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Science

Coherent Laser Control of the Product Distribution Obtained in the Photoexcitation of HI

Langchi Zhu, Valeria D. Kleiman, Xiaonong Li, Shao Ping Lu, Karen A. Trentelman, Robert J. Gordon

Published Oct 6, 1995• 305 citations
Publisher

Abstract

Active control of the distribution of products of a chemical reaction was demonstrated by using a method based on the principle of quantum mechanical interference. Hydrogen iodide (HI) molecules were simultaneously excited above their ionization threshold by two competing pathways. These paths were absorption of three ultraviolet photons of frequency ω 1 and one vacuum ultraviolet photon of frequency ω 3 = 3ω 1 . The HI + and I + signals were modulated as the phase between the lasers was varied, with the HI + signal lagging by 150° ± 15°. A mechanism consisting of autoionization and predissociation is proposed.

DOI
10.1126/science.270.5233.77

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Nature Photonics•Oct 9, 2026•1 citation
Active control of the distribution of products of a chemical reaction was demonstrated by using a method based on the principle of quantum mechanical interference. Hydrogen iodide (HI) molecules were simultaneously excited above their ionization threshold by two competing pathways. These paths were absorption of three ultraviolet photons of frequency ω 1 and one vacuum ultraviolet photon of frequency ω 3 = 3ω 1 . The HI + and I + signals were modulated as the phase between the lasers was varied, with the HI + signal lagging by 150° ± 15°. A mechanism consisting of autoionization and predissociation is proposed.
The phase lag associated with coherent control by two-pathway excitation in the presence of autoionizing and/or predissociating resonances is studied experimentally and theoretically. For a mixture of HI and DI, the phase lag $\ensuremath{\Delta}\ensuremath{\delta}$ between the two-parent ion signals vanishes to within experimental accuracy, whereas for an HI/H2S\mathrm{HI}/{\mathrm{H}}_{2}\mathrm{S}HI/H2​S mixture, $|\ensuremath{\Delta}\ensuremath{\delta}|$ reaches a maximum in the vicinity of the HI $5s\ensuremath{\sigma}$ resonance. These and previous observations are all explained within a single theoretical model which clarifies the origins of the phase lag in different atomic and molecular systems.
The phase lag between the ionization and dissociation products of HI and DI molecules obtained by one- and three-photon excitation was measured as a function of excitation energy in the vicinity of a molecular resonance. The phase lag was observed to have an asymmetric profile with a deep minimum near the center of the resonance. A strong isotope effect was also observed. A theoretical analysis, using projection operators to partition the Lippmann-Schwinger equation, showed that a molecular phase is responsible for the energy dependence of the phase lag.
We present the results of our study on coherent control of photoionization of atomic barium. Our study focused on the understanding of the controllability, especially due to the effect of the coherent interaction between the atomic system and the laser field. The first half of the study investigates the mechanisms of the control behind the previously observed laser phase-insensitive product state control. The controllability of this excitation scheme, two-color two-photon resonantly enhanced excitation, was analyzed from two aspects, the role of ac Stark shift introduced by the strong laser field and the multi-pathway quantum mechanical interferences. We have analyzed the excitation scheme from the analysis of the photoelectron angular distribution measured using the excitation scheme and the monitoring of the intermediate state population. Analysis of the data as well as the numerical simulation showed clear understanding of the role of two mechanisms in the product state control reported. We also investigated the control of the phase lag during the product state control. We conducted the control of the phase lag in the study of asymmetric photoelectron angular distribution, which arises from the concurrent even-odd parity outgoing electron wave excitation. The phase lag was controlled in full range, 2π, and the results were analyzed in terms of the role of autoionizing resonance structures as well as the nature of outgoing electron waves at different locations of the autoionizing resonances.
An ab initio investigation of the excitation of an H(2) resonance, coupled to ionization and dissociation continua, was performed. The excitation occurs via a one- and a three-photon path where both fields have a variable but well-defined phase relation. While both continuum yields show an oscillatory behavior as the phase is varied, a pronounced phase lag between the two oscillations is observed, if the excitation energy is detuned from the resonant transition. The results are discussed in connection to a recent series of experiments performed on HI and the subsequent discussion on the possible origin of the observed phase lag.
Abstract Coherent control uses interference between quantum pathways to enhance or suppress transitions to a target quantum state. In stimulated Raman excitation, such control can be achieved through interference between two indistinguishable pathways connecting the same initial and final states, with the relative phase between the pathways modulating the population transfer to the final state. Extending this strategy to the extreme ultraviolet (XUV) and X-ray regimes would enable control of ultrafast electronic dynamics and site-selective core excitations. However, Raman coupling at short wavelengths competes with photoionization and intermediate-state decay, whereas intense fields with stable and adjustable relative phases remain difficult to produce. Here we use two-colour pulses from a seeded coherent free-electron laser to excite helium from its ground state to a bound metastable state via stimulated XUV Raman excitation. We implement phase-coherent control by creating two interfering Raman pathways through the ionization continuum and modulate the transferred population by varying their relative phases. Blocking one pathway removes the modulation, confirming its interferometric origin, and time-dependent Schrödinger equation calculations reproduce the measured phase dependence. Our results establish phase-controlled stimulated Raman excitation in the XUV regime, providing a strategy for the coherent control of atomic and molecular processes at short wavelengths.