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Physical Review A

Interferometric characterization of Raman redistribution among perturbed Rydberg states of barium

D. I. Duncan, R. R. Jones

Published Jun 1, 1996• 9 citations
Publisher

Abstract

The evolution of a ``dark'' wave packet in Ba is probed using a bound-state interferometric method. The wave packet is created by photoionizing the 5d7d component of a perturbed Rydberg state having predominantly 6snd character, and thereby redistributing population to neighboring states. Information on which Rydberg-Rydberg Raman transitions dominate the transfer of population to each state is obtained. A closed-form expression for the observed strong-field interferograms is developed for the limit of very short pulses. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.4338

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Physical Review A•Jun 1, 1986•10 citations
The evolution of a ``dark'' wave packet in Ba is probed using a bound-state interferometric method. The wave packet is created by photoionizing the 5d7d component of a perturbed Rydberg state having predominantly 6snd character, and thereby redistributing population to neighboring states. Information on which Rydberg-Rydberg Raman transitions dominate the transfer of population to each state is obtained. A closed-form expression for the observed strong-field interferograms is developed for the limit of very short pulses. \textcopyright{} 1996 The American Physical Society.
Laser-induced autoionization, a form of forced autoionization, is discussed. Highly excited atomic Rydberg states generally have a small cross section for photoionization due to their small overlap with the nucleus. The cross section for photoionization can be strongly enhanced if the Rydberg series is perturbed by an atomic state with a relatively large cross section for photoionization. The photoionization dynamics of such a perturbed Rydberg system shows strong similarities to that of an autoionizing Rydberg series. For intense laser pulses, which strongly couple the perturber to the continuum, the photoionization rate is limited by the coupling between the Rydberg states and the perturber or, in other words, the autoionization rate. The model describing this ionization dynamics is applied to the barium 6snd(1,3{(}^{1,3}(1,3D2{\mathit{D}}_{2}D2​) state which is perturbed by the 5d7d(1{(}^{1}(1D2{\mathit{D}}_{2}D2​) state. The calculations show good agreement with experimental results of photoionization of barium Rydberg states with subpicosecond laser pulses of light of 620 nm in wavelength.
We have experimentally observed the time dependence of the population oscillation between a doubly excited state and singly excited Rydberg states subsequent to excitation by a short laser pulse. Specifically, we have examined the case in which the configuration interaction between a doubly excited state and a singly excited Rydberg series is weak. The data are quantitatively reproduced using a quantum defect theory calculation, and we contrast simple classical pictures of the population oscillation for weak and strong configuration interaction. We also observe significant, and unexpected, Raman redistribution by the final laser pulse, and we are able to demonstrate that it occurs primarily by Raman transitions via a lower state, not via the continuum.
The short-pulse dynamics of excitation and stimulated emission between highly excited states (15<n<60)(15<n<60)(15<n<60) is studied with picosecond far-infrared pulses (40--60 $\ensuremath{\mu}\mathrm{m}$) originating from a free electron laser. We measured the excitation of a Rydberg wave packet in rubidium around n=40n=40n=40 excited from a deeper bound $(n\ensuremath{\approx}20)$ Rydberg state by a picosecond far-infrared laser pulse. Moreover, starting from a stationary n=40n=40n=40 state we found that upon irradiating with a short far-infrared laser pulse, adjacent Rydberg states are populated by resonant Raman transfer via a deeper (n=20)(n=20)(n=20) bound state. Finally, we discuss the dynamics of wave-packet excitation at a Cooper minimum in lithium, where sign change of the excitation matrix elements causes excitation at large distances from the atomic core. Possible measurement methods for this effect are discussed and found to yield different information.
Ultrashort, unipolar electric-field pulses have been used to coherently redistribute population between Rydberg states in sodium. The population in several Rydberg states is recorded simultaneously as a function of peak field. In addition, two temporally separated pulses have been used to interferometrically characterize the redistribution process. In the lowest fields, the experimental results are in agreement with the predictions of lowest-order perturbation theory. At slightly higher fields, still well below the ionization threshold, the redistribution process becomes nonperturbative, and a numerical integration of the time-dependent Schr\"odinger equation must be used to achieve partial agreement between theory and experiment.
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.