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Bulletin of the American Physical Society

Laser induced continuum structure of krypton

G.J. Bottrell, Wallace L. Glab

Published May 1, 1993• 0 citations
OpenAlex

Abstract

Ionization, and in particular ionization using lasers, is one of the most important tools we have for examining the structure of atoms and molecules. However, one difficulty is that the bound-free transition probabilities can be rather weak. By taking advantage of resonant structure in the continuum, we can enhance the probability of transition. The idea behind laser induced continuum structure is to create the desired resonant continuum states by using a second laser tuned to an excited state of the atom. A number of interesting effects can be observed due to interference between the direct ionization channel and the channel that involves excitation to the resonant continuum state followed by induced emission to the excited state followed by ionization from this state. We will discuss theoretical calculations of this effect for krypton, as well as its relation to preliminary data involving experiments done at Texas Tech on this system.

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Ionization, and in particular ionization using lasers, is one of the most important tools we have for examining the structure of atoms and molecules. However, one difficulty is that the bound-free transition probabilities can be rather weak. By taking advantage of resonant structure in the continuum, we can enhance the probability of transition. The idea behind laser induced continuum structure is to create the desired resonant continuum states by using a second laser tuned to an excited state of the atom. A number of interesting effects can be observed due to interference between the direct ionization channel and the channel that involves excitation to the resonant continuum state followed by induced emission to the excited state followed by ionization from this state. We will discuss theoretical calculations of this effect for krypton, as well as its relation to preliminary data involving experiments done at Texas Tech on this system.
Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.
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 present a theoretical study of laser-induced autoionization and laser-induced continuum structures, performed with the aim of deriving predictions on the effect of continuum-continuum transitions on the resonance line shape. The model, developed as an extension of models introduced for the description of laser-induced autoionization and laser-induced continuum structures, on one hand, and for above-threshold ionization, on the other hand, provides quantitative predictions. For specific atomic systems, the line shapes of the structures are shown not to be affected by continuum-continuum transitions, in contrast with previously reported results.
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.
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.
In recent years our understanding of how laser fields interact with a continuum of levels has progressed substantially. No longer is the continuum viewed as a dissipative environment, best described by a set of kinetic equations. Recent theoretical* and experimental work has shown that strong laser fields involving a continuum of levels may often maintain perfeet coherence. One of the m ore interesting colterent phenomena involving the continuum is the dressing of a continuum by a bound state, termed laser-induced continuum structure (LICS). In the LICS phenomenon, a strong laser field, connecting an unpopulated bound state to the continuum mixes some bound- state character into the continuum. This optically induced resonance can be detected by a second (weaker) laser field, which induces a transition from a populated bound state to the structured continuum.