Effect Of Molecular Alignment Research Articles

Effect of CsPbBr3 perovskite quantum dots on molecular alignment, dielectric and memory effect of a cyanobiphenyl based liquid crystal

We demonstrate here the impact of doping of CsPbBr3 perovskite quantum dots (CsPbBr3 PQDs, diameter ∼ 7–9 nm) on the molecular alignment, dielectric and memory effect of nematic (N) and smectic A (SmA) phases of a thermotropic liquid crystal material, namely 4-octyl-4′-cyanobiphenyl (8CB) using polarising optical microscope and dielectric spectroscopy. There is no significant change in the alignment of N and SmA phases of 8CB by the doping of CsPbBr3 PQDs, except the color change attributed to the tuned birefringence of 8CB by PQDs. The slight aggregation of PQDs is observed in both LC phases of 1.0 wt% CsPbBr3 PQDs-8CB composite and which could be due to the higher dopant concentration in 8CB matrix. The dielectric parameters (permittivity, loss and anisotropy) of N and SmA phases of 8CB are found to be strongly dependent on the dopant concentration. The dielectric anisotropy (Δε′) of 8CB is observed to decrease with increase in the dopant concentration except 0.5 wt% composite. The temperature dependent Δε′ further confirms that there is no significant change in the clearing temperature of composites as compared to pure 8CB. The short axis molecular relaxation observed in both N and SmA phases of pure 8CB and its composites on the application of 40 V bias. The slight spectral shift of this relaxation towards lower frequency side and decrease in the amplitude of ɛ″ in both SmA and N phases of 1.0 wt% composite as compared to the pure 8CB and composites with concentration lower than 1.0 wt%. The partial memory effect is observed in SmA phase of pure 8CB through voltage-dependent dielectric permittivity and cross-polarised optical textures. The observed memory effect in SmA phase is found to improve with increase in the concentration of PQDs. Most importantly, the complete memory effect is observed in SmA phase of 1.0 wt% CsPbBr3 PQDs-8CB composite. This could be attributed to the holding of smectic layers reorientation achieved at 40 V bias for longer duration after the removal of 40 V bias voltage. A point to be noted here is that no memory effect is observed in the N phase of pure 8CB and its composites with CsPbBr3 PQDs. Our results on CsPbBr3 PQDs-8CB composites are clearly indicative towards their potential applications in next generation tunable electro-optical and long-lasting soft memory devices. Moreover, results on these composites would also serve as the testbeds for theoretical and computational studies to explore the mechanism responsible for tunable optical, dielectric and electro-optical properties of 8CB by the dopant CsPbBr3 PQDs.

Elliptically polarized high-order harmonic generation from N[formula omitted]-CO mixed gases

We theoretically study the elliptically polarized high-order harmonic generation from N2-CO mixed gases. Our simulations show that the polarization of HHG is highly sensitive to both the mixing ratio and molecular alignment angle. In particular, even a small addition of CO gas results in a significant enhancement of harmonic ellipticity compared to pure N2 gas. Under a proper mixing ratio and molecular alignment angle, a large ellipticity of HHG is observed, which is larger than that in reported N2-Ar mixture. Furthermore, when considering the molecular alignment effect, the dependence of ellipticity of HHG on mixing ratio exhibits slight variations. These findings pave the way for generating elliptically polarized extreme ultraviolet pulses.

Effect of Sample Geometry on Graphitization of Polyacrylonitrile.

In this study, it is analyzed how sample geometry (spheres, nanofibers, or films) influences the graphitization behavior of polyacrylonitrile (PAN) molecules. The chemical bonding and changes in the composition of these three geometries are studied at the oxidation, carbonization, and graphitization stages via scanning electron microscopy (SEM), in situ thermogravimetric-infrared (TGA-IR) analysis, elemental analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The influence of molecular alignment on the graphitization of the three sample geometries is investigated using synchrotron wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). The effects of molecular alignment at different draw rates during spinning are explored in detail.

Effect of molecular alignment and orientation on elliptically polarized high-order harmonic generation

Effect of molecular alignment and orientation on elliptically polarized high-order harmonic generation

Modulation of non-adiabatic alignment of N<sub>2</sub> molecule by femtosecond laser pulses

The alignment of molecules in the femtosecond laser field induces the refractive index of the propagation medium to change, resulting in the spectral modulation effect. In this paper, the time evolution of the wavelength offset of a probe laser pulse under the influence of the non-adiabatic molecular alignment and Kerr effect is measured experimentally by the pump-probe method in nitrogen medium, and the alignment degree of N<sub>2</sub> molecules is obtained. By solving the time-dependent Schrödinger equation theoretically, the expression of the degree of molecular non-adiabatic alignment is obtained, and the time evolution of non-adiabatic alignment of N<sub>2</sub> molecules is calculated. Taking into account the combined influence of the non-adiabatic molecular alignment and Kerr effect on the change of refractive index of the propagation medium, the modulation effect of birefringence on the spectrum of the probe pulse is achieved. The experimental result accords well with the theoretical calculation, demonstrating that the spectral modulation results obtained by experimental measurement can be used to characterize the alignment degree. Further, the double pulse pump method is used to control the degree of molecular alignment. It is found that the degree of molecular alignment can be enhanced by the double pulse pump method. Moreover, by adjusting the delay time between the two pump laser pulses, that is, adding the second pump laser pulse at one rotational period and half rotational period, respectively, the enhancement and loss of the alignment of N<sub>2</sub> molecules can be achieved, which is named the “alignment switch” effect. The molecular alignment control induced by the double pulse pump method can also be applied to the other molecular systems with different alignment and anti-alignment times, such as CO<sub>2</sub> molecules and O<sub>2</sub> molecules, indicating that the double pulse pump method can be used universally.

Effect of Taylor vortex wavelength on polymorphic crystallization of L-histidine

Axial wavelength is a key characteristic parameter of Taylor vortex flows. Thus, in the current study, the effect of Taylor vortex wavelength on the polymorphic nucleation and phase transformation of L-histidine was examined. The vortex wavelength was altered by altering the gap width between the outer and inner cylinders. Further, the vortex velocity was changed by controlling the inner cylinder rotation speed. Owing to the promotion of stable nucleation by the periodic fluid motion, the primary polymorphic fraction of the stable Form A of L-histidine in Taylor vortex flow [Couette–Taylor (CT) crystallizer] was much greater than that in a turbulent eddy flow [mixing tank (MT) crystallizer]. As the vortex wavelength decreased, stable nucleation was promoted, thus increasing the initial polymorphic fraction of the stable Form A. Moreover, the initial polymorphic fraction of the stable Form A increased as the vortex velocity increased. These experimental results are described based on molecular alignment effect of Taylor vortex on stable nucleation. The phase transformation of L-histidine was also strongly influenced by the vortex wavelength and vortex velocity. Hence, the rate of phase transformation in CT crystallizer was approximately ten-fold better than MT crystallizer.

Abnormal brittle-ductile transition for glassy polymers after free and constrained melt stretching: The role of molecular alignment

By comparing the brittle-ductile transition (BDT) behavior between uniaxial width-Free Melt pre-Stretched (FMS) and width-Constrained Melt pre-Stretched (CMS) poly(methyl methacrylate) (PMMA), the effect of molecular alignment on the mechanical response of glassy polymers has been investigated. Unexpectedly, during further mechanical characterization, CMS samples exhibit higher ductility than FMS samples along both the machine (stretching) direction (MD) and the transverse direction (TD). Combining the results from molecular orientation and stress relaxation, it is proposed that chain stretching and aligned chain density is responsible for the ductile deformation along the MD and TD, respectively. Along the MD, CMS samples exhibit more significant chain stretching than FMS samples due to width constraint, leading to a lower chain activation energy barrier in CMS samples. Upon further deformation, chain activation is then accelerated and shear yielding can be achieved at lower temperature. Along the TD, the orientation function remains constant for CMS samples but negatively increases for FMS samples, and the lower aligned chain density in FMS samples increases the activation energy barrier for plastic deformation. As a result, crazing prevails during further deformation for FMS samples and causes weaker ductility. The mode of BDT in glassy polymers is considered from multiple scales.

Effect of Flow-Induced Molecular Alignment on Welding and Strength of Polymer Interfaces

Structures formed by fused filament fabrication are often substantially weaker than those made with conventional techniques and fail at the welds between successive layers. One factor that may infl...

Extremely nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wave packet

We report generation of cascaded rotational Raman scattering up to 58th orders in coherently excited CO_2 molecules. The high-order Raman scattering, which produces a quasiperiodic frequency comb with more than 600 sidebands, is obtained using an intense femtosecond laser to impulsively excite rotational coherence and the femtosecond-laser-induced N_2^+ lasing to generate cascaded Raman signals. The novel configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific spectra-temporal structures of N_2^+ lasing, the ideal spatial overlap of femtosecond laser and N2+ lasing, and the guiding effect of molecular alignment. The Raman spectrum extending above 2000 cm^-1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wavepackets.

Surface diffusion in glasses of rod-like molecules posaconazole and itraconazole: effect of interfacial molecular alignment and bulk penetration.

The method of surface grating decay has been used to measure surface diffusion in the glasses of two rod-like molecules posaconazole (POS) and itraconazole (ITZ). Although structurally similar antifungal medicines, ITZ forms liquid-crystalline phases while POS does not. Surface diffusion in these systems is significantly slower than in the glasses of quasi-spherical molecules of similar volume when compared at the glass transition temperature Tg. Between the two systems, ITZ has slower surface diffusion. These results are explained on the basis of the near-vertical orientation of the rod-like molecules at the surface and their deep penetration into the bulk where mobility is low. For molecular glasses without extensive hydrogen bonds, we find that the surface diffusion coefficient at Tg decreases smoothly with the penetration depth of surface molecules and the trend has the double-exponential form for the surface mobility gradient observed in simulations. This supports the view that these molecular glasses have a similar mobility vs. depth profile and their different surface diffusion rates arise simply from the different depths at which molecules are anchored. Our results also provide support for a previously observed correlation between the rate of surface diffusion and the fragility of the bulk liquid.

Adaptive Self-Assembly in 2D Nanoconfined Spaces: Dealing with Geometric Frustration

The impact of lateral confinement on the 2D self-assembly of a normal alkane is investigated via scanning tunneling microscopy-based nanoshaving on covalently modified graphite surfaces. Physical constraints placed on the confined assemblies lead to geometric frustration, which is shown to be released via a suppression of the lamellar order. This result is explained on the basis of competing enthalpic contributions from molecule–molecule and molecule–substrate interactions and is corroborated by a simple coarse-grain model. Furthermore, a pronounced molecular alignment effect results from the in situ nanoshaving procedure.

Interplay between pulse parameters and alignment of in strong laser fields

We present an ab initio study of strong field ionization of aligned by general elliptically polarized laser pulses containing few optical cycles. In particular, we address the effects of laser pulse parameters (light ellipticity and carrier-envelope phase) and molecular alignment on photoelectron momentum distributions (PMDs) of .

Field-free molecular alignment control of filamentation

With an approach of controlling the nonlinearity of medium rather than the light field, the effect of field-free molecular alignment on filamentation and resulting white-light generation is studied. This is done by measuring the rotational wavepacket evolution of nitrogen molecules after passing of a femtosecond laser pump pulse by observing the nonlinear propagation dynamics of a variably delayed filament-producing probe pulse.

Effect of rotational-state-dependent molecular alignment on the optical dipole force

The properties of molecule-optical elements such as lenses or prisms based on the interaction of molecules with optical fields depend in a crucial way on the molecular quantum state and its alignment created by the optical field. However, in previous experimental studies, the effects of state-dependent alignment have never been included in estimates of the optical dipole force acting on the molecules while previous theoretical investigations took the state-dependent molecular alignment into account only implicitly. Herein, we consider the effects of molecular alignment explicitly and, to this end, introduce an effective polarizability which takes proper account of molecular alignment and is directly related to the alignment-dependent optical dipole force. We illustrate the significance of including molecular alignment in the optical dipole force by a trajectory study that compares previously used approximations with the present approach. The trajectory simulations were carried out for an ensemble of linear molecules subject to either propagating or standing-wave optical fields for a range of temperatures and laser intensities. The results demonstrate that the alignment-dependent effective polarizability can serve to provide correct estimates of the optical dipole force, on which a state-selection method applicable to nonpolar molecules could be based. We note that an analogous analysis of the forces acting on polar molecules subject to an inhomogeneous static electric field reveals a similarly strong dependence on molecular orientation.

Spectral modulation of third-harmonic generation by molecular alignment and preformed plasma**Project supported by the National Key Scientific Instrument Project, China (Grant No. 2012YQ150092), the National Basic Research Program of China (Grant No. 2011CB808105), the National Natural Science Foundation of China (Grant No. 11434005), China Postdoctoral Science Foundation (Grant No. 2014M560348), the National Natural Science Foundation of China (Grant No. 11504237),

We demonstrate spectral modulation of third-harmonic generation from molecular alignment effects. The third harmonic spectrum is broadened or narrowed under different influences of cross-phase modulations originating from various molecular alignment revivals. Furthermore, the spectrum and spatial distribution of the generated third harmonic pulse change dramatically in the presence of a preformed plasma. Under the influence of a preformed plasma, a narrower third harmonic spectrum is observed, and the conical third-harmonic pulse increases while the axial part decreases. The investigation provides an effective method to modulate the spectral characteristic and spatial distribution of third-harmonic generation from intense femtosecond filament.

Rotational and vibrational dynamics in the excited electronic state of deprotonated and protonated fluorescein studied by time-resolved photofragmentation in an ion trap.

Excited state dynamics of deprotonated and protonated fluorescein were investigated by polarization dependent femtosecond time-resolved pump-probe photofragmentation in a 3D ion trap. Transients of deprotonated fluorescein exhibit vibrational wavepacket dynamics with weak polarization dependence. Transients of protonated fluorescein show only effects of molecular alignment and rotational dephasing. The time resolved rotational anisotropy of protonated fluorescein is simulated by the calculated orientational correlation function. The observed differences between deprotonated and protonated fluorescein are ascribed to their different higher lying electronically excited states and corresponding structures. This is partially supported by time-dependent density functional theory calculations of the excited state structures.

Molecular alignment effect on the photoassociation process via a pump-dump scheme.

The photoassociation processes via the pump-dump scheme for the heternuclear (Na + H → NaH) and the homonuclear (Na + Na → Na2) molecular systems are studied, respectively, using the time-dependent quantum wavepacket method. For both systems, the initial atom pair in the continuum of the ground electronic state (X(1)Σ(+)) is associated into the molecule in the bound states of the excited state (A(1)Σ(+)) by the pump pulse. Then driven by a time-delayed dumping pulse, the prepared excited-state molecule can be transferred to the bound states of the ground electronic state. It is found that the pump process can induce a superposition of the rovibrational levels |v, j〉 on the excited state, which can lead to the field-free alignment of the excited-state molecule. The molecular alignment can affect the dumping process by varying the effective coupling intensity between the two electronic states or by varying the population transfer pathways. As a result, the final population transferred to the bound states of the ground electronic state varies periodically with the delay time of the dumping pulse.

Thermomagnetic processing of liquid-crystalline epoxy resins and their mechanical characterization using nanoindentation.

A thermomagnetic processing method was used to produce a biphenyl-based liquid-crystalline epoxy resin (LCER) with oriented liquid-crystalline (LC) domains. The orientation of the LCER was confirmed and quantified using two-dimensional X-ray diffraction. The effect of molecular alignment on the mechanical and thermomechanical properties of the LCER was investigated using nanoindentation and thermomechanical analysis, respectively. The effect of the orientation on the fracture behavior was also examined. The results showed that macroscopic orientation of the LC domains was achieved, resulting in an epoxy network with an anisotropic modulus, hardness, creep behavior, and thermal expansion.

Rotations of molecular photoelectron angular distributions in above threshold ionization of H2+ by intense circularly polarized attosecond UV laser pulses

We present molecular photoelectron angular distributions (MPADs) in multi-photon ionization processes by circularly polarized attosecond UV laser pulses. Simulations are performed on the single electron aligned molecular ion H by solving corresponding 3D time-dependent Schrödinger equations. Numerical results of molecular above threshold ionization (MATI) show that rotations of MPADs with respect to the molecular and polarization axes depend on pulse intensities and photoelectron kinetic energies. We attribute the rotation to Γ, the difference between parallel and perpendicular ionization probabilities. It is found that in a resonant ionization process, the rotation angle is also a function of the symmetry of intermediate electronic states. The coherent population transfer between the initial and the resonant electronic states is controlled by pulse intensities. Such dependence of rotations on the pulse intensity is absent in Rydberg resonant ionizations as well as in MATI at large energy photons ℏω > Ip, where ω is angular frequency of photons and Ip is the molecular ionization potential. We describe these processes by a multi-photon perturbation theory model. Effects of molecular alignment and pulse ellipticities on rotations are investigated, confirming the essence of the ionization parameter Γ in rotations of MPADs.

Above-threshold ionization of diatomic molecules in an intense laser field: Molecular alignment effects

In this work, we study the photoelectron spectra of the diatomic molecule O2 in the direction perpendicular to the laser polarization. Compared with atoms, molecules have additional parameters, e. g., the orientation of the molecule and the symmetry of the initial electronic state, and thus give rise to richer strong-field phenomena. Our simulations show that for parallel and perpendicular alignments of the laser polarization with respect to the molecular axis, the photoelectron spectra display above-threshold ionization (ATI) peaks with a specific 2 (h) over bar omega. separation in which the absorption of an even number of photons is absent. However, for other alignment angles, the even-order ATI peaks appear. According to our analysis, the physical mechanism of the alignment effect of the even-order ATI peaks can be attributed to the symmetry of the molecular ground-state wave function with respect to the photoelectron emission direction.