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Physical Review Letters

Photoassociation Spectroscopy of Cold He(23S) Atoms

Norbert Herschbach, Paul J. J. Tol, W. Vassen, W. Hogervorst, G.R. Woestenenk, Jan W. Thomsen, P. van der Straten, Arend Niehaus

Published Feb 28, 2000• 68 citations
Publisher

Abstract

We observe vibrational states by photoassociation spectroscopy of cold He(2 3S) atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 3S1-2 3P2 transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.

DOI
10.1103/physrevlett.84.1874

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We observe vibrational states by photoassociation spectroscopy of cold He(2 3S) atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic 2 3S1-2 3P2 transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.
We study the role of Penning ionization on the photoassociation spectra of He(^3S)-He(^3S). The experimental setup is discussed and experimental results for different intensities of the probe laser are shown. For modelling the experimental results we consider coupled-channel calculations of the crossing of the ground state with the excited state at the Condon point. The coupled-channel calculations are first applied to model systems, where we consider two coupled channels without ionization, two coupled channels with ionization, and three coupled channels, for which only one of the excited states is ionizing. Finally, coupled-channel calculations are applied to photoassociation of He(^3S)-He(^3S) and good agreement is obtained between the model and the experimental results.
We have studied photoassociation of metastable 2 3 S 1 helium atoms near the 2 3 S 1 -2 3 P 2 asymptote by both ion detection in a magneto-optical trap and trap-loss measurements in a magnetic trap. A detailed comparison between the results of the two experiments gives insight into the mechanism of the Penning ionization process. We have identified four series of resonances corresponding to vibrational molecular levels belonging to different rotational states in two potentials. The corresponding spin states become quasi-purely quintet at small interatomic distance, and Penning ionization is inhibited by spin conservation rules. Only a weak rotational coupling is responsible for the contamination by singlet spin states leading to a detectable ion signal. However, for one of these series, Bose statistics does not enable the rotational coupling and the series detected through trap-loss does not give rise to sufficient ionization for detection.
We report the photoassociation spectroscopy of laser-cooled ytterbium atoms in an optical trap. We observed more than 90 photoassociation resonances of vibrational levels in the 1Σu+ state, including 80 consecutive series, up to 490 GHz detuning with respect to the atomic resonance. From the resonance frequencies we derived the atomic radiative lifetime of the (6s6p) 1P1 state to be 5.464±0.005 ns, which is about 2 orders of magnitude improvement over previous results. We also observed line broadening of resonances, which is ascribed to the predissociation to the triplet states, and estimated the transition probability to be 0.2. Furthermore, we observed the decrease of the photoassociation signal intensity, from which the scattering length is estimated to be equal to or less than 3 nm.
We report the photoassociation spectroscopy of laser-cooled ytterbium atoms in an optical trap. We observed more than 90 photoassociation resonances of vibrational levels in the (1)Sigma(+)(u) state, including 80 consecutive series, up to 490 GHz detuning with respect to the atomic resonance. From the resonance frequencies we derived the atomic radiative lifetime of the (6s6p) 1P1 state to be 5.464+/-0.005 ns, which is about 2 orders of magnitude improvement over previous results. We also observed line broadening of resonances, which is ascribed to the predissociation to the triplet states, and estimated the transition probability to be 0.2. Furthermore, we observed the decrease of the photoassociation signal intensity, from which the scattering length is estimated to be equal to or less than 3 nm.
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.
We demonstrate a number of two-colour spectroscopy techniques where the first step is the photoassociation of laser-cooled and trapped sodium atoms to form bound states of . High-resolution spectra of are obtained. Spectra of the `purely long-range' state demonstrates the use of two-colour spectroscopy to open up an ionization channel for detecting the occurrence of photoassociation. The utility of photoassociation as a technique for producing cold molecular samples in a well defined rovibrational state is demonstrated with spectra showing and (Condon internal diffraction) transitions after the photoassociation. Starting from states in the and potentials, these transitions include upward transitions to autoionizing potentials dissociating near 3P + 3P and downward transitions back to the 3S + 3S ground-state potentials. Spectroscopic constants for six levels of a doubly excited state of are given.