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arXiv:2104.00873 · Physical Review A

Resonant control of photoelectron directionality by interfering one- and two-photon pathways

Yimeng Wang, Chris H. Greene

Published May 24, 2021• 8 citations
arXivHTMLPDF

Abstract

Coherent control of interfering one- and two-photon processes has for decades been the subject of research to achieve the redirection of photocurrent. The present study develops two-pathway coherent control of ground-state helium atom above-threshold photoionization for energies up to the $N=2$ threshold, based on a multichannel quantum defect and R-matrix calculation. Three parameters are controlled in our treatment: the optical interference phase $\mathrm{\ensuremath{\Delta}}\mathrm{\ensuremath{\Phi}}$, the reduced electric field strength $\ensuremath{\chi}={\mathcal{E}}_{\ensuremath{\omega}}^{2}/{\mathcal{E}}_{2\ensuremath{\omega}}$, and the final state energy $\ensuremath{\epsilon}$. A small energy change near a resonance is shown to flip the emission direction of photoelectrons with high efficiency, through an example where $90%$ of photoelectrons whose energy is near the $2{p}^{2}{\phantom{\rule{4pt}{0ex}}}^{1}{S}^{e}$ resonance flip their emission direction. However, the large fraction of photoelectrons ionized at the intermediate state energy, which are not influenced by the optical control, make this control scheme challenging to realize experimentally.

DOI
10.1103/physreva.103.053118

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Prospects for the coherent control of optically-induced processes have attracted a great deal of interest of late. In this talk we will discuss our recent demonstration of the first example of two-pathway coherent control on a bound-free transition in a molecular system. In this demonstration, we controlled the asymmetry of the angular distribution of photoelectrons produced by photoionization of NO. Preparation of the NO in its A{sup 2}{Sigma}, state allowed isolation of a single rotational state, which was subsequently ionized via interfering one- and two-photon interactions. We discuss the role of intermediate-state resonances and laser mode quality on two-pathway control processes.
In coherent control schemes, pathways connecting an initial and a final state can be independently controlled by manipulating the complex amplitudes of their transition matrix elements. For paths characterized by the absorption of multiple photons, these quantities depend on the magnitude and phase between the intermediate steps, and are expected to be strongly affected by the presence of resonances. We investigate the coherent control of the photoemission process in neon using a phase-controlled two-color extreme ultraviolet pulse with frequency in proximity of an excited energy state. Using helium as a reference, we show that the presence of such a resonance in neon modifies the amplitude and phase of the asymmetric emission of photoelectrons. Theoretical simulations based on perturbation theory are in fair agreement with the experimental observations.
We experimentally demonstrate one- and two-photon phase sensitive coherent control over the excitation and ionization of Rydberg states of Sr atom in the presence of a static homogeneous electric field. Without the field this excitation scheme can only be employed for the manipulation of angular distributions of photoelectrons and molecular photofragmentation products. Total atomic excitation-ionization yields cannot be modulated because the final states excited by each pathway are orthogonal to each other. When, however, a static electric field is applied, sss, ppp and ddd character is admixed into the final Rydberg state, which has no definite parity anymore. Hence, the excitation of the target Stark state from the ground state is possible with either two (fundamental laser frequency) photons or one (second harmonic frequency) photon and its population and further ionization can be controlled by varying the relative phase between the two radiation fields. The concept is successfully tested below as well as above the classical saddle point and with either mutually crossed or parallel linear laser beam polarizations (while the second harmonic beam polarization vector is always parallel to the static field direction). We examine the behavior of the obtained photoionization signal modulation depth VVV as a function of the static field strength FFF for otherwise identical experimental conditions. The V(F)V(F)V(F) curve exhibits a maximum (typically $\ensuremath{\sim}65%--85%$) at a field strength value that is dictated by the interplay between the employed laser power densities, one- and two-photon transition dipole moments, and relative amounts of field-dependent sss, ppp and ddd character. It is therefore shown that the static field strength may serve as an additional, experimentally adjustable, control parameter in a fashion complementary to both the intensities and relative phase of the two light beams.
A quantitative analysis of part of the experimental data reported by Wang, Chen and Elliott [1,3] who studied in barium coherent control through two-color resonant interfering paths is reported. Dynamics of the two-color photoionization process, described as an adiabatic process in the rotating wave approximation, is governed by the coherent excitation of the 6s6p and 6s7p 1P1 intermediate states. Interference effects are found to play a minor role. The required atomic parameters are obtained from a theoretical approach based on a combination of jj-coupled eigenchannel R-matrix and Multichannel Quantum Defect Theory.
Coherent control provides a quantum interference based method for controlling atomic and molecular dynamics. We report two experimental observations of different ways to achieve such a control in atomic photoionization continuum. One is the photoionization of sodium atoms, initially prepared in the 3P1/2 excited state, by a non resonant three-photon interaction with 1064-nm laser light and by a one-photon interaction with the third-harmonic field, at 355 nm. Interference between the two transition paths allows the control of the total ionization rate by varying the relative phase of the laser fields. The second experiment is the observation of the modification of the ionization continua of xenon through laser induced continuum structure (LICS). The multiphoton radiative decay of the xenon ground state into the two electronic continua, that correspond to the two fine structure levels of the ground state of the xenon ion, is varied by electromagnetically embedding a bound state of the atom into the two continua. Both experiments are performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons.
In this study, we extended the coherent control theory proposed by Brumer and Shapiro to bound-continuum transition in a molecular system where control of the ejection direction of photoelectrons in the ionization of nitric oxide was achieved. Nitric oxide was prepared in its first excited Rydberg state and was then ionized simultaneously by two different paths: a one-photon and a two-photon process. The fundamental and second harmonic frequencies of a dye laser were used for the two different ionization processes. Interference due to the two paths was detected by measuring the photoelectrons ejected in a defined solid angle. Changing the relative phase between the two paths resulted in a destructive or constructive interference leading to a decrease or increase of the measured signal, respectively. This study showed that interference experiments are very sensitive, not only to lasers' properties and the optical components, but also to the properties of the molecular system. Laser properties, such as, spatial and temporal mode qualities, power fluctuations and timing, influenced the relative strength of the signals of the two ionization paths. Optical component properties, such as, surface parallelism and quality, affected laser divergence, and therefore, considerably reduced the depth of modulation. As for the molecular system, in a resonance two-photon excitation, appropriate resonance states must be chosen to ensure that the two processes produce electron waves of opposite parities which can interfere constructively to produce an enhancement of electron ejection in a certain direction. Due to the non-ideal properties of the lasers used, it is best to use a non-resonant two-photon ionization as one of the interfering paths. Resonant two-photon ionization, combined with the multi-mode laser greatly reduced the depth of the modulation.