A universal method has been developed in our group for measuring the spectra of molecular ions in a 22-pole radio frequency trap at low temperatures. It is based on laser induced inhibition of complex growth (LIICG)¹. At low temperatures and high number densities of buffer gas, helium attaches to ions via ternary association. The formation of these weakly bound complexes, however, is inhibited following resonant absorption of the bare molecular ion. The first successful measurements have been demonstrated on the A ²Π_u ← X ²Σ_g⁺ electronic transition of N₂⁺, with some thousand N₂⁺ ions, helium densities of 10¹⁵ cm⁻³, and storage times of 1 s. The reduction in the number of N₂⁺−He complexes is the result of an interplay between excitation, radiative and collisional cooling, ternary association, and collision induced dissociation, and is explained using a kinetic model. The method is also applicable to larger molecular species. In this case internal conversion following electronic excitation produces internally "hot" ions, reducing the attachment of helium. The technique is universal because complex formation can be impeded over a wide wavelength range. [1] S. Chakrbarty, M. Holz, E. K. Campbell, A. Banerjee, D. Gerlich, and J. P. Maier, J. Phys. Chem. Lett. 2013, 4, 4051.

Following the sequential pick-up of OH and CO by helium nanodroplets, the infrared depletion spectrum is measured in the fundamental OH stretching region. Although several potentially accessible minima exist on the associated OH + CO reactive potential energy surface [e.g. J. Ma, J. Li, and H. Guo, J. Phys. Chem. Lett. 3 (2012) 2482], such as the weakly bound OH-OC dimer and the chemically bound HOCO molecule, we only observe the weakly bound OH-CO dimer. The rovibrational spectrum of this complex displays narrow (0.02 cm⁻¹) Lorentzian shaped peaks with spacings that are characteristic of a linear complex with unquenched electronic angular momentum, similar to what was previously observed in the gas phase [M.I. Lester, B.V. Pond, D.T. Anderson, L.B. Harding, and A.F. Wagner, J. Chem. Phys. 113 (2000) 9889]. Analogous spectra involving OD were collected, for which we also only observe the OD-CO isomer. From the Stark spectra, the dipole moments for OH-CO are determined to be 1.85(3) and 1.89(3) D for v=0 and v=1, respectively, while the analogous dipole moments for OD-CO are determined to be 1.88(8) and 1.94(5) D. The computed equilibrium ground state dipole moment at the CCSD(T)/Def2-TZVPD level of theory is 2.185 D, in disagreement with experiment. The role of vibrational averaging is investigated via the solution of a three-dimensional vibrational Schrödinger equation, which is constructed in internal bond-angle coordinates. The computed expectation value of the ground state dipole moment is in excellent agreement with experiment, indicating a floppy molecular complex.

Light polar, paramagnetic molecules, such as magnesium hydride, MgH, are attractive for slowing and trapping experiments because these molecules have both non-zero permanent electric dipole, μ_el, and magnetic dipole, μ_m moments. The permanent electric dipole moment is particularly relevant to Stark deceleration which depends on the ratio of the Stark shift to molecular mass. Here we report on the Stark effect in the (0,0) A²Π – X ²Σ⁺ band system of a cold molecular beam sample of magnesium deuteride, MgD. The lines associated with the lowest rotational levels are detected for the first time. The field-free spectrum was analyzed to produce an improved set of fine structure parameters for the A²Π(v = 0) state. The observed electric field induced splittings and shifts were analyzed to produce permanent electric dipole moments, μ_el,of 2.561(10)D and 1.34(8)D for A²Π(v = 0) and X²Σ⁺(v=0)states, respectively. This is the first molecular beam study of MgD.

We aim to slow a supersonic, molecular beam of ¹¹BH using a Zeeman slower and subsequently cool the molecules to sub-millikelvin temperatures in a magneto-optical trap. Most molecules are not suitable for direct laser cooling because the presence of rotational and vibrational degrees of freedom means there is no closed-cycle transition which is necessary to scatter a large number of photons. As was pointed out by Di Rosa¹ there exists a class of molecules for which the excitation of vibrational modes is suppressed due to highly diagonal Franck-Condon factors. Furthermore, Stuhl et al.² showed that angular momentum selection rules can be used to suppress leakage to undesired rotational states. Here we present a measurement of the radiative branching ratios of the A¹Π→ X¹Σ transition in ¹¹BH - a necessary step towards subsequent laser cooling experiments. We also perform high-resolution mm-wave spectroscopy of the J′=1← J=0 rotational transition in the X¹Σ(v=0) state near 708 GHz. From this measurement we derive new, accurate hyper fine constants and compare these to theoretical descriptions. The measured branching ratios suggest that it is possible to laser cool ¹¹BH molecules close to the recoil temperature of 4 μK using three laser frequencies only.
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¹M. D. Di Rosa, The European Physical Journal D 31, 395, 2004 ²B. K. Stuhl et al., Physical Review Letters 101, 243002, 2008

Reactions of laser-cooled Ca⁺ and Ba⁺ ions with O₂ provide pathways to the formation of cold molecular cations (CaO⁺ and BaO⁺) that may be further manipulated using ion trapping techniques. Spectroscopic data for these ions are needed to facilitate the characterization of internal state population distributions using highly sensitive detection schemes such as resonantly enhanced multi-photon dissociation (REMPD). Ab initio electronic structure calculations predict that both ions have X¹Σ⁺ ground states, accompanied by low-lying A²Π states. We are currently using two-color photoionization techniques to observe the low-lying ro-vibronic states of CaO⁺ and BaO⁺. The neutral molecules are produced by laser ablation of the metals, combined with free-jet expansion driven by He/O₂ (0.1-0.2%) mixtures. Photoionization efficiency curves and zero kinetic energy photoelectron spectra will be reported.

Having three and four electrons, respectively, He₂⁺ and He₂ represent systems for which highly accurate ab-initio calculations might become feasible in the near future¹. With the goal of performing accurate measurements of the rovibrational energy-level structure of He₂⁺ by Rydberg spectroscopy of He₂ and multichannel quantum-defect theory extrapolation techniques², we have produced samples of helium molecules in the a ³Σ_u⁺ state in supersonic beams with velocities tunable down to 100 m/s by combining a cryogenic supersonic-beam source with a multistage Zeeman decelerator³. The molecules are formed at an initial velocity of 500 m/s by striking a discharge in the pulsed expansion of helium gas from a reservoir kept at a cryogenic temperature of 10 K. Using rotationally-resolved PFI-ZEKE (pulsed-field-ionization zero-kinetic-energy) photoelectron spectroscopy, we have probed the rotational-state distribution of the molecules produced in the discharge and found vibrational levels up to ν" = 2 and rotational levels up to N"=21 to be populated. The molecular beam is coupled to a multistage Zeeman decelerator⁴ that employs pulsed inhomogeneous magnetic fields to further reduce the beam velocity. By measuring the quantum-state distribution of the decelerated sample using photoelectron and photoionization spectroscopy we observed no rotational or vibrational state-selectivity of the deceleration process, but found that one of the three spin-rotation components of the He₂ a ³Σ_u⁺ rotational levels is eliminated.
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¹W.-C. Tung, M. Pavanello, L. Adamowicz, J. Chem. Phys. 136, 104309 (2012). ²D. Sprecher, J. Liu, T. Krähenmann, M. Schäfer, and F. Merkt, J. Chem. Phys. 140, 064304 (2014). ³M. Motsch, P. Jansen, J. A. Agner, H. Schmutz, and F. Merkt, arXiv:1401.7774. ⁴N. Vanhaecke, U. Meier, M. Andrist, B. H. Meier, and F. Merkt, Phys. Rev. A 75, 031402(R) (2007).

To laser-cool a species, a closed (or nearly closed) cycle is required to dissipate translational energy through many directed laser-photon absorption and subsequent randomly-directed spontaneous emission events. Many atoms lend themselves to such a closed-loop cooling cycle. Attaining laser-cooled molecular species is challenging because of their inherently complex internal structure, yet laser-cooling molecules could lead to studies in interesting chemical dynamics among other applications. Typically, laser-cooled atoms are assembled into molecules through photoassociation or Feschbach resonance. CaH is one of a few molecules whose internal structure is quite atom-like, allowing a nearly closed cycle without the need for many repumping lasers. We will also present our work-to-date on laser cooling this molecule. We employ traditional pulsed atomic/molecular beam techniques with a laser vaporization source to generate species with well-defined translational energies over a narrow range of velocity. In this way, we can apply laser-cooling to most species in the beam along a single dimension (the beam’s axis). This project is funded by the LDRD program of the Los Alamos National Laboratory.

We perform vibrational spectroscopy on the V_0←10 overtone of a trapped and sympathetically cooled CaH⁺ molecular ion using a resonance enhanced two photon dissociation scheme. Our experiments are motivated by theoretical work that proposes comparing the vibrational overtones of CaH⁺ with electronic transitions in atoms to detect possible time variation of in the mass ratio of the proton to electron ^1

We demonstrate sympathetic Doppler cooling and our progress towards sideband cooling of a CaH⁺ ion co-trapped with a Ca⁺ atomic ion in a linear Paul trap. Molecular ions are generally difficult to laser cool due to a lack of closed electronic transitions as a result of vibrational and rotational states. Despite this challenge, they can be cooled indirectly through their Coulombic interaction with a fluorescent atomic ion that is being directly laser cooled. Ions are firstly Doppler cooled to get to the Lamb-Dicke regime, where the ion motion is small relative to the excitation wavelength and then sideband cooled reaching temperatures below 1 μK. All the ions' axial modes (center of mass and breathing mode) and radial modes ( two center of mass and two tilt modes) are addressed, and the temperature is determined by examining the ratio of sidebands.

We demonstrate that a single spectrally filtered femtosecond laser, tuned to the electronic A-X transition of trapped AlH⁺, can efficiently cool rotations from room temperature to the ground state. The nearly diagonal Franck-Condon-Factors between the electronic X and A states create semi-closed cycling transitions between the vibrational ground states of the X and A states. Parity-preserving electronic cycling cools to the two lowest rotational levels with a 10 μs timescale set by repeated electronic relaxation, and collection into the lowest rotational level relies upon a slower vibrational relaxation event setting the overall cooling timescale to 140 ms. The population distribution among the rotational levels is detected by (1+1’) resonance-enhanced multiphoton dissociation (REMPD) and time-of-flight mass-spectrometry (TOFMS).

Cryogenic buffer gas cooled beams and cells can be used to study many species, from atoms and polar molecules to biomolecules. We report on recent applications of this technique to improve the limit on the electron electric dipole moment [1], load polar molecules into a magnetic trap through optical pumping [2], perform chirally sensitive microwave spectroscopy on polyatomic molecules [3], progress towards magneto-optical trapping of polar molecules [4], and studies of atom-molecule sticking [5]. [1] The ACME Collaboration: J. Baron et al., Science 343, p. 269 (2014) [2] B. Hemmerling et al., arXiv:1310.2669, to appear in Phys. Rev. Lett. [3] D. Patterson, M. Schnell, & J. M. Doyle, Nature 497, p. 475 (2013) [4] H. Lu et al., arXiv:1310.3239, to appear in New. J. Phys. [5] J. Piskorski et al., under preparation

The generation of a fountain of laser-cooled ytterbium fluoride, YbF, has been recently proposed¹ as a method for long coherent observation times, thereby improving the electron electric dipole moment (eEDM) measurement. Understanding the properties of the excited electronic states of YbF is essential for the development of such a scheme for laser cooling. Here we report on the measurement of the radiative lifetimes,τ, permanent electric dipole moments, μ_el, and magnetic g-factors for the [18.6] and [18.7] excited states of YbF. The results are compared with the previously determined values for [18.1] state²³. The [18.1] state is the Ω=1/2 spin-orbit component of the A ²Π(v=0) electronic state arising from the Yb⁺(4f¹⁴6pπ)F⁻(2p¹⁴) configuration. The experimentally determined μ_el, and g-factors will be used to unravel the nature of the [18.6] and [18.7] states, which are known to be admixtures A ²Π and an additional Ω=1/2 state of unknown electronic configuration.
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¹Tarbutt, M R; Sauer, B E; Hudson, J J; Hinds E A, New J. Phys 15, 053034, 2013. ²Zhuang,X; Le,A.;Steimle, T C; Bulleid, N E; Smallman, I J; Hendricks, R J; Skoff, S M ; M R; Hudson, J J; Sauer, B E; Hinds, Tarbutt, M R, PCCP, 13 19103, 2011 ³Condylis,P C; Hudson, J J; Tarbutt, M R; Sauer, B E; Hinds E A, J. Chem. Phys. 123, 231101, 2005

Spectroscopy has taught us that atoms and molecules are the same now as they were in the early Universe. This means that the nature of the fundamental forces, and the chemical bonds that derive from the strength of the electromagnetic force, have not changed over cosmological history. High-resolution spectroscopy of molecules in far-distant galaxies is currently applied to tighten the constraints on the constancy of physical law, which is expressed in terms of the constancy of the fundamental constants, i.e. the fine-structure constant α and the proton-electron mass ratio μ. H₂ observable at redshifts z = 2−4, corresponding to look-back times of 10-12 billion years, is sensitive to probe a possible variation of μ. We have examined a number of high redshift objects, Q0405-443 at z=2.59, Q0347-383 at z=3.02, Q0528-250 at z=2.81, Q2123-005 at z=2.06, Q2348-011 at z=2.42, and Q0642-504 at z=2.66 to constrain ∆μ/μ. Currently work is in progress to analyze spectra of additional objects Q1237+064 at z=2.69, in which also CO is observed, and Q1441+272, which is the highest redshift object (at z=4.22) in which H₂ is abundantly detected. From the combined studies it follows that μ varies by less than 10⁻⁵ for look back times of 90% of the age of the Universe.

A limit on a possible cosmological variation of the proton-to-electron mass ratio μ is derived from observation of methanol lines in the PKS1830-211 lensed galaxy at redshift z  ∼ 0.89 with the Effelsberg 100 m single-dish radio telescope (at frequencies 6.5 - 32 GHz), the IRAM 30 m telescope (at frequencies 80 - 160 GHz), and band-6 of the novel ALMA telescope array (at 260 GHz). Ten different absorption lines of CH₃OH are detected covering a wide range of sensitivity coefficients K_μ. Systematic effects of chemical segregation, excitation temperature, frequency dependence, and time variability of the back- ground source are quantified. A robust constraint of ∆μ/μ = (1.0 ±0.8_stat ±1.0_syst) ×10⁻⁷ is derived from this large sample of lines belonging to a single molecular species. Analysis of additional observations at the E-VLA radio telescope is under way.

Figure Molecular spectroscopy makes very precise applications of quantum theory including GPS, BEC, and laser clocks. Now it can return the favor by shedding some light on modern physics mysteries by further unifying quantum theory and relativity.
We first ask, “What is the simplest molecule?” Hydrogen H₂ is the simplest stable molecule. Positronium is an electron-positron (e⁺e⁻)-pair. An even simpler “molecule” or “radical” is a photon-pair (γ, γ) that under certain conditions can create an (e⁺e⁻)-pair.
To help unravel relativistic and quantum mysteries consider CW laser beam pairs or TE-waveguides. Remarkably, their wave interference immediately gives Minkowski space-time coordinates and clearly relates eight kinds of space-time wave dilations or contractions to shifts in Doppler frequency or wavenumber.
Modern physics students may find this approach significantly simplifies and clarifies relativistic physics in space-time (x,ct) and inverse time-space (ω,ck). It resolves some mysteries surrounding super-constant c=299,792,458m/s by proving “Evenson's Axiom” named in honor of NIST metrologist Ken Evenson (1932-2002) whose spectroscopy established c to start a precision-renaissance in spectroscopy and GPS metrology.
The following Talk II applies this approach to relativistic quantum mechanics.

Figure A “simplest molecule” consisting of CW-laser beam pairs helps to clarify relativity in Talk I. In spite of a seemingly massless evanescence, an optical pair also clarifies classical and quantum mechanics of relativistic matter and anti-matter.
Logical extension of (x,ct) and (ω,ck) geometry gives relativistic action functions of Hamiltonian, Lagrangian, and Poincare that may be constructed in a few ruler-and-compass steps to relate relativistic parameters for group or phase velocity, momentum, energy, rapidity, stellar aberration, Doppler shifts, and DeBroglie wavelength. This exposes hyperbolic and circular trigonometry as two sides of one coin connected by Legendre contact transforms. One is Hamiltonian-like with a longitudinal rapidity parameter ρ (log of Doppler shift). The other is Lagrange-like with a transverse angle parameter σ (stellar aberration). Optical geometry gives recoil in absorption, emission, and resonant Raman-Compton acceleration and distinguishes Einstein rest mass, Galilean momentum mass, and Newtonian effective mass. (Molecular photons appear less bullet-like and more rocket-like.) In conclusion, modern space-time physics appears as a simple result of the more self-evident Evenson's axiom: “All colors go c.”