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Physical Review Letters

Spectroscopy of Na2 by photoassociation of laser-cooled Na

Paul D. Lett, Kristian Helmerson, William D. Phillips, L. P. Ratliff, S. L. Rolston, Mark E. Wagshul

Published Oct 4, 1993• 227 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.71.2200

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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.
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 present a new spectroscopic technique which allows the observation of long-range molecular states of ultracold sodium via the associative ionization (AI) reaction. Due to the ultracold conditions, we are able to achieve very high resolution spectra of free-bound molecular transitions which were previously observable only via bound-bound spectroscopy. Previous experiments using laser cooled sodium have revealed evidence of molecular spectra but were limited to a narrow tuning range and power broadened because the intense trapping laser was also used to excite the molecular transitions. By alternating the beams of a magneto-optical trap with a separate laser, we confine the atoms while leaving them unperturbed by the trapping light during the excitation period. The second laser excites free ground state atoms into the bound molecular states. With this scheme, we have been able to detect the vibrational series of molecular states in the intermediate AI excitation step, from just below threshold to 3 cm{sup {minus}1} deep. By resolving narrow structure within the spectra associated with molecular hyperfine splittings and comparing with theoretical calculations, we can unambiguously identify the molecular states involved.
We report two-color photoassociative ionization of sodium in a magneto-optical trap. The experimental results yield information on both singly and doubly excited states. We find that the highest bound vibrational levels ( v>20v>20v>20) of the singly excited ${0}_{g}^{\ensuremath{-}}$ state predissociate into the 32P3/2+32S1/2(Fg=1){3}^{2}{P}_{3/2}{+3}^{2}{S}_{1/2}({F}_{g}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1)32P3/2​+32S1/2​(Fg​=1) dissociation continuum due to avoided crossings of the hyperfine components of this potential with other molecular symmetries. Based on symmetry and energy consideration we argue that a doubly excited 1u{1}_{u}1u​ state remains autoionizing even when excited only a few GHz above the dissociation continuum.