Figure The rotational spectrum of the chiral molecule CpReCH₃(CO)(NO) (Cp=cyclopentadienyl) has been measured between 2 and 8.5 GHz using a chirped-pulse FTMW spectrometer. Rotational and nuclear quadrupole constants for the nitrogen and both rhenium isotopes (¹⁸⁵Re & ¹⁸⁷Re) have determined. Initial assignment was hampered by the large coupling constants for the rhenium nuclei (χ_xx=720.21 and 680.55 MHz for ¹⁸⁵Re and ¹⁸⁷Re, respectively) and the barely resolved splitting due to the nitrogen nucleus. Large (100-300 MHz) off-diagonal quadrupole coupling terms (χ_ab and χ_bc) for the rhenium nuclei also complicated the fitting. Relativistic corrections were required to reproduce experimental results in ab initio calculations. This molecule is of particular interest due to a high energy difference between enantiomers due to parity violation. For the related molecule CpRe(CO)I(NO), a frequency difference in rotational transitions of a few hertz is anticipated. A high-resolution, cavity-based microwave spectrometer is being commissioned with the aim of approaching this level of precision.

Fourier transform microwave (FTMW) spectroscopy has been used for many years as one of the most accurate methods to determine gas-phase structures of molecules and small molecular clusters. In the last years two pioneering works ushered in a new era applications. First, by exploiting the reduced measurement time and the high sensitivity, the development of chirped-pulse CP-FTMW spectrometers¹ enabled the full structural determination of molecules of increasing size as well as molecular clusters. Second, and more recently, Patterson et al.² showed that rotational spectroscopy can also be used for enantiomer-specific detection. Here we present an experimental approach that combines both in a single spectrometer. This set-up is capable to rapidly obtain the full heavy-atom substitution structure using the CP-FTMW features. The inclusion of an extra set of broadband horns allows for a chirality-sensitive measurement of the sample.^3,4 The measurement we implement is a three-wave mixing experiment that uses time-separated pulses to optimally create the chiral coherence – an approach that was proposed recently.⁵ Using samples of R-, S- and racemic Solketal, the physical properties of the three-wave mixing experiment were studied. This involved the measurement of the corresponding nutation curves (molecular signal intensity vs excitation pulse duration) to demonstrate the optimal pulse sequence. The phase stability of the chiral signal, required to assign the absolute stereochemistry, has been studied as a function of the measurement signal-to-noise ratio using a “phasogram” method.
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¹G. G. Brown, B. C. Dian, K. O. Douglass, S. M. Geyer, S. T. Shipman, B. H. Pate, Rev. Sci. Instrum. 2008, 79, 053103. ²D. Patterson, M. Schnell, J. M. Doyle, Nature 2013, 497, 475–477. ³D. Patterson, J. M. Doyle, Phys. Rev. Lett. 2013, 111, 023008. ⁴V. A. Shubert, D. Schmitz, D. Patterson, J. M. Doyle, M. Schnell, Angew. Chem. Int. Ed. 2014, 53, 1152–1155. ⁵J.-U. Grabow, Angew. Chem. 2013, 125, 11914 – 11916; Angew. Chem. Int. Ed. 2013, 52, 11698 –11700.

Since many years low temperature ion trapping techniques are successfully used in our laboratories in combination with sensitive action spectroscopy schemes (Laser Induced Reactions) to measure high resolution ro-vibrational and rotational spectra of gas-phase molecular ions. Here we present a further development of a LIR method first introduced for recording rotationally resolved electronic spectra of N₂^{+ 1}. This new method, called LIICG (Light Induced Inhibition of Complex Growth), makes use of state specific He-attachment rates to stored cold molecular ions. We have recently demonstrated its applicability to rotational and ro-vibrational spectroscopy of C₃H⁺ and CH₅^{+ 2}. The measurements were performed in recently completed 4K 22-pole ion trap instruments. Ionic species are produced in a storage ion source and are mass selected before they enter the trap. For spectroscopy normally a few thousand ions are stored at 4K together with He at high number densities (around 10¹⁴ cm⁻³). Under these conditions He attaches to the ions via ternary collision processes. As we will show, this attachement process is hindered by exciting a rotational or ro-vibrational transition, likely because the attachment rates for He are slower for higher rotational or ro-vibrational levels. So by exciting the bare ion the number of ion- He complexes at equilibrium is reduced. In this way the spectrum of the bare ion can be recorded by counting the number of ion-He complexes as a function of frequency. To test the new method we chose well known rotational ground state transitions of CO⁺, HCO⁺ and CD⁺. In particular CD⁺ appeared to be a good candidate for understanding the new method in detail, due to its strong LIICG signal and its simple rotational spectrum. In this contribution we will explain the LIICG scheme and its underlying kinetics using the example of CD⁺. We will show effects of different experimental conditions on the signal (e.g. He number density, temperature, radiation power…) to explain our kinetic model. Beside these tests we will present measurements of new rotational transitions of C₃H⁺, CD₂H⁺ and CH₂D⁺, demonstrating LIICG as a general spectroscopic method.
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¹Chakrabarty et al. 2013, J. Phys. Chem. Lett., 4, 4051 ²S. Brünken et al. 2014, ApJL, 783, L4; O. Asvany et al. 2014, ApPhB, 114, 203-211

Room-temperature rotational spectra are dense and difficult to assign, and so we have been working to develop methods to accelerate this process. We have tested two different methods with our waveguide-based spectrometer, which operates from 8.7 to 26.5 GHz. The first method, based on previous work by Medvedev and De Lucia¹, was used to estimate lower state energies of transitions by performing relative intensity measurements at a range of temperatures between -20 and +50 ^°C. The second method employed hundreds of microwave-microwave double resonance measurements to determine level connectivity between rotational transitions. The relative intensity measurements were not particularly successful in this frequency range (the reasons for this will be discussed), but the information gleaned from the double-resonance measurements can be incorporated into other spectral search algorithms (such as autofit or genetic algorithm approaches) via scoring or penalty functions to help with the spectral assignment process.
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¹I.R. Medvedev, F.C. De Lucia, Astrophys. J. 656, 621-628 (2007).

Advances in chip-level microwave technology in the communications field have led to the possibilities of low cost alternatives for current Fourier transform microwave (FTMW) spectrometers. Many of the large, expensive microwave components in a traditional design can now be replaced by robust, mass market monolithic microwave integrated circuits (MMICs). "Spectrometer on a board” designs are now feasible that offer dramatic cost reduction for microwave spectroscopy. These chip-level components can be paired with miniature computers to produce compact instruments that are operable through USB. A FTMW spectrometer design using the key MMIC components that drive cost reduction will be presented. Two dual channel synthesizers (Valon Technology Model 5008), a digital pattern generator (Byte Paradigm Wav Gen Xpress), and a high-speed digitizer arbitrary waveform generator combination unit (Tie Pie HS-5 530 XM) form the key components of the spectrometer for operation in the 18-26.5 GHz range. The design performance is illustrated using a spectrometer that is being incorporated into a museum display for astrochemistry. For this instrument a user interface, developed in Python, has been developed and will be shown.

Frequency measurements are given for the 10⁰0-00⁰0 band of N₂O near 4.5 μm and for pure rotational transitions beyond 151.5 μm. The infrared measurements utilize a periodically-poled lithium niobate (PPLN) based difference frequency generation (DFG) source locked to the saturated absorption center of an N₂O absorption line. The DFG frequency is calibrated by an optical frequency comb and an iodine hyperfine transition. We report 44 transitions ranging from J = 1 - 100 for both the P− and R− branches and the accuracy is better than 10 kHz for most transitions. In addition, 175 pure rotational transitions have been measured including 33 measurement with sub-Doppler precision ( < 3 kHz), and 142 Doppler limited measurements. These are combined with other precision rotational and vibrational measurements to provide improved quantum mechanical parameters as well as frequency calibration tables for the N₂O bands near 4.5 μm.

Submillimeter absorption spectroscopy was used to study semiconductor processing plasmas. Abundances and temperatures of molecules, radicals, and ions can be determined without altering any of the properties of the plasma. The behavior of these measurements provides useful applications in monitoring process steps. A summary of such applications will be presented, including etching and cleaning endpoint detection.

Vibrational sum frequency generation spectroscopy (VSFG) is a popular approach to obtaining molecular information from the interfaces of liquids and submonolayer adsorbates. A new challenge becomes apparent when applying this ultrafast technique to thin films in which two interfaces are present in the focal volume of the experiment. Although the signal levels from these two interfaces can be adjusted to some extent by beam geometries, their overall interferences can remain significant. Rather than viewing this as a two-interface problem, we present experimental work from our group in which we utilize this interference as a tool to determine molecular information from both interfaces. In particular, we present data showing the influence of charge accumulation at a buried interface and the possibilities of using mixed beam polarizations to improve global data fitting.

The buried interface of an organic semiconductor at the dielectric has a large on influence on the function of organic field effect transistors (OFETs). The use of vibrational sum frequency generation (VSFG) to obtain structural and orientational information on the buried interfaces of organic thin films has historically been complicated by the signals from other interfaces in the system. A thin film of N,N'-Dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) was deposited on a SiO₂ dielectric to simulate the interfaces found in OFETs. We will show how probing the sample with a varying mixture of linear polarizations in the experimental setup can deconvolute contributions to the overall signal from multiple interfaces.

The chemical reaction of powder energetic material is of great interest in energy and pyrotechnic applications since the high reaction temperature. Under the shock compression, the chemical reaction appears in the sub-microsecond to microsecond time scale, and releases a large amount of energy. Experimental and theoretical research progresses have been made in the past decade, in order to characterize the process under the shock compression. However, the knowledge of energy release and temperature change of this procedure is still limited, due to the difficulties of detecting technologies. We have constructed a thermal imaging microscopy apparatus, and studied the temperature change in energetic materials under the long-wavelength infrared (LWIR) and ultrasound exposure.¹² Additionally, the real-time detection of the localized heating and energy concentration in composite material is capable with our thermal imaging microscopy apparatus. Recently, this apparatus is combined with our laser driven flyer plate system³ to provide a lab-scale source of shock compression to energetic material. A fast temperature increase of thermite particulars induced by the shock compression is directly observed by thermal imaging with 15-20 μm spatial resolution. Temperature change during the shock loading is evaluated to be at the order of 10⁹K/s, through the direct measurement of mid-wavelength infrared (MWIR) emission intensity change. We observe preliminary results to confirm the hot spots appear with shock compression on energetic crystals, and will discuss the data and analysis in further detail.
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¹M.-W. Chen, S. You, K. S. Suslick, and D. D. Dlott, Rev. Sci. Instr., 85, 023705 (2014), ² M.-W. Chen, S. You, K. S. Suslick, and D. D. Dlott, Appl. Phys. Lett., 104, 061907 (2014) ³K. E. Brown, W. L. Shaw, X. Zheng, and D. D. Dlott, Rev. Sci. Instr., 83, 103901 (2012)