Resonances are metastable states that decay after a finite period of time. These states play a role in many physical processes. For example, in recent cold collision experiments, autoionization from a resonance state was observed. Complementing such observations with theory provides insight into the reaction dynamics under study. Theoretical investigation of autoionization processes is enabled via complex potential energy surfaces (CPESs), where the real and imaginary parts, respectively, provide the energy and decay rate of the system. Unfortunately, calculation of ab initio polyatomic CPESs are cumbersome; hence, they are not in abundance. Here, we present an ab initio polyatomic CPES utilizing a recently developed approach, which makes such calculations feasible. This CPES helps interpret the autoionization process observed in the He(2S) + H collision. From the behavior of the calculated CPES we can conclusively determine the nature of the autoionization process. Moreover, this CPES was used to generate reaction rates for the collision of He with ortho- and para-H. These reaction rates are obtained from first principles. The results show a remarkable agreement with the cold collision experimental measurements, which demonstrates the robustness of our method. Hereby, we provide a computational tool for designing and interpreting new types of experiments that involve resonance states, e.g., in nucleobase damages (DNA or RNA) or in interatomic (intermolecular) Coulombic decay.

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The quantum phenomena of electronic and nuclear resonances are associated with structures in measured cross sections. Such structures were recently reported in a cold chemistry experiment of ground-state hydrogen isotopologues (H2/HD) colliding with helium atoms in the excited triplet P-state (He­(23P)) [Shagam et al. Nature Chem. 2015, 7, 921], but a theoretical explanation of their appearance was not given. This work presents a quantum explanation and simulation of this experiment, which are strictly based on ab initio calculations. We incorporate complex potential energy surfaces into adiabatic variational theory, thereby reducing the multidimensional scattering process to a series of uncoupled 1D scattering “gedanken experiments”. Our theoretical result, which is in remarkable agreement with the experimental data, manifests that the structures in the observed reaction rate coefficient are due to the spatial arrangement of the excited He p-orbitals with respect to the interaction axis, consequently changing the system from a normal two-rotor model to a three-rotor one. This theoretical scheme can be applied to explain and predict cross sections or reaction rate coefficients for any resonance-related phenomenon.

Theoretical description of potential energy curves (PECs) of molecular ions is essential for interpretation and prediction of coupled electron-nuclear dynamics following ionization of parent molecule. However, an accurate representation of these PECs for core or inner valence ionized state is nontrivial, especially at stretched geometries for double- or triple-bonded systems. In this work, we report PECs of singly and doubly ionized states of molecular nitrogen using state-of-the-art quantum chemical methods. The valence, inner valence, and core ionized states have been computed. A double-loop optimization scheme that separates the treatment of the core and the valence orbitals during the orbital optimization step of the multiconfiguration self-consistent field method has been implemented. This technique allows the energy to be converged to any desired ionized state with any number of core or inner-shell holes. The present work also compares the PECs obtained using both delocalized and localized sets of orbitals for the core hole states. The PECs of a number of singly and doubly ionized valence states have also been computed and compared with previous studies. The computed PECs reported here are expected to be of importance for future studies to understand the interplay between photoionization and Auger spectra during the breakup of molecular nitrogen when interacting with intense free electron lasers.

Within the Fock-space multi-reference coupled cluster framework, we have evaluated the electronic transition dipole moments, which determine absorption intensities. These depend on matrix elements between two different wave functions (e.g., ground state to the excited state). We present two different ways, to calculate these transition moments. In the first method, we construct the ground and excited state wave functions with the normal exponential ansatz of Fock-space coupled cluster method and then calculate the relevant off-diagonal matrix elements. In the second approach, we linearize the exponential form of the wave operator which will generate the left vector, by use of Lagrangian formulation. The right vector is obtained from the exponential ansatz. In order to relate the transition moments to oscillator strengths, excitation energies need to be evaluated. The excitation energies are obtained from the Fock-space multi-reference framework. The transition dipole moments of the ground to a few excited states, together with the oscillator strengths of a few molecules, are presented.

Surface engineering of SiO2 dielectric using different self-assembled monolayer (SAM) has been carried out, and its effect on the molecular packing and growth behavior of copper phthalocyanine (CuPc) has been studied. A correlation between the growth behavior and performance of organic field effect transistors is examined. Depth profiling using positron annihilation and X-ray reflectivity techniques has been employed to characterize the interface between CuPc and the modified and/or unmodified dielectric. We observe the presence of structural defects or disorder due to disorientation of CuPc molecules on the unmodified dielectric and ordered arrangement on the modified dielectrics, consistent with the high charge carrier mobility in organic field effect transistors in the latter. The study also highlights the sensitivity of these techniques to the packing of CuPc molecules on SiO2 modified using different SAMs. Our study also signifies the sensitivity and utility of these two techniques in the characterization of buried interfaces in organic devices.

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Nanoparticulate thin films of PdO were prepared using the Langmuir-Blodgett (LB) technique by thermal decomposition of a multilayer film of octadecylamine (ODA)-chloropalladate complex. The stable complex formation of ODA with chloropalladate ions (present in subphase) at the air-water interface was confirmed by the surface pressure-area isotherm and Brewster angle microscopy. The formation of nanocrystalline PdO thin film after thermal decomposition of as-deposited LB film was confirmed by X-ray diffraction and Raman spectroscopy. Nanocrystalline PdO thin films were further characterized by using UV-vis and X-ray photoelectron spectroscopic (XPS) measurements. The XPS study revealed the presence of prominent Pd(2+) with a small quantity (18%) of reduced PdO (Pd(0)) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, the band gap energy of PdO was estimated to be 2 eV, which was very close to that obtained from specular reflectance measurements. Surface morphology studies of these films using atomic force microscopy and field-emission scanning electron microscopy indicated formation of nanoparticles of size 20-30 nm. These PdO film when employed as a chemiresistive sensor showed H2 sensitivity in the range of 30-4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light.