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Physical Review A

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

Fredrik K. Fatemi, K. M. Jones, He Wang, Ian A. Walmsley, Paul D. Lett

Published Aug 17, 2001• 35 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.64.033421

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Pump-probe experiments are performed in which cold colliding Na atoms are photoassociated to form Na2{\mathrm{Na}}_{2}Na2​ 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.
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^{2}2S)+Na∗{\mathrm{Na}}^{\mathrm{*}}Na∗(3 2^{2}2P3/2{\mathit{P}}_{3/2}P3/2​) dissociation limit have been obtained by monitoring an enhanced Na2+{\mathrm{Na}}_{2}^{+}Na2+​ 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.
We have performed photoassociation spectroscopy to study the doubly excited states of Na2{\mathrm{Na}}_{2}Na2​ involved in the associative ionization of colliding Na(3P)\mathrm{Na}(3P)Na(3P) atoms. Rovibrational levels of two doubly excited potentials connected to the P3/2+P3/2{P}_{3/2}{+P}_{3/2}P3/2​+P3/2​ asymptote, one of ${0}_{u}^{\ensuremath{-}}$ and one of 1u{1}_{u}1u​ 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 <50GHz<50\mathrm{GHz}<50GHz have been measured. The asymptotic potentials and short-range ionization probabilities are extracted from the data.
The paper investigates cold molecules formation in the photoassociation of two cold atoms by a strong laser pulse applied at short interatomic distances, which lead to a molecular dynamics taking place in the light-induced (adiabatic) potentials. A two electronic states model in the cesium dimer is used to analyse the effects of this strong coupling regime and to show specific results: i) acceleration of the ground state population to the inner zone due to a non-impulsive regime of coupling at short and intermediate interatomic distances; ii) formation of cold molecules in strongly bound levels of the ground state, where the population at the end of the pulse is much bigger than the population photoassociated in bound levels of the excited state; iii) the final momentum distribution of the ground state wavepacket keeping the signatures of the maxima in the initial wavefunction continuum. It is shown that the topology of the light-induced potentials plays an important role in dynamics.
This paper investigates the formation of cold molecules in the photoassociation of two cold atoms by a strong laser pulse applied at short interatomic distances, which lead to molecular dynamics taking place in the light-induced (adiabatic) potentials. A two-electronic-state model in the caesium dimer is used to analyse the effects of this strong coupling regime and to show specific results: (i) acceleration of the ground-state population to the inner zone due to a non-impulsive regime of coupling at short and intermediate interatomic distances, (ii) formation of cold molecules in strongly bound levels of the ground state, where the population at the end of the pulse is much bigger than the population photoassociated in bound levels of the excited state, and (iii) the final momentum distribution of the ground-state wavepacket keeping the signatures of the maxima in the initial wavefunction continuum. It is shown that the topology of the light-induced potentials plays an important role in dynamics.
We analyze the formation of Rb2{\mathrm{Rb}}_{2}Rb2​ molecules with short photoassociation pulses applied to a cold 85Rb^{85}\mathrm{Rb}85Rb sample. A pump laser pulse couples a continuum level of the ground electronic state $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$ with bound levels in the 0u+(5S+5P1∕2){0}_{u}^{+}(5S+5{P}_{1∕2})0u+​(5S+5P1∕2​) and 0u+(5S+5P3∕2){0}_{u}^{+}(5S+5{P}_{3∕2})0u+​(5S+5P3∕2​) vibrational series. The nonadiabatic coupling between the two excited channels induces time-dependent beatings in the populations. We propose to take advantage of these oscillations to design further laser pulses that probe the photoassociation process via photoionization or that optimize the stabilization in deep levels of the ground state.
The dynamics of the excited state during the photoassociation of cold molecules from cold rubidium atoms is studied in a series of pump-probe experiments. Dipole transitions similar to those of the atoms are observed in the molecular signal. While such behaviour is characteristic of the long-range molecules, the photoassociation of bound molecules is confirmed in additional experiments. The pump-probe signal observed on a 250 ps time scale did not, however, reveal wavepacket oscillations predicted by theory. This result is discussed using numerical simulations of photoassociation and a modification to the current experiments that could lead to the detection of wavepacket dynamics is suggested.