Velocity Slice Research Articles

Predissociation dynamics of negative-ion resonances of H2 near 12 and 14.5 eV using the velocity slice imaging technique

Dissociative electron attachment is an important tool for investigating negative-ion resonances. We have studied the negative-ion resonances of ${\mathrm{H}}_{2}$ at 10 and 14 eV using the improved velocity slice imaging technique. We obtained modulations in the kinetic energy spectrum of ${\mathrm{H}}^{\ensuremath{-}}$ ions obtained at 12 and 14.5 eV electron energy, consistent with the earlier reported vibrational state contributions from the higher-lying bound resonances. We show that structures obtained at 12 eV are due to predissociation of the $C{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ resonance consistent with the current understanding. However, based on our angular distribution measurements, we propose that the structures obtained at 14.5 eV are due to a predissociation of bound resonance of ${}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ symmetry as against ${\mathrm{\ensuremath{\Delta}}}_{g}$ that was proposed earlier. We also report that the bound ${}^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ resonance contributes to the observed inversion symmetry breaking near 14 eV.

Breakdown of dipole Born approximation and the role of Rydberg’s predissociation for the electron-induced ion-pair dissociation to oxygen in the presence of background gases

We study the electron-induced ion-pair dissociation to gas-phase oxygen molecules using a state-of-the-art velocity-map ion-imaging technique. The analysis is entirely based on the conical time-gated wedge-shaped velocity slice images of O-/O2 nascent anionic fragments, and the resulting observations are in favor of Van Brunt et al.'s report [R. J. Van Brunt and L. J. Kieffer, J. Chem. Phys. 60, 3057 (1974)]. A new image reconstruction method, Jacobian over parallel slicing, is introduced to overcome the drawback of ion exaggeration in determining the kinetic energy distribution from the time-gated parallel slicing technique, which offers an alternative approach to the wedge slicing method. Most importantly, the role of the quintet-heavy Rydberg state has been drawn out to the complex ion-pair formalism. The extracted kinetic energy and angular distributions from the wedge slice images reveal a high momentum transfer during the ion-pair dissociation process, which could be the finest rationale to observe the breakdown of dipole Born approximation driven by multipole moment associated with the incident electron beam. Three distinct dissociative momentum bands have been precisely identified for O- dissociation. However, radiationless Rydberg's predissociation continuum (≥15%) has become an inherent character of electron-induced ion-pair dissociation, which could be dealt with using the beyond Born-Oppenheimer treatment. The incoherent sum of Σ and Π symmetric-associated ion-pair final states has been precisely identified by modeling the angular distribution of O-/O2 for each of the kinetic energy bands. A negligibly small amount of forward-backward asymmetry is observed in the angular distribution of O-/O2, which might be explained by the dissociative state-specific quantum coherence mechanism as reported [Krishnakumar et al., Nat. Phys. 14, 149 (2018); Kumar et al., arXiv:2206.15024 (2022)] by Prabhudesai et al.

Structure and dynamics of the negative-ion resonance in H2, D2 , and HD at 10 eV

The angular distribution of the anions formed in the dissociative electron attachment (DEA) to ${\mathrm{H}}_{2}, {\mathrm{D}}_{2}$, and HD is studied for the 10-eV resonance using the velocity slice imaging technique. The angular distributions of ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{\ensuremath{-}}$ from HD are found to be identical and very similar to that of ${\mathrm{H}}^{\ensuremath{-}}$ (${\mathrm{D}}^{\ensuremath{-}}$) from ${\mathrm{H}}_{2}$ (${\mathrm{D}}_{2}$) indicating that the distinct dissociation limits for ${\mathrm{H}}^{\ensuremath{-}}$ and ${\mathrm{D}}^{\ensuremath{-}}$ channels and the small but finite dipole moment of HD do not affect the DEA process. The angular distributions suggest that the main contribution for this resonance is from the capture of $s$ and $d$ partial waves of the attaching electron confirming the resonance to be a ${}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+}$ state. The relative amplitudes of the two partial waves as a function of electron energy also indicate the contribution from a higher-lying ${}^{2}{{\mathrm{\ensuremath{\Sigma}}}_{g}}^{+}$ state through possible predissociation at higher electron energies.

Modeling and mass transfer performance of Dioscorea alata L. slices drying in convection air dryer

AbstractThe drying experiments of Dioscorea alata L. slices were implemented in order to study the thin‐layer drying characteristics and kinetic in hot air forced convection tunnel dryer. The influences of temperatures, velocity, and slice thickness on the drying behavior were researched. Six commonly thin‐layer drying models were used to fit the experimental data to express the mass transfer characteristics. The fitting degree between models and experiment data was analyzed. The effective diffusion coefficients (Deff) and activation energy (Ea) were estimated. The air temperature, air velocity, and sample slice thickness had significant influence on drying time, dehydration rate, and Deff. The drying process had a momentary accelerating rate stage and a long falling rate stage; the Logarithmic model had good degree of fitting with all experiments data; and Arrhenius equation could be well expressed the relationship of Deff and drying temperature. The drying process of hot air thin layer of Dioscorea alata L. can be better described by the Logarithmic model. At the range of research conditions, the Deff value ranged from 4.77 × 10−10 m2/s to 8.90 × 10−10 m2/s; the Ea value was 20.74 kJ/mol.Practical ApplicationsPostharvest processing and storage of agricultural products is very important, and drying is a traditional and practical technology. The common drying method is heat drying. This research discussed the thin‐layer drying of Dioscorea alata L. with hot air and obtained the appropriate drying conditions, applicable drying model, and relevant parameters, which could provide reference to select the drying conditions and design the drying equipment for practical application.

Velocity imaging of H− from formic acid: probing functional group dependence in dissociative electron attachment

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.

Study of S− and SO− ion formation from dissociative electron attachment to SO2

A detailed experimental investigation of the dissociative electron attachment (DEA) process to sulfur dioxide molecules in the gas phase has been performed. The sulfur dioxide molecule is known to produce O−, S− and SO− ions, mainly forming two prominent resonant peaks at around 4.2 and 7.5 eV incident electron energies. The S− and SO− ions are studied over the second resonance around 7.5 eV incident electron energy in detail using the well-established velocity slice imaging (VSI) spectrometer. The time-sliced images, kinetic energy and angular distribution have been reported for the two ions over the second resonance for the first time. Both the ions are seen to be formed with very low kinetic energies with their respective neutral fragments being produced in internally excited states. The angular distribution data also signifies the presence of a linear combination of A1 and B2 temporary negative ion states responsible for the resonant peak at 7.5 eV incident electron energy.

ГРАФОАНАЛИТИЧЕСКИЙ БАЗИС ИССЛЕДОВАНИЯ ПРИЗЕМНОГО ДВИЖЕНИЯ КВАНТОМОБИЛЯ

Introduction. Development of a graphical-and-analytical basis to perform optimization calculations for quantomobile motion control will contribute to studies on vehicle creation. Purpose of the study. It is required to develop a methodology for graphical representation of thrust vector realization as a graphical-and-analytical basis for optimization calculations of quantomobile trajectories. Methods. The thrust vector is decomposed into orthogonal components. A generalized quantomobile force balance equation is used. A mode of partial hovering of a vehicle is distinguished as the basic mode of near-ground motion. 2D modeling of force balance for a particular velocity slice is performed. 3D modeling of force balance with velocity sweeping is carried out. 2D and 3D models of force balance are developed using Maple software. Images of surfaces with regard to road and wind resistance with limits to the maximum thrust are built. Examples of calculation as well as graphical-and-analytical studies are provided. Results. A graphical-and-analytical basis for optimization calculations of quantomobile trajectories, changes in QE thrust and corresponding control actions is developed. Results of calculations with visualization are presented using specific examples. Discussion . Development of a program in the graphical environment required combining knowledge of the software tools and programmed field. Development of the quantomobile simulation model as well as its graphical-and-analytical basis in the direction of increasing complexity is the only way. Corresponding development stages are described. Two hypothetical areas for the minimization of sufficient thrust are distinguished: in the middle of vehicle hovering and after its full hovering. It shall be taken into account both in quantomobile takeoff and its near-ground motion with partial hovering.

DEA dynamics of chlorine dioxide probed by velocity slice imaging.

We report, for the first time, the detailed dynamics of dissociative electron attachment to the atmospherically important chlorine dioxide (OClO) molecule exploring all the product anion channels. Below 2 eV, the production of vibrationally excited OCl- dominates the DEA process whereas at electron energies greater than 2 eV, three-body dissociation is found to result in O- and Cl- production. We find that the internal energy of OCl- and the kinetic energy of Cl- are large enough for them to be relevant in the ozone-depleting catalytic cycle and more investigations on the reaction of these anions with ozone are necessary to completely understand the role of DEA to OClO in ozone depletion. These results also point to an urgent need for comprehensive theoretical calculations of the DEA process to this atmospherically important molecule.

Dissociation dynamics in low energy electron attachment to ammonia using velocity slice imaging.

The complete dissociation dynamics of low energy electron attachment to the ammonia molecule has been studied using velocity slice imaging (VSI) spectrometry. One low energy resonant peak around 5.5 eV and a broad resonance around 10.5 eV incident electron energies have been observed. The resonant states mainly dissociate via H- and NH2- fragments, though for the upper resonant state, the signature of NH- fragments is also predicted due to a three-body dissociation process. Kinetic energy and angular distributions of the NH2- fragment anions are measured simultaneously around the two resonances. Based on our experimental observations, we conclude that a temporary negative ion (TNI) state with A1 symmetry is responsible for the lower resonance. Whereas, we find strong evidence for the existence of a TNI state having A1 symmetry at the 10.5 eV resonance for the first time.

Optimization of convective drying by response surface methodology

In this work, response surface methodology (RSM) is applied to state an optimized system for convective drying of apple slices using desirability function. The interaction of the independent parameters including air temperature (T = 70–90 °C), air velocity (V = 4–5 m/s), and apple slice geometry (G = circle, square, and triangle) with the dependent variables consist of drying time, energy consumption, and shrinkage is determined. Minimum drying time, energy consumption and shrinkage is regarded as the optimizing drying conditions of apple slices. Experimental results are adapted by a second-order polynomial model where analysis of variance is utilized to define model compatibility and optimal drying conditions. The optimal conditions of combined optimized responses were 90 °C inlet temperature, 5 m/s inlet velocity, and square geometry which designated by maximum desirability function (D = 0.781). The air temperature had a notable influence on total responses, but the effect of air velocity and apple slice geometry were important in shrinkage. Generally, the results marked that a greater desirability value could be obtained in the higher air temperature and higher air velocity with the square geometry.

Kinematic study of O− -ion formation from dissociative electron attachment to SO2

We report a complete kinematic study of ${\mathrm{O}}^{\ensuremath{-}}$-ion formation due to dissociative electron attachment to ${\mathrm{SO}}_{2}$ using the velocity slice imaging technique in the incident electron energy range over the resonances. Two resonances are observed at 5.2 and 7.5 eV, respectively. From the kinetic energy distribution, the two resonances are observed to have the same threshold energy, pointing to the fact that the two processes, giving rise to the two resonant peaks, have the same dissociation limit. From the angular distribution results we identified the involvement of an ${A}_{1}$ and a combination of ${A}_{1}+{B}_{2}$ temporary negative-ion state(s) for the first and second resonances, respectively.

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.

Production of electronically excited NO via DEA to NO2

Dissociative electron attachment (DEA) to NO2 in the 7–11 eV range is studied using velocity slice imaging technique. Two distinct channels are observed in the DEA corresponding to O– + NO(A 2 Σ +) and O– + NO(C 2 Π and/or D 2 Σ +). While NO(A 2 Σ +) is found to be formed only in very high vibrational levels, NO(C 2 Π and/or D 2 Σ +) is found to be formed with vibrational distribution starting from v = 0. From the angular distribution of the O– ions leading to the NO(C 2 Π and/or D 2 Σ +) channel, we obtain the symmetry of the negative ion resonance to be dominantly B1 with small contribution from B2.

Dissociative electron attachment to CO molecule probed by velocity slice imaging technique

We have studied dissociative electron attachment to CO molecule using the well-established velocity slice imaging spectrometer. We have conclusively determined the symmetries of the TNI states involved in both the channels producing O− ions. In contrast to a recent report, we observed additional forwards lobes in the angular distribution data and we claim there is no need to invoke coherent interference between different states as introduced previously. Recent R-matrix calculations and momentum imaging study by other groups strongly support our claims.

Dissociation dynamics of transient anion formed via electron attachment to sulfur dioxide.

We report the molecular dynamics of dissociative electron attachment to sulfur dioxide (SO2) by measuring the momentum distribution of fragment anions using the velocity slice imaging technique in the electron energy range of 2-10 eV. The S- channel results from symmetric dissociation which exhibits competition between the stretch mode and bending mode of vibration in the excited parent anion. The asymmetric dissociation of parent anions leads to the production of O- and SO- channels where the corresponding neutral fragments are formed in their ground as well as excited electronic states. We also identify that internal excitation of SO- is responsible for its low yield at higher electron energies.

Dissociation dynamics of transient anion formed via electron attachment to sulphur dioxide (SO2)

SynopsisWe have investigated the dynamics of dissociative electron attachment to sulfur dioxide (SO2) using velocity slice imaging (VSI) technique. The symmetric dissociation of the transient parent anion results in S− formation whereas its asymmetric dissociation results in the O− and SO− channels. We clearly see the competition between bending and stretch mode of vibrations of the parent anions in S− formation whereas the stretch modes dominate in the other two ions production.

Dissociative electron attachment to CO molecule probed by velocity slice imaging technique

SynopsisWe have studied dissociative electron attachment to CO molecule using the well-established velocity slice imaging spectrometer. We have conclusively determined the symmetries of the TNI states involved in both the channels producing O− ions. Also in contrast to a recent report we observed that there is no need to invoke coherent interference between different states. Recent calculations and experimental study by other groups also strongly support our claims.

Dipolar dissociation dynamics in electron collisions with carbon monoxide

SynopsisDipolar dissociation (DD) dynamics of carbon monoxide with electron collision has been studied using velocity slice imaging (VSI) technique. Threshold of the process has been obtained from the ion yield curve. Using the time sliced images kinetic energy and angular distribution have been obtained providing the detailed dynamics involved in the DD process. Two ion-pair states have been identified from the angular distribution measurements.

Perspective: Advanced particle imaging

Since the first ion imaging experiment [D. W. Chandler and P. L. Houston, J. Chem. Phys. 87, 1445-1447 (1987)], demonstrating the capability of collecting an image of the photofragments from a unimolecular dissociation event and analyzing that image to obtain the three-dimensional velocity distribution of the fragments, the efficacy and breadth of application of the ion imaging technique have continued to improve and grow. With the addition of velocity mapping, ion/electron centroiding, and slice imaging techniques, the versatility and velocity resolution have been unmatched. Recent improvements in molecular beam, laser, sensor, and computer technology are allowing even more advanced particle imaging experiments, and eventually we can expect multi-mass imaging with co-variance and full coincidence capability on a single shot basis with repetition rates in the kilohertz range. This progress should further enable "complete" experiments-the holy grail of molecular dynamics-where all quantum numbers of reactants and products of a bimolecular scattering event are fully determined and even under our control.

Finite slice analysis (FINA)-A general reconstruction method for velocity mapped and time-sliced ion imaging.

Since the advent of ion imaging, one of the key issues in the field has been creating methods to reconstruct the initial 3D distribution of particles from its 2D projection. This has led to the development of a number of different numerical methods and fitting techniques to solve this fundamental issue in imaging. In recent years, slice-imaging methods have been developed that permit direct recording of the 3D distribution, i.e., a thin slice of the recoiling fragment distribution. However, in practice, most slice imaging experiments achieve a velocity slice width of around 10%-25% around the center of the distribution. This still carries significant out-of-plane elements that can blur the spectrum, lose fine resolution, and underestimate the contribution from slow recoiling products. To overcome these limitations, we developed a new numerical method to remove these out-of-plane elements from a sliced image. The finite sliced analysis method models the off-axis elements of the 3D particle distribution through the use of radial basis functions. Once applied, the method reconstructs the underlying central slice of the 3D particle distribution. The approach may be applied to arbitrarily sliced or unsliced data and has the further advantage that it neither requires nor enforces full cylindrical symmetry of the data. We demonstrate this reconstruction approach with a broad range of synthetic and experimental data that, at the same time, allows us to examine the impact of finite slicing on the recovered distributions in detail.