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Register a new userQuantum dynamics of a polar rotor acted upon by an electric rectangular pulse of variable duration
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As demonstrated in our previous work [J. Chem. Phys. 149, 174109 (2018)], the kinetic energy imparted to a quantum rotor by a non-resonant electromagnetic pulse with a Gaussian temporal profile exhibits quasi-periodic drops as a function of the pulse duration. Herein, we show that this behaviour can be reproduced with a simple waveform, namely a rectangular electric pulse of variable duration, and examine, both numerically and analytically, its causes. Our analysis reveals that the drops result from the oscillating populations that make up the wavepacket created by the pulse and that they are necessarily accompanied by drops in the orientation and by a restoration of the pre-pulse alignment of the rotor. Handy analytic formulae are derived that allow to predict the pulse durations leading to diminished kinetic energy transfer and orientation. Experimental scenarios are discussed where the phenomenon could be utilized or be detrimental.
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Dimers of carbon disulfide (CS$_2$) molecules embedded in helium nanodroplets are aligned using a moderately intense, 160ps, non-resonant, circularly polarized laser pulse. It is shown that the intermolecular carbon-carbon (C-C) axis aligns along the axis perpendicular to the polarization plane of the alignment laser pulse. The degree of alignment, quantified by $langle cos^2(theta_text{2D}) rangle$, is determined from the emission directions of recoiling CS$_2$$^+$ fragment ions, created when an intense 40fs probe laser pulse doubly ionizes the dimers. Here, $theta_text{2D}$ is the projection of the angle between the C-C axis on the 2D ion detector and the normal to the polarization plane. $langle cos^2(theta_text{2D}) rangle$ is measured as a function of the alignment laser intensity and the results agree well with $langle cos^2(theta_text{2D}) rangle$ calculated for gas-phase CS$_2$ dimers with a rotational temperature of 0.4K.
In this work we develop a simple theory for the green photoluminescence of ZnO quantum dots (QDs) that allows us to understand and rationalize several experimental findings on fundamental grounds. We calculate the spectrum of light emitted in the radiative recombination of a conduction band electron with a deeply trapped hole and find that the experimental behavior of this emission band with particle size can be understood in terms of quantum size effects of the electronic states and their overlap with the deep hole.We focus the comparison of our results on detailed experiments performed for colloidal ZnO nanoparticles in ethanol and find that the experimental evolution of the luminescent signal with particle sizeat room temperature can be better reproduced by assuming the deep hole to be localized at the surface of the nanoparticles. However, the experimental behavior of the intensity and decay time of the signal with temperature can be rationalized in terms of holes predominantly trapped near the center of the nanoparticles at low temperatures being transferred to surface defects at room temperature. Furthermore, the calculated values of the radiative lifetimes are comparable to the experimental values of the decay time of the visible emission signal.We also study the visible emission band as a function of the number of electrons in the conduction band of the nanoparticle, finding a pronounced dependence of the radiative lifetime but a weak dependence of energetic position of the maximum intensity.
The Wigner time delay of slow particles in the process of their elastic scattering by complex targets formed by several zero-range potentials is investigated. It is shown that at asymptotically large distances from the target, the Huygens-Fresnel interference pattern formed by spherical waves emitted by each of the potentials is transformed into a system of spherical waves generated by the geometric center of the target. These functions determine flows of particles in and out through the surface of the sphere surrounding the target. The energy derivatives of phase shifts of these functions are the partial Wigner time delay. General formulas that connect the s-phase shifts of particle scattering by each of the zero-range potentials with the phases of particle scattering by the potential cluster forming the target are obtained. Model targets consisting of two-, three- and 4-centers are considered. It is assumed that these targets are built from identical delta-potentials with equal distances between their centers. The partial Wigner time delay of slow particles by considered targets are obtained. We apply the derived general formulas to consideration of electron scattering by atomic clusters that trap electron near the target, and by calculating the times delay of mesons scattered by few-nucleons systems.
Coherence among rotational ion channels during photoionization is exploited to control the anisotropy of the resulting photoelectron angular distributions at specific photoelectron energies. The strategy refers to a robust and single parameter control using two ultra-short light pulses delayed in time. The first pulse prepares a superposition of a few ion rotational states, whereas the second pulse serves as a probe that gives access to a control of the molecular asymmetry parameter $beta$ for individual rotational channels. This is achieved by tuning the time delay between the pulses leading to channel interferences that can be turned from constructive to destructive. The illustrative example is the ionization of the $E(1Sigma_{g}^{+})$ state of Li$_{2}$. Quantum wave packet evolutions are conducted including both electronic and nuclear degrees of freedom to reach angle-resolved photoelectron spectra. A simple interference model based on coherent phase accumulation during the field-free dynamics between the two pulses is precisely exploited to control the photoelectron angular distributions from almost isotropic, to marked anisotropic.
We show that combined permanent and induced electric dipole interactions of polar and polarizable molecules with collinear electric fields lead to a sui generis topology of the corresponding Stark energy surfaces and of other observables - such as alignment and orientation cosines - in the plane spanned by the permanent and induced dipole interaction parameters. We find that the loci of the intersections of the surfaces can be traced analytically and that the eigenstates as well as the number of their intersections can be characterized by a single integer index. The value of the index, distinctive for a particular ratio of the interaction parameters, brings out a close kinship with the eigenproperties obtained previously for a class of Stark states via the apparatus of supersymmetric quantum mechanics.
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