Spatial Taming and Trapping of Molecules

Molecular beam techniques for study of collisional and spectroscopic processes have recently been enhanced by use of static electric or magnetic fields to orient or align molecules with permanent dipole moments. A more general method is now in prospect, applicable both to alignment and to spatial trapping of molecules. This exploits the anisotropic interaction of the electric field vector of intense laser radiation with the dipole moment induced in a polarizable molecule by the laser field. The interaction creates directional superpositions of field-free states that correspond to oblate spheroidal wavefunctions, with eigenenergies that decrease with increasing field strength. We suggest that this polarizability interaction produces the marked alignment found in laser-induced dissociative ionization of CO by the Saclay group. We also present calculations illustrating the feasibility of spattal trapping. In combination with supermirror focussing and buffer-gas cooling, an intense infrared laser can typically confine molecules for long-times (-hours) within a small (-picoliter) and cold (?1°K) “pocket of light.”     

A mesoscale modelling study of nematic liquid crystals confined to ellipsoidal domains

Director configurations of nematic liquid crystalline molecules packed in ellipsoidal domains have been investigated using mesoscale modelling techniques. Interactions between the directors were described by the Lebwohl-Lasher potential. Four different ellipsoidal shapes (sphere, oblate spheroid, prolate spheroid, and ellipsoid) were studied under homogeneous and homeotropic surface anchoring conditions. The model has been characterized by computing thermodynamic and structural properties as a function of ellipsoidal shape (prolate and oblate) and size. The predicted director configuration in ellipsoids resulting from homeotropic surface anchoring is found to be very different from that in spherical domains. The bipolar configuration involving homogeneous surface anchoring is nearly identical in the four cases. The effect of an external electric field, applied at different orientations with respect to the major axis of the ellipsoid, has been probed as a function of the magnitude of the field and the ellipsoidal size and shape. The orientation of directors is most easily accomplished parallel and perpendicular to the major axis for the oblate and prolate spheroids, respectively, for homeotropic anchoring, and along the bipolar symmetry axis for homogeneous anchoring. In domains with homeotropic surface anchoring, the oblate spheroid and elongated ellipsoid are predicted to be the most efficient geometries for PDLC applications; for homogeneous anchoring conditions, the prolate spheroid and elongated ellipsoid are predicted to be the most efficient.     

A mesoscale modelling study of nematic liquid crystals confined to ellipsoidal domains

Director configurations of nematic liquid crystalline molecules packed in ellipsoidal domains have been investigated using mesoscale modelling techniques. Interactions between the directors were described by the Lebwohl-Lasher potential. Four different ellipsoidal shapes (sphere, oblate spheroid, prolate spheroid, and ellipsoid) were studied under homogeneous and homeotropic surface anchoring conditions. The model has been characterized by computing thermodynamic and structural properties as a function of ellipsoidal shape (prolate and oblate) and size. The predicted director configuration in ellipsoids resulting from homeotropic surface anchoring is found to be very different from that in spherical domains. The bipolar configuration involving homogeneous surface anchoring is nearly identical in the four cases. The effect of an external electric field, applied at different orientations with respect to the major axis of the ellipsoid, has been probed as a function of the magnitude of the field and the ellipsoidal size and shape. The orientation of directors is most easily accomplished parallel and perpendicular to the major axis for the oblate and prolate spheroids, respectively, for homeotropic anchoring, and along the bipolar symmetry axis for homogeneous anchoring. In domains with homeotropic surface anchoring, the oblate spheroid and elongated ellipsoid are predicted to be the most efficient geometries for PDLC applications; for homogeneous anchoring conditions, the prolate spheroid and elongated ellipsoid are predicted to be the most efficient.     

Statistical evaporation of rotating clusters. IV. Alignment effects in the dissociation of nonspherical clusters

Unimolecular evaporation in rotating, nonspherical atomic clusters is investigated using phase space theory in its orbiting transition state version. The distributions of the total kinetic energy release epsilon(tr) and the rotational angular momentum J(r) are calculated for oblate top and prolate top main products with an arbitrary degree of deformation. The orientation of the angular momentum of the product cluster with respect to the cluster symmetry axis has also been obtained. This statistical approach is tested in the case of the small eight-atom Lennard-Jones cluster, for which comparison with extensive molecular dynamics simulations is presented. The role of the cluster shape has been systematically studied for larger, model clusters in the harmonic approximation for the vibrational densities of states. We find that the type of deformation (prolate versus oblate) plays little role on the distributions and averages of epsilon(tr) and J(r) except at low initial angular momentum. However, alignment effects between the product angular momentum and the symmetry axis are found to be significant, and maximum at some degree of oblateness. The effects of deformation on the rotational cooling and heating effects are also illustrated.     

Time-dependent alignment of molecules trapped in octahedral crystal fields

The hindered rotational states of molecules confined in crystal fields of octahedral symmetry, and their time-dependent alignment obtained by pulsed nonresonant laser fields, are studied computationally. The control over the molecular axis direction is discussed based on the evolution of the rotational wave packet generated in the cubic crystal-field potential. The alignment degree obtained in a cooperative case, where the alignment field is applied in a favorable crystal-field direction, or in a competitive direction, where the crystal field has a saddle point, is presented. The investigation is divided into two time regimes where the pulse duration is either ultrashort, leading to nonadiabatic dynamics, or long with respect to period of molecular libration, which leads to synchronous alignment due to nearly adiabatic following. The results are contrasted to existing gas phase studies. In particular, the irregularity of the crystal-field energies leads to persistent interference patterns in the alignment signals. The use of nonadiabatic alignment for interrogation of crystal-field energetics and the use of adiabatic alignment for directional control of molecular dynamics in solids are proposed as practical applications.     

On the flow-phase diagram for discotic liquid crystals in uniaxial extension and compression

A mesoscopic, extended Doi theory for flows of nematic liquid crystals (LCs) has been successfully applied by Rey to study extensional flow-induced, homogeneous phase transitions both for rod-like and disc-like molecular geometry. Rey analysed the two order parameters (eigenvalues) of the orientation tensor. Recently the authors generalized the flow-phase diagram (nematic concentration vs. flow rate) for rod-like nematics by analysing all tensor degrees of freedom, i.e. by coupling the three director (eigenvector) degrees of freedom. Here we record and discuss subtleties of the corresponding diagram for discotic LCs in uniaxial extension and uniaxial compression. We focus here on the induced stable orientation configurations. Uniaxial extension (an idealization of fibre flow) yields a low concentration region of unique oblate uniaxial states at every flow rate; a very small finite region of bi-stable oblate and biaxial states; and the predominant region, encompassing all concentrations above the pure I-N transition and all flow rates, where the only stable steady state is a biaxial pattern. Furthermore, whereas uniaxial states are 'unique', all biaxial states occur in a continuous family, corresponding to an arbitrary positioning of the director pair in the plane transverse to the flow axis of symmetry. Uniaxial compression (an idealization of film stretching flow) of discotic LCs exclusively yields stable prolate uniaxial patterns.     

On the flow-phase diagram for discotic liquid crystals in uniaxial extension and compression

A mesoscopic, extended Doi theory for flows of nematic liquid crystals (LCs) has been successfully applied by Rey to study extensional flow-induced, homogeneous phase transitions both for rod-like and disc-like molecular geometry. Rey analysed the two order parameters (eigenvalues) of the orientation tensor. Recently the authors generalized the flow-phase diagram (nematic concentration vs. flow rate) for rod-like nematics by analysing all tensor degrees of freedom, i.e. by coupling the three director (eigenvector) degrees of freedom. Here we record and discuss subtleties of the corresponding diagram for discotic LCs in uniaxial extension and uniaxial compression. We focus here on the induced stable orientation configurations. Uniaxial extension (an idealization of fibre flow) yields a low concentration region of unique oblate uniaxial states at every flow rate; a very small finite region of bi-stable oblate and biaxial states; and the predominant region, encompassing all concentrations above the pure I-N transition and all flow rates, where the only stable steady state is a biaxial pattern. Furthermore, whereas uniaxial states are 'unique', all biaxial states occur in a continuous family, corresponding to an arbitrary positioning of the director pair in the plane transverse to the flow axis of symmetry. Uniaxial compression (an idealization of film stretching flow) of discotic LCs exclusively yields stable prolate uniaxial patterns.     

Diabatic potential energy surfaces of H+ + CO

Ab initio adiabatic and diabatic surfaces of the ground and the first excited electronic states have been computed for the H⁺ + CO system for the collinear (γ = 0°) and the perpendicular (γ = 90°) geometries employing the multi-reference configuration interaction method and Dunning’s cc-pVTZ basis set. Other properties such as mixing angle before coupling potential and before coupling matrix elements have also been obtained in order to provide an understanding of the coupling dynamics of inelastic and charge transfer process.     

Deflection of rotating symmetric top molecules by inhomogeneous fields

We consider deflection of rotating symmetric top molecules by inhomogeneous optical and static electric fields, compare results with the case of linear molecules, and find new singularities in the distribution of the scattering angle. Scattering of the prolate/oblate molecules is analyzed in detail, and it is shown that the process can be efficiently controlled by means of short and strong femtosecond laser pulses. In particular, the angular dispersion of the deflected molecules may be dramatically reduced by laser-induced molecular prealignment. We first study the problem by using a simple classical model, and then find similar results by means of more sophisticated methods, including the formalism of adiabatic invariants and direct numerical simulation of the Euler-Lagrange equations of motion. The suggested control scheme opens new ways for many applications involving molecular focusing, guiding, and trapping by optical and static fields.     

MECHANISM OF ORIENTATION AND LINEAR DICHROISM OF PHOTOSYNTHETIC PARTICLES IN ELECTRIC FIELDS: CHLOROPLASTS AND CHLOROPHYLL-PROTEIN COMPLEXES

Abstract— The mechanisms of orientation in pulsed and alternating electric fields of thylakoids (derived from the sonication of spinach chloroplasts) and of light-harvesting chlorophyll a/b-protein complexes (CPII) were investigated by utilizing linear dichroism techniques. Comparisons of the linear dichroism spectra of thylakoids and CPII particles suggest that the latter are oriented with their directions of largest electronic polarizabilities (and thus probably their largest dimensions) within the thylakoid membrane planes. At low electric field strengths (< 12 V cm^?1), and at low frequencies of alternating electric fields (< 0.25 Hz), thylakoid membranes tend to align with their normals parallel to the direction of the applied electric field; the mechanism of orientation involves a permanent dipole moment of the thylakoids which is oriented perpendicular to the planes of the membranes. However, at high field strengths and high frequencies of the applied alternating electric fields, the thylakoids tend to orient with their planes parallel to the applied field, thus exhibiting an inversion of the sign of the linear dichroism as the electric field strength is increased. At the higher frequencies and at higher field strengths, the orientation mechanisms of the thylakoids involve induced dipole moments related to anisotropies in the electronic polarizabilities. The polarizability is higher within the plane than along a normal to the plane, thus accounting for the inversion of the dichroism as the electric field strength is increased. The CPII particles align with their largest dimension parallel to the applied field at all field strength, indicating that the induced dipole moment dominates the orientation mechanisms in pulsed electric fields. The magnitude of the absolute linear dichroism of CPII suspensions increases with increasing dilution, indicating that aggregates of lower symmetry are formed at higher concentrations of the CPII complexes.     

Electric field transport of biphilic ions and anchoring transitions in nematic liquid crystals

Anchoring interaction of MBBA and MBBA + 5CB nematic layers with monomolecular films of CTAB on ITO glass supports is studied by videomicroscopy in the presence of an electric field. Planar-planar or homeotropichomeotropic electrode substrates are used to make the nematic cells. The substrate symmetry is broken by coating only one electrode with self-assembled CTAB film. In a DC electric field we observe some new effects; (i) polarity-dependent breaking of anchoring and switching to two oblique states in dielectrically stable planar cells; (ii) a polarity-dependent flow-induced metastable anchoring transition in homeotropic cells to a planar or tilted alignment after the field is switched off. These results are discussed in terms of a surface transition assisted by electric transport of biphilic CTAB ions and by a surface memorization of the flow-induced planar alignment.     

Orientation of dipole molecules and clusters upon adiabatic entry into an external field

The induced polarization of a beam of polar clusters or molecules passing through an electric or magnetic field region differs from the textbook Langevin-Debye susceptibility. This distinction, which is important for the interpretation of deflection and focusing experiments, arises because instead of acquiring thermal equilibrium in the field region, the beam ensemble typically enters the field adiabatically, i.e., with a previously fixed distribution of rotational states. We discuss the orientation of rigid symmetric top systems with a body-fixed electric or magnetic dipole moment. The analytical expression for their "adiabatic-entry" orientation is elucidated and compared with exact numerical results for a range of parameters. The differences between the polarization of thermodynamic and "adiabatic-entry" ensembles of prolate and oblate tops, and of symmetric top and linear rotators, are illustrated and identified.     

The effect of a nonresonant radiative field on low-energy rotationally inelastic collisions

We examine the effects of a linearly polarized nonresonant radiative field on the dynamics of rotationally inelastic Na⁺+N₂ collisions at eV collision energies. Our treatment is based on the Fraunhofer model of matter wave scattering and its recent extension to collisions in electric fields [M. Lemeshko, B. Friedrich, J. Chem. Phys. 129 (2008) 024301]. The nonresonant radiative field changes the effective shape of the target molecule by aligning it in the space-fixed frame. This markedly alters the differential and integral scattering cross-sections. As the cross-sections can be evaluated for a polarization of the radiative field collinear or perpendicular to the relative velocity vector, the model also offers predictions about steric asymmetry of the collisions.     

Electric birefringence of discotic benzene-hexa-n-heptanoate in isotropic solution phase

Abstract

The electric birefringence of solutions of oblate discogen and prolate nematogen in dioxane has been examined. Discotic benzene-hexa-n-heptanoate (BH7) exhibits negative birefringence, while nematic 5CB and MBBA exhibit positive birefringences at low concentrations. Possible modes of solute orientation, compatible with the observed birefringence sign, are discussed. For BH7, it is suggested that the in-plane axes with larger optical polarizabilities align perpendicular to the field direction. The dependence of the specific Kerr constant on the solution concentration is also reported.     

Static magnetization of V15 cluster at ultra-low temperatures: precise estimation of antisymmetric exchange

In this article, the low-temperature static (adiabatic) magnetization data of the nanoscopic V15 cluster present in K6[VIV15As6O42(H2O)] x 8H2O is analyzed. The cluster anion, which attracted much attention in the past, contains a triangular VIV3 array causing frustration as a function of applied field and temperature. In the analysis, a three-spin (S = 1/2) model of V15 was employed that includes isotropic antiferromagnetic exchange interaction and antisymmetric (AS) exchange in the most general form compatible with the trigonal symmetry of the system. It was shown that, along with the absolute value of AS exchange, the orientation of the AS vector plays a significant physical role in spin-frustrated systems. In this context, the role of the different components of the AS in the low-temperature magnetic behavior of V15 was analyzed, and we were able to reach a perfect fit to the experimental data on the staircaselike dependence of magnetization versus field in the whole temperature range including extremely low temperature. Furthermore, it was possible for the first time to precisely estimate the two components of the AS vector coupling constant in a triangular unit, namely, the effective in-plane component, Dperpendicular, and the perpendicular part, Dn.     

Bimodal form distribution from modelling biaxial lyotropic liquid crystal solutions through a polydisperse Maier–Saupe model

Biaxial nematic phases have been the subject of a long list of studies. In particular, they were found for a few lyotropic micellar solutions. There is a debate in the literature on whether the micellar aggregates should be biaxial, or if biaxiality could be the result of perpendicular alignment of uniaxial particles of cylinder-like and disc-like geometry in a mixture. Based on recent studies on the phase stability of such mixtures, we have investigated a polydisperse distribution of uniaxial particles interacting through a Maier–Saupe potential. Our calculations were developed for a general distribution of micellar anisometries. The distribution was obtained from the fitting of our results to the experimental data of Yu and Saupe’s well-known 1980 paper, near the Landau point, yielding a bimodal distribution, with the presence of two quadrupoles referred to objects of opposite symmetry, that is to prolate and oblate micelles. This result lends support to the rationalization of the biaxial phase for lyotropic systems in terms of a polydisperse mixture of rod-like and disc-like micelles.     

Simulated SERS spectra of 4,4′-bipyridine/gold single molecule junction in different conductance states: ab initio molecular dynamics approach

The SERS spectra of 4,4′-bipyridine/gold single molecule junction in different conductance states at room temperature are calculated by performing ab initio molecular dynamics simulations, in connection with a Fourier transform of the polarizability autocorrelation function, to illustrate that the Raman peaks of the ON state are enhanced by an additional one or two orders of magnitude in comparison with the OFF state. Considering the relative intensities of SERS spectra, the largest enhanced peak for the OFF state is the ring breathing mode due to the simultaneous contribution from structural change, vibrational coupling, and charge transfer. For the ON state, the C–H bending mode has the largest enhancement due to structural change and charge transfer effects. Finally, the SERS spectra of 4,4′-bipyridine junction in the the ON state with perpendicular and tilted orientations are examined to understand the reason why the tilted junction has lower SERS spectrum intensity with doublet feature of the C–C stretching mode.     

Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3

Spectra of ethylene dimers and trimers are studied in the ν(11) fundamental band region of C(2)D(4) (≈2200 cm(-1)) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D(2d) symmetry. An ethylene trimer has not previously been observed with rotational resolution. The spectrum is that of an oblate symmetric top parallel band. It leads to a proposed trimer structure which is barrel shaped and has C(3h) or C(3) symmetry, with the ethylene monomer C-C axes approximately aligned along the trimer symmetry axis.