Velocity Map Imaging Study Research Articles

Ultraviolet Photodissociation of the N,N-Dimethylformamide Cation.

N,N-Dimethylformamide (DMF) provides a useful small-molecule model for studying features of the peptide bond that forms the backbone of proteins. We report results from a comprehensive multimass velocity-map imaging study into the ultraviolet (UV) photolysis of the N,N-dimethylformamide cation (DMF+) at wavelengths of 225, 245, and 280 nm. Electronic structure calculations on DMF and DMF+ were employed to help interpret the experimental results. DMF+ ions are generated by 118 nm single-photon ionization of neutral DMF. Subsequent UV photolysis is found to lead to selective cleavage of the N-CO amide bond. This yields HCO + NC2H6+ as major products, with virtually all of the excess energy released into internal modes of the fragments. The data also indicate a small branching ratio into the HCO+ + NC2H6 product pair, which can be accessed from the 32A' electronic state of DMF+. N-CO bond dissociation can also be accompanied by simultaneous intramolecular hydrogen transfer from the oxygen to the nitrogen end of the amide bond, in which case NCH4+ can be formed efficiently at all three wavelengths. The primary NC2H6+ product is relatively long-lived, but the high degree of internal excitation often results in secondary fragmentation via a variety of pathways to form CH3+, NH4+, NCH2+, and NC2H4+, with secondary dissociation more likely at higher photon energies. The isotropic velocity-map images recorded for the various fragments attest to the long lifetime of NC2H6+ and also imply that dissociation most probably occurs from the same set of electronic states at all wavelengths studied; these are thought to be the 12A' ground state and 22A' first excited state of the DMF+ cation.

A multimass velocity-map and covariance-map imaging study of the dissociative electron ionisation dynamics of carbonyl sulphide (OCS)

Abstract We present the results of a velocity-map imaging study into the dissociative electron ionisation dynamics of OCS at electron energies of 50, 75, and 100 eV, supported by new EOM-CCSD and CASSCF potential energy surface calculations for the OCS+ cation and OCS2+ dication. We report partial ionisation cross-sections and scattering distributions for the observed ionic fragments, and interpret these in terms of the dissociation dynamics of the singly and doubly charged OCS parent ions formed in the electron-molecule collisions. For many of the dissociation channels, consideration of the calculated potential energy surfaces supplemented by comparison with data from photoionisation studies provides insight into the electronic potential energy surfaces involved. For fragmentation products arising from dissociation of the dication, time-of-flight and recoil-frame covariance maps provide mechanistic insight into the stepwise dissociation mechanisms leading to these products and support previous conclusions by other authors.

Ion Velocity Map Imaging Study of the Reactive Collisions between Carbon Dioxide and Helium Ion.

The reactive collision between He+ and CO2 plays an important role in substance evolutions of the planetary CO2-rich atmosphere. Using a three-dimensional ion velocity map imaging technique, we investigate the low-energy ion-molecule reactions He+ + CO2 → He + CO2+/He + CO+ + O/He + CO + O+. The velocity images of the products CO+ and O+ of dissociative charge-exchange reactions are distinctly different from those of charge-exchange product CO2+. The remarkable features of stereodynamics are observed in the dissociative charge-exchange reaction and are attributed to the spatial alignment of the initially random target CO2 during the He+ approach. Branching ratios of different channels of dissociative charge exchange are further obtained with the Doppler kinematics model, indicating a high preference for the energy-resonant channel.

Photodissociation dynamics of tetrahydrofuran at 193 nm.

Tetrahydrofuran (THF), a cyclic ether with the chemical formula C4H8O, can be considered the simplest analog of the deoxyribose backbone component of deoxyribonucleic acid (DNA). As such, it provides a useful model for probing the photochemistry of such biomolecular motifs. We present a velocity-map imaging study into the ultraviolet dissociation of THF at a wavelength of 193 nm. Excitation to the S1 state occurs via a 3s ← n transition involving a lone-pair electron on the oxygen atom, and has been shown by other authors to result in rapid ring opening via cleavage of one of the C-O bonds to form a ring-opened C4H8O diradical, followed by C-C bond cleavage over a longer timescale to form either OCH2 + C3H6 products (Channel 1a), HOCH2 + C2H5 products (Channel 1b), or OCH2CH2 + C2H4 products (Channel 2). The C2H4O products formed via Channel 2 are unstable on the timescale of our experiment and dissociate further to form CH3 and CHO. We also observe a number of minor products resulting from H or H2 loss from the primary photofragments. The speed distributions observed for all photofragments are broad, indicating excitation of a range of rotational and vibrational states of the products. The angular distributions of the photofragments show an interesting speed dependence: the slowest products have almost isotropic angular distributions, but the magnitude of the recoil anisotropy increases monotonically with photofragment speed. The fastest products exhibit highly anisotropic angular distributions, with the recoil anisotropy parameter β approaching its limiting value of -1 (-0.75 for Channel 1 and -0.5 for Channel 2). This behaviour is attributed to the range of timescales over which the diradical intermediate dissociates into the observed photofragments. Rapid dissociation leads to fast photofragments which retain the correlation between the transition dipole moment for the S1 ← S0 excitation (which lies perpendicular to the ring) and the photofragment velocities (which lie predominantly in the plane of the ring). Slow dissociation results in a high degree of energy redistribution into internal modes, slower photofragments, and loss of correlation between the photofragment velocities and the transition dipole. The higher barrier associated with dissociation via Channel 2 suggests somewhat longer lifetimes for the diradical intermediate and is consistent with a corresponding reduction in the maximum observed value for β.

Predissociation Dynamics of Br2 in the [2Π1/2]c5d; 0g + and [2Π3/2]c6d; 0g + Rydberg States by Velocity Map Imaging Study.

We investigated the predissociation dynamics from the [2Π1/2]c5d; 0g+ and [2Π3/2]c6d; 0g+ Rydberg states of Br2 using the velocity map imaging technique. Two-dimensional scattering images of the fragmented Br+ exhibited an isotropic feature upon the excitation of these Rydberg states. Analysis of the total kinetic energy release suggested the existence of the predissociation pathways to the dissociation limits of Br(5s, 4P3/2) + Br(4p, 2P3/2) and Br(5s, 4P5/2) + Br(4p, 2P3/2) via the 0g+ ion-pair states that interact with the lower and/or excited-core Rydberg states lying at long internuclear distance regions thorough the avoided crossing.

Regulating and Directionally Controlling Electron Emission from Gold Nanorods with Silica Coatings.

Dielectric coatings offer a versatile means of manipulating hot carrier emission from nanoplasmonic systems for emerging nanocatalysis and photocathode applications, with uniform coatings acting as regulators and nonuniform coatings providing directional photocurrent control. However, the mechanisms for electron emission through dense and mesoporous silica (SiO2) coatings require further examination. Here, we present a systematic investigation of photoemission from single gold nanorods as a function of dense versus mesoporous silica coating thicknesses. Studies with dense coatings on gold nanostructures clarify the short (∼1 nm) attenuation length responsible for severely reduced transmission through the silica conduction band. By contrast, mesoporous silica is much more transmissive, and a simple geometric model quantitatively recapitulates the electron escape probability through nanoscopic porous channels. Finally, photoelectron velocity map imaging (VMI) studies of nanorods with coating defects verify that photoemission occurs preferentially through the thinner regions, illustrating new opportunities for designing photocurrent distributions on the nanoscale.

Multi-mass velocity map imaging study of the 805 nm strong field ionization of CF3I.

Multi-mass velocity map imaging studies of charged fragments formed by near infrared strong field ionization together with covariance map image analysis offer a new window through which to explore the dissociation dynamics of several different highly charged parent cations, simultaneously - as demonstrated here for the case of CF3IZ+ cations with charges Z ranging from 1 to at least 5. Previous reports that dissociative ionization of CF3I+ cations yields CF3+, I+ and CF2I+ fragment ions are confirmed, and some of the CF3+ fragments are deduced to undergo secondary loss of one or more neutral F atoms. Covariance map imaging confirms the dominance of CF3+ + I+ products in the photodissociation of CF3I2+ cations and, again, that some of the primary CF3+ photofragments can shed one or more F atoms. Rival charge symmetric dissociation pathways to CF2I+ + F+ and to IF+ + CF2+ products and charge asymmetric dissociations to CF3 + I2+ and CF2I2+ + F products are all also identified. The findings for parent cations with Z ≥ 3 are wholly new. In all cases, the fragment recoil velocity distributions imply dissociation dynamics in which coulombic repulsive forces play a dominant role. The major photoproducts following dissociation of CF3I3+ ions are CF3+ and I2+, with lesser contributions from the rival CF2I2+ + F+ and CF32+ + I+ channels. The CF32+ fragment ion images measured at higher incident intensities show a faster velocity sub-group consistent with their formation in tandem with I2+ fragments, from photodissociation of CF3I4+ parent ions. The measured velocity distributions of the I3+ fragment ions contain features attributable to CF3I5+ photodissociation to CF32+ + I3+ and the images of fragments with mass to charge (m/z) ratio ∼31 show formation of I4+ products that must originate from parent ions with yet higher Z.

Photodissociation Dynamics of CH2OO on Multiple Potential Energy Surfaces: Experiment and Theory.

UV excitation of the CH2OO Criegee intermediate across most of the broad span of the (B1A')-(X1A') spectrum results in prompt dissociation to two energetically accessible asymptotes: O (1D) + H2CO (X1A1) and O (3P) + H2CO (a3A''). Dissociation proceeds on multiple singlet potential energy surfaces that are coupled by two regions of conical intersection (CoIn). Velocity map imaging (VMI) studies reveal a bimodal total kinetic energy release (TKER) distribution for the O (1D) + H2CO (X1A1) products with the major and minor components accounting for ca. 40% and ca. 20% on average of the available energy (Eavl), respectively. The unexpected low TKER component corresponds to highly internally excited H2CO (X1A1) products accommodating ca. 80% of Eavl. Full dimensional trajectory calculations suggest that the bimodal TKER distribution of the O (1D) + H2CO (X1A1) products originates from two different dynamical pathways: a primary pathway (69%) evolving through one CoIn region to products and a smaller component (20%) sampling both CoIn regions enroute to products. Those that access both CoIn regions likely give rise to the more highly internally excited H2CO (X1A1) products. The remaining trajectories (11%) dissociate to O (3P) + H2CO (a3A'') products after traversing through both CoIn regions. The complementary experimental and theoretical investigation provides insight on the photodissociation of CH2OO via multiple dissociation pathways through two regions of CoIn that control the branching and energy distributions of products.

Effects of π*−σ* coupling on dissociative-electron-attachment angular distributions in vinyl, allyl, and benzyl chloride and in chlorobenzene

We report on a velocity map imaging study of ${\mathrm{Cl}}^{\ensuremath{-}}$ anions resulting from the dissociative electron attachment to four unsaturated chlorohydrocarbons. In all four molecules, this process is mediated by the formation of the lowest shape resonance. The choice of the molecules was motivated by the different character of this resonance. In the planar compounds chlorobenzene and vinyl chloride, it is a ${\ensuremath{\pi}}^{*}$ resonance, which is not dissociative along the C-Cl bond without distortion of the planar geometry. In the nonplanar compounds benzyl chloride and allyl chloride, the shape resonance has a mixed ${\ensuremath{\pi}}^{*}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{*}$ character and is directly dissociative without the need for any additional distortion. Our motivation was to find out whether the dissociation-allowing nuclear motion has a common imprint in the resulting fragment angular distributions. In spite of the expected similarities between the two classes of compounds, the resulting images are quite different for all four molecules. We interpret the results, especially the imprints of the bending dynamics, with the aid of a single-electronic-state model in the axial recoil approximation.

A velocity map imaging study of multiphoton photodissociation and photoionisation dynamics in niobium oxide, NbO

Velocity map imaging has transformed our capabilities in terms of determining photofragmentation and photoionisation thresholds. Here, we present an investigation into the multi-photon dissociation and photoionisation dynamics of niobium monoxide (NbO) probed using velocity-map imaging via the C 4Σ– ← X 4Σ– and B 4Π ← X 4Σ- transitions. Analysis of the kinetic energy release spectra permits determination of refined values of the dissociation energies for NbO and NbO+ of 60,650 ± 124 and 57,414 ± 124 cm−1, respectively, in good agreement with previous values using other methods. With the same instrument, we have further recorded photoelectron images which exhibit a long progression assigned to the production of the ground state, X 3Σ− NbO+ (v +), for which a vibrational constant of 1002 ± 11 cm−1 is determined.

Covariance-map imaging study into the fragmentation dynamics of multiply charged CF3I formed in electron-molecule collisions

We have previously presented a comprehensive velocity-map imaging study into the dissociative electron-ionisation dynamics of CFI at energies ranging from 20 to 100 eV, focusing on dissociation channels of singly charged parent ions. Here we extend this work to include channels arising from doubly and triply charged CFI ions formed in collisions with 40–100 eV electrons. Both the dication and trication can dissociate either via two-body Coulomb explosion to form CF + I/I, or via a variety of many-body dissociation channels involving the loss of one or more neutral fluorine atoms. Covariance mapping of the charged fragments allows us to distinguish between the various possible dissociation mechanisms that could account for these products.

Velocity map imaging studies of the photodissociation of CS2 by two-photon excitation at around 303–315 nm

ABSTRACT Two-photon dissociation dynamics of carbon disulfide (CS2) have been studied by using the time-sliced velocity map ion imaging technique. Images of the S (1D2) and S (3P0) photoproducts formed in the CS2 photodissociation are acquired at four photolysis wavelengths from 303 nm to 315 nm. Vibrational states of the CS co-products are partially resolved and identified in the images. The CS (X1Σ+) products are highly vibrationally excited with moderate rotational excitation. The spin–orbit state-specific dissociation dynamics are also investigated by measuring the images of three S (3PJ) spin–orbit states (J=0, 1, and 2) at photolysis wavelength 303.878 nm. The branching ratios of CS (a3Φ)/CS (X1Σg +) are determined to be 0.05 ± 0.02, 0.17 ± 0.04, and 0.26 ± 0.05 for the three spin–orbit states S (3P0), S (3P1), and S (3P2) respectively, implying a strong spin–orbit coupling exists in the dissociation process. The averaged anisotropy parameters β 2>0 and β 4∼0 suggest that the CS2 molecules undergo a sequential transition 21 B 2(21Σ+)←11 B 2(1Δu)←X 1Σg + with the intermediate state 11 B 2 having a long lifetime, followed by nonadiabatic and spin–orbit couplings to other electronic states and then dissociate.

Velocity map imaging study of the photodissociation dynamics of the allyl radical

The photodissociation of the allyl radical (CH2[double bond, length as m-dash]CH-CH2˙) following excitation between 216 and 243 nm has been investigated employing velocity map imaging in conjunction with resonance enhanced multiphoton ionization to detect the hydrogen atom and CH3(ν = 0) produced. The translational energy distributions for the two fragments are reported and analyzed along with the corresponding fragment ion angular distributions. The results are discussed in terms of the different reactions pathways characterizing the hydrogen atom elimination and the minor methyl formation. On one hand, the angular analysis provides evidence of an additional mechanism, not reported before, leading to prompt dissociation and fast hydrogen atoms. On the other hand, the methyl elimination channel has been characterized as a function of the excitation energy and the contribution of three reaction pathways: single 1,3-hydrogen shift, double 1,2-hydrogen shift and through the formation of vinylidene have been discussed. Contrary to previous predictions, the vinylidene channel, which plays a significant role at lower energies, seems to vanish following excitation on the 2B1(3px) excited state at λ ≤ 230 nm.

Correction to “Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells”

Correction to “Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells”

A Velocity Map Imaging Study of the Reactions of O+ (4S) With CH4.

We present a velocity map imaging study of the key ion-molecule reactions occurring in the O+(4S3/2) + CH4 (X 1A1) system at collision energies of 1.84 and 2.14 eV. In addition to charge transfer to form CH (X 2B2), we also present data on formation of CH (X 1A1'), for which the experimentally determined images provide clear confirmation that the products arise from dissociative charge transfer rather than hydride transfer. Experimental data are also presented on the formation of HCO+ through a transient [OCH4]+ complex living many rotational periods. Plausible reaction pathways and intermediate structures are presented to give insight into the routes for formation of these reaction products.

Multi-mass velocity-map imaging studies of photoinduced and electron-induced chemistry.

Multi-mass velocity-map imaging (VMI) is becoming established as a promising method for probing the dynamics of a variety of gas-phase chemical processes. We provide an overview of velocity-map imaging and multi-mass velocity-map imaging techniques, highlighting examples in which these approaches have been used to provide mechanistic insights into a range of photoinduced and electron-induced chemical processes. Multi-mass detection capabilities have also led to the development of two new tools for the chemical dynamics toolbox, in the form of Coulomb-explosion imaging and covariance-map imaging. These allow details of molecular structure to be followed in real time over the course of a chemical reaction, offering the tantalising prospect of recording real-time 'molecular movies' of chemical dynamics. As these new methods become established within the reaction dynamics community, they promise new mechanistic insights into chemistry relevant to fields ranging from atmospheric chemistry and astrochemistry through to synthetic organic photochemistry and biology.

Communication: Multi-mass velocity map imaging study of the ultraviolet photodissociation of dimethyl sulfide using single photon ionization and a PImMS2 sensor.

This study of the photodissociation of dimethyl sulfide at λ = 227.5 nm demonstrates the opportunities (and some of the challenges) of product detection using vacuum ultraviolet photoionization combined with recently developed multi-mass imaging methods. The capability of imaging different charged products simultaneously allows determination of the primary fragmentation dynamics through, for example, product fragment momentum and angular distribution matching and reveals potential complications from dissociative ionization, product alignment-dependent photoionization probabilities, and the effects of space charging.

Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells

Transverse (2D) photoelectron velocity distributions are directly measured on 10 nm Au film and single Au nanoshells following multiphoton excitation/photoemission. This unique capability is achieved by combining scanning photoemission microscopy with velocity map imaging, yielding photoelectron spectra as a function of diffraction-limited position on a sample. Detailed 3D photoelectron velocity distributions are retrieved for Au film by fitting the 2D data with a ballistic (three-step) photoemission model, where contributions from two-photon, three-photon, and d-band processes are identified and further characterized as a function of photon energy using a broadly tunable, visible femtosecond optical parametric oscillator. These techniques are further applied to investigate the more complex behaviors of single plasmonic Au nanoshells with silica cores. The strong plasmonic near- and far-field signatures are first characterized via optical and photoemission measurements, along with theoretical methods (Mie...

Photodissociation dynamics of bromoiodomethane from the first and second absorption bands. A combined velocity map and slice imaging study.

The photodissociation dynamics of bromoiodomethane (CH2BrI) have been investigated at the maximum of the first A and second A' absorption bands, at 266 and 210 nm excitation wavelengths, respectively, using velocity map and slice imaging techniques in combination with a probe detection of both iodine and bromine fragments, I(2P3/2), I*(2P1/2), Br(2P3/2) and Br*(2P1/2) via (2 + 1) resonance enhanced multiphoton ionization. Experimental results, i.e. translational energy and angular distributions, are reported and discussed in conjunction with high level ab initio calculations of potential energy curves and absorption spectra. The results indicate that in the A-band, direct dissociation through the 5A' excited state leads to the I(2P3/2) channel while I*(2P1/2) atoms are produced via the 5A' → 4A'/4A'' nonadiabatic crossing. The presence of Br and Br* fragments upon excitation to the A-band is attributed to indirect dissociation via a curve crossing between the 5A' with upper excited states such as the 9A'. The A'-band is characterized by a strong photoselectivity leading exclusively to the Br(2P3/2) and Br*(2P1/2) channels, which are likely produced by dissociation through the 9A' excited state. Avoided crossings between several excited states from both the A and A' bands entangle however the possible reaction pathways.

Electron-impact-ionization dynamics of SF6

A detailed understanding of the dissociative electron ionization dynamics of $\mathrm{S}{\mathrm{F}}_{6}$ is important in the modeling and tuning of dry-etching plasmas used in the semiconductor manufacture industry. This paper reports a crossed-beam electron ionization velocity-map imaging study on the dissociative ionization of cold $\mathrm{S}{\mathrm{F}}_{6}$ molecules, providing complete, unbiased kinetic energy distributions for all significant product ions. Analysis of these distributions suggests that fragmentation following single ionization proceeds via formation of $\mathrm{S}{{\mathrm{F}}_{5}}^{+}$ or $\mathrm{S}{{\mathrm{F}}_{3}}^{+}$ ions that then dissociate in a statistical manner through loss of F atoms or ${\mathrm{F}}_{2}$, until most internal energy has been liberated. Similarly, formation of stable dications is consistent with initial formation of $\mathrm{S}{{\mathrm{F}}_{4}}^{2+}$ ions, which then dissociate on a longer time scale. These data allow a comparison between electron ionization and photoionization dynamics, revealing similar dynamical behavior. In parallel with the ion kinetic energy distributions, the velocity-map imaging approach provides a set of partial ionization cross sections for all detected ionic fragments over an electron energy range of 50--100 eV, providing partial cross sections for ${\mathrm{S}}^{2+}$, and enables the cross sections for $\mathrm{S}{{\mathrm{F}}_{4}}^{2+}$ from $\mathrm{S}{\mathrm{F}}^{+}$ to be resolved.