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Physical Review Letters

Photoassociation of Cold Ca Atoms

G. Zinner, T. Binnewies, F. Riehle, E. Tiemann

Published Sep 11, 2000• 114 citations
Publisher

Abstract

We present the first measurement of a photoassociative spectrum of an alkaline earth element near the dissociation limit. The observed spectrum of Ca2 formed from cold atoms shows the regular vibrational series with the characteristic spacing of the 1/R3 asymptotic potential. The interpretation is in principle simplified compared to previous measurements on alkali metals by the nondegenerate ground state and the missing hyperfine structure of 40Ca. As an example, we derive the natural decay rate of the excited atomic 4p 1P1 state from the positions of the observed vibrational and rotational resonances with reduced uncertainty compared to previous measurements.

DOI
10.1103/physrevlett.85.2292

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We present the first measurement of a photoassociative spectrum of an alkaline earth element near the dissociation limit. The observed spectrum of Ca2 formed from cold atoms shows the regular vibrational series with the characteristic spacing of the 1/R3 asymptotic potential. The interpretation is in principle simplified compared to previous measurements on alkali metals by the nondegenerate ground state and the missing hyperfine structure of 40Ca. As an example, we derive the natural decay rate of the excited atomic 4p 1P1 state from the positions of the observed vibrational and rotational resonances with reduced uncertainty compared to previous measurements.
We have observed the spectrum of long range molecular states of singly excited Na2{\mathrm{Na}}_{2}Na2​ 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/R3{\mathit{R}}^{3}R3 long range portion of the 1g{1}_{\mathit{g}}1g​ potential (asymptotically S1/2{\mathit{S}}_{1/2}S1/2​+P3/2{\mathit{P}}_{3/2}P3/2​) as well as molecular hyperfine structure within these peaks. We extract potential parameters and compare with recent theory.
In the molecular photoassociation process, two free cold atoms absorb one photon by a resonant way, to create an excited molecule in a well defined ro-vibrational state. The photoassociative spectroscopy of alkaline dimers yields accurate determinations for the long-range parts of molecular potential curves correlated to the first s + p asymptote. In contrast to atoms, laser cooling of molecules is very difficult because of the lack of a closed two-level scheme for recycling the population. The molecular photoassociation of cold atoms opens also a promising alternative for the formation of cold ground state molecules. Photoassociation of cold cesium atoms has permitted the first observation of translationally cold ground state molecules. The case of the cesium atom is very favorable because the 0 g - and 1 u purely long-range states below the dissociation limit 6s 1/2 +6p 3/2 present Condon points at intermediate distances. Such configurations offer rather efficient channels for the creation of singlet or triplet ground state Cs 2 molecules after spontaneous decay.
The properties of photoassociation (PA) spectra near the intercombination line (the weak transition between and states) of group II atoms are theoretically investigated. As an example, we have carried out a calculation for calcium atoms colliding at ultralow temperatures of , , and . Unlike in most current photoassociation spectroscopy, the Doppler effect can significantly affect the shape of the investigated lines. Spectra are obtained using and short-range potentials and long-range van der Waals and resonance dipole potentials. The similar van der Waals coefficients of ground and excited states cause the PA to differ greatly from those of strong, allowed transitions with resonant dipole interactions. The density of spectral lines is lower, the Condon points are at relatively short range, the reflection approximation for the Franck-Condon factors is not applicable, and the spontaneous decay to bound ground-state molecules is efficient. Finally, the possibility of efficient production of cold molecules is discussed.
We study the photoassociative collisional loss of laser-cooled Rb atoms from a far-off resonance optical dipole force atom trap. We obtain a well-resolved photoassociation spectrum from 50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to 980 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ below the first excited dissociation limit of the Rb2{\mathrm{Rb}}_{2}Rb2​ molecule. Two vibrational series associated with excited Rb2{\mathrm{Rb}}_{2}Rb2​ $^{3}\mathrm{\ensuremath{\Sigma}}_{\mathit{g}}^{+}$ states are clearly visible. Oscillations in the associated Franck-Condon factors reflect the structure of the triplet ground state wave function. Our results clearly demonstrate the potential of photoassociation spectroscopy as a new probe of molecular structure.
Photoassociation spectroscopy experiments on 40Ca{}^{40}\mathrm{Ca}40Ca atoms close to the dissociation limit ${4s4s}^{1}{S}_{0}\ensuremath{-}{4s4p}^{1}{P}_{1}$ are presented. The vibronic spectrum was measured for detunings of the photoassociation laser ranging from 0.6 to 68 GHz with respect to the atomic resonance. In contrast to previous measurements, the rotational splitting of the vibrational lines was fully resolved. Full quantum-mechanical numerical simulations of the photoassociation spectrum were performed which allowed us to put constraints on the possible range of the calcium scattering length between 50a0{50a}_{0}50a0​ and 300a0.{300a}_{0}.300a0​.