Inverting ion images without Abel inversion: maximum entropy reconstruction of velocity maps

Dick, Bernhard

doi:10.1039/c3cp53673d

Cited by 191 publications

(149 citation statements)

References 28 publications

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“…It is essential to bin centroids into a grid that is fine enough not to limit the resolution of the spectrometer. The radial and angular electron distributions are reconstructed from the accumulated image, using the maximum-entropy velocity Legendre reconstruction method (51).…”

Section: Methodsmentioning

confidence: 99%

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

Weichman

DeVine

Levine

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Polycyclic aromatic hydrocarbons, in various charge and protonation states, are key compounds relevant to combustion chemistry and astrochemistry. Here, we probe the vibrational and electronic spectroscopy of gas-phase 9-, 1-, and 2-anthracenyl radicals (C 14 H 9 ) by photodetachment of the corresponding cryogenically cooled anions via slow photoelectron velocity-map imaging (cryo-SEVI). The use of a newly designed velocity-map imaging lens in combination with ion cooling yields photoelectron spectra with <2 cm −1 resolution. Isomer selection of the anions is achieved using gasphase synthesis techniques, resulting in observation and interpretation of detailed vibronic structure of the ground and lowest excited states for the three anthracenyl radical isomers. The groundstate bands yield electron affinities and vibrational frequencies for several Franck-Condon active modes of the 9-, 1-, and 2-anthracenyl radicals; term energies of the first excited states of these species are also measured. Spectra are interpreted through comparison with ab initio quantum chemistry calculations, Franck-Condon simulations, and calculations of threshold photodetachment cross sections and anisotropies. Experimental measures of the subtle differences in energetics and relative stabilities of these radical isomers are of interest from the perspective of fundamental physical organic chemistry and aid in understanding their behavior and reactivity in interstellar and combustion environments. Additionally, spectroscopic characterization of these species in the laboratory is essential for their potential identification in astrochemical data.polycyclic aromatic hydrocarbons | anion photoelectron spectroscopy | velocity-map imaging | vibronic structure P olycyclic aromatic hydrocarbons (PAHs) are an important class of species in many areas of chemistry. They are major components in coal (1) and in soot formed from combustion of organic matter (2, 3). PAHs are therefore common environmental pollutants and have well-documented mutagenic and carcinogenic biological activity (4, 5). PAHs are also believed to be abundant in the interstellar medium (6) and may be carriers of the anomalous IR emission bands (7-9). Recent molecular beam studies indicate that PAH growth can proceed through cold collisions of smaller hydrocarbons under interstellar conditions (10, 11). Individual PAH molecules can subsequently provide nucleation sites for amorphous graphitic grains (9). Interstellar PAHs and their clusters therefore bridge the gap between small carbonaceous molecules and larger particles, analogous to their role in soot condensation in combustion environments (12).In space, PAH species are likely to exist as an equilibrium of neutral and ionic charge states, with varying degrees of hydrogenation and dehydrogenation (13-15). Models of dense interstellar clouds find that anionic PAHs are the major carriers of negative charge, rather than free electrons (16). Closed-shell, singly deprotonated PAH carbanions have large electron affinities compared with ...

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Section: Methodsmentioning

confidence: 99%

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

Weichman

DeVine

Levine

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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“…In the main detector chamber, the He droplet beam crosses the synchrotron beam perpendicularly in the center of a combined VMI and time-of-flight (TOF) detector. By detecting either electrons or ions with the VMI detector in coincidence with the corresponding particles of opposite charge with the TOF detector, we obtain either ion mass-correlated electron spectra or mass-selected ion kinetic energy (KE) distributions by Abel inversion of the VMIs [20]. The XUV photon energy is tuned near the first excited level of He + , hν E(He + * , n = 2) = 65.4 eV [2].…”

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confidence: 99%

Interatomic Coulombic decay in helium nanodroplets

et al. 2017

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Interatomic Coulombic decay (ICD) is induced in helium (He) nanodroplets by photoexciting the n = 2 excited state of He + using XUV synchrotron radiation. By recording multiple coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic He + ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow He + ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of He + k complexes.Isolated atoms or molecules excited by energetic radiation typically decay through intramolecular processes such as the emission of an electron or photon. In contrast, in weakly bound complexes, locally generated electrons can additionally interact with neighboring atoms or molecules, leading to new interatomic or intermolecular interactions. Interatomic Coulombic decay (ICD) is a particularly interesting decay process which occurs when local electronic decay is energetically forbidden [1]. Thus, ICD offers a new, ultrafast decay path where energy is exchanged with a neighboring atom leading to its ionization. Since its discovery, ICD has been observed in a wide variety of weakly-bound systems from He dimers [2,3] and rare-gas clusters to biologically relevant systems such as water clusters; for reviews see [4,5]. Today, the focus is on condensed-phase systems where ICD is involved in complex relaxation mechanisms [6][7][8], which can generate genotoxic low-energy electrons and radical cations [9]. Recently, it was suggested to utilize this property of ICD for cancer treatment [10,11].Here we present the first study of ICD in helium (He) nanodroplets. He nanodroplets are generally considered as an ultracold, inert spectroscopic matrix for embedded, isolated molecules and clusters [12,13]. Upon ionization by intense or energetic radiation, however, He droplets turn into a highly reactive medium, inducing reactions and secondary ionization processes of the embedded species [14]. Their homogeneous quantum liquid density profile, and the simple structure of atomic constituents, make He droplets particularly beneficial as benchmark systems for elucidating correlated decay processes. Recent examples include the collective autoionization of multiply excited pure He droplets [15,16] and the creation of doubly charged species by one-photon ionization of doped He droplets [17]. In this work we fully characterize the product states generated by ICD and secondary processes in He nanodroplets using coincidence imaging techniques.The experiments were performed using a He droplet machine attached to a velocity map imaging photoelectron-photoion coincidence (VMI-PEPICO) detector at the GasPhase beamline of Elettra-Sincrotrone Trieste, Italy. The apparatus has been described in detail elsewhere [18,19]. Briefly, a beam of He nanodroplets is produced by continuously expanding pressurized He (50 bar) of high purity He out of a c...

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“…The instrument is not configured for slice imaging [19,32] and image reconstruction was performed using a polar onion peeling algorithm developed by Roberts et al. [33] We also used the maximum entropy method of Dick [34] which we find to perform more reliably especially at low signal intensities or in identifying minor dissociation channels.…”

Section: Methodsmentioning

confidence: 99%

Dissociation energies of Ag–RG (RG = Ar, Kr, Xe) and AgO molecules from velocity map imaging studies

Cooper¹,

Kartouzian

Gentleman³

et al. 2015

The Journal of Chemical Physics

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The near ultraviolet photodissociation dynamics of silver atom -rare gas dimers have been studied by velocity map imaging. Ag-RG (RG = Ar, Kr, Xe) species generated by laser ablation are excited in the region of the C ( 2  + )  X ( 2  + ) continuum leading to direct, near-threshold dissociation generating Ag* ( 2 P 3/2 ) + RG ( 1 S 0 ) products. Images recorded at excitation wavelengths throughout the C ( 2  + )  X ( 2  + ) continuum, coupled with known atomic energy levels, permit determination of the ground X ( 2  + ) state dissociation energies of 85.9 ± 23.4 cm -1 (Ag-Ar), 149.3 ± 22.4 cm -1 (Ag-Kr) and 256.3 ± 16.0 cm -1 (Ag-Xe). Three additional photolysis processes, each yielding Ag atom photoproducts, are observed in the same spectral region. Two of these are markedly enhanced in intensity upon seeding the molecular beam with nitrous oxide, and are assigned to photodissociation of AgO at the two-photon level. These features yield an improved ground state dissociation energy for AgO of 15965  81 cm -1 , which is in good agreement with high level calculations. The third process results in Ag atom fragments whose kinetic energy shows anomalously weak photon energy dependence and is assigned tentatively to dissociative ionization of the silver dimer Ag 2 .3

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Inverting ion images without Abel inversion: maximum entropy reconstruction of velocity maps

Cited by 191 publications

References 28 publications

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging

Interatomic Coulombic decay in helium nanodroplets

Dissociation energies of Ag–RG (RG = Ar, Kr, Xe) and AgO molecules from velocity map imaging studies

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