Isomers of nitrophenolate can serve as models for flourophores commonly found in fluorescent proteins. Here we report electronic spectra for mass-selected 2- and 4-nitrophenolate ions prepared in a cryogenic ion trap, measured by photodissociation spectroscopy. The features in the spectra remain broad with no resolvable vibrational structure down to 25 K. We discuss the width of the experimental spectral features in the framework of excited state lifetime and spectral congestion, based on time-dependent density functional theory calculations.

Isomers of nitrophenolate can serve as models for flourophores commonly found in fluorescent proteins. Here we report electronic spectra for mass-selected 3-nitrophenolate ions prepared in a cryogenic ion trap, measured by photodissociation spectroscopy. Different from the two other isomers, the spectrum shows sharp vibrational bands at low temperatures. We present a Franck-Condon analysis of the spectrum, and discuss the differences between the spectra of the different isomers.

Isoprene (C₅H₈) is an abundant, reactive tropospheric hydrocarbon, derived from biogenic emissions. A detailed understanding of the spectroscopy of isoprene is therefore desirable. Isoprene monomer is isolated in helium droplets and its infrared spectrum is measured in the CH stretching region. Anharmonic frequencies are predicted by VPT2+K simulations employing CCSD(T) force fields with quadratic (cubic and quartic) force constants computed using the ANO1 (ANO0) basis set. The vast majority of the spectral features can be assigned to trans-isoprene on the basis of these computations. Some features of the higher energy gauche conformer are also assignable, by comparison to experiments using heated isoprene. Convergent ab initio thermochemistry is presented for the isomerization pathway, for which the partition function explicitly accounts for the eigenstates associated with separate, uncoupled one-dimensional potential surfaces for methyl torsion and internal rotation between rotamers. The respective 0 and 298.15 K trans/gauche energy differences are 2.82 and 2.52 kcal/mol, which implies a room temperature gauche population of 2.8%. Additionally, preliminary spectroscopic results for the OH-π complexes between hydroxyl radical and isoprene are presented.

We report the infrared spectra of propene in the C–H stretching region measured in helium nanodroplets (HENDI) at 0.4 K and solid para-hydrogen (p-H₂) matrices at 3.2 K, in order to probe the effects of the matrix host environments on the experimental spectra. Propene is an ideal test molecule to study these matrix effects, due to the many anharmonic resonance polyads present in the C–H stretching region of the spectrum. We observe a 4 – 5 cm⁻¹ on average red-shift of the bands in p-H₂ relative to HENDI. Moreover, the choice of matrix environment influences the positions and intensity ratios of transitions within each resonance polyad, leading to qualitatively different spectra. To better understand the nuances involved, simulations were performed that capture the important resonance interactions in a VPT2+K effective Hamiltonian. Certain elements of the Hamiltonian were adjusted to model the impact that different matrix environments have on the anharmonic couplings. In addition, propene reacted with hydrogen atoms via electron bombardment of a p-H₂ matrix during sample deposition, producing propyl radicals. i-Propyl radicals were produced in greater proportion than n-propyl radicals, indicating that for hydrogen addition to the double bond, the rate of addition to the terminal carbon (i-propyl) is faster than the rate of addition to the center carbon (n-propyl). Because the barriers for addition are approximately 700 cm⁻¹ – 1500 cm⁻¹ (1000 K – 2000 K), the only available mechanism for reaction in the p-H₂ matrix (3.2 K) is tunneling. Ab initio calculations were used to compute the tunneling probabilities for the formation of the n-propyl and i-propyl radicals. The rate of addition to the terminal carbon (i-propyl) was calculated to be faster, in agreement with experiment.

We are building a cryogenic source of polyatomic molecules that will be used for tests of fundamental physicsB. Darquié et al., Chirality 22, 870-884 (2010)^,S.K. Tokunaga et al. New Journal of Physics 19, 053006 (2017) The molecules are cooled by a buffer gas of 4 K He inside a copper cell mounted on the cold stage of a cryo−cooler. For the development of this source we are using 1,3,5−trioxane. Although a solid at room temperature, it has a high vapour pressure. We inject this vapour into the buffer gas cell through a room temperature tube. We probe the cooled molecules inside the cell using wavelength modulation (WM) spectroscopy, driving vibration−rotation transitions using 10.2 m wavelength radiation from a quantum cascade laser.I will present data from recent experiments in which we performed WM spectroscopy close to the Q−branch origin of the v_5 S.K. Tokunaga et al. New Journal of Physics 19, 053006 (2017).

Variation of fundamental constants would indicate physics beyond the Standard Model. Astronomical evidence and proposed theoretical models suggest a possible variation in the proton to electron mass ratio (μ). Detection of μ variation will require high precision measurements. Historically, the highest precision measurements have been performed on ultracold atoms. Atomic transitions, however, have limited sensitivity to μ compared to what is found in molecules. Unfortunately, the additional motional degrees of freedom in molecules that give them this sensitivity also lead to more complex internal structure, making it difficult to control them using powerful techniques such as optical pumping. To achieve laser control of the molecular degrees of freedom we need an accurate knowledge of transition energies, state lifetimes and radiative branching ratios. In addition, we need a method to reliably produce molecular ions. In this talk, I will discuss spectroscopic techniques developed in our lab to probe molecular ions.

Measuring spin-rotation intervals in molecular cations is challenging, particularly so when the ions do not have electric-dipole-allowed rovibrational transitions. We present a method to determine the spin-rotational fine structure of molecular ions from the fine structure of high Rydberg statesP. Jansen, L. Semeria, and F. Merkt, Phys. Rev. Lett. 120, 043001 (2018). The method is illustrated by the determination of the so far unknown spin-rotation fine structure of the fundamentally important He₂⁺ ion in the X^{+ 2}Σ_\textu⁺ ground electronic state. The interaction that is responsible for the level structure in the high Rydberg states of He₂ that were probed in our experiment is the n-independent spin-rotation interaction of the ion core. As a consequence, the fine-structure splittings in He₂⁺ can be related to the fine-structure of the Rydberg states by applying an angular-momentum basis transformation from Hund's case (e[b]) to Hund's case (d). The experiment relies on the use of single-mode cw radiation to record spectra of high Rydberg states of He₂ from the a ³Σ_\textu⁺ metastable state. Metastable helium molecules are produced by striking a discharge in a pulsed expansion of neat helium gasM. Motsch, P. Jansen, J.A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. A 89, 043420 (2014). Cooling the valve body to a temperature of 10 K and using continuous-wave excitation results in an observed Doppler-limited linewidth of 25 MHz. The fine structure of Rydberg states of He₂ is determined from strict selection rules by comparing the observed splitting of the Rydberg spectrum with the spin-rotational intervals of the initial metastable state. The fine-structure splittings of the v⁺=0, N⁺=1, 3, and 5 levels of He₂⁺ are 7.96(14)MHz, 17.91(32) MHz and 28.0(6) MHz, respectively.

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P. Jansen, L. Semeria, and F. Merkt, Phys. Rev. Lett. 120, 043001 (2018).. M. Motsch, P. Jansen, J.A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. A 89, 043420 (2014)..

The a ³Σ_u⁺ state of ⁴He₂ is a metastable state with a lifetime of about 18 s. The spin-spin and spin-rotation interactions result in a splitting of each rotational level N into three components J=N, N±1. The fine structure intervals of the N=1, 3, 5, 7 − 11 and 25 - 29 have been measured by radio frequency (rf) spectroscopyW. Lichten, M.V. McCusker and T. L. Vierima, J. Chem. Phys., 61, 2200 (1974).W. Lichten and T. Wik, J. Chem. Phys., 69, 98 (1978). M. Kristensen and N. Bjerre, J. Chem. Phys., 93, 983 (1990).I. Hazell, A. Nørregaard and N. Bjerre, J. Mol. Spectrosc., 172, 135 (1995). and were included in a global analysis of the a ³Σ_u⁺ state C. Focsa, P. F. Bernath and R. Colin, J. Mol. Spectrosc., 191, 209, (1998). A new measurement of the fine structure of all rotational levels between N=1 and 21 of the a ³Σ_u⁺ (v=0) state will be presented. The J=N fine-structure components, which are high-field seeking in magnetic fields, have been eliminated using a multistage Zeeman decelerator, and repopulated from the low-field-seeking J=N±1 components using rf radiation prior to detection by excitation to Rydberg states followed by pulsed-field ionization. The low velocity of the Zeeman decelerated beamM. Motsch, P. Jansen, J. A. Agner, H. Schmutz and F. Merkt, Phys. Rev. A, 89, 043420 (2014).P. Jansen, L. Semeria, L. E. Hofer, S. Scheidegger, J. A. Agner, H. Schmutz and F. Merkt, Phys. Rev. Lett., 115, 133202 (2015). enabled long interaction times of the molecules with the rf radiation and therefore a reduction of the transit-time broadening down to 10 kHz (FWHM), allowing the transition frequencies to be determined very accurately. The fine structure has been analyzed using an effective Hamiltonian to obtain improved values of the spin-spin and spin-rotation coupling constants for the a ³Σ_u⁺ (v=0) metastable state of ⁴He₂, including centrifugal distortion corrections.

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Footnotes:

W. Lichten, M.V. McCusker and T. L. Vierima, J. Chem. Phys., 61, 2200 (1974).

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M. Kristensen and N. Bjerre, J. Chem. Phys., 93, 983 (1990).

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C. Focsa, P. F. Bernath and R. Colin, J. Mol. Spectrosc., 191, 209, (1998).. M. Motsch, P. Jansen, J. A. Agner, H. Schmutz and F. Merkt, Phys. Rev. A, 89, 043420 (2014).

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SiO⁺ was proposed as a candidate for ultracold molecule experiments. Cooling schemes required to prepare SiO⁺ in its ground state require knowledge of state energies, lifetimes and branching of selected SiO⁺ transitions. Knowledge of dissociative transitions is needed to probe state populations of SiO⁺ in the proposed experiments. Finally, efficient loading of SiO⁺ into a trap by photoionization requires studying spectroscopy of neutral SiO. In this talk, I will discuss recent progress in study of SiO and SiO⁺ spectroscopy in our lab and approaches used to address these studies.

Chemical reactions at ultralow temperatures can proceed surprisingly efficiently due to their quantum mechanical nature. However, the detailed chemical physics to describe and predict the distribution of final states is still unclear. We will discuss a barrier-less, likely 4-center reaction, 2 KRb → K₂ + Rb₂, in the temperature regime below 1 μK. Due to the low exothermic energy of this reaction,  ∼ 10 cm⁻¹, we aim to resolve individual product quantum states through ionization detection. Our approach combines the physical chemistry techniques of REMPI spectroscopy and velocity-map imaging for ion and quantum state detection with AMO physics techniques for preparation of the ultracold molecular reagents.