Frustrated charge transfer in vibrationally inelastic Ar++N2 collisions via hard collision glory scattering

Vibrational energy transfer in collisions between ions and neutrals is a fundamental process in interstellar media, planetary atmospheres, and plasmas. The conventional wisdom is that glancing collisions with large impact parameters are forward-scattered with low vibrational excitation, while hard collisions with small impact parameters are sideway- or backward-scattered with relatively high vibrational excitation. Here, we report experimental observations with a three-dimensional velocity-map imaging crossed-beam apparatus in the inelastic scattering process Ar++N2(v'' = 0, J'')→Ar++N2(v', J'), where all the vibrationally excited N2 products are dominated by forward scattering, contradicting the textbook model. Trajectory surface hopping calculations not only reproduced the experimental observation qualitatively, but also revealed that the vibrational excitation mainly occurs through a transient charge-transfer process. The hard collision glory mechanism, which has so far only been observed in inelastic rotational energy transfer between neutrals, is shown to play a major role for vibrational excitation in the inelastic Ar++N2 collision, via the frustrated charge transfer process.

Photodetachment and Tunneling Dissociation of Cryogenic Double-Rydberg Anions NH4. J Phys Chem Lett 2024;15:5612-5617. [PMID: 38758204 DOI: 10.1021/acs.jpclett.4c01168] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The Rydberg radical NH4 and the double Rydberg anion (DRA) NH4- have long aroused researchers' interests due to their potential for exploring the reaction dynamics of the H + NH3 → H2 + NH2 reaction, a prototypical penta-atomic system. In this study, we present high-resolution photodetachment spectroscopy of DRA NH4- and ion-molecule complex H-(NH3). We observed multiple new photodetachment channels of DRA NH4-. The energy level of the excited state (3p 2T2) of the Rydberg radical NH4 was determined to be 15052(94) cm-1, in excellent agreement with the principal Schüler band (15061.61 cm-1). Additionally, we observed the tunneling dissociation of NH4- in a cryogenic ion trap with its dissociation lifetime determined to be 19(2) ms.

Photodissociation of deuterated pyrrole-ammonia clusters: H-atom transfer or electron coupled proton transfer?

Several years ago the discovery of a conical intersection offered an explanation for the ultafast photodissociation of pyrrole. Subsequently, the photodissociation of pyrrole ammonia complexes PyH*(NH3)n with n ≥ 3 was studied in the gas phase as a model for a hydrogen-bond forming solvent. Two alternative mechanisms, electron coupled proton transfer (ECPT) and hydrogen atom transfer (HAT, also called the impulsive model, IM), have been proposed. The parent 1 : 1 complex was never studied, due to the short lifetime of the NH4 radical fragment. Here we report experiments on the deuterated species PyD*(ND3)n, including the 1 : 1 complex (n = 1). The velocity distribution of the ND4 radical is well approximated by a Maxwell-Boltzmann distribution of T ≈ 530 K, with a negative anisotropy parameter of β = -0.3. The impulsive model predicts a much narrower velocity distribution with larger negative anisotropy. The ECPT model predicts a long lived intermediate that should allow thermal equilibration of the vibrational energy but should also destroy the rotational memory of the initially excited state. The average kinetic energy agrees with the prediction of the impulsive model, whereas the wide range of kinetic energies is more in line with ECPT. Hence the mechanism seems to be more complex and requires further theoretical modelling.

On the electronic structure and spin-orbit coupling of BiB from photoelectron imaging of cryogenically-cooled BiB- anion

We report a study on the electronic structure and chemical bonding of the BiB molecule using high-resolution photoelectron imaging of cryogenically cooled BiB- anion. By eliminating all the vibrational hot bands, we can resolve the complicated detachment transitions due to the open-shell nature of BiB and the strong spin-orbit coupling. The electron affinity of BiB is measured to be 2.010(1) eV. The ground state of BiB- is determined to be 2Π(3/2) with a σ2π3 valence electron configuration, while the ground state of BiB is found to be 3Σ-(0+) with a σ2π2 electron configuration. Eight low-lying spin-orbit excited states [3Σ-(1), 1Δ(2), 1Σ+(0+), 3Π(2), 3Π(1), 1Π(1)], including two forbidden transitions, [3Π(0-) and 3Π(0+)], are observed for BiB as a result of electron detachment from the σ and π orbitals of BiB-. The angular distribution information from the photoelectron imaging is found to be critical to distinguish detachment transitions from the σ or π orbital for the spectral assignment. This study provides a wealth of information about the low-lying electronic states and spin-orbit coupling of BiB, demonstrating the importance of cryogenic cooling for obtaining well-resolved photoelectron spectra for size-selected clusters produced from a laser vaporization cluster source.

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a high-resolution variant of anion photoelectron spectroscopy that has been applied with considerable success over the years to the study of radicals, size-selected clusters, and transition states for unimolecular and bimolecular reactions. Cryo-SEVI retains the versatility of conventional anion photoelectron spectroscopy while offering sub-meV resolution, thereby enabling the resolution of vibrational structure in the photoelectron spectra of complex anions. This Feature Article describes recent experiments in our laboratory using cryo-SEVI, including a new research direction in which anions are vibrationally pre-excited with an infrared laser pulse prior to photodetachment.

Electron affinity of atomic scandium and yttrium and excited states of their negative ions

The latest experimental electron affinity (EA) values of atomic scandium and yttrium were 0.189(20) and 0.308(12) eV as reported by Feigerle et al. in 1981. The measurement accuracy of these was far lower than that of other transition elements, and no conclusive result had been made on the excited states of their negative ions. In the current work, we report more accurate EA values of Sc and Y and the electronic structure of their negative ions using the slow-electron velocity-map imaging method. The EA values of Sc and Y are determined to be 0.179 378(22) and 0.311 29(22) eV, respectively. The ground state of Sc^- is identified as 3d4s²4p ¹D₂, and the ground state is 4d5s²5p ¹D₂ for Y^-. Furthermore, several excited states of Sc^- and Y^- are observed: Sc^- (³D₁) and Y^- (³D₁, ³D₂, ³D₃, ³F₂, and ³F₃), and their energy levels are determined to be 1131.8(28), 1210.0(13), 1362.3(30), 1467.7(26), 1747(16), and 1987(33) cm^-1, respectively.

Tomographic reconstruction techniques optimized for velocity-map imaging applications

Examples of extracting meaningful information from image projection data using tomographic reconstruction techniques can be found in many areas of science. Within the photochemical dynamics community, tomography allows for complete three-dimensional (3D) charged particle momentum distributions to be reconstructed following a photodissociation or photoionization event. This permits highly differential velocity- and angle-resolved measurements to be made simultaneously. However, the generalized tomographic reconstruction strategies typically adopted for use with photochemical imaging-based around the Fourier-slice theorem and filtered back-projection algorithms-are not optimized for these specific types of problems. Here, we discuss pre-existing alternative strategies-namely, the simultaneous iterative reconstruction technique and Hankel Transform Reconstruction (HTR)-and introduce them in the context of velocity-map imaging applications. We demonstrate the clear advantages they afford, and how they can perform considerably better than approaches commonly adopted at present. Most notably, with HTR we can set a bound on the minimum number of projections required to reliably reconstruct 3D photoproduct distributions. This bound is significantly lower than what is currently accepted and will help make tomographic imaging far more accessible and efficient for many experimentalists working in the field of photochemical dynamics.

Structural Versatility and Energy Difference of Salt-Water Complex NaCl(H2O) Encoded in Cryogenic Photoelectron Spectroscopy

A weakly bound complex usually has multiple structural isomers with small energy differences. The sophisticated ab initio calculations are the main workhorse for providing theoretical results of different isomers. In contrast, the experimental determination of the energy difference is very rare. We report the energy-difference measurement of a model complex: salt-water complex NaCl(H₂O). We measured the energy difference among the structural isomers of the negatively charged NaCl(H₂O) complex and the neutral counterpart using cryogenic photoelectron spectroscopy. The temperature-dependent photoelectron spectra (15-300 K) revealed that the negatively charged NaCl(H₂O) and the neutral counterpart both have three isomers. The two higher-lying isomers are 186(22) and 481(48) cm^-1, respectively, above the most stable isomer for the negatively charged and 123(10) and 1821(24) cm^-1 for the neutral. These results provide a benchmark for the development of theoretic methods of weakly bound complexes. The experimental technique demonstrated here can be employed to investigate other weakly bound complexes with multiple isomers.

Arbitrary image reinflation: A deep learning technique for recovering 3D photoproduct distributions from a single 2D projection

Many charged particle imaging measurements rely on the inverse Abel transform (or related methods) to reconstruct three-dimensional (3D) photoproduct distributions from a single two-dimensional (2D) projection image. This technique allows for both energy- and angle-resolved information to be recorded in a relatively inexpensive experimental setup, and its use is now widespread within the field of photochemical dynamics. There are restrictions, however, as cylindrical symmetry constraints on the overall form of the distribution mean that it can only be used with a limited range of laser polarization geometries. The more general problem of reconstructing arbitrary 3D distributions from a single 2D projection remains open. Here, we demonstrate how artificial neural networks can be used as a replacement for the inverse Abel transform and-more importantly-how they can be used to directly "reinflate" 2D projections into their original 3D distributions, even in cases where no cylindrical symmetry is present. This is subject to the simulation of appropriate training data based on known analytical expressions describing the general functional form of the overall anisotropy. Using both simulated and real experimental data, we show how our arbitrary image reinflation (AIR) neural network can be utilized for a range of different examples, potentially offering a simple and flexible alternative to more expensive and complicated 3D imaging techniques.

Study of He*/Ne*+Ar, Kr, N2, H2, D2 Chemi-Ionization Reactions by Electron Velocity-Map Imaging

The chemi-ionization of Ar, Kr, N₂, H₂, and D₂ by Ne(³P₂) and of Ar, Kr, and N₂ by He(³S₁) was studied by electron velocity map imaging (e-VMI) in a crossed molecular beam experiment. A curved magnetic hexapole was used to state-select the metastable species. Collision energies of 60 meV were obtained by individually controlling the beam velocities of both reactants. The chemi-ionization of atoms and molecules can proceed along different channels, among them Penning ionization and associative ionization. The evolution of the reaction is influenced by the internal redistribution of energy, which happens at the first reaction step that involves the emission of an electron. We designed and built an e-VMI spectrometer in order to investigate the electron kinetic energy distribution, which is related to the internal state distribution of the ionic reaction products. The analysis of the electron kinetic energy distributions allows an estimation of the ratio between the two-reaction channel Penning and associative ionization. In the molecular cases the vibrational or electronic excitation enhanced the conversion of internal energy into the translational energy of the forming ions, thus influencing the reaction outcome.

Chirality detection of surface desorption products using photoelectron circular dichroism

Chirality detection of gas-phase molecules at low concentrations is challenging as the molecular number density is usually too low to perform conventional circular dichroism absorption experiments. In recent years, new spectroscopic methods have been developed to detect chirality in the gas phase. In particular, the angular distribution of photoelectrons after multiphoton laser ionization of chiral molecules using circularly polarized light is highly sensitive to the enantiomeric form of the ionized molecule [multiphoton photoelectron circular dichroism (MP-PECD)]. In this paper, we employ the MP-PECD as an analytic tool for chirality detection of the bicyclic monoterpene fenchone desorbing from a Ag(111) crystal. We record velocity-resolved kinetics of fenchone desorption on Ag(111) using pulsed molecular beams with ion imaging techniques. In addition, we measure temperature-programmed desorption spectra of the same system. Both experiments indicate weak physisorption of fenchone on Ag(111). We combine both experimental techniques with enantiomer-specific detection by recording MP-PECD of desorbing molecules using photoelectron imaging spectroscopy. We can clearly assign the enantiomeric form of the desorption product fenchone in sub-monolayer concentration. The experiment demonstrates the combination of MP-PECD with surface science experiments, paving the way for enantiomer-specific detection of surface reaction products on heterogeneous catalysts for asymmetric synthesis.