Physical Review A

Photoassociation of atoms in ultracold collisions probed by wave-packet dynamics

Mette Machholm, Annick Giusti-Suzor, Frederick H. Mies

Published Dec 1, 1994• 50 citations

Abstract

Photoassociation in laser-assisted collisions of ultracold alkali atoms has been used recently to perform high-precision spectroscopy of very-long-range states. Rovibrational spectra close to the Na(3 $^{2}$S)+${\mathrm{Na}}^{\mathrm{*}}$(3 $^{2}$${\mathit{P}}_{3/2}$) dissociation limit have been obtained by monitoring an enhanced ${\mathrm{Na}}_{2}^{+}$ ion signal. We propose the use of pump-probe techniques to study the dynamics of the ionization process. At very large distances (typically 70 a.u.) two colliding Na atoms absorb a photon and form a coherent vibrational wave packet. We follow the packet in time and probe two possible mechanisms that are proposed to yield molecular ions: photoionization, where the second photon is absorbed at a small internuclear distance R directly ionizing the molecule, and associative ionization with excitation at intermediate R to the nuclear continuum of a doubly excited state followed by a half collision and autoionization at small R. The latter event can also lead to dissociation of the complex. The two mechanisms lead to distinct differences in the ion signals as a function of delay time between the pump and the probe pulses. Estimates of the ion rates to be expected in actual pump-probe experiments are given, which tend to favor the autoionization mechanism as the dominant reaction path.

DOI: 10.1103/physreva.50.5025

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Single-color photoassociative ionization of ultracold sodium: The region from 0 to -5 GHz

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Physical Review A

Photoassociation of atoms in ultracold collisions probed by wave-packet dynamics

Mette Machholm, Annick Giusti-Suzor, Frederick H. Mies

Published Dec 1, 1994• 50 citations

Publisher

Abstract

^{2}

S)+

{\mathrm{Na}}^{\mathrm{*}}

^{2}$${\mathit{P}}_{3/2}

) dissociation limit have been obtained by monitoring an enhanced

{\mathrm{Na}}_{2}^{+}

ion signal. We propose the use of pump-probe techniques to study the dynamics of the ionization process. At very large distances (typically 70 a.u.) two colliding Na atoms absorb a photon and form a coherent vibrational wave packet. We follow the packet in time and probe two possible mechanisms that are proposed to yield molecular ions: photoionization, where the second photon is absorbed at a small internuclear distance R directly ionizing the molecule, and associative ionization with excitation at intermediate R to the nuclear continuum of a doubly excited state followed by a half collision and autoionization at small R. The latter event can also lead to dissociation of the complex. The two mechanisms lead to distinct differences in the ion signals as a function of delay time between the pump and the probe pulses. Estimates of the ion rates to be expected in actual pump-probe experiments are given, which tend to favor the autoionization mechanism as the dominant reaction path.

DOI: 10.1103/physreva.50.5025

Related papers

Dynamics of photoinduced collisions of cold atoms probed with picosecond laser pulses

Fredrik K. Fatemi, K. M. Jones, He Wang +2 more

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Spectroscopy of autoionizing doubly excited states in ultracold Na2 molecules produced by photoassociation

Arjen Amelink, K. M. Jones, Paul D. Lett +2 more

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Ultrafast Dynamics and Nonlinear Interactions in Alkali Atoms and Diatomics

Pramod Charles John

Ultrafast laser pump-probe techniques have been used to study the rovibrational dynamics of the B, C, and D states of the Cesium dimer and to study atomic wavepackets and nonlinear effects in Rubidium and Potassium. Two color, pump-probe techniques have been used to probe the time evolution of wavepackets in the B, C, and D rovibrational levels of Cs$\sb2$. The temporal history of the wavepacket is inferred from the time and energy-integrated photoelectron signal produced when the excited state is photoionized by a time-delayed probe pulse. Oscillatory motion is observed in the photoionization signals, corresponding to the vibrational energy differences of the state begin studied. The period of oscillation ranges between 2.2 and 1.1 ps depending on the state. Quantum mechanical simulations of the wavepacket motion show excellent agreement with the experimental data. Experiments have been carried out in which atomic wavepackets interact with a four-wave mixing process in Rb and K to produce spatially and temporally coherent pulses of UV radiation. Wave packets are produced by coherently exciting an ensemble of states using the broad bandwidth of a femtosecond pulse. Oscillations corresponding to the energy differences between the levels in the wavepacket are superimposed on the UV signal when two identical pump and delayed probe pulses are introduced into the alkali vapor. Analyses of the UV emission line profiles show excellent agreement with the four-wave mixing process.Ultrafast laser pump-probe techniques have been used to study the rovibrational dynamics of the B, C, and D states of the Cesium dimer and to study atomic wavepackets and nonlinear effects in Rubidium and Potassium. Two color, pump-probe techniques have been used to probe the time evolution of wavepackets in the B, C, and D rovibrational levels of Cs$\sb2$. The temporal history of the wavepacket is inferred from the time and energy-integrated photoelectron signal produced when the excited state is photoionized by a time-delayed probe pulse. Oscillatory motion is observed in the photoionization signals, corresponding to the vibrational energy differences of the state begin studied. The period of oscillation ranges between 2.2 and 1.1 ps depending on the state. Quantum mechanical simulations of the wavepacket motion show excellent agreement with the experimental data. Experiments have been carried out in which atomic wavepackets interact with a four-wave mixing process in Rb and K to produce spatially and temporally coherent pulses of UV radiation. Wave packets are produced by coherently exciting an ensemble of states using the broad bandwidth of a femtosecond pulse. Oscillations corresponding to the energy differences between the levels in the wavepacket are superimposed on the UV signal when two identical pump and delayed probe pulses are introduced into the alkali vapor. Analyses of the UV emission line profiles show excellent agreement with the four-wave mixing process.

Dec 1, 19970 citations

Ultracold photoassociation spectroscopy: Long-range molecules and atomic scattering

K. M. Jones, Eite Tiesinga, Paul D. Lett +1 more

Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold $(<1\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$ temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the ``complications'' of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold $(<1\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$ temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the ``complications'' of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.

Reviews of Modern PhysicsMay 22, 2006873 citations

Single-color photoassociative ionization of ultracold sodium: The region from 0 to -5 GHz

Arjen Amelink, K. M. Jones, Paul D. Lett +2 more

We have measured the rate of production of ${\mathrm{Na}}_{2}^{+}$ ions in collisions of ultracold Na atoms held in a magneto-optical trap (MOT) as a function of probe laser detuning using a single-color probe beam. The ion rate is measured with the atoms mainly in the $3S(f=2)$ level (``bright MOT'') or the $3S(f=1)$ level (``dark MOT''). Using recent experimental information about the doubly excited autoionizing states of ${\mathrm{Na}}_{2},$ we find that the large structures in the first 5 GHz red of the atomic transition frequency are due to a doubly resonant excitation process with levels in the ${0}_{g}^{\ensuremath{-}}$ potential (asymptotically connected to ${3S}_{1/2}{+3P}_{3/2})$ as the intermediate states, and levels in the ${0}_{u}^{\ensuremath{-}}$ and ${1}_{u}$ potentials (asymptotically connected to ${3P}_{3/2}{+3P}_{3/2})$ as the final autoionizing states. We can account for nearly all of the observed structure in this region of the spectra for both the bright and dark MOT.We have measured the rate of production of ${\mathrm{Na}}_{2}^{+}$ ions in collisions of ultracold Na atoms held in a magneto-optical trap (MOT) as a function of probe laser detuning using a single-color probe beam. The ion rate is measured with the atoms mainly in the $3S(f=2)$ level (``bright MOT'') or the $3S(f=1)$ level (``dark MOT''). Using recent experimental information about the doubly excited autoionizing states of ${\mathrm{Na}}_{2},$ we find that the large structures in the first 5 GHz red of the atomic transition frequency are due to a doubly resonant excitation process with levels in the ${0}_{g}^{\ensuremath{-}}$ potential (asymptotically connected to ${3S}_{1/2}{+3P}_{3/2})$ as the intermediate states, and levels in the ${0}_{u}^{\ensuremath{-}}$ and ${1}_{u}$ potentials (asymptotically connected to ${3P}_{3/2}{+3P}_{3/2})$ as the final autoionizing states. We can account for nearly all of the observed structure in this region of the spectra for both the bright and dark MOT.

Physical Review AJun 13, 20008 citations

^{2}

S)+

{\mathrm{Na}}^{\mathrm{*}}

^{2}$${\mathit{P}}_{3/2}

) dissociation limit have been obtained by monitoring an enhanced

{\mathrm{Na}}_{2}^{+}

Pump-probe experiments are performed in which cold colliding Na atoms are photoassociated to form

{\mathrm{Na}}_{2}

molecules, and subsequently ionized. The experiments are performed in a regime where the pulsed photoassociation to an intermediate potential takes place at long range, and to several electronic, vibrational, and rotational states. A probe pulse then produces an ionization signal by further exciting these molecules to autoionizing doubly excited states. The time dependence of the ion signal shows a dramatic flux-enhancement effect on a nanosecond time scale due to the motion on the intermediate potentials. This time dependence can be viewed as monitoring the inward motion and dephasing of a population wave packet formed at long range.

We have performed photoassociation spectroscopy to study the doubly excited states of

{\mathrm{Na}}_{2}

involved in the associative ionization of colliding

\mathrm{Na}(3P)

atoms. Rovibrational levels of two doubly excited potentials connected to the

{P}_{3/2}{+P}_{3/2}

asymptote, one of ${0}_{u}^{\ensuremath{-}}$ and one of

{1}_{u}

symmetry, have been identified. These two doubly excited potentials dominate the route to ionization used in photoassociation experiments of cold Na. Binding energies and rotational constants for all the vibrational levels with binding energies

<50\mathrm{GHz}

have been measured. The asymptotic potentials and short-range ionization probabilities are extracted from the data.

We have observed the spectrum of long range molecular states of singly excited

{\mathrm{Na}}_{2}

formed by photoassociation during ultracold associative ionization collisions of laser-cooled Na atoms. The low kinetic energy of the collision allows us to obtain very high resolution spectra and to observe states close to the dissociation limit. The spectrum shows regular vibrational series characteristic of the 1/

{\mathit{R}}^{3}

long range portion of the

{1}_{\mathit{g}}

potential (asymptotically

{\mathit{S}}_{1/2}

{\mathit{P}}_{3/2}

) as well as molecular hyperfine structure within these peaks. We extract potential parameters and compare with recent theory.

Photoassociation is the process in which two colliding atoms absorb a photon to form an excited molecule. The development of laser-cooling techniques for producing gases at ultracold

(<1\phantom{\rule{0.3em}{0ex}}\mathrm{mK})

temperatures allows photoassociation spectroscopy to be performed with very high spectral resolution. Of particular interest is the investigation of molecular states whose properties can be related, with high precision, to the properties of their constituent atoms with the ``complications'' of chemical binding accounted for by a few parameters. These include bound long-range or purely long-range vibrational states in which two atoms spend most or all of their time at large internuclear separations. Low-energy atomic scattering states also share this characteristic. Photoassociation techniques have made important contributions to the study of all of these. This review describes what is special about photoassociation spectroscopy at ultracold temperatures, how it is performed, and a sampling of results including the determination of scattering lengths, their control via optical Feshbach resonances, precision determinations of atomic lifetimes from molecular spectra, limits on photoassociation rates in a Bose-Einstein condensate, and briefly, production of cold molecules. Discussions are illustrated with examples on alkali-metal atoms as well as other species. Progress in the field is already past the point where this review can be exhaustive, but an introduction is provided on the capabilities of photoassociation spectroscopy and the techniques presently in use.

We have measured the rate of production of

{\mathrm{Na}}_{2}^{+}

ions in collisions of ultracold Na atoms held in a magneto-optical trap (MOT) as a function of probe laser detuning using a single-color probe beam. The ion rate is measured with the atoms mainly in the

3S(f=2)

level (``bright MOT'') or the

3S(f=1)

level (``dark MOT''). Using recent experimental information about the doubly excited autoionizing states of

{\mathrm{Na}}_{2},

we find that the large structures in the first 5 GHz red of the atomic transition frequency are due to a doubly resonant excitation process with levels in the ${0}_{g}^{\ensuremath{-}}$ potential (asymptotically connected to

{3S}_{1/2}{+3P}_{3/2})

as the intermediate states, and levels in the ${0}_{u}^{\ensuremath{-}}$ and

{1}_{u}

potentials (asymptotically connected to

{3P}_{3/2}{+3P}_{3/2})

as the final autoionizing states. We can account for nearly all of the observed structure in this region of the spectra for both the bright and dark MOT.