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Annual Meeting Optical Society of America

Step-like drops in two-photon resonant three photon ionization in atomic potassium

John D. Corless, John A. Yeazell, Carlos R. Stroud

Published Jan 1, 1992• 0 citations
Publisher

Abstract

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.

DOI
10.1364/oam.1992.ff2

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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.
One-photon ionization from the 6s27d 1^{1}1D2{\mathit{D}}_{2}D2​ state in barium is measured with short (0.25--2.7 ps), high-intensity laser pulses. Fermi's golden rule predicts that only the fluence (time-integrated intensity) determines the yield. We observed a decrease in the yield for fixed-fluence pulses shorter than the Kepler orbit time of the Rydberg electron (2.2 ps). This is explained semiclassically: The wave function of a Rydberg electron performs a Kepler-like orbit. Only the wave function near the core can be ionized. Not all of the wave function nears the core during a short pulse, and therefore the wave function far away from the core is stable against ionization. A quantum-mechanical calculation based on Raman transitions over the continuum agrees well with experimental observations and the semiclassical explanation.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
Quantum mechanical wave packet states can approach the classical ideal of a spatially localized particle traveling along a well-defined trajectory. These states offer a system in which the classical limit of quantum atomic theory can be studied. In this work, a wave packet that is radially localized and whose radial motion occurs at the classical orbital period is studied. Despite these strong classical features, the long-term evolution displays the quantum nature of the state. As the wave packet evolves, it becomes dispersed, and the radial motion becomes irregular. However, the wave packet does not remain dispersed, and classical motion returns with the return of the spatial localization. Both the collapse and revival of the wave packet occur because of the quantized nature of the atom. In this experiment, the wave packet was excited to the Rydberg slates by a short, ultraviolet pulse from the ground state of potassium. The probe pulse delay was scanned, and the resulting photo-ionization signal displayed the evolution of the wave packet. The collapse and revival of the radial wave packet were observed.1
Three-photon ionization spectra of Ba were studied which result from two-photon resonances between the 6s2{s}^{2}s2 1{\mathrm{}}^{1}1S0{S}_{0}S0​ ground state and the 6snd J=2,6s(n+1)s 1{\mathrm{}}^{1}1S0{S}_{0}S0​ (16\ensuremath{\le}n40) Rydberg states and 5d7d 1{\mathrm{}}^{1}1D2{D}_{2}D2​,3{\mathrm{}}^{3}3P0{P}_{0}P0​,2 perturber states. A tunable laser with field strength ${10}^{\mathrm{\ensuremath{-}}(4\mathrm{\ensuremath{-}}5)}$ a.u. was used to drive the excitation in a low-density atomic beam (n\ensuremath{\lesssim}3\ifmmode\times\else\texttimes\fi{}108{10}^{8}108 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). Resonant line shapes were systematically studied as a function of light polarization and intensity. Studies with linearly polarized light find all resonant line shapes manifest the same peak shift and asymmetry which is traced to the ac Stark shift of the ground state. The resonant peak height, shift, and linewidth scale linearly with light intensity which is accounted for with a rate-equation formulation.For the case of circular polarization, a distinct minimum of ionization occurs between each J=2 doublet of the 6snd series. One level (generally 3D2^{3}\mathrm{D}_{2}3D2​) is significantly broadened in comparison to the laser linewidth (0.1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) with increasing intensity while its sister level remains narrow (\ensuremath{\sim}0.2 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Further, light shifts of the 3D2^{3}\mathrm{D}_{2}3D2​ states are generally \ensuremath{\gtrsim}1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to the red while shifts of 1D2^{1}\mathrm{D}_{2}1D2​ states are negligible. This behavior switches for the n=27 doublet. Lastly, the dynamic shifts are not quadratic with the field strength. These effects are attributed in part to cancellation due to channel interference and a two-photon Raman process which mixes each J=2 doublet. These processes act in conjunction with the ac Stark effect to produce the observed profile behavior. The shifts and coupling of the excited states are attributed to the influence of the 6pnd ${J}^{\ensuremath{\pi}}$${=3}^{\mathrm{\ensuremath{-}}}$ autoionization series and are shown to be sensitive to the singlet-triplet mixing coefficient \ensuremath{\beta} of the J=2 doublets. A perturbation analysis is introduced which accounts for the salient features of the data, although it does not include bandwidth and saturation effects. Calculations of light shifts of the 6snd J=2 levels based on this analysis require \ensuremath{\beta} be modified from previous assignments based on hfs measurements for n in the vicinity of the 5d7d 1{\mathrm{}}^{1}1D2{D}_{2}D2​ perturber.
The resonantly enhanced multiphoton ionization of atomic hydrogen is studied for the case of four-photon ionization (three-photon resonant). The time-dependent probabilities, the shape of the resonance profile at different intensities, and the nonlinear index are computed for coherent light at frequencies around the resonance (365 nm). The systematic dependence of the cross section as a function of the principal quantum number of the resonant state (as this tends towards the Rydberg states) is investigated at zero detuning. The probabilities of absorption of one photon more than necessary for the four-photon ionization process (resonantly enhanced above-threshold ionization) is computed and the relevant branching ratios are examined. Then it is shown that the effect of the spatio-temporal structures of the laser pulse is to red shift the maximum of the resonance profile, whereas that of an ideal chaotic light is to blue shift and broaden the resonance peak. The shape, shift, and width of the profile resulting from both these effects are in very fair agreement with the experimental ones.
A joint experimental and theoretical study of four-photon ionization of Li is performed using a pump-probe setup with the laser frequency tuned in between the excitation energies of the 2s2s2s--4p4p4p and 2s2s2s--4f4f4f three-photon transitions. The three-dimensional photoelectron momentum pattern is recorded as a function of the delay time between two otherwise identical laser pulses. The process is modeled by numerically solving the time-dependent Schr\"odinger equation. While complicated interference patterns arise in the electron spectrum when the two pulses partially overlap in time, the expected quantum-beat phenomenon between Li atoms in either the 4p4p4p or the 4f4f4f state are seen when the fast oscillations involving the 2s2s2s initial state are averaged over. The qualitative agreement between the predicted and observed beating patterns is very good.