B., Elkharrat, C., Catoire, F., Houver, J. - C., & Dowek, D. (2008). Resonant double Auger decay in carbon K-shell excitation of CO. PHYSICAL REVIEW A, 77(4).
Résumé: We have studied double Auger decay after C 1s -> 2 pi(*) photoexcitation in gas phase carbon monoxide. Two distinct processes, namely direct double Auger decay and cascade double Auger decay, are identified and studied in detail using multiple coincidence techniques. Cascade Auger decay is shown to be the overall dominant process. Decay channels involving the dissociation of the molecule followed by autoionization of the oxygen fragments are observed.
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