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Physical Review A

Branching ratios in the radiative decay of helium doubly excited states

Marcello Coreno, Kevin Charles Prince, Robert Richter, Monica de Simone, K. Bučar, M. Žitnik

Published Nov 23, 2005• 18 citations
Publisher

Abstract

The doubly excited singlet states of He below the $N=2$ 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 $(2,{0}_{\mathrm{n}})$, $(2,{1}_{\mathrm{n}})$ and $(2,\ensuremath{-}{1}_{\mathrm{n}})$ $^{1}\mathrm{P}$, $n=3--7$, 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.

DOI
10.1103/physreva.72.052512

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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−1n)(2,\ensuremath{-}{1}_{\mathrm{n}})(2,\ensuremath−1n​) 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 have measured the lifetimes of 1Pe^{1}P^{e}1Pe (n=9−−12)(n=9--12)(n=9−−12) doubly excited states in static electric fields (1−−6kV∕cm)(1--6\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm})(1−−6kV∕cm) by observing the decay of the fluorescence signal as a function of time. The effects of the field on these helium states below the second ionization threshold are twofold: their excitation becomes possible due to the Stark mixing with the optically allowed 1Po^{1}P^{o}1Po series, and their lifetime is strongly modified by the opening of the autoionization channel, not accessible in zero field. Although the electric field represents only a tiny perturbation of the atomic potential, a substantial shortening of the lifetimes below 100ps100\phantom{\rule{0.3em}{0ex}}\mathrm{ps}100ps is observed. This is the simplest quantum system where the ratio of autoionization to fluorescence decay probability can be effectively controlled by an electric field of moderate strength.
Doubly excited states below the N=2N=2N=2 ionization threshold are populated by exciting helium atoms in a supersonic beam with monochromatized synchrotron radiation. The fluorescence decay of these states triggers a radiative cascade back to the ground state with large probability to populate long lived singlet and triplet helium metastable states. The yield of metastables is measured using a multichannel plate detector after the beam has passed a singlet-quenching discharge lamp. The variation of the yield observed with the lamp switched on or off is related to the triplet-singlet mixing of the doubly excited states.
The asymmetry parameter β for fluorescence photons emitted during the radiative decay of the sp2, n +( 1 P o ), sp2, n −( 1 P o ), 2p n d( 1 P o ) and 2p n d( 3 D o ) series of helium doubly excited states, formed by the absorption of linearly polarized synchrotron radiation, has been measured. The results are compared with recent theoretical calculations (Žitnik et al 2002 Phys. Rev. A 65 032520). At high n the β parameter is shown to increase and oscillate due to the excitation of triplet states, optically accessible from the helium ground state due to spin–orbit effects.
We have observed fluorescence and ionization signals resulting from two-photon resonant excitation of thallium in an atomic beam. The output of a pulsed dye laser is used to excite the resonance between the ground state (6('2)P(,1/2)) and an excited n('2)P state of thallium. Both two-photon absorption and three-photon absorption occur simultaneously; the latter results in photoionization of the atom. The excited states populated by two-photon absorption decay via cascade fluorescence back to the ground state; one or more of these spectral components constitute our fluorescence signal. The lifetimes of the excited n('2)P states of thallium have been measured directly by observing the exponential decay of this fluorescence. Calculations are included to ascertain the time required for the fluorescence signal to reach within 2% of a simple exponential decay. This decay is a measure of the lifetime of the initial state, under the condition that none of the fluorescence components arise from states which are populated by cascade through longer-lived states. The lifetimes of the n('2)P(,1/2) and n('2)P(,3/2) states of thallium for n = 7 to 11 are determined in this manner and compared with the values deduced from semi-empirical calculations of oscillator strengths by various authors. For calculations employing the effects of core polarizability in the electron wavefunctions and in the transition matrix elements, the agreement is very good--to within our experimental uncertainty, which is typically(, )<5%. Our measurements are also compared to the four other known experimental values for such excited states of thallium. Some of these previous experiments and calculations were carried out in conjunction with parity non-conservation experiments in thallium. Our measurements may provide useful checks on the accuracy of some of the calculations performed in the evaluation of such experiments. The ionization and fluorescence signals are also studied under conditions of strong saturation of the two-photon resonance with the 7('2)P states. The manifestation of saturation in the ionization signal is a broadening of the excitation spectrum, which is found to greatly exceed that of the fluorescence signal. Furthermore, the ratio of the ionization signal to the fluorescence signal at the peak of the resonance is found to vary linearly at intensities below about 10('9) W/cm('2); this too saturates at higher intensities. Various models are employed to simulate the observed behavior, encompassing both rate equations as well as density matrix formulations. The numerical solutions to these models are compared with the observed behavior; the agreement is found to depend strongly on the assumed experi- mental conditions to which the ensemble of atoms is subjected.
We present a detailed theoretical study of two-photon above-threshold excitation to the lowest dipole forbidden 1Se{}^{1}{S}^{e}1Se and 1De{}^{1}{D}^{e}1De autoionizing states of helium with free-electron-laser pulses. We consider the case where the formation of these transient states is detected by monitoring the yield of atoms in the metastable 1s2s1s2s1s2s states, which are reached in the radiative cascade decays of the autoionizing states. We describe the two-photon near-resonant driving with an effective Hamiltonian operator within the density matrix formalism, and we report the relevant parameters of the model, such as the field-induced shifts, the effective Rabi frequencies, and the cascade branching ratios of both the autoionizing states and the $1sn\ensuremath{\ell}$ states, to which the autoionizing states decay. We compare the calculated metastable atom yield to the vacuum ultraviolet florescence yield and to the electron yield. We discuss the effects of the intensity of the source, admixture of higher harmonics in the incident beam, polarization of the incident light, and the pulse chirp on the metastable atom yield.
The vibrational branching ratios of SrOH for radiative decay to the ground electronic state, $X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$, from the first two electronically excited states, $A{\phantom{\rule{0.16em}{0ex}}}^{2}\mathrm{\ensuremath{\Pi}}$ and $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$, are determined experimentally at the $\ensuremath{\sim}{10}^{\ensuremath{-}5}$ level. The observed small branching ratios enable the design of a full, practical laser-cooling scheme, including magnetooptical trapping and sub-Doppler laser cooling, with >104>{10}^{4}>104 photon scatters per molecule. Ab initio calculations sensitive to weak vibronic transitions are performed to facilitate the experimental measurement and analysis, and show good agreement with the experiment.