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Branching ratios of multiphoton excited autoionizing states

L. D. Van Woerkom

Published Jan 1, 2015• 0 citations
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Abstract

Recent experiments have shown that it is possible to excite a specific character of an autoionizing state with high selectivity, even far from that character's resonance. When a state is excited in this manner, the branching ratio with which it decays into various continua may acquire an energy dependence. Dramatic variations were observed in the branching ratio for the decay of the 6p/sub 3/2nl, J = 1 states of barium as a function of energy when they are excited far off resonance. Using a formalism based on the Multichannel Quantum Defect Theory, simple expressions were derived to explain these variations. Essentially, it is shown that the decay of a state is determined by its couplings to the continua only near its resonance. Far from its resonance, however, its branching ratio is determined primarily by the couplings of the continua to other, near-resonant states.

DOI
10.25549/usctheses-c36-308344

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Recent experiments have shown that it is possible to excite a specific character of an autoionizing state with high selectivity, even far from that character's resonance. When a state is excited in this manner, the branching ratio with which it decays into various continua may acquire an energy dependence. Dramatic variations were observed in the branching ratio for the decay of the 6p/sub 3/2nl, J = 1 states of barium as a function of energy when they are excited far off resonance. Using a formalism based on the Multichannel Quantum Defect Theory, simple expressions were derived to explain these variations. Essentially, it is shown that the decay of a state is determined by its couplings to the continua only near its resonance. Far from its resonance, however, its branching ratio is determined primarily by the couplings of the continua to other, near-resonant states.
The doubly excited singlet states of He below the N=2N=2N=2 threshold may decay by autoionization or fluorescence. In the fluorescence decay channel, most decay cascades consist of emission of three photons, of which the first is a VUV photon, the second is in or near the visible range, and the last is another VUV photon. We have studied the fluorescence channel decay dynamics of the (2,0n)(2,{0}_{\mathrm{n}})(2,0n​), (2,1n)(2,{1}_{\mathrm{n}})(2,1n​) and $(2,\ensuremath{-}{1}_{\mathrm{n}})$ 1P^{1}\mathrm{P}1P, n=3−−7n=3--7n=3−−7, states by wavelength dispersed photon-induced fluorescence spectroscopy. We have detected the photons in the second step of the cascade and determined the branching ratios for the strongest lines in this step. From these data we are able to calculate the branching ratios of the first step in the cascade. The results are in good agreement with calculations of the main decay channels of the higher resonances, but about 20--30 % lower, and so we are able to describe quantitatively the whole fluorescence cascade of the above-mentioned doubly excited states.
We measure photoion spectra and angular distributions of photoelectrons from the two-color, two-photon resonant ionization of barium atoms in order to probe the effect of the photoionization pathway on autoionization processes. The (6s6p)1{)}^{1})1P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ and the (5d6p)1{)}^{1})1P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ states are used as intermediates in the photoexcitation of barium to energies in the range of a series of 5dnd autoionizing states. Dramatic differences in the profiles of autoionizing lines are observed for excitation pathways involving different intermediate states. Angular distributions of photoelectrons collected across the range of autoionizing resonances demonstrate strong variations that follow constraints imposed by the angular momentum of the autoionizing peaks and also suggest significant spin-orbit interactions in the photoionization step.
The authors have measured the population in each of the 5d 3/2 (m j ) ion states following autoionization of the (6p 3/2 nd) J=MJ=3 state by using a circularly polarized laser to excite the Ba + 5d 3/2 to 6p 1 2/ transition, and observing the polarized fluorescence of the Ba + 6p 1/2 to 6s 1/2 transition. The branching ratios for autoionization into each of the 5d 3/2 in l j channels can be related to these ion populations by using angular momentum algebra. The fractions of ions found in the m j =+3/2, +1/2, -1/2 and -3.2 state of the 5d 3/2 ion states were 5.9(+or-2.6)%, 4.5(+or-1.9)%, 3.2(+or-1.3)% and 1.9(+or-0.6)% respectively. This method can easily be generalized to measure the branching ratio for autoonization into other ion states.
We study, using a combination of experiment and theory, the excitation and decay of very-high-$n\phantom{\rule{4pt}{0ex}}(n\ensuremath{\sim}280\text{--}430)$ strontium autoionizing Rydberg states formed by near-resonant driving of the $5s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$ core-ion transition. The branching ratio between decay through radiative transitions and through autoionization is explored. Autoionization rates are measured as a function of both the nnn and $\ensuremath{\ell}$ quantum numbers of the Rydberg electron. The nonstationary decay dynamics is studied by creating and manipulating Rydberg wave packets and by varying the laser pulse that drives the core excitation.
We present a formal theory and detailed calculations for phase-dependent laser-atom interactions involving autoionizing states. First, through simple models, we demonstrate that the simultaneous one- and three-photon excitation of one or two neighboring autoionizing states can exhibit profound changes of the line shape, as the relative phase of the two fields is varied from 0 to \ensuremath{\pi}. Through a proper choice of the field intensities and the phase, we obtain analytical results showing that one can cancel the transition to the discrete or the continuum part of the wave function, thereby leading to a flat or a completely symmetric line shape, respectively. At higher intensities, additional effects come into play, providing additional coupling between the discrete and continuum parts, which also exhibits a phase dependence. Finally, our theory is applied to a much more complex situation in Xe, involving many channels, not amenable to simple analytical expressions, but exhibiting nevertheless equally profound effects, including a modification of the branching ratio of two different products. The theory, which is here developed for atomic autoionization, is in fact fairly general and should pertain to related problems in any system involving discrete states embedded in continua.