Physics

Complete dissociation dynamics of low energy electron attachment to ammonia molecule has been studied using velocity slice imaging (VSI) spectrometer. One low energy resonant peak around 5.5 eV and a broad resonance around 10.5 eV incident electron energy has been observed. The resonant states mainly dissociate via H − and NH − 2 fragments, though for the upper resonant state, signature of NH − fragments are also predicted due to three body dissociation process. Kinetic energy and angular distributions of the NH − 2 fragment anions are measured simultaneously using VSI technique. Based on our experimental observations, we find the signature of A 1 symmetry in the 10.5 eV resonance energy whereas, the 5.5 eV resonance is associated with the well known A 1 symmetry.

The near ultraviolet photodissociation dynamics of silver atom rare gas dimers have been studied by velocity map imaging. AgRG (RG = Ar, Kr, Xe) species generated by laser ablation are excited in the region of the C <- X continuum leading to direct, near threshold dissociation generating Ag* (2P3/2) + RG (1S0) products. Images recorded at excitation wavelengths throughout the C <- X continuum, coupled with known atomic energy levels, permit determination of the ground X (2SIGMA+) state dissociation energies of 85.9 +/- 23.4 cm-1 (AgAr), 149.3 +/- 22.4 cm-1 (AgKr) and 256.3 +/- 16.0 cm-1 (AgXe). 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 Ag2.

The formation of O − and O − 2 ions via dissociative electron attachment to a pulsed supersonic jet of O 2 molecules containing weakly bound small van der Waals clusters seeded in a beam of argon is reported. The energy dependence of the O − and O − 2 yield exhibits three peaks near 7, 11 and 16 eV incident electron energies. The 7 eV peak arises from the 2 Π u state of O − 2 whereas, the 11 and 16 eV peaks are ascribed to two distinct resonance states: 2 Σ + g and 2 Σ + u states of O − 2 , respectively, via a violation of the Σ + ⇌ Σ − selection rule. The dependence of the cross-section of these two new peaks at ∼ 11 and ∼ 16 eV on the proportion of the carrier gas is also investigated and an optimum proportion has been observed experimentally which gives the lowest temperature of 14.86 K and highest Mach number of 72.31 for the pulsed supersonic jet.

In this article, density functional theory (DFT) and natural bond orbital (NBO) calculations are performed to understand experimental observations of dissociative electron attachment (DEA) to SO 2 . The molecular structure, fundamental vibrational frequencies with their corresponding intensities and molecular electrostatic potential (MEP) map of SO 2 and SO − 2 are interpreted from respective ground state optimized electronic structures calculated using DFT. The quantified MEPs and the second order perturbation energies for different oxygen lone pair (n) to σ ∗ and π ∗ interactions of S-O bond orbitals have been calculated by carrying out NBO analysis. The change in the electronic structure of the molecule after the attachment of a low-energy ( ≤ 15 eV) electron, thus forming a transient negative ion, can be interpreted from the n→ σ ∗ and n→ π ∗ interactions. The results of the calculations are used to interpret the dissociative electron attachment process. The dissociation of the anion SO − 2 into negative and neutral fragments has been explained by interpreting the infrared spectrum and different vibration modes. It could be observed that the dissociation of SO_{2}^{-} into S^{-} occurs as a result of simultaneous symmetric stretching and bending modes of the molecular anion. While the formation of O − and SO − occurs as a result of anti-symmetric stretching of the molecular anion. The calculated symmetries of the TNI state contributing to the first resonant peak at around 5.2 eV and second resonant peak at around 7.5 eV was observed from time-dependent density functional theory calculations to be an A 1 and a combination of A 1 +B 2 states for the two resonant peaks, respectively. These findings strongly support our recent experimental observations for DEA to SO 2 [Jana and Nandi, Phys. Rev. A, 97, 042706 (2018)].

The present study shows that the positrophilic electrons of a molecule dock into the positron attractive potential region in the annihilation process under the plane-wave approximation. The positron-electron annihilation processes of both polar and non-polar fluorinated methanes (CH4-nFn, n=0, 1,..., 4) are studied under this role. The predicted gamma-ray spectra of these fluorinated methanes agree well with the experiments. It further indicates that the positrophilic electrons of a molecule docking at the negative end of a bond dipole are independent from the molecular dipole moment in the annihilation process.

TThe stability of Ngn@B12N12 and Ngn@B16N16 systems is assessed through a density functional study and ab initio simulation. Although they are found to be thermodynamically unstable with respect to the dissociation of individual Ng atoms and parent cage, ab initio simulation reveals that except Ne2@B12N12 they are kinetically stable to retain their structures intact throughout the simulation time (500 fs). The Ne2@B12N12 cage dissociates and the Ne atoms get separated as the simulation proceeds. The He-He unit undergoes translation, rotation and vibration inside the cavity of B12N12 and B16N16 cages. Electron density analysis (Atoms-in-Molecule (AIM)) shows that there is some degree of covalent character in He-He bond (Wc type) of He2@B12N12. In case of He2@B16N16, the He-He interaction is mostly of noncovalent type (Wn). In many cases, especially for the heavier Ng atoms the Ng-N/B bonds are found to be of covalent type or at least having some degree of covalent character. But Wiberg bond indices show zero bond order in He-He bond and very low bond order in cases of Ng-N/B bonds. Energy decomposition analysis (EDA) provides further insights into the bonding among the Ng atoms and the cage. The change in charge distribution, radius, hardness, electrophilicity and polarizability implies that they possess different properties and reactivity from their parent moieties. The HOMO energies of He2 units in He2@B12N12 and He2@B16N16 are significantly higher than that in free He atoms. Their kinetic stability and different calculated properties imply that the He-He interaction and Ng-N/B interaction may be considered as chemical bonds according to the IUPAC definition.

Helium (He) nanodroplets irradiated by intense near-infrared laser pulses form a nanoplasma by avalanche-like electron impact ionizations even at lower laser intensities where He is not directly field ionized, provided that the droplets contain a few dopant atoms which provide seed electrons for the electron impact ionization avalanche. In this theoretical paper on calcium and xenon doped He droplets we elucidate the mechanism which induces ionization avalanches, termed ignition. We find that the partial loss of seed electrons from the activated droplets starkly assists ignition, as the Coulomb barrier for ionization of helium is lowered by the electric field of the dopant cations, and this deshielding of the cation charges enhances their electric field. In addition, the dopant ions assist the acceleration of the seed electrons (slingshot effect) by the laser field, supporting electron impact ionizations of He and also causing electron loss by catapulting electrons away. The dopants' ability to lower the Coulomb barriers at He as well as the slingshot effect decrease with the spatial expansion of the dopant, causing a dependence of the dopants' ignition capability on the dopant mass. Here, we develop criteria (impact count functions) to assess the ignition capability of dopants, based on (i) the spatial overlap of the seed electron cloud with the He atoms and (ii) the overlap of their kinetic energy distribution with the distribution of Coulomb barrier heights at He. The relatively long time delays between the instants of dopant ionization and ignition (incubation times) for calcium doped droplets are determined to a large extent by the time it takes to deshield the dopant ions.

We report on the Doppler-free saturation spectroscopy of the nitrous oxide (N 2 O) overtone transition at 1.28~ μ m. This measurement is performed by the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) technique based on the quantum-dot (QD) laser. A high intra-cavity power, up to 10~W, reaches the saturation limit of the overtone line using an optical cavity with a high finesse of 113,500. At a pressure of several mTorr, the saturation dip is observed with a full width at half-maximum of about 2~MHz and a signal-to-noise ratio of 71. To the best of our knowledge, this is the first saturation spectroscopy of molecular overtone transitions in 1.3~ μ m region. The QD laser is then locked to this dispersion signal with a stability of 15 kHz at 1 sec integration time. We demonstrate the potential of the N 2 O as markers because of its particularly rich spectrum at the vicinity of 1.28-1.30 μ m where lies several important forbidden transitions of atomic parity violation measurements and the 1.3 μ m O-band of optical communication.

Double and triple ionization spectra of isocyanic acid have been measured using multi-electron and ion coincidence techniques combined with synchrotron radiation and compared with high-level theoretical calculations. Vertical double ionization at an energy of 32.8+/-0.3 eV forms the 3A'' ground state in which the HNCO2+ ion is long-lived. The vertical triple ionization energy is determined as 65+/-1 eV. The core-valence double ionization spectra resemble the valence photoelectron spectrum in form, and their main features can be understood on the basis of a simple and rather widely applicable Coulomb model based on the characteristics of the molecular orbitals from which electrons are removed. Characteristics of the most important dissociation channels are examined and discussed.

We report on a study of Dynamic Nuclear Polarization and electron and nuclear spin relaxation of atomic hydrogen and deuterium in solid molecular matrices of H 2 , D 2 , and HD mixtures. The electron and nuclear spin relaxation times ( T 1e and T 1N ) were measured within the temperature range 0.15-2.5 K in a magnetic field of 4.6 T, conditions which ensure a high polarization of electron spins. We found that T 1e is nearly temperature independent in this temperature range, while T 1N decreased by 2 orders of magnitude. Such strong temperature dependence is typical for the nuclear Orbach mechanism of relaxation via the electron spins. We found that the nuclear spins of H atoms in solid D 2 and D 2 : HD can be efficiently polarized by the Overhauser effect. Pumping the forbidden transitions of H atoms also leads to DNP, with the efficiency strongly dependent on the concentration of D atoms. This behaviour indicates the Cross effect mechanism of the DNP and nuclear relaxation, which turns out to be well resolved in the conditions of our experiments. Efficient DNP of H atoms was also observed when pumping the middle D line located in center of the ESR spectrum. This phenomenon can be explained in terms of clusters or pairs of H atoms with strong exchange interaction. These clusters have partially allowed transitions in the center of the ESR spectrum and DNP may be created via the resolved Cross effect.