Orientation Of Molecular Axis Research Articles

Improved Chiral Photoelectron Spectroscopy via Selection of Chirality-Selective Molecular Axis Orientations: A Theoretical Analysis of Non-Negative Smooth Functions

Photoelectron spectroscopy for chiral discrimination is routinely performed for low photoelectron kinetic energies (PKEs), whereas it is considered impossible for high PKEs. We demonstrate theoretically that chiral photoelectron spectroscopy for high PKEs is possible using chirality-selective molecular orientation. The photoelectron angular distribution associated with one-photon ionization by unpolarized light can be characterized by a single parameter, β. We show that most other anisotropy parameters are zero when β is 2, as is often the case in the high PKEs. Exceptionally, odd-order anisotropy parameters are increased by a factor of 20 by orientation, even for high PKEs.

XUV ionization of the H2 molecule studied with attosecond angular streaking

We study orientation and two-center interference effects in attosecond time-resolved photoionization of the H2 molecule. Time resolution of extreme-ultraviolet ionization of H2 is gained through the phase retrieval capability of attosecond angular streaking demonstrated earlier by Kheifets et al (2022 Phys. Rev. A 106 033106). Once applied to H2, this technique delivers an anisotropic phase and time delay which both depend sensitively on the molecular axis orientation. In addition, the photoelectron momentum distribution displays a very clear two-center interference pattern. When the interference formula of Walter and Briggs (1999 J. Phys. B 32 2487) is applied, an effective photoelectron momentum appears to be greater than the asymptotic momentum at the detector. This effect is explained by a molecular potential well surrounding the photoemission center.

Effect of molecular axis orientation of 3.6 MeV Si2+ projectiles on the ion-induced secondary electron emission from a carbon foil

We study ion-induced secondary electron emission from solid surfaces. This work focuses on the dependence of the energy distribution of secondary electrons on the orientation of Si-dimers impinging on thin carbon foils. The orientation was determined by the foil-induced Coulomb explosion method. Furthermore, the electron energy was measured with a retarding field analyzer. The correlation between the electron energy distribution and the cluster orientation was obtained through simultaneous measurements. A comparison of the electron energy distribution for parallel and perpendicular orientations indicated that the electron yields at an energy below 20 eV are lower for parallel orientation. This reduction can be explained by the orientation dependence of the energy loss of Si2+ in carbon. It was suggested that the orientation effect is due to the distant collision between the cluster and the target atom, in which low-energy electrons are produced.

Model-free analysis of molecular orientation in amorphous organic semiconductor films for understanding its formation dynamics: Methods and systematic investigation

To achieve high-performance organic light-emitting diodes (OLEDs), various amorphous organic semiconductor materials were developed, and the higher-order structure of their films and their formation dynamics were investigated. In particular, their molecular orientations were widely analyzed because it significantly affects optical and electrical properties of the OLEDs. However, conventional methods of the analysis of the orientation order parameter ( S ) need the construction of an anisotropic optical model, which brings a certain degree of analytical error and thus makes it difficult to assess the slight changes in S values arising from differences in experimental conditions. In this study, we present a simple model-free analytical method to estimate S values, named "QACIOS", that avoids the significant analytical errors and applies to various OLED materials, including materials that easily crystallize. In applying the method to UV/visible and IR spectral regions, the S values of electronic and vibrational transition dipole moments in the films can be simply estimated. The orientations of functional groups in single-layer and multilayer samples are obtainable separately, and in situ measurements are also demonstrated. Furthermore, we present a systematic analysis of the dependence of S on experimental conditions during sample fabrication such as deposition rate, substrate temperature, and substrate surface. From this analysis, the difference in formation dynamics of the molecular orientation of two structural isomers of OLED materials is quantitatively clarified, further demonstrating the usefulness of the method. A model-free simple analytical method by quantitative absorbance comparison with an isotropically oriented standard (QACIOS) is applied to systematic investigation of orientations of long molecular axis and functional groups in amorphous organic semiconductor films and their formation dynamics. • A model-free simple analytical method for reliable estimation of orientation order parameter is proposed and applied to many kinds of amorphous OLED materials. • Orientation analysis of functional groups in single-layer and multilayer samples are performed. • The difference in formation dynamics of molecular orientation of two OLED isomers is clarified quantitatively. • The method can be a commonly-used standard method for orientation analysis of OLED films.

Anomalous dependence of ionization probability and electron angular distributions on orientation of molecular axis in photoionization of : effect of two-center interference

A theoretical and computational study of photoionization of the one-electron molecular ion initially in the 1σ u state is performed. The laser pulse is linearly polarized with the carrier wavelength in the extreme ultraviolet and soft x-ray regions. The electron wave function is obtained by solving the time-dependent Schrödinger equation with the help of the generalized pseudospectral method. The dependence of the total ionization probability and photoelectron spectra on the orientation of the molecular axis is analyzed. Within the wavelength interval 6 to 23 nm, anomalous behavior of the ionization probability is found, where it increases with the angle between the polarization vector of the external field and the molecular axis and reaches a maximum at the perpendicular orientation of the molecule. The phenomenon is explained as resulting from the two-center interference in the wave function of the emitted electron.

Understanding stress-induced disorder and breakage in organic crystals: beyond crystal structure anisotropy.

Crystal engineering has advanced the strategies for design and synthesis of organic solids with the main focus being on customising the properties of the materials. Research in this area has a significant impact on large-scale manufacturing, as industrial processes may lead to the deterioration of such properties due to stress-induced transformations and breakage. In this work, we investigate the mechanical properties of structurally related labile multicomponent solids of carbamazepine (CBZ), namely the dihydrate (CBZ·2H2O), a cocrystal of CBZ with 1,4-benzoquinone (2CBZ·BZQ) and the solvates with formamide and 1,4-dioxane (CBZ·FORM and 2CBZ·DIOX, respectively). The effect of factors that are external (e.g. impact stressing) and/or internal (e.g. phase transformations and thermal motion) to the crystals are evaluated. In comparison to the other CBZ multicomponent crystal forms, CBZ·2H2O crystals tolerate less stress and are more susceptible to breakage. It is shown that this poor resistance to fracture may be a consequence of the packing of CBZ molecules and the orientation of the principal molecular axes in the structure relative to the cleavage plane. It is concluded, however, that the CBZ lattice alone is not accountable for the formation of cracks in the crystals of CBZ·2H2O. The strength and the temperature-dependence of electrostatic interactions, such as hydrogen bonds between CBZ and coformer, appear to influence the levels of stress to which the crystals are subjected that lead to fracture. Our findings show that the appropriate selection of coformer in multicomponent crystal forms, targetting superior mechanical properties, needs to account for the intrinsic stress generated by molecular vibrations and not solely by crystal anisotropy. Structural defects within the crystal lattice, although highly influenced by the crystallisation conditions and which are especially difficult to control in organic solids, may also affect breakage.

Effects of molecular axis orientation of MeV diatomic projectiles on secondary ion emission from biomolecular targets

We report first experimental results on the orientation effect of fast diatomic molecular projectiles on secondary ion emission processes for 3.6-MeV C2+ traversing a self-supporting target of phenylalanine (Phe) film evaporated on a carbon foil. Coincidence measurements were performed on the image of fragments resulting from the Coulomb explosion of C2+ projectiles and the time-of-flight mass spectrometry of secondary ions emitted from the Phe film. We compared the secondary ion yield for C2+ projectiles with parallel and perpendicular orientations to the beam direction. The parallel orientation was found to enhance the secondary ion yield by a factor of approximately 1.1 compared with that for the perpendicular orientation. This enhancement corresponds to an increase in the average charge of Coulomb-exploded fragments. This finding implies that there is a linear correlation between the electronic energy deposition causing secondary ion production and the effective charge number of the incident molecular ions which interact with biomolecular targets.

Checkered Films of Multiaxis Oriented Nanocelluloses by Liquid-Phase Three-Dimensional Patterning.

It is essential to build multiaxis oriented nanocellulose films in the plane for developing thermal or optical management films. However, using conventional orientation techniques, it is difficult to align nanocelluloses in multiple directions within the plane of single films rather than in the thickness direction like the chiral nematic structure. In this study, we developed the liquid-phase three-dimensional (3D) patterning technique by combining wet spinning and 3D printing. Using this technique, we produced a checkered film with multiaxis oriented nanocelluloses. This film showed similar retardation levels, but with orthogonal molecular axis orientations in each checkered domain as programmed. The thermal transport was enhanced in the domain with the oriented pattern parallel to the heat flow. This liquid-phase 3D patterning technique could pave the way for bottom-up design of differently aligned nanocellulose films to develop sophisticated optical and thermal materials.

Surface effects in the model of polymer-stabilized ferroelectric liquid crystal cells

The interplay between volume and surface interactions in polymer-stabilized ferroelectric liquid crystals often results in the so-called quasi-bookshelf or tilted layer structure. Unambiguous description of the most probable orientation of the long molecular axes within a ferroelectric liquid crystal cell (director profile) stabilized by polymer network implies the consideration of the confined volume effects. The model of polymer-stabilized ferroelectric liquid crystal cell with a quasi-bookshelf layer structure was investigated by using the liquid crystal continuum theory. Promising applications of the biaxial surface potential for preparation of ferroelectric liquid crystal cells with the desired surface parameters has motivated us to embark upon the proposed model. To fully consider the surface effects, the proposed model accounts splay deformation of the spontaneous polarization. This enables us to estimate the effective applied voltage across the cell. The effect of polymer stabilization on the director orientation profiles across the cell was examined. We have found that the director-polymer network interaction coefficient induces insignificant difference between the director orientation profiles. We believe that this theoretical model can be useful for fabrication of experimental ferroelectric liquid crystal cells.

13C NMR Investigations of Hairy-Rod-Like π-Conjugated Mesogens

Two hairy-rod π-conjugated mesogens comprising dihexylfluorene and didecyloxyterphenyl are synthesized, and their mesophase properties are examined by hot-stage optical polarizing microscope and differential scanning calorimetry techniques. Both mesogens exhibit enantiotropic nematic mesophase with broad mesophase range. A detailed 13C NMR study is carried out in solution as well as in nematic mesophase to understand changes in the orientation of the aliphatic chains. Accordingly, the unusual NMR chemical shift value of one of the methylene carbons of the hexyl chain of fluorene moiety in solution is ascribed to ring current effect changes in nematic mesophase owing to variation in the orientation of the hexyl chain. The change in conformation of lateral and alkyl chains in the nematic phase is clearly noticed compared to isotropic solution as the molecular orientation in mesophase is governed by the orientation of the long molecular axis. Furthermore, the 13C-1H dipolar couplings obtained from 2D separated local field experiments in nematic phase aided the assignments of all of the carbons of the molecules besides offering the local order. To comprehend the orientation of the fluorene unit, three order parameters are necessary whereas for the phenyl rings, two order parameters are found to be sufficient. The molecular biaxiality ( S xx - S yy) for fluorene-based mesogen is found to be higher due to the fused nature of the moiety and the presence of dihexyl chains. The orientational ordering of π-conjugated mesogens is crucial as the optoelectronic properties of them critically depend on the orientation of the constituent chromophores. The complete mapping of order parameters of fluorene, the phenyl rings and the alkyl chains showed that despite both molecules exhibiting nematic mesophase, the orientational constraints are governed by the molecular structure as well as the rigidity of the core unit, i.e., fused versus linearly connected rings.

Circular dichroism in photoionization of degenerate orbitals: Spin-polarized photoelectrons and spontaneous separation of oriented photoions.

This work investigated the circular dichroic effect on the photoionization integral cross section of molecules in conjunction with irreducible tensor theory and effective operator formalism. The results show that the dichroic effect can be non-zero for complex orbitals, but becomes zero for all real orbitals due to time-reversal symmetry, within the electric dipole and Born-Oppenheimer approximations. Calculations were performed for carbon monoxide, boric acid, and fullerene, and implications of the first-order coefficient for the spin polarization of photoelectrons and the molecular axis orientation of photoions are discussed herein. The results of this work demonstrate that the photoionization of complex orbitals can cause photoions to become oriented such that photoions originating from complex conjugate orbitals are oriented opposite to one another. Due to electron-ion recoil, the spontaneous separation of these two kinds of photoions is expected for the point groups C n , , , C nh , and S n with n ≥ 3.

Biaxial potential of surface-stabilized ferroelectric liquid crystals.

A biaxial surface potential Φ_{s} of smectic-C^{*} surface-stabilized ferroelectric liquid crystals (SSFLCs) is introduced in this paper to explain the experimentally observed electric-field dependence of polarization P[over ̃]_{cell}(E), in particular the shape of the static hysteresis loops. Our potential consists of three independent parts. The first nonpolar part Φ_{n} describes the deviation of the prime director n (which is the most probable orientation of the long molecular axes) from the easy alignment axis R, which is located in the boundary surface plane. It is introduced in the same manner as the uniaxial Rapini potential. The second part Φ_{p} of the potential is a polar term associated with the presence of the polar axis in a FLC. The third part Φ_{m} relates to the inherent FLC biaxiality, which has not been taken into consideration previously. The Φ_{m} part takes into account the deviations of the secondary director m(which is the most probable orientation of the short molecular axes) from the normal to the boundary surface. The overall surface potential Φ_{s}, which is a sum of Φ_{n},Φ_{p}, and Φ_{m}, allows one to model the conditions when either one, two, or three minima of the SSFLC cell free energy are realized depending on the biaxiality extent. A monodomain or polydomain structure, as well as the bistability or monostability of SSFLC cells, depends on the number of free-energy minima, as confirmed experimentally. In this paper, we analyze the biaxiality impact on the FLC alignment. We also answer the question of whether the bistable or monostable structure can be formed in an SSFLC cell. Our approach is essentially based on a consideration of the biaxial surface potential, while the uniaxial surface potential cannot adequately describe the experimental observations in the FLC.

The alignment of dichroic dyes in a nematic liquid crystal: a molecular dynamics investigation

ABSTRACTThe absorption properties of a dye molecule depend on its orientation relative to the light source. Thus the ability to predict how well a particular dye aligns with a liquid crystal host will improve the design of smart materials. One measurement of this alignment is the order parameter, which can be calculated from molecular dynamics simulations. The results for three dyes are presented here. The orientation of the long molecular axis of the dye relative to the liquid crystal director can range from perpendicular to parallel, with the dyes studied having an average order parameter of the dye similar to the average order parameter of the host.

Strong Field dissociative ionization of the

SynopsisStrong field dissociative ionization of the molecule in the multiphoton regime is investigated employing ab initio calculations for fixed molecular axis orientations. The resulting joint (photoelectron and nuclear kinetic) energy spectra are explored. The number of photons absorbed in each multiphoton peak of these spectra is determined analyzing the angular distribution of the photoelectrons, while the exact path followed by is identified studying the time evolution of the nuclear wave packets.

Strong field dissociative ionization of the [formula omitted]: Nuclear wave packet analysis

Theoretical ab initio investigation of strong field dissociative ionization of the D2+ molecule in the multiphoton regime is reported. The dynamics is initiated by ultrashort laser pulses for fixed molecular axis orientations. Nuclear wave packet calculations are performed to provide the joint energy spectra (JES): ionization-dissociation probability density via electron (Ee) and nuclear (En) kinetic energy. Analyzing the time-dependent nuclear wave packet densities we have successfully identified the exact path followed by the D2+ target for each multiphoton peak.

Principal molecular axis and transition dipole moment orientations in liquid crystal systems: an assessment based on studies of guest anthraquinone dyes in a nematic host.

An assessment of five different definitions of the principal molecular axis along which molecules align in a nematic liquid crystal system has been made by analysing fully atomistic molecular dynamics (MD) simulations of a set of anthraquinone dyes in the cyanobiphenyl-based nematic host mixture E7. Principal molecular axes of the dyes defined by minimum moment of inertia, minimum circumference, minimum area, maximum aspect ratio, and surface tensor models were tested, and the surface tensor model was found to give the best description. Analyses of MD simulations of E7 alone showed that the surface tensor model also gave a good description of the principal molecular axes of the host molecules, suggesting that this model may be applicable more generally. Calculated dichroic order parameters of the guest-host systems were obtained by combining the surface tensor analysis with fixed transition dipole moment (TDM) orientations from time-dependent density functional theory (TD-DFT) calculations on optimised structures of the dyes, and the trend between the dyes generally matched the trend in the experimental values. Additional analyses of the guest-host simulations identified the range of conformers explored by the flexible chromophores within the dyes, and TD-DFT calculations on corresponding model structures showed that this flexibility has a significant effect on the TDM orientations within the molecular frames. Calculated dichroic order parameters that included the effects of this flexibility gave a significantly improved match with the experimental values for the more flexible dyes. Overall, the surface tensor model has been shown to provide a rationale for the experimental alignment trends that is based on molecular shape, and molecular flexibility within the chromophores has been shown to be significant for the guest-host systems: the computational approaches reported here may be used as a general aid in the predictive design of dyes with appropriate molecular shapes and flexibilities for guest-host applications.

Toward the Full Quantum Dynamical Description of Photon Induced Processes in D2.

The dissociative ionization (multiphoton regime) of the D2+ ion by ultrashort laser pulses has been studied theoretically using ab initio calculations. The combined ionization and dissociation spectrum was explored for fixed molecular axis orientations. In accordance with previous investigations, the dominant features in the obtained joint energy spectrum were multiphoton peaks. In addition to this, in the present work, photoelectron angular distributions were analyzed as well. By performing a partial wave analysis for each multiphoton peak, we have identified the number of absorbed photons. Moreover, we also found that the angular distribution can significantly change inside a multiphoton peak as a function of electron and nuclear kinetic energy.

Possible Formation of Metastable PAH Dimers upon Pickup by Helium Droplets.

Using path-integral molecular dynamics simulations and two quantum-mechanical-based force fields, we have investigated the conformational stability of dimers of a polycyclic aromatic hydrocarbon, perylene (C20H12), produced under typical experimental conditions of successive pick-up under helium nanodroplet environment. The most stable configurations are found to be of the stacked form with different relative orientations of the main molecular axes, perpendicular or T-shaped dimers being energetically much disfavored; however, in the presence of helium our simulations suggest that the time for rearrangement and π-stacking may be rather long and exceed hundreds of picoseconds. In addition, highly metastable dimers that are stacked but with a helium monolayer sandwiched between the two molecules are also found as likely products upon successive pickup. This stabilization occurs owing to the stronger localization of the helium atoms facing the aromatic rings, which is further enhanced in the dimer. The implications of the present results are discussed in the perspective of possible identification by spectroscopic methods.

Interference structures in nonlinear processes in strong infrared laser fields

Molecular strong-field approximation in the frame of the S-matrix theory is applied in order to investigate above-threshold ionization (ATI) and high-order harmonic generation (HHG) for diatomic molecules. By analyzing the HHG and ATI spectra for different orientations of molecular axis with respect to the direction of the laser polarization vector we have observed interference structures. The positions of the interference minima depend on the internuclear distance and on the relative contributions of the atomic orbitals in the linear combination of the atomic orbitals of the highest occupied molecular orbitals. The HHG and ATI spectra have been obtained for infrared laser wavelengths and linearly polarized laser field. The calculated results may also provide additional information about the molecular structure .

Molecular Lifting, Twisting, and Curling during Metal-Assisted Polycyclic Hydrocarbon Dehydrogenation

The atomistic understanding of the dissociation mechanisms for large molecules adsorbed on surfaces is still a challenge in heterogeneous catalysis. This is especially true for polycyclic aromatic hydrocarbons, which represent an important class of organic compounds used to produce novel graphene-based architectures. Here, we show that coronene molecules adsorbed on Ir(111) undergo major conformational changes during dissociation. They first tilt upward with respect to the surface, still keeping their planar configuration, and subsequently experience a rotation, which changes the molecular axis orientation. Upon lifting, the internal C-C strain is initially relieved; as the dehydrogenation proceeds, the molecules experience a progressive increase in the average interatomic distance and gradually settle to form dome-shaped nanographene flakes. Our results provide important insight into the complex mechanism of molecular breakup, which could have implications in the synthesis of new carbon-based nanostructured materials.