[Paper Review] Observation of Cs Rydberg atom macrodimers

K. Richard Overstreet, Arne Schwettmann+3 more|ArXiv.org|Jan 13, 2009

Cold Atom Physics and Bose-Einstein Condensates16 references3 citations

TL;DR

This paper reports the first experimental observation of cold cesium Rydberg atom macrodimers bound at internuclear separations of 3–9 μm, formed via avoided crossings in Rydberg pair interaction potentials under an applied electric field. The molecules are detected by mapping ion recoil velocities after pulsed field ionization, revealing a stable radial separation distribution consistent with bound states, with excellent agreement between experiment and Monte Carlo simulations using no adjustable parameters.

ABSTRACT

We report the observation of cold Cs Rydberg atom molecules bound at internuclear separations of R~3-9 um. The bound states result from avoided crossings between Rydberg atom pair interaction potentials in an applied electric field. The molecular states can be modified by changing the applied electric field. The molecules are observed by mapping the radial separation of the two Rydberg atoms as a function of time delay between excitation and detection using the Coulomb repulsion of the ions after pulsed field ionization. Measurements were performed for 63D+65D, 64D+66D, 65D+67D, and 66D+68D pairs. The experiment is in good agreement with calculations of the pair interactions for these states.

Motivation & Objective

• To unambiguously demonstrate the existence of bound macrodimers formed by two cold Rydberg cesium atoms at large internuclear distances.
• To resolve the long-standing experimental challenge of detecting macrodimers due to their quasi-continuum ro-vibrational spectrum and broad linewidths.
• To investigate the role of an external electric field in stabilizing, modifying, or destroying the potential wells that support these bound states.
• To validate theoretical predictions of Rydberg pair interactions and potential wells using time-resolved ion detection.

Proposed method

• Detection of macrodimers via time-delayed pulsed field ionization (PFI), where Coulomb repulsion between ionized atoms produces recoil velocities proportional to their radial separation at ionization.
• Measurement of time-of-flight (TOF) distributions of ion fragments to infer the radial separation distribution of Rydberg atom pairs as a function of time delay between excitation and ionization.
• Use of Monte Carlo simulations to model ion trajectories, including spectrometer geometry, electric field strength, PFI pulse timing, and initial radial separation drawn from calculated potential wells.
• Comparison of simulated TOF distributions with experimental data to validate the presence of bound states, using no adjustable parameters.
• Systematic variation of the electric field to tune avoided crossings and control the depth and width of potential wells supporting bound states.
• Controlled measurement of TOF distributions with laser polarization perpendicular to the TOF axis to test for molecular alignment effects.

Experimental results

Research questions

• RQ1 Can bound macrodimers of cold Rydberg cesium atoms be experimentally observed at internuclear separations of 3–9 μm?
• RQ2 Does the radial separation distribution of Rydberg atom pairs remain stable over time delay when bound, as opposed to dissociating?
• RQ3 Can an external electric field be used to control the formation and characteristics of macrodimer states via avoided crossings in pair interaction potentials?
• RQ4 To what extent do experimental TOF distributions of ion recoil velocities agree with simulations based on calculated Rydberg pair potentials and no adjustable parameters?
• RQ5 Is there evidence of molecular alignment in the macrodimer signal, and if so, is it induced by the applied electric field?

Key findings

• The experiment provides the first unambiguous observation of bound macrodimers in Cs Rydberg atoms, with radial separations of 3–9 μm, confirmed by stable recoil velocity distributions over time delay.
• The measured TOF distributions for molecular states show excellent agreement with Monte Carlo simulations using no adjustable parameters, validating the theoretical model of Rydberg pair interactions.
• The macrodimer signal is strongly dependent on the applied electric field, with potential wells and bound states appearing only at specific field strengths due to avoided crossings.
• The recoil velocity of the ion pairs is significantly lower than the thermal velocity of the cold gas, indicating that the atoms remain bound and do not dissociate freely.
• The experimental data rule out selective excitation of low-velocity pairs as the origin of the signal, given the laser spectral bandwidth and intensity dependence.
• No significant change in TOF distribution width or shape was observed when laser polarization was rotated perpendicular to the TOF axis, suggesting any molecular alignment is likely induced by the electric field rather than the laser.