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Dissociative electron attachment (DEA) to formic acid is investigated using velocity slice imaging of H− ions. From the momentum distributions, we infer that the broad peak observed in absolute cross section of H− between 6 and 12 eV has contribution from three resonances. The resonance below 7 eV shows angular distribution fairly close to that seen from the B1 resonance in water and the first peak of the two peaks from the hydroxyl site of acetic acid. The resonance observed around 9 eV appears to have definite contribution from the C-site of the molecule, as identified from the momentum distribution and similarity with other organic molecules including methane. The analysis of the kinetic energy distribution shows that the two resonances below 8 eV are dominated by the contribution from hydroxyl site of the molecule. The results indicate that formic acid tends to behave like other small carboxylic acids and alcohols and displays functional group dependence and corresponding site specificity in the DEA process, to a large extent.

We report on the implementation of a thin wire Paul trap with tungsten wire electrodes for trapping ions. The ion trap geometry, though compact, allows large optical access enabling a moderate finesse Fabry–Perot cavity to be built along the ion trap axis. The design allows a vapor-loaded magneto-optical trap of alkali atoms to be overlapped with trapped atomic or molecular ions. The construction and design of the trap are discussed, and its operating parameters are determined, both experimentally and numerically, for Rb+. The macromotion frequencies of the ion trap for 85Rb+ are determined to be f r = 43 kHz for the radial and f z = 54 kHz for the axial frequencies, for the experimentally determined optimal operating parameters. The destructive off axis ion extraction and detection by ion counting is demonstrated. Finally, evidence for the stabilization and cooling of trapped ions, due to ion–atom interactions, is presented by studying the ion-atom mixture as a function of interaction time. The utility and flexibility of the whole apparatus, for a variety of atomic physics experiments, are discussed in conclusion.

The dynamics of the formation and decay of negative ion resonance of A1 symmetry at 8.5 eV electron energy in the dissociative electron attachment (DEA) process in H2O and D2O are investigated using the velocity slice imaging technique. While the highest energy hydride ions formed by DEA show angular distributions characteristic to the A1 symmetry, those formed with low-kinetic energy show considerably different angular distributions indicating changes in the orientation of the dissociating bond due to bending mode vibrations. Our observations are quite different from the recently reported measurements, but consistent with the fully quantum calculations.

The velocity slice imaging technique is used to obtain the kinetic energy and angular distributions of H− and CH 2 - ions formed by dissociative electron attachment (DEA) to methane. The data show two distinct anion states in the broad peak between 8 and 12 eV with the lower energy one undergoing two-body fragmentation, while the higher energy one undergoing a three-body fragmentation. The dissociation process is seen to be influenced by the Jahn–Teller effect similar to that seen in methane cation and its neutral counterpart.

Dissociative electron attachment (DEA) cross sections for NH 3 and CH 4 are measured in a crossed beam apparatus with special care to eliminate discrimination due to kinetic energy and angular distribution of the fragment ions. The cross sections are put on absolute scale using the relative flow technique. The absolute cross sections for the formation of H − and NH 2 − from ammonia and H − and CH 2 − from methane are compared with available data from literature. It is seen that the present results are considerably different from what has been reported before. We also compare the cross sections of the H − channel from these molecules along with that from H 2 O to those from organic molecules containing alkyl, amino and hydroxyl groups to examine the extent to which the recently observed functional group dependence in the DEA contributes.

Simultaneous, enantiomer-specific identification of chiral molecules in multi-component mixtures is extremely challenging. Many established techniques for single-component analysis fail to provide selectivity in multi-component mixtures and lack sensitivity for dilute samples. Here we show how enantiomers may be differentiated by mass-selected photoelectron circular dichroism using an electron–ion coincidence imaging spectrometer. As proof of concept, vapours containing ∼1% of two chiral monoterpene molecules, limonene and camphor, are irradiated by a circularly polarized femtosecond laser, resulting in multiphoton near-threshold ionization with little molecular fragmentation. Large chiral asymmetries (2–4%) are observed in the mass-tagged photoelectron angular distributions. These asymmetries switch sign according to the handedness (R- or S-) of the enantiomer in the mixture and scale with enantiomeric excess of a component. The results demonstrate that mass spectrometric identification of mixtures of chiral molecules and quantitative determination of enantiomeric excess can be achieved in a table-top instrument., Detecting enantiomers in dilute mixtures of volatile organic compounds is a challenge. Here, the authors demonstrate a method to identify enantiomers and enantiomeric excess in a multi-component mixture containing two chiral species using laser mass spectrometry and photoelectron circular dichroism.

The Journal of Physical Chemistry A, 119 (49), ISSN:1089-5639, ISSN:1520-5215

We have studied a coupled electronic-nuclear wave packet in nitric oxide using time-resolved strong-field photoelectron holography and rescattering. We show that the electronic dynamics mainly appears in the holographic structures whereas nuclear motion strongly modulates the angular distribution of the rescattered photoelectrons., Journal of Physics: Conference Series, 635, ISSN:1742-6588, ISSN:1742-6596

Detection of molecular chirality with high sensitivity and selectivity is important for many analytical and practical applications. Photoionization has emerged as a very sensitive probe of chirality in molecules. We show here that a table top setup with a femtosecond laser and a single imaging detector for both photoelectrons and photoions enables detection of chirality up to 3 orders of magnitude better than the existing conventional absorption based techniques.

Negative ion resonance states of ammonia are accessed upon capture of electrons with energy 5.5 eV and 10.5 eV, respectively. These resonance states dissociate to produce H(-) and NH(2)(-) fragment anions via different fragmentation channels. Using the velocity slice imaging technique, we measured the angular and kinetic energy distribution of the fragment H(-) and NH(2)(-) anions with full 0-2π angular coverage across the two resonances. The scattered H(-) ions at both resonances show variation in their angular distribution as a function of the kinetic energy indicating geometric rearrangement of NH(3)(-*) ion due to internal excitations and differ from the equilibrium geometry of the neutral molecule. The second resonance at 10.5 eV shows strong forward-backward asymmetry in the scattering of H(-) and NH(2)(-) fragment ions. Based on the angular distributions of the H(-) ions, the symmetry of the resonances at 5.5 eV and 10 .5 eV are determined to be A(1) and E, respectively, within C(3v) geometry.

Here we report on a new simplified setup for velocity map photoelectron-photoion coincidence imaging using only a single particle detector. We show that both photoelectrons and photoions can be extracted toward the same micro-channel-plate delay line detector by fast switching of the high voltages on the ion optics. This single detector setup retains essentially all the features of a standard two-detector coincidence imaging setup, viz., the high spatial resolution for electron and ion imaging, while only slightly decreasing the ion time-of-flight mass resolution. The new setup paves the way to a significant cost reduction in building a coincidence imaging setup for experiments aiming to obtain the complete correlated three-dimensional momentum distribution of electrons and ions. © 2012 American Institute of Physics.

A Comment on the Letter by H. Adaniya et al., Phys. Rev. Lett. 103, 233201 (2009). The authors of the Letter offer a Reply.

The electron-correlation effects largely determine the structure and dynamics of anions. Theoretical calculations in the literature indicate the influence of electron correlations on the absolute photodetachment cross section for Cu − , for photon energies above 3.5 eV, but has not been confirmed experimentally. In the present work we probe the influence of channel coupling on the photoelectron angular distributions for photon energies above 3.5 eV. A velocity map imaging spectrometer was tested and employed in this experiment. The experimental results for the asymmetry parameter show no deviation from the single-electron model. The results indicate no significant influence of the channel-coupling effect on the photoelectron angular distribution in the photon energy range of 3.46-4.6 eV.

Here, we provide a detailed account of novel experiments employing electron-ion coincidence imaging to discriminate chiral molecules. The full three-dimensional angular scattering distribution of electrons is measured after photoexcitation with either left or right circular polarized light. The experiment is performed using a simplified photoelectron-photoion coincidence imaging setup employing only a single particle imaging detector. Results are reported applying this technique to enantiomers of the chiral molecule camphor after three-photon ionization by circularly polarized femtosecond laser pulses at 400 nm and 380 nm. The electron-ion coincidence imaging provides the photoelectron spectrum of mass-selected ions that are observed in the time-of-flight mass spectra. The coincident photoelectron spectra of the parent camphor ion and the various fragment ions are the same, so it can be concluded that fragmentation of camphor happens after ionization. We discuss the forward-backward asymmetry in the photoelectron angular distribution which is expressed in Legendre polynomials with moments up to order six. Furthermore, we present a method, similar to one-photon electron circular dichroism, to quantify the strength of the chiral electron asymmetry in a single parameter. The circular dichroism in the photoelectron angular distribution of camphor is measured to be 8% at 400 nm. The electron circular dichroism using femtosecond multiphoton excitation is of opposite sign and about 60% larger than the electron dichroism observed before in near-threshold one-photon ionization with synchrotron excitation. We interpret our multiphoton ionization as being resonant at the two-photon level with the 3s and 3p Rydberg states of camphor. Theoretical calculations are presented that model the photoelectron angular distribution from a prealigned camphor molecule using density functional theory and continuum multiple scattering X alpha photoelectron scattering calculations. Qualitative agreement is observed between the experimental results and the theoretical calculations of the Legendre moments representing the angular distribution for the two enantiomers. The electron-ion coincidence technique using multiphoton ionization opens new directions in table-top analytical mass-spectrometric applications of mixtures of chiral molecules.

The dissociation dynamics of negative ion resonance states in H(2)S formed upon electron attachment are studied using momentum imaging of the fragment H(-) and S(-) ions and compared with similar resonances in water. The H(-) momentum images show that dissociation dynamics at the 5.2 eV resonance are very similar to those of the 6.5 eV (B(1)) resonance in water. Unlike the 8.5 eV resonance in water, which has A(1) symmetry but is found to display considerable deviation from the axial recoil approximation in the momentum distribution of H(-) ions, the distribution from the corresponding resonance in H(2)S at 7.5 eV is found to follow the axial recoil approximation fairly well. The resonance state with B(2) symmetry at 10 eV is found to decay via four dissociation channels viz.-H(-) + H + S, H(-) + SH(A(2)Σ), H(-) + SH(X(2)Π) and S(-) + H + H channels, similar to those that were seen in the B(2) resonance in water at 12 eV, including sequential fragmentation in the multiple fragmentation channels. However, the angular distributions for the fragment ions from this resonance are found to be distinctly different from those in water, even while displaying considerable deviation from the axial recoil approximation similar to that in water.

The control of multichannel ionic fragmentation dynamics in CF3I is studied by femtosecond pulse shaping and velocity map photoelectron photoion coincidence imaging. When CF3I is photoexcited with femtosecond laser pulses around 540 nm there are two major ions observed in the time-of-flight mass spectrum, the parent CF3I+ ion and the CF3+ fragment ion. In this first study we focussed on the influence of LCD-shaped laser pulses on the molecular dynamics. The three-dimensional recoil distribution of electrons and ions were imaged in coincidence using a single time-of-flight delay line detector. By fast switching of the voltages on the various velocity map ion lenses after detection of the electron, both the electron and the coincident ion are measured with the same imaging detector. These results demonstrate that a significant simplification of a photoelectron-photoion coincidence imaging apparatus is in principle possible using switched lens voltages. It is observed that shaped laser fields like chirped pulses, double pulses, and multiple pulses can enhance the CF3+/CF3I+ ratio by up to 100%. The total energetics of the dynamics is revealed by analysis of the coincident photoelectron spectra and the kinetic energy of the CF3+ and I fragments. Both the parent CF3I+ and the CF3+ fragment result from a five-photon excitation process. The fragments are formed with very low kinetic energy. The photoelectron spectra and CF3+/CF3I+ ratio vary with the center wavelength of the shaped laser pulses. An optimal enhancement of the CF3+/CF3I+ ratio by about 60% is observed for the double pulse excitation when the pulses are spaced 60 fs apart. We propose that the control mechanism is determined by dynamics on neutral excited states and we discuss the results in relation to the location of electronically excited (Rydberg) states of CF3I.

Dissociative electron attachment to H 2 O and D 2 O is studied using the recently developed ion momentum imaging spectrometer. We obtain the kinetic energy distribution and the angular distribution in the entire 2π range in the plane of the electron beam for H - and O - ions at the three well-known resonances. While the results at the first resonance at 6.5 eV are in excellent agreement with existing experimental and theoretical results, the angular distributions of the ions at the second (8.5 eV) and third resonance (11.8 eV) are considerably different from the available data. The angular distribution at 11.8 eV shows a very unique distribution of the H - ions in the backward hemisphere, while that of the O - ions is in the forward hemisphere. Based on our results, we reassign the symmetry of the resonance seen at 8.5 eV as a B 1 state instead of A 1 as believed till now. The kinetic energy distribution of H - at the third resonance clearly indicate three dissociation channels leading to the formation of the OH radical in the ground and first electronically excited state as well as a complete three-body breakup.

Kinetic energy and angular distribution of the fragment anions produced due to dissociative electron attachment resonances in simple polyatomic molecules like methane, ammonia, and hydrogen sulphide are measured using a new technique of velocity map imaging. These measurements aim at determining the electronic symmetry and dissociation dynamics of the resonance states.

The velocity map imaging (VMI) technique developed recently for dissociative electron attachment (DEA) experiments is improvised further to study the formation of H− ions. The experiment is used to study DEA to water molecules at the 6.5 eV and 8.5 eV resonances. The kinetic energy and angular distribution of the H− ions arising from this resonance are compared with those in the existing experimental and theoretical data.

Site selective fragmentation is observed at the hydrogen site in small organic molecules like simple carboxylic acids and alcohols in the dissociative attachment (DA) process. We have investigated this site selectivity in the acetic acid and methanol molecules by measuring the kinetic energy and angular distribution of the hydride ions using Velocity Slice Imaging (VSI) technique. The results are explained in terms of the formation of valance excited Feshbach resonances. The kinetic energy and angular distribution measurements also reveal the dynamics associated with the process.