André T. J. B. Eppink

Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and ...

Photoelectron and photofragment velocity map imaging of state-selected molecular oxygen dissociation/ionization dynamics

A substantial improvement in the photofragment imaging technique is illustrated in a study of molecular oxygen photodynamics. In this method, labeled velocity map imaging, electrostatic ion lenses are shown to allow mapping of all particles with the same initial velocity vector onto the same point on a 2D detector, irrespective of their position of creation in the ionization volume. This leads to a dramatic increase in image resolution. Velocity map imaging of photoelectrons from molecular ionization is also demonstrated and applied along with O+ imaging to identify the processes leading to O+ formation when using (2+1) resonantly enhanced multiphoton ionization (REMPI) detection for O2. Oxygen molecules prepared in the (v=2, N=2) level of the 3dπ(3Σ1g−) Rydberg state by two-photon excitation at 11.02 eV are excited by a third photon to an energy near v=24 of ground-state O2+ (equivalent to one-photon excitation at 75 nm). All energetically accessible excited oxygen atoms and an extensive range of vibrati...

ENERGY PARTITIONING FOLLOWING PHOTODISSOCIATION OF METHYL IODIDE IN THE A BAND : A VELOCITY MAPPING STUDY

Translational and internal energy partitioning in the methyl and iodine fragments formed from photodissociation of methyl iodide in the A-band region is measured using velocity mapping. State-selective detection combined with the very good image quality afforded by the two-dimensional imaging technique allow a detailed analysis of the kinetic energy and angular distributions. Product vibrational energy is, as previously known, mainly partitioned into ν2, the umbrella mode of the methyl fragment, but a substantial fraction of molecules is also excited with one quantum of ν1, the symmetric C–H stretch, especially at higher dissociation energies. Preliminary evidence is also presented for excitation of several quanta of ν4, the asymmetric deformation mode. Rotational energy partitioning is similar for CH3 products formed in both the ground-state I(2P3/2) and the spin–orbit excited I*(2P1/2) channel for photodissociation across the full A-band spectrum. Dissociation of vibrationally excited molecules plays an...

METHYL IODIDE A-BAND DECOMPOSITION STUDY BY PHOTOFRAGMENT VELOCITY IMAGING

The methyl iodide A-band photodissociation process CH3I+hν→CH3(v,N,K)+I(2P3/2), I*(2P1/2) has been studied in a cold molecular beam. Full three-dimensional state-specific speed and angular distributions of the nascent fragments were recorded using (2+1) resonance-enhanced multi-photon ionization (REMPI) and velocity imaging, a new variant of ion imaging. By combining the I* quantum yield and anisotropy parameters for both I and I* channels, the relative absorption strength to the contributing electronic states (3Q0, 3Q1 and 1Q1) as well as the probability for curve crossing (3Q0→1Q1) are determined for excitation wavelengths across the full A band (240–334 nm). Parallel excitation to the 3Q0 state turns out to dominate the A band even more than previously thought.

State-to-state cross sections for rotational excitation of OH by collisions with He and Ar

Parity resolved state‐to‐state cross sections for rotational excitation of OH(X 2Π) colliding with He and Ar, have been obtained in a crossed molecular beam experiment. The OH radicals were produced in a pulsed dc discharge in a H2O/Ar gas mixture. Adiabatic expansion into vacuum resulted in an effective rotational cooling yielding a 94% population of the lowest Λ‐doublet (X 2Π3/2, J=3/2). Further state preparation could be achieved via electrostatic state selection in a hexapole electric field, resulting in a 93.5% population of the upper Λ‐doublet component (f,+). Experiments were performed both with and without the state selector to provide detailed information about the rotational excitation from both Λ‐doublet states. The OH rotational state distribution was probed, before and after the collision event, by means of laser‐induced fluorescence (LIF) spectroscopy of the A←X electronic band at 308 nm. The OH–Ar and OH–He scattering behave very similar with the exception of the J=3/2 Λ‐doublet transition ...

Angular distributions for photodissociation of O2 in the Herzberg continuum

Photodissociation in the Herzberg continuum of molecular oxygen has been studied at 236, 226 and 204 nm. Using ion-imaging and monitoring of O(3Pj), j=0, 1, and 2 product-atom angular distributions, the amount of parallel character of the transition was measured. In order to interpret these data, analyses of the photoabsorption oscillator strengths and the parallel-perpendicular nature of the Herzberg I, II and III bands, and extrapolation of these properties into the Herzberg-continuum region have been performed. Our measured fine-structure-averaged angular distributions are found to be consistent with this photoabsorption model. In addition, the dynamics of the dissociation process is discussed, based on the O-atom fine-structure distributions.

Production of maximally aligned O(1D) atoms from two-step photodissociation of molecular oxygen

Two-step photodissociation of O2 with a total excitation energy of 7.7 eV yields O(1D) atoms with their angular momenta aligned perpendicular to the fragment recoil axis. Correlation rules linking the bound molecule with the separated atoms predict such strong alignment only for an adiabatic process.

Application of a time-resolved event counting technique in velocity map imaging

We illustrate the use of a three-dimensional (x,y,t) charge-coupled-device (CCD) camera detection system in an ion imaging experiment. The time measurement is based on the decay characteristics of the phosphor screen, which is recorded in two successive images by a double exposure CCD camera. The strength of the method is illustrated in a velocity map imaging experiment on iodine molecules that are ionized and dissociated by intense femtosecond laser pulses. Singly and doubly charged iodine fragments are detected and their (x,y) coordinates and arrival time are recorded in an event counting routine. We estimate the time resolution of the system to be 1.3 ns. We show that the fragment velocity distribution derived from the (x,y,t) data is similar and in some conditions more accurate than the distribution obtained by a mathematical inversion of the (x,y) data only. This principle of detection can be used in all situations in which inversion methods are impossible, for example, when the particle distribution does not have an axis of symmetry.

Photofragment alignment from the photodissociation of HCl and HBr

Abstract Slice imaging is used to measure the alignment of Br ( 2 P 3/2 ) and Cl ( 2 P 3/2 ) photofragments from the photodissociation of HBr and HCl, respectively, at 193 nm. In both cases the A 1 Π 1 state is predominantly optically excited, which correlates to mJ=±3/2 for the halogen atom photofragments. However, both the Br and Cl photofragments are measured to populate mostly mJ=±1/2, showing that for both HBr and HCl nonadiabatic transitions transfer more population to the a 3 Π 1 state (which correlates to mJ=±1/2) during dissociation than remains in the A 1 Π 1 state. The interference between these two pathways is also measured.

The sequential two photon dissociation of NO as a source of aligned N(2D), N(4S) and O(3P) atoms

Abstract Velocity map imaging is used to characterise the angular distributions of atoms which are formed from the sequential two photon excitation of NO via the A 2 Σ + state. Molecules excited at 226 nm and dissociated at 339 nm from the same laser source yield two sets of products, ground state O( 3 P) atoms in conjunction with N( 2 D) and N( 4 S). The angular distributions are found to be well described by β parameters. The use of this method as a source of monoenergetic aligned atoms for studies of reaction dynamics is discussed.