High Resolution Spectroscopy

Molecular Spectroscopy at Ultracold Temperatures

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

Published Jan 1, 1993• 0 citations

Abstract

Associative ionization (AI) in sodium (Na + Na + 2 hv → Na2+ + e−) has been a convenient reaction in which to study excited state collisions under "ultracold" conditions (≤1 mK) because the reaction product is easily identified and collected with high signal-to-noise. Previous studies [1] have revealed evidence of molecular intermediate-state resonances under high-intensity laser conditions. Theoretical analysis [2] has associated this structure with singly- and doubly-excited Na2 intermediate states. In this work, we present a new technique utilizing a switched magneto-optical trap (MOT) to make clean measurements of the spectrum of associative ionization in a regime where we can identify the peaks as being associated with states of the Na2 molecule. In addition, our technique reveals new structure in the spectrum of AI versus laser frequency, pointing out the previously unappreciated importance of atomic hyperfine structure in the reaction.

DOI: 10.1364/hrs.1993.pd12

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High Resolution Spectroscopy

Molecular Spectroscopy at Ultracold Temperatures

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

Published Jan 1, 1993• 0 citations

Publisher

Abstract

DOI: 10.1364/hrs.1993.pd12

Related papers

High resolution molecular spectroscopy using laser cooled sodium atoms

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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 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.

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We report new calculations of two-body associative ionization collisions between sodium atoms confined in a magneto-optic trap at ultracold temperatures. These collisions represent a new kind of {open_quotes}open{close_quotes} or dissipative collision for which the energy of the atom + applied light field need not be conserved due to spontaneous emission coupling to the vacuum modes of the radiation field and require density matrix methods rather than wavefunction methods to describe the collision. In these collisions the Na atoms must absorb a photon when they are in close enough proximity that a significant fraction of the excited population survives relaxation back to the ground state. The Na atoms accelerate towards each other on the excited molecular state and absorbs a second photon to a doubly excited state which then proceeds to associatively ionize at close range. Our calculations incorporate a molecular picture of the atomic collision, laser optical-field dressing of the molecular states participating in the dynamics, and population and coherence decay due to spontaneous emission. The effect of excited state hyperfine structure, which plays a critical role in the dynamics, is simulated by simplified models with a reduced number of states based on the full set of molecular-hyperfine adiabatic potentialmore » curves. Comparison is made with recent measurements of photo-associative ionization. rates as a function of optical field intensity from about 20 to 130 mW cm{sup -2}.« lessWe report new calculations of two-body associative ionization collisions between sodium atoms confined in a magneto-optic trap at ultracold temperatures. These collisions represent a new kind of {open_quotes}open{close_quotes} or dissipative collision for which the energy of the atom + applied light field need not be conserved due to spontaneous emission coupling to the vacuum modes of the radiation field and require density matrix methods rather than wavefunction methods to describe the collision. In these collisions the Na atoms must absorb a photon when they are in close enough proximity that a significant fraction of the excited population survives relaxation back to the ground state. The Na atoms accelerate towards each other on the excited molecular state and absorbs a second photon to a doubly excited state which then proceeds to associatively ionize at close range. Our calculations incorporate a molecular picture of the atomic collision, laser optical-field dressing of the molecular states participating in the dynamics, and population and coherence decay due to spontaneous emission. The effect of excited state hyperfine structure, which plays a critical role in the dynamics, is simulated by simplified models with a reduced number of states based on the full set of molecular-hyperfine adiabatic potentialmore » curves. Comparison is made with recent measurements of photo-associative ionization. rates as a function of optical field intensity from about 20 to 130 mW cm{sup -2}.« less

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

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Physical Review AJun 13, 20008 citations

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.

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.