Physical Review Letters

Ionization suppression of Stark states in intense laser fields

R. R. Jones, P. H. Bucksbaum

Published Dec 2, 1991• 105 citations

Abstract

We have observed suppression of ionization in Rydberg atoms irradiated by an intense short-pulse laser field. We find that single-photon ionization of the outer electron in a Ba 6snk Stark state can be severely suppressed relative to high-order (\ensuremath{\ge}3 photon) nonresonant multiphoton ionization of the tightly bound 6s core electron. This population trapping occurs in intense short laser pulses, where the bandwidth of the ionizing laser is greater than the spacings between adjacent Stark states, and may be due to a substantial modification of the initial Stark state via stimulated Raman processes through the continuum. The ${\mathrm{Ba}}^{+\mathrm{*}}$ ion should be left in a higly excited superposition state, or wave packet.

DOI: 10.1103/physrevlett.67.3215

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Physical Review Letters

Ionization suppression of Stark states in intense laser fields

R. R. Jones, P. H. Bucksbaum

Published Dec 2, 1991• 105 citations

Publisher

Abstract

{\mathrm{Ba}}^{+\mathrm{*}}

ion should be left in a higly excited superposition state, or wave packet.

DOI: 10.1103/physrevlett.67.3215

Related papers

Population trapping in Kr and Xe in intense laser fields

R. R. Jones, Douglass W. Schumacher, P. H. Bucksbaum

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Ionization suppression of Rydberg atoms by short laser pulses

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We have investigated multiphoton ionization of Ba $6snl$ Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the $6sns$ and $6snd$ 15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of ${\mathrm{Ba}}^{+}$ Rydberg states and ${\mathrm{Ba}}^{2+}$ photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto ${\mathrm{Ba}}^{+}$ Rydberg states.We have investigated multiphoton ionization of Ba $6snl$ Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the $6sns$ and $6snd$ 15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of ${\mathrm{Ba}}^{+}$ Rydberg states and ${\mathrm{Ba}}^{2+}$ photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto ${\mathrm{Ba}}^{+}$ Rydberg states.

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Autoionizing states of negative ions in strong resonant laser fields: The negative rubidium ion

Henrik Stapelfeldt, Philip Trøst Kristensen, Ulric B. Ljungblad +2 more

We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5 ${\mathit{s}}^{2}$ is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{} ${10}^{10}$ W/ ${\mathrm{cm}}^{2}$ ) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5${\mathit{s}}^{2}$ is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}${10}^{10}$ W/${\mathrm{cm}}^{2}$) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.

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{\mathrm{Ba}}^{+\mathrm{*}}

ion should be left in a higly excited superposition state, or wave packet.

One-photon ionization from the 6s27d

^{1}

{\mathit{D}}_{2}

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 investigated multiphoton ionization of Ba

6snl

Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the

6sns

and

6snd

15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of

{\mathrm{Ba}}^{+}

Rydberg states and

{\mathrm{Ba}}^{2+}

photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto

{\mathrm{Ba}}^{+}

Rydberg states.

{\mathrm{Ba}}^{+}

ion in a Rydberg state. For low-n states the outer electron is ejected and the residual

{\mathrm{Ba}}^{+}

ion can subsequently be multiphoton ionized to

{\mathrm{Ba}}^{++}

We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5

{\mathit{s}}^{2}

is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}

{10}^{10}

{\mathrm{cm}}^{2}

) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.