Journal of Chemical Physics最新文献

The photodissociation and photoionization of O2 and the subsequent photodissociation of O2+ in the wavelength region of 200 to 240 nm are reported using resonance enhanced multiphoton ionization (REMPI) and velocity map imaging detection. A series of two-photon allowed Rydberg states with principle quantum number n = 3-11 converging to the ground electronic state of O2+X2Πg are used as doorway states to reach the region of superexcited states of O2 in the three-photon energy range of 15.8-18.6 eV. A detailed analysis of the kinetic energy release and anisotropy parameters of photofragments extracted from velocity map images reveals competition between neutral dissociation and autoionization and leads to the identification of different O+ formation channels. Moreover, the measurement of anisotropy parameters for each channel gives additional information on the symmetry of electronic states involved in the absorption process. Formation followed by the dissociation of vibrationally excited O2+ is the strongest channel over the full wavelength range studied. Ground and vibrationally excited O2+(X2Πg, a4Πu, A2Πu) are formed and dissociated to ionic products via one and two-photon processes. Neutral dissociation to form electronically excited atoms is important at the longer wavelengths studied and becomes noticeably less important at shorter wavelengths. These results agree with and expand on a previous study from our lab of O+ formation at a single (2 + 1) REMPI wavelength, and the results obtained in this study are found to complement our study of the electronically analogous counterpart S2, where most of the S+ ions arise from electronically excited S* atoms. The results of this study will also be of use in the pixel-to-velocity calibration of any velocity map imaging apparatus in the wide ultraviolet wavelength regions. Because O2 is a common reactant or product in many molecular dynamics studies, knowledge of its ionization/dissociation pathways at commonly used wavelengths should also be useful in avoiding signal overlap problems.

We have developed an efficient scheme for the calculation of transition properties within the four-component relativistic equation-of-motion coupled cluster (EOM-CC) method using the expectation value approach. The calculation of transition properties within the relativistic EOM-CC framework requires the solution of both right and left eigenvectors. The accuracy of the approach has been investigated by calculating low-lying transitions of a Xe atom, a HI molecule, and spin forbidden 1S0 → 3P1 and spin allowed 1S0 → 1P1 transitions in a few closed shell cations. In addition to the valence spectra, the relativistic EOM-CCSD expectation value approach is particularly suitable for simulating the L-edge x-ray absorption spectrum (XAS). The calculated results show good agreement with the earlier reported theoretical studies and experimental values.

Utilizing an atomistic computational model, which handles both translational and spin degrees of freedom, combined molecular and spin dynamics simulations have been performed to investigate the effect of vacancy defects on spin wave excitations in ferromagnetic iron. Fourier transforms of space- and time-displaced correlation functions yield the dynamic structure factor, providing characteristic frequencies and lifetimes of the spin wave modes. A comparison of the system with a 5% vacancy concentration with pure lattice data shows a decrease in frequency and a decrease in lifetime for all transverse spin wave excitations observed. In addition, the clearly defined transverse spin wave excitations are distorted with the introduction of vacancy defects, and we observe reduced excitation lifetimes due to increased magnon-magnon scattering. We observe further evidence of increased magnon-magnon scattering, as the peaks in the longitudinal spin wave spectrum become less distinct. Similar impacts are observed in the vibrational subsystem, with a decrease in characteristic phonon frequency and flattening of lattice excitation signals due to vacancy defects.

Singlet fission (SF) in molecular semiconductors is a photophysical process of spontaneous splitting of the excited singlet state into a pair of triplet excitons (TT-pair). This process is usually strongly influenced by spin-selective back geminate TT-annihilation (TTA). Spin selectivity manifests itself in magnetic field effects (MFEs) on both TTA and SF kinetics, the study of which allows us to reveal some specific features of this kinetics. In our work, we analyze the mechanism of MFE generation in TTA and SF processes in amorphous molecular semiconductors. In this mechanism, the MFEs are assumed to be determined by magnetic field dependent spin-lattice relaxation (SLR) in TT-pairs, generated by the zero-field splitting interaction (in T-excitons), fluctuating due to T-exciton hopping over arbitrarily oriented molecules in amorphous semiconductors. The SLR-transitions are described with a semiempirical model, which makes it possible to obtain the SF-kinetic functions in analytical form. The mechanism of SLR-assisted MFEs is found to be very efficient in TTA and SF processes. The obtained results are analyzed in detail and applied to interpret experimentally observed SF-kinetic dependences in various magnetic fields. In particular, it is shown that the proposed model of SLR-generated MFEs enables one to describe the effect of crossing of SF-kinetic functions, corresponding to different magnetic fields.

Fluorine-containing flavin derivatives can be used as probes in flavin-binding proteins forming radical pairs to exploit the photo-chemically induced dynamic nuclear polarization (photo-CIDNP) effect. Knowledge of the hyperfine structure is crucial for studying the mechanism of intramolecular radical-pair formation in proteins. Transient 19F photo-CIDNP NMR has so far not been used to determine the isotropic hyperfine coupling constants of 19F nuclei. Here, we show that this method provides reliable results by studying three monofluorinated flavin mononucleotide (FMN) derivatives in conjunction with 6-fluoro-tryptophan. Combining this method with transient 1H photo-CIDNP spectroscopy leads to a more accurate interpretation of the intermediate radical species forming a radical pair. The gathered information can be used to identify the most promising FMN derivative for usage as a probe for formation of radical pairs in proteins.

First steps toward a molecular dynamics (MD) implementation in a cluster of field-programmable gate arrays (FPGAs) are presented, reaching a simulation speed of a few microseconds/day. The nodes in this cluster are programmed into a mid-ranged FPGA (Artix 7 XC7A200T), interconnected as a 3D torus by fast optical links. The implemented MD algorithm is highly parallelized and highly pipelined internally. The FPGA cluster is freely scalable in terms of size, i.e., a larger MD system requires more nodes, however, without compromising simulation speed. The performance in terms of energy stability and simulation speed is analyzed. At present, the focus lies on the fast networking, while only minimal MD functionality has been implemented so far, i.e., Lennard-Jones interactions and a thermostat, which were needed to demonstrate the feasibility of the FPGA cluster to run multi-microsecond simulations. To that end, the nucleation of a super-cooled Lennard-Jones liquid is investigated by unbiased MD simulations, which is a difficult MD problem since a high nucleation barrier has to be overcome. Finally, the pathways toward a full MD implementation are outlined. The current implementation will be made available as an open-source development project.

We present an analytical description of the Alchemical Transfer Method (ATM) for molecular binding using the Potential Distribution Theory (PDT) formalism. ATM models the binding free energy by mapping the bound and unbound states of the complex by translating the ligand coordinates. PDT relates the free energy and the probability densities of the perturbation energy along the alchemical path to the probability density at the initial state, which is the unbound state of the complex in the case of a binding process. Hence, the ATM probability density of the transfer energy at the unbound state is first related by a convolution operation of the probability densities for coupling the ligand to the solvent and coupling it to the solvated receptor-for which analytical descriptions are available-with parameters obtained from maximum likelihood analysis of data from double-decoupling alchemical calculations. PDT is then used to extend this analytical description along the alchemical transfer pathway. We tested the theory on the alchemical binding of five guests to the tetramethyl octa-acid host from the SAMPL8 benchmark set. In each case, the probability densities of the perturbation energy for transfer along the alchemical transfer pathway obtained from numerical calculations match those predicted from the theory and double-decoupling simulations. The work provides a solid theoretical foundation for alchemical transfer, offers physical insights on the form of the probability densities observed in alchemical transfer calculations, and confirms the conceptual and numerical equivalence between the alchemical transfer and double-decoupling processes.

We report a rotationally resolved spectroscopic detection scheme for vibrationally excited molecular oxygen with high sensitivity. Two-color (2 + 1') resonance-enhanced multiphoton ionization (REMPI) spectra of O2 hot bands were recorded for the first time via the 3dπ(v'=0)←X3Σg- (v″ = 1) Rydberg transitions. Spectroscopic constants and relative Franck-Condon factors were extracted and compared to simulations. This new access to quantum-state-resolved diagnostics of vibrationally excited O2 promises to shed light on the physical and chemical dynamics of many processes.

We explore the impact of point defects, including oxygen vacancies (Ov), cerium interstitials (Ce-int), and hydroxyl groups (Hy), on the electronic and optical properties of bulk CeO2 using many-body Green's function theory (GW method and Bethe-Salpeter equation). Although these three defects all produce occupied electronic levels near the conduction band minimum, they impose quite different effects. Ov and Ce-int induce strong peaks in the low-energy region of the imaginary part of the microscopic dielectric function, indicating stronger electronic screening compared to the pristine CeO2. This causes pronounced narrowing of the bandgap, e.g., by 0.8 eV in G0W0 and 1.6 eV in the eigenvalue self-consistent GW for Ov. Comparatively, Hy affects little electronic screening and bandgap at different levels of GW calculations. For the lowest several 4f orbitals, the exchange part of the self-energy (|Σx| > 9 eV) in GW is much stronger than the correlation part (|Σc| < 5 eV) for Ov and Ce-int, while |Σc| is much stronger than |Σx| instead for the pristine CeO2 and Hy. Quasiparticle weights in Ov and Ce-int decrease by a large quantity compared to the pristine CeO2. Consideration of Ov and Ce-int might to some extent relieve the discrepancy between the GW bandgap of the pristine CeO2 and the experimental gap. Ov and Ce-int could reduce the excitonic binding energy several times and result in optical absorption, which corresponds to the experiments.

An exchange-correction to the fixed diagonal matrices (FDM) method is introduced to improve accuracy when employing a single reference wavefunction. In addition, the performance of the Becke-Roussel exchange-hole in approximating the pair density-mediated integrals was explored. With the exchange-correction, the FDM procedure yields dispersion coefficients for closed-shell atoms on par with highly correlated methods when using only Hartree-Fock or Kohn-Sham pair density. Conversely, the Becke-Roussel exchange-hole results in an overestimation of the dispersion coefficients for closed-shell atoms; however, the performance of the Becke-Roussel model can be improved by scaling the multipole moment integrals by a fixed amount. For both the exchange-correction and the Becke-Roussel model, the FDM method continues to fail for open-shell atoms and ions by consistently underestimating the dispersion coefficients.