Polarized laser pulses

Recently, an interesting correlation between the laser pulse polarization and the ellipticity of the electron beam profile has been observed [71]. However, no major influence of laser polarization on the efficiency of the electron acceleration processes has been observed so far, nor this influence has been predicted by theory and simulations, differently from the proton acceleration. For proton acceleration, a great improvement on bunch charge and quality are expected by using circularly polarized laser pulses focused on thin foils at ultra-high intensities [72-74]. 

An ultrafast time-resolved near- and mid-IR absorption spectrometer was designed to achieve high sensitivity, ultrafast time resolution, and broad tunability in the near- and mid-IR regions (see Fig. 2). The details of this spectrometer are described elsewhere (9). Briefly, MbCO was photolyzed with a linearly polarized laser pulse, whose polarization direction was controlled electronically by a liquid crystal polarization rotator. The photolyzed sample was probed with an optically delayed, linearly polarized IR pulse whose transmitted intensity was spectrally resolved with a monochromator and detected with either a Si photodiode (near-IR RilO cm-1 bandpass) or a liquid nitrogen-cooled InSb photodetector (mid-IR 3 cm-1 bandpass). To measure the sample transmission, this signal was divided by a corresponding signal from a reference IR pulse... 

Keywords Chiral control, Current transfer, Dephasing, Molecules, Junctions, Polarized laser pulses, Quantum model simulations... 

If a polarized laser pulse is used to excite the triplet state then a photo-selected excited state population is created, since the excitation probability of a dipole goes as ... 

Barth et al. performed a quantum simulation of laser-driven electron dynamics in Mg porphyrin, which is an aromatic molecule of symmetry [14]. The results of their simulation showed that n electrons of Mg porphyrin can be rotated around its aromatic ring by an ultrashort circularly polarized UV laser pulse propagating along its C4 axis. The rotation direction of n electrons is predetermined in a laboratory frame by that of the polarization plane of the circularly polarized laser pulse, that is, by photon angular momentum. 

In Sect. 6.3, we first provide the pulse-design scheme to induce and control 7T-electron rotation in a chiral aromatic molecule. Next, on the basis of dynamical simulations in a semiempirical model, we demonstrate that the initial direction of 7T-electron rotation depends on the spatial configuration of each enantiomer with respect to the polarization direction of a linearly polarized laser pulse and then 7T electrons continue to rotate clockwise and counterclockwise (or counterclockwise and clockwise) in turn. Moreover, a pump-dump control scheme to prevent the switching of the rotation direction and realize a consecutive unidirectional JT-electron rotation is presented. 

When a circularly polarized laser pulse is applied to Mg porphyrin, the spin angular momentum of a photon selects Eu+) or -), and n electrons start to rotate in the clockwise or counterclockwise direction. In other words, a linearly polarized laser pulse, which has no spin angular momentum, cannot induce n-electron rotations in Mg porphyrin. In general, a linearly polarized pulse cannot rotate n electrons in an aromatic ring molecule with degenerate excited states. 

L l are close to the eigenvalues fi. We denote H)) by ft)L (o)h)- From the energy gap between the quasi-degenerate states, - -) or —) produced by a linearly polarized laser pulse subsequently evolves in time as a coherent nonstationary state ... 

In this section, a pulse-design scheme to induce and control jt-electron rotation in a chiral aromatic molecule is provided within a frozen-nuclei approximation. We perform electron WP simulations and show that the initial direction of rr-electron rotation in a chiral aromatic molecule depends on the polarization direction of a linearly polarized laser pulse. A pump-dump method for performing unidirectional rotation of n electrons is also presented [15]. An ansa (planar-chiral) aromatic molecule with a six-membered ring, 2,5-dichloro[n](3,6)pyrazinophane (DCPH Fig. 6.1), was chosen. 

Let us design a linearly polarized laser pulse to transfer as much of the population as possible from the ground state G) to either of the approximate angular momentum eigenstates - -) or —). The linearly polarized laser pulse e(r) is assumed to have the form... 

T/4 TI2 3TI4 T where T = nIAco. In the first quarter period of T after excitation, L) T i H), namely, T), is created. Thus, the initial direction of K-electron rotation depends on the polarization direction. Since the system is a simple two-level system, the rotation changes its direction between elockwise and counterclockwise with the period T. If a molecule is highly symmetric, for example, benzene, takes an infinite time to reach —i since Aa) = 0. This means that lowering the molecular symmetry is essential for the selective generation of either I-h) or —) by a linearly polarized laser pulse. 

Fig. 6.2 (a) The linearly polarized laser pulse e(r) to initially create —) in an R enantiomer of DCPH. The polarization vector of (/) is e+. (b) Temporal behavior in the populations of G) thick solid line), +) thin solid line), and —) thin dotted line) denoted as PgO). P+ t), and P- t), respectively, (c) Expectation value of angular momentum L f). (d) Expectation value of rotational angle t) (Reprinted from Ref. [15]. Copyright (2006) by John l ey and Sons)... 

In the previous section, we treated rr-electron rotation within a frozen-nuclei approximation. However, the effects of nonadiabatic coupling should not be ignored when the duration of n-electron rotations becomes close to the period of molecular vibrations. Therefore, in this section, we explicitly take into account vibrational degrees of freedom and perform nuclear WP simulations in a model chiral aromatic molecule irradiated by a linearly polarized laser pulse. The potentials of the vibrational modes were determined by ab initio MO methods [12]. For reducing computational time, while maintaining properties of jt-electronic structures, we used 2,5-dichloropyrazine (DCP, Fig. 6.4) instead of 2,5-dichloro[n](3,6)pyrazinophane (DCPH), in which the ansa group is replaced by hydrogen atoms. 

The results shown in Figs. 6.2 and 6.3 indicate that the system can be treated as a three-level one consisting of G), L), and H) and therefore, we expanded the state vector of the system in terms of the three adiabatic states. The initial nuclear WP was set to be the vibrational ground-state wave function of G), and the system is then electronically excited by a linearly polarized laser pulse e(f) of the form in Eq. 6.14. The time evolution of the expansion coefficients for k) k = G, L, and H), i/k (Q, t), where Q is the two-dimensional mass-weighted normal coordinate vector, can be obtained from the following coupled equations [32] ... 

Here, Ho is the molecular Hamiltonian in the BO approximation, and V is the nonadiabatic coupling operator. U(t) = —/x e(r)cos(a)/t) is the pulse excitation operator. Here, e t) is the amplitude of the laser pulse with photon polarization vector e, and coi is laser central frequency. In Eq. 6.18, i] denotes the parameter depending on photon polarization direction of the linearly polarized laser pulse = 1 for the polarization vector e+, while r] = -l for e. ... 

The vector potential for a linearly polarized laser pulse employed here is of a Gaussian form... 

Here we consider the ideal case of three-dimensionally aligned molecule in interaction with a linearly polarized laser pulse. We compare the results obtained with the effective model Hamiltonian described in Sect. 8.2.3 to those obtained with the exact vibrational Hamiltonian described in Sect. 8.2.2. 

Barth I, Manz J (2006) Periodic electron circulation induced by circularly polarized laser pulses quantum model simulations for Mg porphyrin. Angew Chem Int Ed 45 2962... 

The polarized laser pulse which induces the one-photon photofragmentation also ionizes the I( Pi/2) product atoms via a 2+1 REMPI process [64] at 281.74 nm. 

Ionization and High-Order Harmonic Generation in Aligned Benzene by a Short Intense Circularly Polarized Laser Pulse. 

An alternative experimental approach is presented by Hyde, et al.(S), who examined reorientation via holographic grating methods. In these experiments, picosecond polarized laser pulses passing through the sample along two nonparallel... 

Such measurements have been performed with polarized laser pulses to measure the orientation relaxation [12.22] and to measure the total rate of inelastic collisions [12.23]. With single-mode lasers even velocity changing elastic collisions can be investigated [12.24]. 

Pulsed fluorescence spectroscopy can be applied to investigations of dynamics and structure of biopolymers [14.28]. If the fluorescence emission, excited by a linearly polarized laser pulse, is probed with respect to its polarization, the anisotropy is related to the orientation... 

A useful and common way of describing the reorientation dynamics of molecules in the condensed phase is to use single molecule reorientation correlation functions. These will be described later when we discuss solute molecular reorientational dynamics. Indirect experimental probes of the reorientation dynamics of molecules in neat bulk liquids include techniques such as IR, Raman, and NMR spectroscopy. More direct probes involve a variety of time-resolved methods such as dielectric relaxation, time-resolved absorption and emission spectroscopy, and the optical Kerr effect. The basic idea of time-resolved spectroscopic techniques is that a short polarized laser pulse removes a subset of molecular orientations from the equifibrium orientational distribution. The relaxation of the perturbed distribution is monitored by the absorption of a second time-delayed pulse or by the time-dependent change in the fluorescence depolarization. 

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