Photoexcitation Of Atoms Research Articles

Atomic photoexcitation as a tool for probing purity of twisted light modes

Atomic photoexcitation as a tool for probing purity of twisted light modes

Photoexcitation of atoms near the center of vortex light

We investigate the interaction of atoms with a Laguerre-Gaussian beam near the vortex center where an annular nodal area exists on and around the beam propagation axis. To grasp the essential features related to the orbital angular momentum transfer to a bound electron, we first show that it suffices to include in the interaction Hamiltonian the leading (second) term in the usual expansion of e−ik.r to be associated with the transverse (longitudinal) field component of a beam with orbital angular momentum ℓ=±1. We compare the obtained results for the quadrupole transition 4S1/2 to 3D5/2 in Ca+ with published experimental data. For orbital angular momenta |ℓ|>1, this approach predicts that quadrupole transitions are only possible for the configuration where the orbital angular momentum with |ℓ|=2 is antiparallel to the spin degree of freedom. These results are confirmed by another approach based on the multipolar Hamiltonian which, additionally, provides rich information on the emergence of different selection rules for arbitrary value of ℓ including their spatial dependence in the region of interest. This work also extends the discussion of the excitation of target atoms by twisted light to two-electron atomic systems by deriving selection rules for single-electron transitions.

Photoexcitation of atoms by cylindrically polarized Laguerre-Gaussian beams

We analyze the photo-excitation of atoms with a single valence electron by cylindrically polarized Laguerre-Gaussian beams. Theoretical analysis is performed within the framework of first-order perturbation theory and by expanding the vector potential of the Laguerre-Gaussian beam in terms of its multipole components. For cylindrically polarized Laguerre-Gaussian beams, we show that the (magnetic) sub-components of electric-quadrupole field vary significantly in the beam cross-section with beam waist and radial distance from the beam axis. We discuss the influence of varying magnetic multipole component in the beam cross-section on the sub-level population of a localized atomic target. In addition, we calculate the total excitation rate of electric quadrupole transition ($4s \;^{2}S_{1/2} \rightarrow 3d \;^{2}D_{5/2}$) in a mesoscopic target of Ca$^{+}$ ion. These calculations shows that the total rate of excitation is sensitive to the beam waist and the distance between center of the target and the beam axis. However, the excitation by cylindrically polarized Laguerre-Gaussian beam is found more efficient in driving electric quadrupole transition in the mesoscopic atomic target than the circularly polarized beams.

Photoexcitation of Atoms Near the Center of Vortex Light

Photoexcitation of Atoms Near the Center of Vortex Light

Linear-response range-separated density-functional theory for atomic photoexcitation and photoionization spectra.

We investigate the performance of the range-separated hybrid (RSH) scheme, which combines long-range Hartree-Fock (HF) and a short-range density-functional approximation (DFA), for calculating the photoexcitation/photoionization spectra of the H and He atoms, using a B-spline basis set in order to correctly describe the continuum part of the spectra. The study of these simple systems allows us to quantify the influence on the spectra of the errors coming from the short-range exchange-correlation DFA and from the missing long-range correlation in the RSH scheme. We study the differences using the long-range HF exchange (nonlocal) potential and the long-range exact exchange (local) potential. Contrary to the former, the latter supports a series of Rydberg states and gives reasonable photoexcitation/photoionization spectra, even without applying linear-response theory. The most accurate spectra are obtained with the linear-response time-dependent RSH (TDRSH) scheme. In particular, for the He atom at the optimal value of the range-separation parameter, TDRSH gives slightly more accurate photoexcitation and photoionization spectra than the standard linear-response time-dependent HF. More generally, this work shows the potential of range-separated density-functional theory for calculating linear and nonlinear optical properties involving continuum states.

Femtosecond dopant-to-solvent energy transfer inside helium nanodroplets

Photoexcitation of atoms solvated inside superfluid helium nanodroplets is investigated with femtosecond photoelectron spectroscopy. Expansion of the solvation shell within about 600 fs is observed as transient photoelectron peak shift. For larger photoexcitation energies within the blue-shifted absorption band, more energy is transferred to the He droplet and the expansion proceeds faster.

Atomic spectroscopy with twisted photons: Separation of M1−E2 mixed multipoles

We analyze atomic photoexcitation into the discrete states by twisted photons, or photons carrying extra orbital angular momentum along their direction of propagation. From the angular momentum and parity considerations, we are able to relate twisted-photon photoexcitation amplitudes to their plane-wave analogues, independently of the details of the atomic wave functions. We analyzed the photo-absorption cross sections of mixed-multipolarity $E2-M1$ transitions in ionized atoms and found fundamental differences coming from the photon topology. Our theoretical analysis demonstrates that it is possible to extract the relative transition rates of different multipolar contributions by measuring the photo-excitation rate as a function of the atom's position (or the impact parameter) with respect to the optical vortex center. The proposed technique for separation of multipoles can be implemented if the target's atom position is resolved with sub-wavelength accuracy, for example, with Paul traps. Numerical examples are presented for Boron-like highly-charged ions (HCI).

Circular dichroism of twisted photons in non-chiral atomic matter

We calculate the circular dichroism (CD) for the absorption of the twisted photons, or optical vortices, by atoms, caused by atomic excitation into discrete energy levels. The effects of photon spin on the rates and cross sections of atomic photo-excitation are considered. It is demonstrated that, although for electric dipole transitions the atomic excitation rates depend on the relative orientation of photon spin and orbital angular momentum (OAM), the resulting CD is zero. However, CD is nonzero for atomic transitions of higher multipolarity, and it is predicted to peak near the phase singularity in the optical vortex center. The effects remain large in a paraxial limit, where analytic expressions are provided. The predicted spin asymmetries are equivalent to OAM dichroism for the fixed photon spin.

Photoexcitation of atoms by Laguerre-Gaussian beams

In a recent experiment, Schmiegelow et al. [Nat. Commun. 7, 12998 (2016)] investigated the magnetic sublevel population of Ca + ions in a Laguerre-Gaussian light beam if the target atoms were just centered along the beam axis. They demonstrated in this experiment that the sublevel population of the excited atoms is uniquely defined by the projection of the orbital angular momentum of the incident light. However, little attention has been paid so far to the question of how the magnetic sublevels are populated when atoms are displaced from the beam axis by some impact parameter b. Here, we analyze this sublevel population for different atomic impact parameters in first-order perturbation theory and by making use of the density-matrix formalism. Detailed calculations are performed especially for the 4s 2 S 1/2 → 3d 2 D 5/2 transition in Ca + ions and for the vector potential of a Laguerre-Gaussian beam in Coulomb gauge. It is shown that the magnetic sublevel population of the excited 2 D 5/2 level varies significantly with the impact parameter and is sensitive to the polarization, the radial index, as well as the orbital angular momentum of the incident light beam.

Two properties of twisted-light absorption

We discuss novel features of twisted-light absorption both by hydrogen-like atoms and by micro-particles. First, we extend the treatment of atomic photoexcitation by twisted photons to include atomic recoil, derive generalized quantum selection rules and consider phenomena of forbidden atomic transitions. Second, using the same electromagnetic potential for the twisted light beams, we analyze the radiation pressure from these beams on micro-sized particles, and verify that while the Poynting vector can in some circumstances point back toward the source, a complete analysis nonetheless gives a repulsive radiation pressure.

Electron localization following attosecond molecular photoionization

For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10(-15)-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10(-18)-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H(2) and D(2) was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump-probe experiment based on that work: H(2) and D(2) are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.

Photoexcitation of atom with ultrashort laser pulses

The uncoupled basis set is used to describe the interaction between the ultrashort pulses and the Na atom. Several polarization configurations of laser pulses have been investigated, which produce different effects on the two-step photoexcitation process. The study shows that the final states of the transitions driven by ultrashort laser pulses have a strong dependence on the laser polarization configuration. A comparison of the two different basis sets shows that the uncoupled basis set may provide more information than the coupled basis set, such as angular distribution of the atom in the final states of the transitions.

Resonant Laser Ablation of Lanthanides: Eu and Lu Resonances in the 450−470 nm Region

Resonant laser ablation (RLA) of solids with a pulsed dye laser achieves wavelength-specific enhancements in yields of particular metal ions (M+) by resonant photoionization or photoexcitation of atoms in the ablation plume. In the present study, RLA was performed for λ = 450−470 nm on solids comprising selected lanthanide elements. Enhancements in yields of Eu+ and Lu+ occurred at wavelengths corresponding to one-photon resonant absorptions for the neutral atoms: Ln + hν → Ln*. The excited Ln* were subsequently preferentially ionized by the quasi-thermal collisional processes normally operative in the ablation plume: Ln* + kT → Ln+ + e-. The excessive atomic absorption line widths of up to 40 cm-1 (fwhm) were attributed primarily to collisional and Stark broadening in the dense plume. The strongest Eu+ resonances correspond to transitions from ground Eu (4f76s2 8S7/20) to 4f76s6p 8PJ levels at 21 445 (J = 5/2), 21 605 (J = 7/2), and 21 761 cm-1 (J = 9/2). Primary Lu+ resonances were for excitation from...

Inner shell photoexcitation of atoms with correlation effects on autoionization : application to the argon 2p shell

For atoms with closed shells in their ground state, a Hartree-Fock (HF) equation is obtained for the electron moving in the field of an inner shell vacancy which decays through autoionization. In this equation, the following processes are taken into account: 1) change of the HF field of the remaining atom as a result of autoionization decay of the inner shell vacancy; 2) interaction of the photoelectron with a virtual Auger electron, thus the radial correlation in their movement is accounted for. The developed method is applied to study the L II,III -photoexcitation in Ar atom

Multiconfiguration Tamm-Dancoff approximation applied to photoionization of the outer shells of Be and Mg.

A multiconfiguration version of the Tamm-Dancoff approximation (MCTD) is developed to study photoexcitation of atoms in cases where the simultaneous excitation of two atomic electrons is important. Starting from a multiconfiguration Dirac-Fock description of the ground state, the variational principle is applied to give equations for a final-state wave function consisting of both singly excited and doubly excited configurations. To illustrate the method, the MCTD equations are applied to determine the 2pns and 2pnd autoionizing resonances in the low-energy photoionization cross section of Be and the corresponding 3pns and 3pnd resonances in Mg. The predicted cross sections are found to be in good agreement with previous theoretical results as well as with the available experimental data.