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Journal of Physics B Atomic Molecular and Optical Physics

Observation of laser-induced continuum structure in the NO molecule

O. Faucher, E. Hertz, B. Lavorel, R. Chaux, Thomas Dreier, H. Berger, Dimitrios Charalambidis

Published Sep 13, 1999• 31 citations
Publisher

Abstract

We present experimental results on the modification of a molecular ionization continuum through a laser-induced continuum structure. The effect is demonstrated in nitric oxide where the resonant 2 + 1 multiphoton-ionization process of the X 2 3/2 electronic ground state is modified by dressing the continuum with the electromagnetically embedded M 2 + Rydberg state. Through selection of a two-photon rovibronic transition via the D electronic state, a single rotational ground state level is excited to the continuum with one laser field. By adding an extra dressing field, a coupling is then established between this three-photon excited ground state level and one of the M state rotational levels. The modification due to the dressed continuum results in a modification of the ground state level ionization signal in one of the decay channels where an asymmetric line-shape resonance is electromagnetically induced. Limiting effects due to ac-Stark shifts are discussed.

DOI
10.1088/0953-4075/32/18/308

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We present experimental results on the modification of a molecular ionization continuum through a laser-induced continuum structure. The effect is demonstrated in nitric oxide where the resonant 2 + 1 multiphoton-ionization process of the X 2 3/2 electronic ground state is modified by dressing the continuum with the electromagnetically embedded M 2 + Rydberg state. Through selection of a two-photon rovibronic transition via the D electronic state, a single rotational ground state level is excited to the continuum with one laser field. By adding an extra dressing field, a coupling is then established between this three-photon excited ground state level and one of the M state rotational levels. The modification due to the dressed continuum results in a modification of the ground state level ionization signal in one of the decay channels where an asymmetric line-shape resonance is electromagnetically induced. Limiting effects due to ac-Stark shifts are discussed.
We report on the observation of the modification of the two spin-orbit ionization continua of Xe through laser-induced continuum structure (LICS). The radiative decay of the Xe ground state into the two electronic continua that correspond to the two fine-structure levels of the ground state of the Xe+{\mathrm{Xe}}^{+}Xe+ ion can be varied by electromagnetically embedding a bound state of the atom into the two continua. The dressed 2P1/2{}^{2}{P}_{1/2}2P1/2​ continuum, probed from the atomic ground state through three-photon absorption, exhibits a clear induced structure, while the 2P3/2{}^{2}{P}_{3/2}2P3/2​ continuum possesses no observable structure under the present experimental conditions, as manifested through high-resolution photoelectron spectra. The modified ionization ratio is controlled by the wavelength of the dressing field.
We report on the observation of the modification of the final status products in a photoionization of the Xe atom through the coherent process of laser-induced continuum structure (LICS). The photoionization decay of the Xe ground state into the two electronic continua, corresponding to the two fine structure levels of the ground state of the Xe ion, is varied by embedding a bound state into the two continua through a laser field (dressing laser). The dressed continua, probed from the atomic ground state through three-photon absorption, exhibit different induced structures. The experiment is performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analyzing the emitted photoelectrons. The modified final state amplitudes are controlled by the wavelength of the dressing laser. No significant modification of the 2P3/2 continuum has been observed, in the limit of the detection efficiency. The effect is characterized by a window resonance in the slow electron continuum as the dressing laser is tuned over the LICS resonance.
We report the observation of pronounced, spectrally narrow laser-induced continuum structures (LICS) in the two ionization continua of xenon. A nanosecond laser pulse with nearly transform-limited bandwidth induces a LICS by coupling the initially unpopulated 5p59p[1/2]0{5p}^{5}9p[1/2{]}_{0}5p59p[1/2]0​ state to the continuum. The LICS is probed by one-photon ionization of the 5p56p[1/2]0{5p}^{5}6p[1/2{]}_{0}5p56p[1/2]0​ state. This state is populated in an independent excitation step by an additional laser pulse acting prior to the probe and dressing pulses. The two ionization channels that leave the xenon ion in either the 2P1/2{}^{2}{P}_{1/2}2P1/2​ or the 2P3/2{}^{2}{P}_{3/2}2P3/2​ fine-structure state are detected separately by monitoring the kinetic energy of the photoelectrons in a time-of-flight electron spectrometer. A suppression of the photoelectron signal of up to 80% as compared to the far off resonance value is achieved. The line shapes reveal a Fano parameter close to zero. The LICS is identical for the coupling into the two continua. Therefore the branching ratio into the two ionization channels does not vary with the dressing-laser detuning for the coupling scheme implemented here.
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.
Experimental investigations of the laser-induced continuum structure in the vicinity of autoionizing states (AIS's) of Ca are reported for both ionization and third-harmonic generation. The shape of the induced structures is shown to be strongly affected by the presence of the AIS's, as well as exhibiting a detuning- and laser-intensity-dependent transition from a window resonance dip to a peak. The observed modification of the continuum is discussed in terms of interfering excitation channels.