We report the observation and study of the excess-photon absorption (EPA) phenomenon in the region of short-lived autodetaching states of negative ions at low intensities . A sensitive resonant-ionization-based technique is utilized here for the first time to investigate nonlinear processes in the continuum, including detailed studies of signal response to laser power. We demonstrate that the sensitivity of the new method is sufficient to obtain actual positions and widths of the states at the limit of low intensities. Higher intensities (i.e. above ) can lead to significant displacement of the EPA signals from the autodetaching state positions and, depending on the shape of the one-photon detachment cross section, to either broadening or narrowing of the signal peaks. The amplitude of the signal at these higher intensities typically displays a sublinear dependence on laser power.
We report the observation and study of the excess-photon absorption (EPA) phenomenon in the region of short-lived autodetaching states of negative ions at low intensities . A sensitive resonant-ionization-based technique is utilized here for the first time to investigate nonlinear processes in the continuum, including detailed studies of signal response to laser power. We demonstrate that the sensitivity of the new method is sufficient to obtain actual positions and widths of the states at the limit of low intensities. Higher intensities (i.e. above ) can lead to significant displacement of the EPA signals from the autodetaching state positions and, depending on the shape of the one-photon detachment cross section, to either broadening or narrowing of the signal peaks. The amplitude of the signal at these higher intensities typically displays a sublinear dependence on laser power.
The negative ion of lanthanum, La–, has one of the richest bound state spectra observed for an atomic negative ion and has been proposed as a promising candidate for laser-cooling applications. In the present experiments, La– was investigated using tunable infrared photodetachment spectroscopy. The relative signal for neutral atom production was measured with a crossed ion-beam--laser-beam apparatus over the photon energy range 585--920 meV (2120--1350 nm) to probe the continuum region above the La neutral atom ground state. Eleven prominent peaks were observed in the La– photodetachment cross section due to resonant excitation of quasibound transient negative ion states in the continuum, which subsequently autodetach. In addition, thresholds were observed for photodetachment from several bound states of La– to both ground and excited states of La. The present results provide information on the excited state structure and dynamics of La– that depend crucially on multielectron correlation effects.
Efficient excess-photon detachment to lowest nonvanishing order is observed with long (\ensuremath{\sim}0.7 ns) optical pulses and intensities of \ensuremath{\le}6\ifmmode\times\else\texttimes\fi{}109 W/cm2. A Feshbach (``window'') resonance in ${\mathrm{Cs}}^{\mathrm{\ensuremath{-}}}$ suppresses single-photon detachment, while simultaneously enhancing the probability of absorption of one or more excess photons. The perspectives for future spectroscopic excess-photon detachment and studies of direct two-electron multiphoton detachment are briefly discussed.
We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5s2 is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}1010 W/cm2) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.
By applying a sufficiently intense beam of off-resonant light, simultaneously with a conventional excitation source beam, the efficiencies of one- and two-photon absorption processes may be significantly modified. The nonlinear mechanism that is responsible, known as laser modified absorption, is fully described by a quantum electrodynamical analysis. The origin of the process, which involves stimulated forward Rayleigh-scattering of the auxiliary beam, relates to higher order terms which are secured by a time-dependent perturbation treatment. These terms, usually inconsequential when a single beam of light is present, become prominent under the secondary optical stimulus – even with levels of intensity that are moderate by today’s standards. Distinctive kinds of behaviour may be observed for chromophores fixed in a static arrangement, or for solution- or gas-phase molecules whose response is tempered by a rotational average of orientations. In each case the results exhibit an interplay of factors involving the beam polarisations and the molecular electronic response. Special attention is given to interesting metastable states that are symmetry forbidden by one- or two-photon absorption. Such states may be accessible, and thus become populated, on input of the auxiliary beam. For example, in the one-photon absorption case, terms arise that are more usually associated with three-photon processes, corresponding to very different selection rules. Other kinds of metastable state also arise in the two-photon process, and measuring the effect of applying the stimulus beam to absorbances of such character adds a new dimension to the information content of the associated spectroscopy. Finally, based on these novel forms of optical nonlinearity, there may be new possibilities for quantum non-demolition measurements.
Three-photon ionization is measured as the frequency of an optical pulse is tuned through two-photon resonance with the Rydberg series of potassium. When the intermediate Rydberg resonance is far below the ionization threshold, the standard resonance profile indicates the enhancement due to a single isolated level. However, as the frequency is tuned closer to threshold, continuous behavior is reached because of the coherent bandwidth of the short pulse and power broadening of the bound-bound transition line. Between these two limiting regions a transition region is observed where discrete and continuous features are manifested. The discrete nature is seen through resonance peaks that blend together as the threshold is approached. The continuous nature manifests itself through the creation of a coherent superposition state that produces localized radial motion. The laser pulse effectively probes the radial orbital motion of this wave packet. Since the ionization rate is drastically reduced while the wave packet is far from the core, drops in the ion signal are seen when the number of times the wavepacket can return to the core is reduced by one. These drops are abrupt because the electron travels much faster near the core in these highly elliptical orbits.
Efficient harmonic generation at laser-induced autoionizing state in continuum, were observed for the first time by our laboratory [1,2,3]. In this paper, nonlinear frequency mixing in alkali metal vapors at induced continuum structure in Na, are studied in detail. Besides, measurement of nonlinear optical susceptibility at induced autoionizing resonance, is performed. Tunable ultraviolet radiations by four-photon processes near induced resonances are obtained. Estimations of the real and imaginary parts of the nonlinearities, are estimated. We will specially note, that the resonant photoabsorption of ions is experimentally investigated since it gives information about the autoionizing states, which is impossible to obtain on the base only of the emission spectra. We use also laser inducing for obtaining of powerful radiation source in VUV. Thus, many actual problems as the selective impact on matter, laser photochemistry and laser isotope separation are connected directly with the induced autoionizing nonlinear resonances. The main interest is to control the spectral characteristics of continuum. The discrete level from the continuum structure leads to appearance of asymmetrical autoionizing resonance onto the background of the broad-band absorption line.