Physical Review Letters

Far-Infrared Multiphoton Ionization of Lithium Rydberg Atoms Bypassing a Cooper Minimum

J. H. Hoogenraad, Rutger B. Vrijen, P. Wim van Amersfoort, Alexander F. G. van der Meer, Lambertus D. Noordam

Published Dec 18, 1995• 16 citations

Abstract

Multiphoton excitation and ionization studies of the Li $23s$ state by 20 ps far-infrared laser pulses originating from the free-electron laser FELIX are presented. The two-photon ionization cross section of highly excited Rydberg $s$ states is small, due to a newly discovered Cooper minimum in the bound-bound $ns\ensuremath{-}{n}^{\ensuremath{'}}p$ transitions. Instead of this lowest-order process, ionization proceeds via a remarkable multiphoton bypass starting with stimulated emission to a lower-lying state. The measurements are interpreted with calculations of the Cooper minimum and a Floquet analysis of the states involved in the bypass.

DOI: 10.1103/physrevlett.75.4579

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Physical Review Letters

Far-Infrared Multiphoton Ionization of Lithium Rydberg Atoms Bypassing a Cooper Minimum

J. H. Hoogenraad, Rutger B. Vrijen, P. Wim van Amersfoort, Alexander F. G. van der Meer, Lambertus D. Noordam

Published Dec 18, 1995• 16 citations

Publisher

Abstract

Multiphoton excitation and ionization studies of the Li

23s

state by 20 ps far-infrared laser pulses originating from the free-electron laser FELIX are presented. The two-photon ionization cross section of highly excited Rydberg

s

states is small, due to a newly discovered Cooper minimum in the bound-bound

ns\ensuremath{-}{n}^{\ensuremath{'}}p

transitions. Instead of this lowest-order process, ionization proceeds via a remarkable multiphoton bypass starting with stimulated emission to a lower-lying state. The measurements are interpreted with calculations of the Cooper minimum and a Floquet analysis of the states involved in the bypass.

DOI: 10.1103/physrevlett.75.4579

Related papers

Rydberg atoms in far-infrared radiation fields. II. Wave packet dynamics

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

The short-pulse dynamics of excitation and stimulated emission between highly excited states $(15<n<60)$ 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 $n=40$ excited from a deeper bound $(n\ensuremath{\approx}20)$ Rydberg state by a picosecond far-infrared laser pulse. Moreover, starting from a stationary $n=40$ 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 $(n=20)$ 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.The short-pulse dynamics of excitation and stimulated emission between highly excited states $(15<n<60)$ 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 $n=40$ excited from a deeper bound $(n\ensuremath{\approx}20)$ Rydberg state by a picosecond far-infrared laser pulse. Moreover, starting from a stationary $n=40$ 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 $(n=20)$ 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.

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Floquet description of multiphoton processes in Li

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We have made several different types of measurements of the three-photon ionization of Li produced by 3-ps laser pulses and describe the results using a Floquet picture. Over the photon frequency range 15 000 to 15 800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, Li represents a strongly coupled three-state system with the 2s ground state coupled to the 2p and 3d states by one and two photons, respectively. Energy analysis of the photoelectrons allows the measurement of the intensity dependent shift of the 2s Floquet state during the laser pulse. The shift shows a strong frequency dependence that is not predicted by first-order perturbation theory. We have also measured the total ionization spectrum over several ranges of frequency, as well as the angular distribution of the ionization and the first above-threshold ionization peak for frequencies where the ground state is near resonance with the 4s and 4d excited states. Calculations based on the Floquet Hamiltonian indicate that all of these processes may be understood in terms of a Floquet description.We have made several different types of measurements of the three-photon ionization of Li produced by 3-ps laser pulses and describe the results using a Floquet picture. Over the photon frequency range 15 000 to 15 800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, Li represents a strongly coupled three-state system with the 2s ground state coupled to the 2p and 3d states by one and two photons, respectively. Energy analysis of the photoelectrons allows the measurement of the intensity dependent shift of the 2s Floquet state during the laser pulse. The shift shows a strong frequency dependence that is not predicted by first-order perturbation theory. We have also measured the total ionization spectrum over several ranges of frequency, as well as the angular distribution of the ionization and the first above-threshold ionization peak for frequencies where the ground state is near resonance with the 4s and 4d excited states. Calculations based on the Floquet Hamiltonian indicate that all of these processes may be understood in terms of a Floquet description.

Physical Review ASep 1, 199510 citations

Inner-electron multiphoton ionization of barium Rydberg states with picosecond pulses

Henrik Stapelfeldt, Dimitrios G. Papaioannou, Lambertus D. Noordam +1 more

We have studied multiphoton ionization of Ba 6snl Rydberg states by intense ps pulses and have shown that the duration of the laser pulse compared to the orbit time of the Rydberg electron determines the laser-atom interaction. For high-n states with a long orbit time, the loosely bound Rydberg electron is not affected by the intense laser pulse. Instead the inner electron is ejected by absorption of five photons, leaving the ${\mathrm{Ba}}^{+}$ 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 studied multiphoton ionization of Ba 6snl Rydberg states by intense ps pulses and have shown that the duration of the laser pulse compared to the orbit time of the Rydberg electron determines the laser-atom interaction. For high-n states with a long orbit time, the loosely bound Rydberg electron is not affected by the intense laser pulse. Instead the inner electron is ejected by absorption of five photons, leaving the ${\mathrm{Ba}}^{+}$ 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 ${\mathrm{Ba}}^{++}$.

Physical Review LettersDec 2, 199156 citations

Multiphoton ionization dynamics of barium Rydberg states in intense femtosecond pulses

Duncan Tate, Thomas F. Gallagher

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.

Physical Review AOct 1, 19985 citations

Resonant inhibition of multiphoton ionization

J. G. Story, D. I. Duncan, Thomas F. Gallagher

When exposed to intense light of \ensuremath{\sim}580 nm, the ground state of K shifts up in energy, passing through two-photon resonances with Rydberg states and finally crossing the two-photon ionization limit. Using laser pulses of 0.5 to 13 ps duration, we have shown experimentally that ionization occurs for short pulses, but for long pulses the population is diverted into the Rydberg states where some population survives the peak intensity of the pulse, in excellent agreement with a dynamic Floquet model.When exposed to intense light of \ensuremath{\sim}580 nm, the ground state of K shifts up in energy, passing through two-photon resonances with Rydberg states and finally crossing the two-photon ionization limit. Using laser pulses of 0.5 to 13 ps duration, we have shown experimentally that ionization occurs for short pulses, but for long pulses the population is diverted into the Rydberg states where some population survives the peak intensity of the pulse, in excellent agreement with a dynamic Floquet model.

Physical Review LettersMay 17, 199357 citations

Ionization suppression of Rydberg atoms by short laser pulses

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

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.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.

Physical Review ANov 1, 199485 citations

Multiphoton excitation and ionization studies of the Li

23s

state by 20 ps far-infrared laser pulses originating from the free-electron laser FELIX are presented. The two-photon ionization cross section of highly excited Rydberg

s

states is small, due to a newly discovered Cooper minimum in the bound-bound

ns\ensuremath{-}{n}^{\ensuremath{'}}p

The short-pulse dynamics of excitation and stimulated emission between highly excited states

(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=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=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)

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.

{\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 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.

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.