"Broadband optical cooling of molecular rotors," C.-Y. Lien, C.S. Seck, and. B.C. Odom. arXiv:1402.3918 [physics.atom-ph] (2014)
Contrary to intuition, laser excitation of bound electrons can decrease the temperature of a system, with electronic relaxation times as fast as nanoseconds allowing for rapid cooling to far below ambient temperature. Although laser cooling of atoms is routine owing to their relatively simple internal structure, laser cooling of molecular translational speeds, vibrations, or rotations is challenging because a different laser frequency is required to electronically excite each populated vibrational and rotational state. Partially addressing these challenges, a broadband laser has recently been used to cool molecular vibrations, and narrowband lasers have been used to cool translational speeds of certain molecules that electronically relax without undesirable vibrational excitation. Since many molecular rotational states are populated at room temperature, it is of great interest to also cool rotations; however, laser excitation of molecular electrons has not previously been used to cool rotations of warm molecules. Here, we show that molecules with decoupled vibrational and electronic modes can be rotationally cooled using a single spectrally filtered broadband laser to simultaneously address many rotational states. We optically cool singly-charged aluminum monohydride (AlH+) molecules held in a room-temperature radiofrequency Paul trap to collect 95% of the population in the ground quantum state, corresponding to a rotational temperature of 4 K. In our current implementation, parity-preserving electronic cycling cools to the two lowest rotational levels with a timescale set by repeated electronic relaxation, and collection into the lowest level utilizes a slower vibrational relaxation event setting the overall cooling timescale to 100 ms; straightforward modifications could allow vibrationless cooling to the ground state in 10 ms. We anticipate that the ability to quickly cool trapped molecular rotors and to repeatedly reset their quantum state will impact several areas of research, including quantum information processing, precision tests of fundamental symmetries, and searches for time-varying constants.
  "Rotational State Analysis of AlH+ by Two-Photon Dissociation," C.S. Seck, E.G. Hohenstein, C.-Y. Lien, P. R. Stollenwerk, and. B.C. Odom. arXiv:1402.0123 [physics.atom-ph] (2014)
We perform ab initio calculations needed to predict the cross-section of an experimentally accessible (1+1') resonance-enhanced multiphoton dissociation (REMPD) pathway in AlH+. Experimenting on AlH+ ions held in a radiofrequency Paul trap, we confirm dissociation via this channel with analysis performed using time-of-flight mass spectrometry. We demonstrate the use of REMPD for rotational state analysis, and we measure the rotational distribution of trapped AlH+ to be consistent with the expected thermal distribution. AlH+ is a particularly interesting species for ion trap work because of its electronic level structure, which makes it amenable to proposals for rotational optical pumping, direct Doppler cooling, and single-molecule fluorescence detection. Potential applications of trapped AlH+ include searches for time-varying constants, quantum information processing, and ultracold chemistry studies.
  "Resonant Few-Photon Excitation of a Single-Ion Oscillator," Y.-W. Lin, S. Williams, and B. C. Odom. Phys. Rev. A 87, 011402(R) (2013)

We study the motion of an undamped single-ion harmonic oscillator, resonantly driven with a pulsed radiation pressure force.We demonstrate that a barium ion, initially cooled to the Doppler limit, quickly phase locks to the drive and builds up coherent oscillations above the thermal distribution after scattering of order 100 photons. In our experiment, this seeded motion is subsequently amplified and then analyzed by Doppler velocimetry. Since the coherent oscillation is conditional upon the internal quantum state of the ion, this motional excitation technique could be useful in atomic or molecular single-ion spectroscopy experiments, providing a simple protocol for state readout of nonfluorescing ions with partially closed-cycle transitions.
  "Suitability of linear quadrupole ion traps for large Coulomb crystals," D.A. Tabor, V. Rajagopal, Y-W. Lin and B. Odom. Appl. Phys. B. 107, 1097-1104 (2012).

Growing and studying large Coulomb crystals, composed of tens to hundreds of thousands of ions, in linear quadrupole ion traps presents new challenges for trap implementation. We consider several trap designs, first comparing the total driven micromotion amplitude as a function of location within the trapping volume; total micromotion is an important point of comparison since it can limit crystal size by transfer of radiofrequency drive energy into thermal energy. We also compare the axial component of micromotion, which leads to first-order Doppler shifts along the preferred spectroscopy axis in precision measurements on large Coulomb crystals. Finally, we compare trapping potential anharmonicity, which can induce nonlinear resonance heating by shifting normal mode frequencies onto resonance as a crystal grows. We apply a non-deforming crystal approximation for simple calculation of these anharmonicity-induced shifts, allowing a straightforward estimation of when crystal growth can lead to excitation of different nonlinear heating resonances. In the axial micromotion and anharmonicity points of comparison, we find significant differences between the compared trap designs, with an original rotated-endcap trap performing slightly better than the conventional in-line endcap trap.
  "Optical pulse-shaping for internal cooling of molecules," C. Lien, S. Williams and B. Odom. Physical Chemistry Chemical Physics, 13, 18825-18829 (2011)

We consider the use of pulse-shaped broadband femtosecond lasers to optically cool rotational and vibrational degrees of freedom of molecules. Since this approach relies on cooling rotational and vibrational quanta by exciting an electronic transition, it is most easily applicable to molecules with similar ground and excited potential energy surfaces, such that the vibrational state is usually unchanged during electronic relaxation. Compared with schemes that cool rotations by exciting vibrations, this approach achieves internal cooling on the orders-of-magnitude faster electronic decay timescale and is potentially applicable to apolar molecules. For AlH+, a candidate species, a rate- equation simulation shows that rovibrational equilibrium should be achievable in 7 μs. In addition, we report laboratory demonstration of optical pulse shaping with sufficient resolution and power for rotational cooling of AlH+.
  "Challenges of laser-cooling molecular ions," J.H.V. Nguyen, C.R. Viteri, E.G. Hohenstein, C.D. Sherrill, K.R. Brown and B. Odom. New J. Phys. 13, 063023 (2011).

The direct laser cooling of neutral diatomic molecules in molecular beams suggests that trapped molecular ions can also be laser cooled. The long storage time and spatial localization of trapped molecular ions provides an opportunity for multi-step cooling strategies, but also requires careful consideration of rare molecular transitions. We briefly summarize the requirements that a diatomic molecule must meet for laser cooling, and we identify a few potential molecular ion candidates. We then carry out a detailed computational study of the candidates BH+ and AlH+, including improved ab initio calculations of the electronic state potential energy surfaces and transition rates for rare dissociation events. On the basis of an analysis of the population dynamics, we determine which transitions must be addressed for laser cooling, and compare experimental schemes using continuous-wave and pulsed lasers.
  "Prospects for Doppler cooling of three-electronic-level molecules," J.H.V. Nguyen and B. Odom, Phys Rev. A. 83, 053404 (2011).

Analogous to the extension of laser cooling techniques from two-level to three-level atoms, Doppler cooling of molecules with an intermediate electronic state is considered. In particular, we use a rate-equation approach to simulate cooling of SiO+ , in which population buildup in the intermediate state is prevented by its short lifetime. We determine that Doppler cooling of SiO+ can be accomplished without optically repumping from the intermediate state, at the cost of causing undesirable parity flips and rotational diffusion. Since the necessary repumping would require a large number of continuous-wave lasers, optical pulse shaping of a femtosecond laser is proposed as an attractive alternative. Other candidate three-electron-level molecules are also discussed.


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