Photoelectron spectroscopy of above-threshold ionization of xenon with circularly and linearly polarized light

Above-threshold multiphoton ionization (photon energy 1.17 eV and 2.33 eV) of xenon was studied with circularly and linearly polarized light. Pronounced differences in shape have been observed for photoelectron spectra taken with linearly and circularly polarized light. With circularly polarized light a strong suppression of low-energy electrons was observed; the total electron yield was reduced by factors of approximately 4 (photon energy 2.33 eV) and, depending on the laser intensity, between 13 and 80 (photon energy 1.17 eV).     

Resonance-enhanced multiphoton ionization with circularly polarized light: chiral carbonyls

An introduction to the principle and possibilities of the new method of circular dichroism laser mass spectrometry is given and its state of development is reviewed. This method allows enantiosensitive, mass-selective probing of chiral molecules. It is based on the combination of resonance-enhanced multiphoton ionization with circularly polarized light and specially modified time-of-flight mass spectrometry. As an example, application to carbonyls is presented.

Absolute asymmetric synthesis driven by circularly polarized light

Circularly polarized light (CPL) is an inherently chiral entity and is regarded as one of the possible deterministic signals that led to the evolution of homochirality in earth. Thus, CPL as an external physical field has been widely used in a technique known as absolute asymmetric synthesis, because a product enriched in one enantiomer is formed from racemic precursor molecules without the intervention of a chiral catalyst. In this review, we retrospect the historical research of CPL-induced absolute asymmetric synthesis, including chiral organic molecules, helical polymers, supramolecular assemblies, noble metal nanostructures. However, based on these results, we concluded that the chiral photon-matter interaction is very faint due to the arrangement of molecular bonds giving rise to chiral features, is over a smaller distance than the helical pitch of CPL, leading extremely small enantiomeric excess for product. Therefore, we highlight the recently emerged technology called superchiral field, in which the superchiral far-field and near-field could enhance the dissymmetry of optical field and near-field, respectively. In sum, we hope this review could bring some enlightenment to researchers and further improve the enantioselectivity of CPL-induced absolute asymmetric synthesis.     

Laser mass spectrometry with circularly polarized light: two-photon circular dichroism

Extensive studies of two-photon circular dichroism (TPCD) on 3-methylcyclopentanone are presented following the first TPCD experiment of gas phase molecules by Compton et al. [J. Chem. Phys.125 (2006) 144304]. (2 + 1)-Multiphoton ionization in a specially designed time-of-flight mass analyzer has been used to perform these studies. CD of two-photon transitions from the molecular ground state to low lying Rydberg states is strongly enhanced with respect to corresponding one-photon transitions in good agreement with Compton. Differences between CD values determined via the molecular ion and via fragment ions indicate strong molecular ion CD effects. This would be the first time that circular dichroism of isolated molecular ions has been measured.     

Small-angle scattering of circularly polarized light from an anisotropic sphere

A theory of the small-angle scattering of circularly polarized light from an anisotropic sphere has been derived. The validity of the theory has been verified, and a relationship between the structural information thus obtained and the structural information obtained with linearly polarized light has been demonstrated.     

Quasienergy formalism for intense field multiphoton ionization of atoms induced by circularly polarized radiation

A practical non-perturbative approach is presented for multiphoton ionization of atoms induced by circularly polarized radiation. By use of co-ordinate rotation transformation and L²-discretization of the atomic continuum, the complex energy spectrum of a stationary quesienergy operator can be located and multiphoton ionization rates determined as a function of time and arbitrary photon intensity. The theory is applied to the two-photon ionization of the H atom in intense fields.     

Induction of molecular chirality by circularly polarized light in cyclic azobenzene with a photoswitchable benzene rotor

New phototriggered molecular machines based on cyclic azobenzene were synthesized in which a 2,5‐dimethoxy, 2,5‐dimethyl, 2,5‐difluorine or unsubstituted‐1,4‐dioxybenzene rotating unit and a photoisomerizable 3,3′‐dioxyazobenzene moiety are bridged together by fixed bismethylene spacers. Depending upon substitution on the benzene moiety and on the E/Z conformation of the azobenzene unit, these molecules suffer various degrees of restriction on the free rotation of the benzene rotor. The rotation of the substituted benzene rotor within the cyclic azobenzene cavity imparts planar chirality to the molecules. Cyclic azobenzene 1 , with methoxy groups at both the 2‐ and 5‐positions of the benzene rotor, was so conformationally restricted that free rotation of the rotor was prevented in both the E and Z isomers and the respective planar chiral enantiomers were resolved. In contrast, compound 2 , with 2,5‐dimethylbenzene as the rotor, demonstrated the property of a light‐controlled molecular brake, whereby rotation of the 2,5‐dimethylbenzene moiety is completely stopped in the E isomer (brake ON, rotation OFF), while the rotation is allowed in the Z isomer (brake OFF, rotation ON). The cyclic azobenzene 3 , with fluorine substitution on the benzene rotor, was in the brake OFF state regardless of E/Z photoisomerization of the azobenzene moiety. More interestingly, for the first time, we demonstrated the induction of molecular chirality in a simple monocyclic azobenzene by circular‐polarized light. The key characteristics of cyclic azobenzene 2 , that is, stability of the chiral structure in the E isomer, fast racemization in the Z isomer, and the circular dichroism of enantiomers of both E and Z isomers, resulted in a simple reversible enantio‐differentiating photoisomerization directly between the E enantiomers. Upon exposure to r‐ or l‐circularly polarized light at 488 nm, partial enrichment of the (S)‐ or (R)‐enantiomers of 2 was observed.     

Theory of multi-photon ionization and autoionization of SI

We present a theory and calculations of 3-photon ionization and autoionization of the silicon atom. We have employed a configuration interaction Hartree-Fock-Dirac method (CIHFD) in order to calculate energies and wavefunctions. The continuum wavefunctions have been obtained numerically and matched to Coulomb functions in the asymptotic region. Our results include energies of bound and autoionizing levels, dipole transition matrix elements, ionization and autoionization transition rates, obtained on the basis of perturbation theory.     

Photo-induced alignment of a photo-crosslinkable liquid-crystalline copolymer film by linearly polarized UV light

The anisotropic photoreaction of a liquid-crystalline (LC( copolymer comprising one co-monomer being anisotropically photo-crosslinkable by linearly polarized (LP( -UV light and the other being insensitive to UV-irradiation is presented. Polarized UV and IR studies suggested a photo-induced alignment of the mesogenic side groups during the LP-UV photoreaction at the LC temperature range of the copolymer. The direction of the aligned mesogenic groups is parallel to the electric vector of the incident LP-UV light and the induced birefringence is 0.07.     

Cholesteric liquid crystalline siloxanes with azo dye. Generation of additional reflection bands with linearly polarized light

Photosensitive cholesteric polysiloxanes, which contain an azo dye, were irradiated with linearly polarized light. The cholesteric samples were oriented in the Grandjean texture. Before irradiation they reflected circularly polarized light in the near infrared region. For perpendicular incidence, only one order of reflection was observed. Upon irradiation with linearly polarized light, which is absorbed by the azo dye, additional reflection bands appeared in the visible part of the spectrum. It turned out that the additional reflection is caused by a new Bragg type grating which shows higher reflection orders. The formation of the grating is based on the periodic deformation of the helical ordering of the molecules. The deformation is periodic, as due to photoselection, only dye molecules in equidistant layers with a suitable orientation absorb radiation. For low exposure, the grating reflects linearly polarized light. After continued irradiation, the reflection bands disappear almost completely. High birefringence, strongly dichroic dye absorption and the loss of the reflecting properties prove that a planar nematic texture has developed. The formation of this texture from the Grandjean texture is a new example for photoinduced rotational diffusion.     

Enantioselective synthesis of near enantiopure compound by asymmetric autocatalysis triggered by asymmetric photolysis with circularly polarized light

Right- and left-handed circularly polarized light (CPL) has been proposed as one of the origins of homochirality of biomolecules. However, the enantiomeric excess induced by CPL has been only very low (<2% ee). We found the unprecedented example of asymmetric autocatalysis triggered directly by a chiral physical factor, that is, right- and left-handed CPL, leading to a near enantiopure compound. Asymmetric photolysis of racemic pyrimidyl alkanol by r-CPL irradiation followed by asymmetric autocatalysis affords (R)-pyrimidyl alkanol with >99.5% ee. On the other hand, irradiation with l-CPL affords (S)-pyrimidyl alkanol with >99.5% ee. Thus, chiral physical power, such as CPL, in conjunction with asymmetric autocatalysis, provides a highly enantioenriched compound.     

Optical anisotropy of a photoreacted side-chain liquid-crystalline polymer induced by linearly polarized UV light

Linearly polarized (LP) UV photoreaction of a photo-crosslinkable side-chain liquid-crystalline polymer (SLCP) containing photoreactive cinnamoyl and biphenyl mesogenic groups ( 1 ) was studied. The optical anisotropy of the polymer film was induced by the LP-UV photoreaction and was investigated by the temperature-controlled polarized UV absorption spectroscopy and polarized FT-IR measurements. The reorientation of the nonreacted mesogenic groups along to the Ê direction of the incident LP-UV light during the photoreaction occurred at the LC temperature range of the polymer, and the induced birefringence Δn was about 0.02. Because of the high-density photo-crosslinking, the LP-UV photoreacted film showed orientational stability up to 160°C. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1521–1526, 1998     

Competitive ionization processes of anthracene excited with a femtosecond pulse in the multi-photon ionization regime

To clarify the ionization mechanism of large molecules under multi-photon ionization conditions, photo-electron spectroscopic studies on anthracene have been performed with electron imaging technique. Electron kinetic energy distributions below a few eV reveal that three kinds of ionization channels coexist, viz., vertical ionization, ionization from Rydberg states, and thermionic hot electron emission. Their relative yield is determined by the characteristic of the laser pulse. The duration in particular influences the ratio between the first two processes, while for higher intensities the last process dominates. Our results provide strong evidence that internal conversion plays an important role for the ionization of the molecule.     

Synthesis and efficient circularly polarized light emission of an optically active hyperbranched poly(fluorenevinylene) derivative

An optically active, hyperbranched poly(fluorenevinylene) derivative was synthesized by polymerization of (-)-2,4,7-tris(bromomethyl)-9-neomenthyl-9-pentylfluorene. A spin-coated film sample of the polymer exhibited efficient circularly polarized light (CPL) emission in the visible range of 400-700 nm on photo excitation even without annealing processes leading to chain ordering.     

Probing excitation delocalization in supramolecular chiral stacks by means of circularly polarized light: experiment and modeling

Photoexcitations in helical aggregates of a functionalized, chiral oligophenylenevinylene (MOPV) are described going beyond the Born-Oppenheimer approximation, in the form of dressed (polaronic) Frenkel excitons. This allows for accurate modeling of the experimentally observed wavelength dependence of the circular polarization in fluorescence, which directly probes the non-adiabatic nature of the electron-vibration (EV) coupling in this system. The fluorescence photon is emitted from a nuclear geometry in which one MOPV and its two nearest neighbors have a nuclear equilibrium that differs appreciably from the ground state due to the presence of the excited state. The absorption and emission band shape and the circular dichroism are consistent with a coherence range of the emitting excitation of approximately two neighboring molecules. Random fluctuations in the zero-order excited-state energy of the MOPVs (disorder) limit the exciton delocalization and can be described by a Gaussian distribution of energies with a width sigma=0.12 eV and a spatial correlation length l0 approximately 5 molecules. We find that disorder and EV coupling act synergistically in localizing the emitting exciton to a single MOPV in the aggregate with 95% probability.     

Scattering of circularly polarized light from the isotropic to the cholesteric phase of cholesteryl oleyl carbonate

The Rayleigh intensity and linewidth of circularly polarized incident light have been measured in the isotropic phases and blue phases of cholesteryl oleyl carbonate. In the isotropic phase the intensity data show mean field behaviour. In the blue phase region the scattering of right handed or left handed circularly polarized light shows a different q dependence for the intensity as well as for the linewidth. The correlation function of the intensity fluctuations can be fitted to two exponentials. The experimental results are compared with the predictions of the theory proposed by Hornreich and Shtrikman.     

Scattering of circularly polarized light from the isotropic to the cholesteric phase of cholesteryl oleyl carbonate

The Rayleigh intensity and linewidth of circularly polarized incident light have been measured in the isotropic phases and blue phases of cholesteryl oleyl carbonate. In the isotropic phase the intensity data show mean field behaviour. In the blue phase region the scattering of right handed or left handed circularly polarized light shows a different q dependence for the intensity as well as for the linewidth. The correlation function of the intensity fluctuations can be fitted to two exponentials. The experimental results are compared with the predictions of the theory proposed by Hornreich and Shtrikman.     

Photochemistry with circularly polarized light—III : Synthesis of helicenes using bis(arylvinyl) arenes as precursors

A systematic study of the photochemical ring-closure of bis(arylvinyl)arenes, using circularly polarized light, is described and commented. In the two series of compounds 5a–e and 6a–e, the observed optical yields follow a similar trend; no asymmetric syntheses were observed in the case of the higher benzologues of [10] helicene, using a 290–370 nm irradiation band, circularly polarized at 313 nm.     

Liquid crystal alignment generated by linearly polarized UV light on photoactive low molecular mass compounds

Alignment films prepared from low molar mass photo-crosslinkable materials containing the cinnamate group can be used for aligning LCs after irradiating the films with linearly polarized UV light. The high contrast observed in the polarizing optical microscope between dark and bright images indicates that the alignment is quite uniform. As the photoreaction progresses, the average roughness of the films is increased. All the aggregate structures, 'lamellar crystals', produced by the photo-crosslinking reaction are of a square shape.