Extreme ultraviolet (XUV) spectroscopy is an inner shell technique that probes the M_2,3-edge excitation of atoms. Absorption of the XUV photon causes a 3p→3d transition, the energy and shape of which is directly related to the element and ligand environment. This technique is thus element-, oxidation state-, spin state-, and ligand field specific. A process called high-harmonic generation (HHG) enables the production of ultrashort (20fs) pulses of collimated XUV photons in a tabletop instrument. This allows transient XUV spectroscopy to be conducted as an in-lab experiment, where it was previously only possible at accelerator-based light sources. Additionally, ultrashort pulses provide the capability for unprecedented time resolution (70fs IRF). This technique has the capacity to serve a pivotal role in the study of electron and energy transfer processes in materials and chemical biology. I will present the XUV transient absorption instrument we have built over the past two years, along with preliminary data and simulations of the M_2,3-edge absorption data of a battery of small inorganic molecules to demonstrate the high specificity of this ultrafast tabletop technique.

r0pt Figure A powerful method in spectroscopy to characterize the structure of large, gas phase molecules is to probe the ionization yield upon irradiating the molecules with infrared (IR) and/or ultraviolet (UV) radiation. When this spectroscopic technique is employed, the photodetached electrons are usually ignored, although they contain information on, for example, the ionization threshold of the molecule and the excited states of the formed ions. Here, the novel combination of a molecular beam mass spectrometer equipped with a laser desorption source, the free electron laser FELIX and the powerful velocity map imaging (VMI) technique is presented. With this extended set of tools we can bring large molecules intact into the gas phase and prepare them in specific vibrationally excited states. UV or VUV radiation can subsequently be used to ionize the molecules. The kinetic energy and the radial distribution of the photoelectrons can be measured using VMI combined with ion detection using a time-of-flight mass spectrometer.

We report an optically accessible pyrolysis micro-reactor suitable for in situ laser spectroscopic measurements. A radiative heating design allows for completely unobstructed views of the micro-reactor along two axes. The maximum temperature demonstrated here is only 1300 K (as opposed to 1700 K for the usual SiC micro-reactor) because of the melting point of fused silica, but alternative transparent materials will allow for higher temperatures. Laser induced fluorescence measurements on nitric oxide are presented as a proof of principle for spectroscopic characterization of pyrolysis conditions. (This work has been published in J. H. Baraban, D. E. David, G. B. Ellison, and J. W. Daily. An Optically Accessible Pyrolysis Micro-Reactor. Review of Scientific Instruments, 87(1):014101, 2016.)

Recently, we have installed a helium nanodroplet machine [1,2] at the free electron beamline FELIX in Nijmegen. The current setup allows to study neutral molecules and molecular complexes in the full spectral range from 500-3000 cm⁻¹. First proof of principle experiments using the strong absorber SF₆ were used to verify the overall alignment between helium nanodroplet beam and the FELIX radiation source. Applications so far included the study of small water clusters and the investigation of microsolvation of small solutes. These results will be presented and compared to recent theoretical predictions of the Bowman group.[3]
[1] K. von Haeften et al., Phys. Rev. B. 73, 054502 (2006) [2] Choi et al., Int. Rev. Phys. Chem. 25, 15 (2006) [3] Samantha et al., Acc. Chem. Res. 47, 2700 (2014)

Water and heavy water is always a combination of liquids that we would like to know their concentrations in mixtures. In this work, we are trying to make a cavity enhanced absorption spectroscopy (CEAS) setup in liquid phase in the near infrared region. By combining a self-built supercontinuum light source with a fiber loop, we are able to build a setup that has a very broad wavelength coverage to work in the liquid phase. A side-polished-fiber is used as a sensing region on the loop. Some H₂O/D₂O sample pairs are tested first for its properties. The results show that this new setup has the ability in liquid phase detection, and a detection limit of less than 10% H₂O in D₂O solutions can be reached so far.

p0pt Figure We demonstrate a new approach to mid-infrared dual-comb spectroscopy. It opens up new opportunities for accurate real-time spectroscopic diagnostics and it significantly simplifies the technique of dual-comb spectroscopy. Two mid-infrared frequency combs of slightly different repetition frequencies and moderate, but rapidly tunable, spectral span are generated in the 2800-3200 cm⁻¹ region. The generators rely on electro-optic modulators, nonlinear fibersG. Millot, S. Pitois, M. Yan, T. Hovannysyan, A. Bendahmane, T.W. Hänsch, N. Picqué, Frequency−agile dual−comb spectroscopy, Nature Photonics 10, 27−30 (2016). for spectral broadening and difference frequency generation and do not involve mode−locked lasers. Flat−top frequency combs span up to 10 cm^-1 with a comb line spacing of 100 MHz (310^-3 cm^-1

Heterodyne detection, velocity modulation, and cavity enhancement are useful tools for observing rovibrational transitions of important molecular ions.K.N. Crabtree, J.N. Hodges, B.M. Siller, A.J. Perry, J.E. Kelly, P.A. Jenkins II, and B.J. McCall, Chem. Phys. Lett. 551 (2012) 1-6.e have utilized these methods to investigate a number of molecular ions, such as H₃⁺, CH₅⁺, HeH⁺, and OH⁺.A.J. Perry, J.N. Hodges, C.R. Markus, G.S. Kocheril, and B.J. McCall, J. Mol. Spec. 317 (2015) 71-73.J.N. Hodges, A.J. Perry, P.A. Jenkins II, B.M. Siller, and B.J. McCall, J. Chem. Phys. 139 (2013) 164291.A.J. Perry, J.N. Hodges, C.R. Markus, G.S. Kocheril, and B.J. McCall. 2014, J. Chem. Phys. 141, 101101C.R. Markus, J.N. Hodges, A.J. Perry, G.S. Kocheril, H.S.P. Müller, and B.J. McCall, Astrophys. J. 817 (2016) 138. In the past, parasitic etalons and the lack of fast and sensitive detectors in the mid-infrared have limited the number of transitions we could measure with MHz-level precision. Recently, we have significantly reduced the amplitude of unwanted interference fringes with a Brewster-plate spoiler. We have also developed a detection scheme which up-converts the mid-infrared light with difference frequency generation which allows the use of a faster and more sensitive avalanche photodetector. The higher detection bandwidth allows for optimized heterodyne detection at higher modulation frequencies. The overall gain in signal-to-noise from both improvements will enable extensive high-precision line lists of molecular ions and searches for previously unobserved transitions.

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K.N. Crabtree, J.N. Hodges, B.M. Siller, A.J. Perry, J.E. Kelly, P.A. Jenkins II, and B.J. McCall, Chem. Phys. Lett. 551 (2012) 1-6.W A.J. Perry, J.N. Hodges, C.R. Markus, G.S. Kocheril, and B.J. McCall, J. Mol. Spec. 317 (2015) 71-73.

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A.J. Perry, J.N. Hodges, C.R. Markus, G.S. Kocheril, and B.J. McCall. 2014, J. Chem. Phys. 141, 101101

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We have recently demonstrated the integration of cavity-enhanced direct frequency comb spectroscopy with buffer gas cooling to acquire high resolution infrared spectra of translationally and rotationally cold ( ∼ 10 K) gas-phase molecules.B. Spaun, et al. Probing buffer-gas cooled molecules with direct frequency comb spectroscopy in the mid-infrared, WF02, 70^th International Symposium on Molecular Spectroscopy, Champaign-Urbana, IL, 2015.ere, we extend this method to significantly larger systems, including naphthalene (C₁₀H₈), a prototypical polyaromatic hydrocarbon, and adamantane (C₁₀H₁₆), the fundamental building block of diamonoids. To the authors' knowledge, the latter molecule represents the largest system for which rotationally resolved spectra in the CH stretch region (3 μm) have been obtained. In addition to the measured spectra, we present several details of our experimental methods. These include introducing non-volatile species into the cold buffer gas cell and obtaining broadband spectra with single comb mode resolution. We also discuss recent modifications to the apparatus to improve its absorption sensitivity and time resolution, which facilitate the study of both larger molecular systems and cold chemical dynamics.

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B. Spaun, et al. Probing buffer-gas cooled molecules with direct frequency comb spectroscopy in the mid-infrared, WF02, 70^th International Symposium on Molecular Spectroscopy, Champaign-Urbana, IL, 2015.H

Astrochemistry, a relatively young field of research, addresses a gap in our understanding of molecular evolution in space. With many space missions gathering data, the number of unresolved spectral lines is growing rapidly. Each year there are about three new molecules that are identified in the interstellar medium (ISM). However, our understanding of molecular processes, branching ratios, and rates are at a beginner level. For instance, we do not yet understand the chemical processes associated with the creation and evolution of even the most basic molecules such as water and methanol in space. One of the important steps toward understanding the chemistry of the ISM is to identify, through laboratory and theoretical work, a list of potential target molecules that are likely to exist in the ISM. This work describes experimental progress towards building a spectrometer that is able to produce complex cold ions that will react with cooled neutral molecules under conditions similar to those in space. I plan to present the astrochemical needs that motivated my research, how the new instrument will meet those needs, and the present status of the instrument and measurements in my lab.

Ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D-spectroscopy, are widely used across many disciplines. However, these techniques are typically restricted to optically thick samples, such as solids and liquid solutions. Using a frequency comb laser and optical cavities, we present a new technique for performing ultrafast optical spectroscopy with high sensitivity, enabling work in dilute gas-phase molecular beams. Resonantly enhancing the probe pulses, we demonstrate transient absorption measurements with a detection limit of ∆OD = 2 ×10⁻¹⁰ (1 ×10⁻⁹/√{Hz}). Resonantly enhancing the pump pulses allows us to produce a high excitation fraction at high repetition-rate, so that signals can be recorded from samples with optical densities as low as OD ≈ 10⁻⁸, or column densities < 10¹⁰ molecules/cm². To our knowledge, this represents a 5,000-fold improvement of the state-of-the-art. r0pt Figure