Physical Review A

Enhancement of the autoionization rate of two-photon excited states of Ba (6d_{5/2}nd_{5/2(3/2)})_{J=4} near the Ba^{+} 6d_{3/2} limit

Ravinder Kachru, N. Tran, H. B. van Linden van den Heuvell, Thomas F. Gallagher

Published Jul 1, 1984• 17 citations

Abstract

We report the excitation of highly excited $6{d}_{\frac{5}{2}(\frac{3}{2})}\mathrm{nd}(\mathrm{ns})$ states of barium, which lie ~5.6 eV above the first ionization limit of Ba, using two-photon excitation from the bound $6snd(\mathrm{ns})$ Rydberg states. The two-photon excitation scheme requires rather modest laser power (~0.5 mJ), and the ${(6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}$ states exhibit a sharp enhancement in the autoionization rate beyond $n=25$ which is accompanied by the onset of ejection of very-low-energy electrons ( 0.02 eV). We observe that the ${6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}(n25)$ states predominantly autoionize to the nearly degenerate $6{d}_{(\frac{3}{2})}$ state of ${\mathrm{Ba}}^{+}$. This result has important implications for the development of autoionization lasers and the phenomenon of dielectronic recombination.

DOI: 10.1103/physreva.30.667

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One- and two-photon ionization cross sections of the laser-excited 6s6p P11 state of barium

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Physical Review A

Enhancement of the autoionization rate of two-photon excited states of Ba (6d_{5/2}nd_{5/2(3/2)})_{J=4} near the Ba^{+} 6d_{3/2} limit

Ravinder Kachru, N. Tran, H. B. van Linden van den Heuvell, Thomas F. Gallagher

Published Jul 1, 1984• 17 citations

Publisher

Abstract

We report the excitation of highly excited

6{d}_{\frac{5}{2}(\frac{3}{2})}\mathrm{nd}(\mathrm{ns})

states of barium, which lie ~5.6 eV above the first ionization limit of Ba, using two-photon excitation from the bound

6snd(\mathrm{ns})

Rydberg states. The two-photon excitation scheme requires rather modest laser power (~0.5 mJ), and the

{(6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}

states exhibit a sharp enhancement in the autoionization rate beyond

n=25

which is accompanied by the onset of ejection of very-low-energy electrons ( 0.02 eV). We observe that the

{6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}(n25)

states predominantly autoionize to the nearly degenerate

6{d}_{(\frac{3}{2})}

state of

{\mathrm{Ba}}^{+}

. This result has important implications for the development of autoionization lasers and the phenomenon of dielectronic recombination.

DOI: 10.1103/physreva.30.667

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Enhancement of dielectronic recombination by an electric field

Lung Ko, Victor Klimenko, Thomas F. Gallagher

Dielectronic recombination from a continuum of finite bandwidth has been examined down to very low electric fields, 60 mV/cm, with very high resolution. A tunable laser is used to excite atoms to the continuum of finite bandwidth, the broad $\mathrm{Ba} {6p}_{3/2}11d$ autoionizing state, which straddles the ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit. Atoms which make transitions into the high-lying $\mathrm{Ba} {6p}_{1/2}\mathrm{nd}$ states and radiatively decay to the bound $6snd$ states are detected by field ionization. The recombination, integrated over energy, has been measured for fields from 60 mV/cm to 28 V/cm, showing a peak at 0.5 V/cm, in agreement with expectation based on calculations done for other systems. The final-state distribution has been measured, showing that the outer electron remains a spectator in the radiative decay. At fields below 1.0 V/cm and binding energies less than 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit we find a deviation of our experimental results from the isolated resonance approximation.Dielectronic recombination from a continuum of finite bandwidth has been examined down to very low electric fields, 60 mV/cm, with very high resolution. A tunable laser is used to excite atoms to the continuum of finite bandwidth, the broad $\mathrm{Ba} {6p}_{3/2}11d$ autoionizing state, which straddles the ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit. Atoms which make transitions into the high-lying $\mathrm{Ba} {6p}_{1/2}\mathrm{nd}$ states and radiatively decay to the bound $6snd$ states are detected by field ionization. The recombination, integrated over energy, has been measured for fields from 60 mV/cm to 28 V/cm, showing a peak at 0.5 V/cm, in agreement with expectation based on calculations done for other systems. The final-state distribution has been measured, showing that the outer electron remains a spectator in the radiative decay. At fields below 1.0 V/cm and binding energies less than 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the ${\mathrm{Ba}}^{+} {6p}_{1/2}$ limit we find a deviation of our experimental results from the isolated resonance approximation.

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Physical Review LettersOct 24, 198315 citations

Short-pulse photoionization of perturbed Rydberg states: Laser-induced autoionization

Rutger B. Vrijen, J. H. Hoogenraad, Lambertus D. Noordam

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 ${(}^{1,3}$ ${\mathit{D}}_{2}$ ) state which is perturbed by the 5d7d ${(}^{1}$ ${\mathit{D}}_{2}$ ) 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.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}$${\mathit{D}}_{2}$) state which is perturbed by the 5d7d${(}^{1}$${\mathit{D}}_{2}$) 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.

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Two-Photon Excitation of Autoionizing States through Zeros in the Continuum

Thomas F. Gallagher, Ravinder Kachru, Nam Huu Tran +1 more

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One- and two-photon ionization cross sections of the laser-excited 6s6p P11 state of barium

John R. Tolsma, Daniel J. Haxton, Chris H. Greene +2 more

Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned $6s6p\text{ }{^{1}P}_{1}$ state of barium in the energy range between the $5{d}_{3/2}$ and $5{d}_{5/2}$ states of ${\text{Ba}}^{+}$ . We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel $R$ -matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned $6s6p\text{ }{^{1}P}_{1}$ state of barium in the energy range between the $5{d}_{3/2}$ and $5{d}_{5/2}$ states of ${\text{Ba}}^{+}$. We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel $R$-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.

arXiv:0906.1638Physical Review ASep 1, 20099 citations

We report the excitation of highly excited

6{d}_{\frac{5}{2}(\frac{3}{2})}\mathrm{nd}(\mathrm{ns})

states of barium, which lie ~5.6 eV above the first ionization limit of Ba, using two-photon excitation from the bound

6snd(\mathrm{ns})

Rydberg states. The two-photon excitation scheme requires rather modest laser power (~0.5 mJ), and the

{(6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}

states exhibit a sharp enhancement in the autoionization rate beyond

n=25

which is accompanied by the onset of ejection of very-low-energy electrons ( 0.02 eV). We observe that the

{6{d}_{\frac{5}{2}}n{d}_{j})}_{J=4}(n25)

states predominantly autoionize to the nearly degenerate

6{d}_{(\frac{3}{2})}

state of

{\mathrm{Ba}}^{+}

. This result has important implications for the development of autoionization lasers and the phenomenon of dielectronic recombination.

\mathrm{Ba} {6p}_{3/2}11d

autoionizing state, which straddles the

{\mathrm{Ba}}^{+} {6p}_{1/2}

limit. Atoms which make transitions into the high-lying

\mathrm{Ba} {6p}_{1/2}\mathrm{nd}

states and radiatively decay to the bound

6snd

states are detected by field ionization. The recombination, integrated over energy, has been measured for fields from 60 mV/cm to 28 V/cm, showing a peak at 0.5 V/cm, in agreement with expectation based on calculations done for other systems. The final-state distribution has been measured, showing that the outer electron remains a spectator in the radiative decay. At fields below 1.0 V/cm and binding energies less than 12 ${\mathrm{cm}}^{\ensuremath{-}1}$ below the

{\mathrm{Ba}}^{+} {6p}_{1/2}

limit we find a deviation of our experimental results from the isolated resonance approximation.

A two-step process using internal conversion and fluorescence to stabilize the

6{P}_{\frac{3}{2}}11d

autoionizing state of barium has been observed. The internal conversion produces very highly excited autoionizing Rydberg states, which then emit fluorescence to produce singly excited, bound states. Inclusion of this process should bring calculations closer to agreement with recent measurements of dielectronic recombination.

The ionization of

6sns^{1}S_{0}

Rydberg states of

^{138}\mathrm{Ba}

has been investigated by high-resolution electron spectroscopy. The Rydberg states were excited by two cw dye lasers via the

6s6p^{1}P_{1}

level. The electron spectra show rich structure between 0.025 and 3 eV, largely attributable to collisional (Penning) ionization, photoionization, and autoionizing transitions. The first process might be interpreted as superelastic scattering of the Rydberg electron by optically pumped

\mathrm{Ba}(6s6p^{1}P_{1})

\mathrm{Ba}(6s5d^{1}D_{2})

atoms.

{(}^{1,3}

{\mathit{D}}_{2}

) state which is perturbed by the 5d7d

{(}^{1}

{\mathit{D}}_{2}

) 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.

Stimulated by a recent measurement of coherent control in photoionization of atomic barium, we have calculated one- and two-photon ionization cross sections of the aligned

6s6p\text{ }{^{1}P}_{1}

state of barium in the energy range between the

5{d}_{3/2}

and

5{d}_{5/2}

states of

{\text{Ba}}^{+}

. We have also measured these photoionization spectra in the same energy region, driving the one- or two-photon processes with the second or first harmonic of a tunable dye laser, respectively. Our calculations employ the eigenchannel

R

-matrix method and multichannel quantum defect theory to calculate the rich array of autoionizing resonances in this energy range. The nonresonant two-photon process is described using lowest-order perturbation theory for the photon-atom interactions, with a discretized intermediate-state one-electron continuum. The calculations provide an absolute normalization for the experiment, and they accurately reproduce the rich resonance structures in both the one- and two-photon cross sections, and confirm other aspects of the experimental observations. These results demonstrate the ability of these computationally inexpensive methods to reproduce experimental observables in one- and two-photon ionization of heavy alkaline earths, and they lay the groundwork for future studies of the phase-controlled interference between one- and two-photon ionization processes.