Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session T7: Photoionization or Photofragmentation of Heteronuclear Molecules |
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Chair: Ali Belkacem, Lawrence Berkeley National Laboratory Room: Terrace |
Friday, June 8, 2012 8:00AM - 8:12AM |
T7.00001: Orientation dependence of the ionization of CO and NO in an intense femtosecond two-color laser field Hui Li, Dipanwita Ray, Sankar De, Wei Cao, Guillaume Laurent, Zhenhua Wang, Anh Thu Le, C. Lewis Cocke, Irina Znakovskaya, Matthias Kling Two-color (800 nm and 400 nm) ultrashort (30$\pm $10 fs) laser pulses were used to ionize and dissociate CO and NO. The emission of C$^{+q}$, N$^{+q}$ and O$^{+}$ fragments were measured with a velocity-map-imaging (VMI) system. The data show that the ionization rate is dependent on the orientation of the molecules with respect to the laser polarization. Both molecules ionize more easily when the electric field points from C to O in CO and from N to O in NO. The asymmetry of emission is much higher for CO than for NO. The sign of the asymmetry is not strongly dependent on kinetic energy release (KER). The favored ionization orientation is in agreement with the expectation of the molecular orbital Ammosov-Delone-Krainov (MO-ADK) [1] theory and with a Stark-corrected version of a strong-field-approximation (SFA) calculation [2]. \\[4pt] [1] X.M. Tong, et al., Phys. Rev. A \textbf{66}, 033402 (2002).\\[0pt] [2] H. Li, et al., Phys. Rev. A \textbf{84}, 043429 (2011). [Preview Abstract] |
Friday, June 8, 2012 8:12AM - 8:24AM |
T7.00002: Molecular dissociation of HD$^+$ by broad bandwidth chirped laser pulses: a molecular bandwidth filter M. Zohrabi, U. Ablikim, K.D. Carnes, B.D. Esry, I. Ben-Itzhak We employ a coincidence 3D momentum imaging method to study the fragmentation of HD$^+$ following interaction with an intense, 800 nm, 25 fs Fourier transform-limited (FTL) laser pulse. The broad bandwidth of our FTL pulse prevents us from observing vibrational peaks that one would expect to see using longer FTL laser pulses $\sim$100 fs. However, by chirping the pulse either positively or negatively, while maintaining a fixed bandwidth, we were able to measure vibrational structure. The kinetic energy release of these vibrational peaks are shifted up or down depending on the sign of the chirp.\footnote{V.~S.~Prabhudesai \textit{et al.}, \textit{Phys. Rev. A} \textbf{81}, 023401 (2010).} We will address the question of why the vibrational structure is observed in spite of the broad bandwidth of the chirped laser pulses. [Preview Abstract] |
Friday, June 8, 2012 8:24AM - 8:36AM |
T7.00003: Auger spectrum of a water molecule after single and double core ionization Ludger Inhester, Carl F. Burmeister, Gerrit Groenhof, Helmut Grubmueller The high intensity of Free Electron Lasers (FEL) opens up the possibility to perform single-shot molecule scattering experiments. However, even for small molecules radiation damage induced by absorption of intense x-ray radiation is not yet fully understood. To provide insight into this process, we have studied the dynamics of water molecules in single and double core ionized states by means of electronic transition rate calculations and ab initio molecular dynamics (MD) simulations. From MD trajectories photoionization and Auger transition rates were computed based on electronic continuum wavefunctions obtained by explicit integration of the coupled radial Schr\"odinger equations. To account for the nuclear dynamics during the core hole lifetime, the calculated electron emission spectra for different molecular geometries were accumulated according to the obtained time-dependent populations. We find that, in contrast to the single core ionized water molecule, the nuclear dynamics for the double core ionized water molecule during the core hole lifetime leaves a clear fingerprint on the electron emission spectra. In addition, the lifetime of the double core ionized water was found to be significantly shorter than half of the single core hole lifetime. [Preview Abstract] |
Friday, June 8, 2012 8:36AM - 8:48AM |
T7.00004: Molecular Frame Photoelectron Angular Distributions as a Probe of Geometry and Auger Dissociation Dynamics Cynthia S. Trevisan, Thomas N. Rescigno, C. William McCurdy Compex Kohn variational calculations of the molecular frame photoelectron distributions (MFPADs) for 1s core ionization of CH$_4$, NH$_3$, and H$_2$O are presented for ejected electron energies below 25 eV. Surprisingly, in these three cases there are energy ranges in which the photoelectron MFPADs effectively form ``images'' of the molecular geometry. Comparison with recent momentum imaging experiments on methane at the Advanced Light Source verify this effect. Simultaneous double Auger decay in these molecules can produce dissociation into three charged fragments, e.g., CH$_2^+$ + 2 H$^+$, allowing the complete orientation of the molecule and therefore the measurement of 3D MFPADs that test these predictions. In other Auger decay channels the measurement of 3D MFPADs verifies axial recoil (prompt dissociation) or probes its absence in the Auger dissociation dynamics of small molecules. [Preview Abstract] |
Friday, June 8, 2012 8:48AM - 9:00AM |
T7.00005: Photo double ionization of acetylene and subsequent fragmentation pathways B. Gaire, P. Braun, I. Bocharova, F. Sturm, D. Haxton, A. Belkacem, Th. Weber, C.L. Cocke, A. Landers, R. Dorner We have investigated the photo double ionization of acetylene (C$_{2}$H$_{2})$ molecules using photons of 42eV energy. Coincident measurements of both ions and electrons with the COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) method make kinematically complete study possible. We extract the 3d-momentum vectors of each particle using the measured flight time to the detector and the position on the detector. We identify the fragmentation pathways by using the measured energy of the photo electrons, kinetic energy release of the nuclear fragments, photon energy, and the ionization potential for a specific dication state evaluated from theory. We mainly discuss the pathways leading to the symmetric breakup (CH$^{+}$/CH$^{+})$, the deprotonation (H$^{+}$/C$_{2}$H$^{+})$ and the quasi-symmetric fragmentation (C$^{+}$/H$_{2}$C$^{+})$. We explain the importance of such pathways for the study of the time dependent dynamics of the fragmentation. [Preview Abstract] |
Friday, June 8, 2012 9:00AM - 9:12AM |
T7.00006: Imaging Molecular Isomerization Using Molecular-Frame Photoelectron Angular Distributions T.N. Rescigno, A.E. Orel, Nicolas Douguet Techniques such as X-ray diffraction and ultrafast electron diffraction can potentially be taken to the time domain to image chemical reactions on their natural timescale. Photoelectron diffraction from fixed-in-space molecules, where an electron is launched from an inner shell by photoabsorption, offers a similar promise. We illustrate the idea here with the results of {\it ab initio} calculations using the complex Kohn variational method of molecular-frame photoelectron angular distributions (MFPADs) on the acetylene monocation (HCCH$^+$). Photoionization of neutral acetylene, which is linear at equilibrium, in the 20-40 eV range produces ground (X) and excited (A) HCCH$^+$ in roughly equal amounts. The electronically excited A-state cation can follow a downhill path to a conical intersection with the X-state near a trans-symmetric geometry and from there to a vinylidene (H$_2$CC) isomeric structure. We will show that the MFPADs produced by C k-shell photoionization of HCCH$^+$, while relatively insensitive to the electronic configuration of the valence electrons at a given photoelectron energy, are much more sensitive to nuclear geometry and can therefore be used to track the acetylene to vinylidene isomerization. [Preview Abstract] |
Friday, June 8, 2012 9:12AM - 9:24AM |
T7.00007: Time-resolved molecular frame photoelectron angular distribution: Snapshots of acetylene and ethylene isomerizations Nicolas Douguet, Thomas Rescigno, Ann Orel It has been proposed that chemical reactions can be imaged by detection of a photoelectron in the molecular fixed body frame (MFPAD), following either valence or K-shell photoionization. We will contrast these two techniques using acetylene and ethylene isomerization as examples. The \textit{ab initio} calculations were carried out using the complex Kohn variational method. Our results indicate that while K-shell MFPADs are only sensitive to molecular geometry whereas valence MFPADs are sensitive to both geometry and the initial electronic state of the target. Both examples show that isomerization takes place via conical intersections. However, acetylene requires an excitation/ionization step, while ethylene undergoes isomerization following direct excitation of the neutral V state. [Preview Abstract] |
Friday, June 8, 2012 9:24AM - 9:36AM |
T7.00008: Intense-field Ionization and Fragmentation of Heterocyclic Organic Molecules: the Azabenzenes Timothy Scarborough, Collin McAcy, David Foote, Cornelis Uiterwaal We report on the ultrafast intense-field photoionization and fragmentation of pyridine, pyridazine, pyrimidine and pyrazine. These four molecules represent a systematic series of perturbations into the structure of a benzene ring which explores the substitution of a C-H entity with a nitrogen atom, creating a heterocyclic structure which remains isoelectronic with benzene. Other than pyridine, each molecule has the same chemical formula, with the only difference being the relative placement of the two perturbing nitrogen atoms (ortho-, meta- or para-substitutions). Data is recorded under intense-field, single-molecule conditions. 50 fs, 800 nm pulses are focused into the molecular vapor, and ion mass spectra are recorded for intensities of 10$^{13}$ to 10$^{15}$ W/cm$^{2}$ in the absence of the focal volume effect. For all targets, stable singly- and doubly-charged parent ions (C$_{6-n}$H$_{6-n}$N$_{n}^{+(+)})$ are observed with features suggesting resonance enhancement (REMPI). Fragmentation dynamics differ greatly between molecules, with each species showing evidence of metastable decay processes. [Preview Abstract] |
Friday, June 8, 2012 9:36AM - 9:48AM |
T7.00009: Large Molecules Reveal a Linear Length Scaling for Double Photoionization Ralf Wehlitz, Tim Hartman, Pavle Jurani\'c, Kelly Collins, Bethany Reilly, Narayana Appathurai We have determined the ratio of doubly-charged to all parent ions of partially deuterated benzene, naphthalene, anthracene, and pentacene up to 30 eV above their double-ionization thresholds. These ratios increase linearly with the length of the molecule.\footnote{T. Hartman {\it et al.}, Phys.\ Rev.\ Lett. {\bf 108}, 023001 (2012)} This means that the general structure (here: the length) of a molecule can have a significant influence on the ratio and that the origin of the two emitted electrons can be as far apart as the length of the molecule. Moreover and quite surprisingly, the overall energy dependences of the molecules' ratios are very similar to that of He and the structure of these molecules does not affect the shape of the energy dependence of the ratio. We interpret this as direct evidence for the validity of the knock-out model even for large molecules and not just for atoms. We want to mention that the increase of the ratio may not only be valid for molecules of increasing lengths but also for molecules that increase in size in two dimensions such as pyrene and coronene. [Preview Abstract] |
Friday, June 8, 2012 9:48AM - 10:00AM |
T7.00010: Strong field Molecular ionization and control viewed with Velocity Map Imaging Dominik Geissler, Tam\'as Rozgonyi, Jes\'us Gonz\'alez-V\'azquez, P\'eter Sandor, Leticia Gonz\'alez, Philipp Marquetand, Thomas C. Weinacht We employ velocity map imaging of electrons and fragment ions to characterize molecular wave packets after interaction with a strong field ultrashort laser pulse (central wavelength 780nm). Our measurements reveal superpositions of electronic states created by strong field ionization. They also demonstrate how dynamic Stark shifts can alter the PESs for a given electronic state sufficiently to change the propagation of a vibrational wavepacket. In the experiment, we prepare the molecules in an excited state and apply a strong IR pulse, whose basic parameters (intensity, time delay and chirp) are systematically varied. The results of the measurements are compared with ab-inito structure and dynamics calculations in order to verify our interpretation. [Preview Abstract] |
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