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.

Integral equation approach to multiphoton ionization by intense fields

We discuss the integral equation for an electron bound by a potential in a monochromatic, spatially homogeneous laser field which is turned on adiabatically. We outline a method for the solution of this equation, and, as a numerical example, present results for the photoionization of hydrogen in the weak field limit.     

Enhancement of anthracene fragmentation by circularly polarized intense femtosecond laser pulse

The authors compared circularly and linearly polarized lights in the ionization and fragmentation of anthracene, using 800 nm femtosecond laser pulses at intensities of 10(13)-10(15) W cm-2. Singly and doubly charged intact molecular ions as well as numerous fragment ions were observed in the mass spectra, which were investigated as a function of laser intensity and polarization. At comparable intensities above the saturation threshold for complete ionization, the fragmentation pathways are enhanced with a circularly polarized field compared to a linearly polarized field. Resonant excitation of the molecular cation through the 2Au<--2Bg transition is proposed to be the initial step to ion fragmentation. The circularly polarized field interacts with a larger fraction of the randomly oriented molecules than the linearly polarized field, and this is considered to be the reason for the enhanced fragmentation brought about by circularly polarized light.     

Molecular photoelectron interference effects by intense circularly polarized attosecond x-ray pulses

We present photoelectron interference effects in molecular photoionization by intense circularly polarized attosecond X-ray laser pulses. Simulations are performed on single electron molecular systems, H\(_{2}^{+}\), HeH²⁺, and H\(_{3}^{2+}\), by numerically solving the corresponding three-dimensional time dependent Schrödinger equations. Photoelectron spectra show that both momentum ring and stripe patterns are obtained, arising respectively from the interference of the direct ionized and scattered electron wave packets. Multi-center electron interference models in molecular ultrafast photoionization are used to describe the ionized electron dynamics. The interference patterns are shown to be dependent on molecular orbital symmetry, thus offering tools for attosecond imaging molecular structure at different molecular geometries.     

Circular dichroism effect for linear molecules induced by a resonant circularly polarized pumping optical field

In this paper we propose and discuss the laser-induced circular dichroism (LICD) effect, which is expected to occur in linear molecules pumped by a strong circular resonant light beam. The effect is to be detected via the absorption of a weak circularly polarized probe beam on another transition. Analogous to the external magnetic field in magnetic circular dichroism the resonant circular polarized pumping optical field can induce the nonzero antisymmetric rotational polarizabilities of a linear molecule, and cause the LICD effect. LICD contains three distinct contributions from M-dependent splittings of the sublevels mid R:JM due to the ac Stark effect, from the differences of Boltzmann statistical distributions among the ground state sublevels mid R:JM due to the ac Stark splittings, and from the changes of occupation probability in rotational sublevels mid R:JM due to the pumping effect. The fundamental formulas for the above three terms of LICD have been deduced by the density matrix method. As an example, the LICD for CO molecules have been calculated. The results indicate that in comparison with the rotationally resolved magnetic vibrational circular dichroism experiment, LICD may be measurable and form a basis of a different kind of CD spectroscopy.     

Real-time multiphoton ionization detection of iodine atoms produced by infrared multiphoton dissociation of perfluoroalkyl iodides

We report the direct detection of iodine atoms following infrared multiphoton dissociation of perfluoroalkyl iodides. The technique, three-photon resonant two-photon ionization, shows great promise as an actinometer for primary dissociation yield in IRMPD.     

Resonant multiphoton ionization of xenon in intense sub-ps-laser pulses

We observe that resonant ionization plays a dominant role in the multiphoton ionization (MPI) of xenon at wavelengths around 600 nm, irrespective of the detuning of multiples of the photon energy from excited states of the undisturbed atom. Angular distributions of energy resolved photoelectrons allow to identify excited states which are strongly ac-Stark shifted in the intense laser field and which serve as intermediate resonances in MPI processes. Angular distributions measured at different wavelengths show that the ac-Stark shift of excited states can be larger than the photon energyhω. Our results support the model proposed by Freeman et al.     

Spin polarization of Auger- and of photoelectrons from barium atoms exposed to circularly polarized radiation and their cross comparison

New results of spin polarization of both photoelectrons and Auger electrons are reported after 5p photoionization of free Ba atoms with circularly polarized light. A substantial polarization transfer from the spin polarized photons to the spin polarized photoelectrons and via the hole state orientation to the spin polarized Auger-electrons is observed. The cross comparison of the results for photoelectrons and Auger-electrons allows a quantitative test of the assumed two step model where both electron-emission processes occur in sequence.     

Generation of harmonic radiation by helium atoms in intense laser pulses

Measurements of natural radiative lifetimes in atomic tellurium in the 5p ³7p configuration are reported. Two-step laser excitation was applied on tellurium vapour in a cell that was differentially heated and pumped through a capillary. The decay of the fluorescence signal was recorded at different vapour pressures. The following four radiative lifetime values were obtained: 7p ⁵ P ₁: 128(11) ns, 7p ⁵ P ₂: 99(8) ns, 7p ³ P ₁: 82(10) ns and 7p ³ P ₂: 124(8) ns.     

Multiphoton ionization of magnesium and calcium atoms by short and intense laser pulses

Single and double ionization of magnesium and calcium atoms following Nd: YAG laser multiphoton excitation at 1064 and 532 nm have been studied by employing pulses of 35 ps and 200 ps duration at intensities of the order of 10¹⁰–2×10¹³ W/cm². The dependence of ion formation on the laser intensity has been measured and the slopes of the linear parts of the log-log plots and the ratios of saturation intensities for two pulse durations have been compared with the predictions of the scaling law. No evidence for a pure direct double ionization process has been obtained.     

Rotationally resolved optical rotation and circular dichroism effects for symmetric top molecules induced by a resonant circularly polarized pumping optical field

The rotationally resolved laser-induced optical activity including the laser-induced optical rotation (LIOR) and laser-induced circular dichroism (LICD) effects of an IR probing light pumped by a collinear intense resonant circularly polarized light dependent on the third-order susceptibility due to the pure electric dipole interaction for achiral symmetric top molecules in the gas phase is discussed theoretically. The laser-induced optical activity contains four distinct contributions named A, B, C, and D terms: the B term of the LIOR and LICD arising from the rotational wave function perturbed by the pumping light is deduced using the semiclassical perturbation theory, and the expressions for A, C, and D terms, respectively, due to the ac Stark effect, the Boltzmann statistical redistribution, and the alteration of occupation polability, are obtained from previous results [Zheng, R.-H.; Chen, D.-M.; Wei, W.-M.; He, T.-J.; Liu, F.-C. J. Chem. Phys. 2004, 121, 6835]. The microwave-IR double resonant spectrum is proposed to detect the LIOR and LICD effects. As an example, the LIOR and LICD for the HCF(3) molecules in the conditions of 298.15 K and 0.3 Torr when the IR probing light sweeps over the rotational-vibrational transition of the v(5) and v(1) modes and the right circularly polarized microwave pumping light with the intensity of 1 kW cm(-2) at the resonant frequency 40.84 GHz are calculated on the basis of the B3LYP/6-311++G* computations. The theoretical results indicate that the B term can be of the same order of magnitude as the A and D terms, and the LIOR and LICD can be measurable in comparison to the rotationally resolved MVCD. The laser-induced optical activity may provide useful new information and form a basis for a different kind of optical activity spectroscopy.     

Photoelectron-spectroscopy of multi-photon ionization of rare gases with circularly and linearly polarized light

Above-threshold multiphoton ionization of xenon, krypton, and argon was studied with circularly and linearly polarized light (photon energy 1.17 eV and 2.33 eV). With linearly polarized light photoelectrons are preferentially ejected along the direction of the polarization vector. With circularly polarized light a strong suppression of ejected photoelectrons was observed; the measured yield of photoelectrons was reduced by factors of up to 80 depending on the laser intensity and the photon energy. The experimental results are compared with theoretical calculations based on a multiphoton-detachment model.     

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).     

Theoretical design of an aromatic hydrocarbon rotor driven by a circularly polarized electric field

An aromatic hydrocarbon rotor without functional groups is theoretically designed. Such a molecular rotor is free from long-range electrostatic interactions. Induced dipole interactions are the rotor-driving forces under a nonresonant excitation condition. As an example, a molecular rotor with a condensed aromatic ring, a pentacene moiety mounted on a phenyl-acetylene axle that is driven by a circularly polarized electric field is considered. Results of simulations of the quantum dynamics of a rotor that take into account short-range rotor-bath interactions are presented by numerically solving the density matrix equations of the rotational motions.     

Molecular dynamics of the bromochlorofluoromethanes in a right circularly polarized field

The liquid state molecular dynamics of the chiral bromochloro-fluoromethanes have been investigated with computer simulation under the influence of an external right circularly polarized field of force. The field promotes the existence direct in the laboratory frame of time cross correlation functions which have a different symmetry for the enantiomer and racemic mixture. This may be used to measure cross correlation functions indirectly through the experimental observation of laboratory frame differences between the dynamical properties of the enantiomer and racemic mixture in a right circularly polarized field.     

Investigation of laser induced structures in multiphoton ionization of magnesium

We examine the effect of the excitation of an autoionizing state on electron spectra resulting from multiphoton-ionization. We study the case of above threshold three-photon ionization of magnesium in the region of the 3p3d ¹ P ⁰ resonance, with conditions similar to a two-colour experiment recently performed. The electron spectrum is compared to the partial spectrum corresponding to the region located just above the threshold (which is populated through two-photon absorption from a probing laser). The calculations confirm the experimental observations, a laser induced continuum structure (LICS) is observed in this partial spectrum. The interference process in examined in detail and a Fano-like parametrization of the two-photon ionization electron profile is proposed.     

Dynamical symmetries of a hydrogen atom in a circularly polarized electromagnetic field

We study dynamical symmetries for a hydrogen atom exposed to a circularly polarized electromagnetic wave in the weak field limit.     

Photoionization of epichlorohydrin enantiomers and clusters studied with circularly polarized vacuum ultraviolet radiation

The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.     

Coupled dressed-states formalism for multiphoton excitation and population inversion by coherent pulses

We show how the single-mode Floquet theory, valid only for sinusoidal field problems, may be generalized to a coupled quasienergy (or dressed-) states formalism, allowing non-perturbative treatment of multiphoton excitation of multilevel quantum systems driven by non-sinusoidal coherent pulse fields. The method is illustrated by a case study of the adiabatic population inversion in a CO Morse oscillator.