Single-molecule adsorption scanning tunneling microscopy (SMA-STM) is a powerful spectroscopy method capable of imaging absorption at sub-nanometer spatial resolution. Herein, we use SMA-STM to investigate electronically excited quantum dots (QDs). Absorption images of individual QDs vary significantly from dot-to-dot, resulting from heterogeneity and defects. Single QD absorption is strongly dependent on the applied electric field, reflecting different excited states being probed. Details on the three-dimensional geometry of the QD excited states are obtained by using the STM tip to nudge and roll the QDs on the surfaces, then image at different angles. Orientation-dependent imaging, in combination with density functional theory calculations of a model QD, reveals presence of surface localized defects. Finally, the energy transfer in arrays of QDs is imaged and manipulated in real space at individual dot level. This study establishes SMA-STM as a powerful method to study electronically excited nanostructures and energy transfer at sub-nm spatial resolution.

Early detection of cancer has a drastic impact on the successful treatment of the disease. However, detection of early signs of cancer is a challenge especially for a type of cancer such as epithelial ovarian cancer (EOC), with few or no symptoms at the early-stages. Development of a noninvasive method that can improve the detection of biomarkers with sufficient selectivity, sensitivity, and reproducibility is a promising approach to overcome the challenges of early detection. This study aims to develop novel optical and mass spectrographic techniques to detect biomolecules in complex matrices. To accomplish this, Laser Ablation-Resonance Enhanced Photoionization Mass Spectrometry is combined with nano- and micro-particle immunoassay to improve the detectability in a complex media. While there are many commercial mass spectrometry configurations available, none of them meet our specific needs, so a significant portion of the effort in this research to date has been dedicated to designing and building the custom apparatus to meet our needs. We present an overview of the design, testing, and preliminary studies on biomolecules.

We present our recent progress in the development and applications of cavity enhanced absorption spectroscopy technique based on light emitting diode (LED) for optical monitoring of chemically reactive atmospheric species (HONO, NO3, NO2) in intensive campaigns and in smog chamber studies.

l1.58in Cavity enhanced absorption techniques derive their sensitivity from an increase in the effective light-matter interaction length provided by a high finesse cavity. However, absorption measurements are often affected by spectrally interfering molecular species, which can hinder selectivity. This issue is addressed by Faraday rotation spectroscopy (FRS), which selectively probes the molecular dispersion of paramagnetic gaseous species (e.g. O₂, NO, NO₂, OH, etc.) subjected to an external magnetic field. Immunity to interfering diamagnetic compounds is thereby obtained allowing reliable quantitative concentration assessments of paramagnetic species in the presence of spectrally interfering molecules, such as H₂O and CO₂. Recently, white-noise limited performance over extended averaging times (minutes/hours) was achieved by combining cavity ring-down (CRD) and FRS. While CRD-FRS provides excellent sensitivities down to noise-equivalent rotation angles of 1.3x10⁻⁹ rad rtHz⁻¹, it requires fast detectors, high bandwidth digitization electronics and high throughput data analysis which significantly increases the system complexity and cost. To address these limitations, cavity attenuated phase shift (CAPS) FRS and integrated cavity output spectroscopy (ICOS) FRS has been developed. Here, CAPS-FRS ICOS-FRS, and CRD-FRS systems are compared by detecting oxygen at the ^PP₁(1) transition in the A-electronic band around 762.3 nm. The FRS-based techniques are fully self-referencing and require no additional off-resonance calibration, which provides a powerful, yet simple alternative for cavity-enhanced spectroscopy targeting paramagnetic species. The FRS techniques allow for continuous measurements in a line-locked mode, which further increases the system effective duty-cycle and improves the sensing performance. A comparison of long-term system performance, system modeling, as well as system improvements will be presented in detail. Acknowledgments: The authors acknowledge funding from the National Science Foundation CBET grant #1507358 and from Thorlabs Inc. LP acknowledges the National Science Foundation Graduate Fellowship support.

Coherent anti-Stokes Raman scattering (CARS) is a non-linear spectroscopic combustion diagnostic technique used for measurement of temperature and species concentration. Broadband CARS spectra can be acquired with a single laser shot with high spatial and temporal resolution. We present two distinct applications of a nanosecond dual-pump vibrational CARS system. The first experiment aimed to study the effect of simulated exhaust-gas-recirculation, via addition of CO₂ to the fuel stream, on the flame structure of lean CH₄-air pilot-assisted turbulent premixed flames. For this experiment over 20,000 single-shot spectra were acquired and spectrally fitted to develop a detailed temperature map of the flame flow-field. In the second experiment, laminar flames with varying soot loading were stabilized over a “Yale burner”. This burner, in the combustion community, is a canonical system for the development of soot models. The principal challenge in this experiment was obtaining a CARS signal with an adequate signal to noise ratio in the presence of strong soot-emission background. Our measurements in both experiments will serve as benchmark data for the development of combustion computational models.

Monitoring gas emission becomes an important issue in the livestock sector. For example, industrial animal farming is responsible for substantial part of total anthropogenic emission of methane and ammonia. Here we present practical aspects of molecular spectroscopy by demonstrating a laser-based system for sensing of methane (near 1651 nm), ammonia (near 1531 nm) and hydrogen sulfide (near 1575 nm) using wavelength modulation spectroscopy (WMS) and a multi-pass cell. This instrument is designed for sequential detection of three species emitted in pig farming facility. Laser-based molecular spectroscopy in the near-infrared region provides unique opportunity for maintenance-free continuous operation at relatively low cost, and with sensitivity and accuracy at single ppmv levels for all three gases. System characterization in laboratory conditions will be presented. We will also demonstrated results of field tests and discuss technical challenges when moving spectroscopic systems from laboratory conditions to real-world environments. Authors acknowledge support by the National Centre for Research and Development (NCBiR) award LIDER/023/379/L-5/13/NCBR/2014.

Molecular rovibrational transitions can potentially provide high-resolution frequency references for the visible to the mid infrared including the telecommunications window at 1.5μm (C₂H₂ or HCN). Going towards compact miniaturized molecular spectroscopy is an extreme challenge as molecular transition probabilities are weak and long propagation length is required. Compact systems based on hollow core fibers filled with acetylene gas (C₂H₂) at high pressures have been presentedF. Benabid, Compact, stable and efficient all-fibre gas cells using hollow-core photonic crystal fibres, Nature 434, 488 (2005).ut the propagation length remains macroscopic. Additionally, collisional broadening (high pressures) or the Doppler effect hinder the available resolution. Here we present selective reflection (SR) spectroscopy measurements on a molecular gas of NH₃ and SF₆ molecules at 10.6μm. Frequency modulated SR is a high-resolution (sub-Doppler) technique, linear in laser power that is essentially only sensitive to a layer of molecules whose depth is defined by the wavelength of optical excitation ( ∼ λ/2π), even when the cell remains macroscopic. Initial measurements, were performed with a CO₂ laser, subsequently changed for a more user friendly QCL laser rendered compatible with high-resolution spectroscopy. The core of our experiments is performed on the P(1) line of ammonia at 948.23cm⁻¹ with best resolution limited by the laser linewidth ( ∼ 0.5MHz). For pressures below 50mTorr (collisional broadening  ∼ 1.4MHz) the hyperfine structure of ammonia can be clearly resolved. Our experiments pave the way towards miniaturized molecular frequency references such as nanocellsG. Dutier et al., Collapse and revival of a Dicke-type coherent narrowing in a sub-micron thick vapor cell, Europhys. Lett. 63, 35 (2003)^_2

In this talk, we present a new hybrid laser-mass spectrometer with two instrument arms: one for infrared multiphoton dissociation (IRMPD) and the other for helium nanodroplet isolation (HENDI) cold ion spectroscopic experiments. The spectrometer features an electrospray source for ion production, using either traditional atmospheric nanospray or a novel Subambient Pressure Ionization with Nanoelectrospray (SPIN) design.[1] Ionic species of interest are isolated in a quadrupole mass filter and stored in a Paul trap for OPO IR laser irradiation. The ion trap is coupled to a TOF tube for fast wide-range mass spectrometry at high resolution. The second arm contains in addition an ion deflector and a linear quadruple ion trap which is currently being integrated into an existing HENDI spectrometer. This will enable higher resolution investigations of mass-selected ionic species in an ultra-cold environment. IRMPD spectroscopy has emerged as a powerful asset in the spectroscopist’s toolbox to investigate the isomerization and aggregation behavior of small molecules.[2] For example, this technique has been coupled with mass spectrometry to elucidate the structures of some amino acid complexes of interest, most notably perhaps the “magic number” serine octamer.[3] To demonstrate the current capabilities of the instrument, we extended previous studies on the gas-phase structures of protonated and sodiated asparagine monomers[4] into the IR regime between 3300 and 4000 cm–1. In addition, a theoretical re-assessment of the structural and spectroscopic properties of these species was undertaken with modern quantum-chemical approaches. We present this study as a stepping stone for further research into the structural and energetic preferences of small non-covalently bound amino acid aggregates. [1] J. S. Page, K. Tang, R. T. Kelly, R. D. Smith, Anal. Chem. 2008, 80, 1800; I. Marginean, et al. Anal. Chem. 2012, 84, 9208. [2] N. C. Polfer, Chem. Soc. Rev., 2011, 40, 2211. [3] F. X. Sunahori, G. Yang, E. N. Kitova, J. S. Klassen, Y. Xu, Phys. Chem. Chem. Phys. 2013, 15, 1873. [4] A. L. Heaton, V. N. Bowman, J. Oomens, J. D. Steill, P. B. Armentrout, J. Phys. Chem. A 2009, 113, 5519.

A microwave-(sub)millimeter wave double resonance experiment was developed in order to improve the ease with which one is able to assign spectral lines by displaying the interconnectivity of states. This experiment combines chirped-pulse microwave spectroscopy and fast-sweep (sub)millimeter spectroscopy to increase the speed of data acquisition. During the experiment, the splitting of spectral lines was detected instead of the traditional movement of population seen in other double resonance experiments. The line splitting was determined to be caused by the AC Stark effect. The experimental design along with the resulting double resonance methanol spectra will be presented.

l0pt Figure The distinct rotational signatures of gas-phase molecular species in the millimeter (mm) and sub- millimeter (sub-mm) spectral regions have long assisted remote sensing communities in the interrogation of atmospheric and astrophysical media. In situ studies employing highly-mobile instrumentation have not been able to reproduce the success of their remote-based counterparts largely due to the unaccommodating size and power requirements of traditional mm and sub-mm wave hardware. The Laboratory Studies and Atmospheric Observations group at JPL has embraced the marriage of novel custom-designed CMOS source and heterodyne detection electronics, which often leverage advances in the mobile phone industry, and traditional cavity enhanced laboratory techniques to combat the issues that have plagued the deployment of in situ mm wave sensors. One device emerging from these efforts is a freestanding CMOS-based transmitter tunable to sub-500 Hz resolution over the operational bandwidth of 90 – 105 GHz. For prototyping purposes this transmitter, the output of which can be both frequency and amplitude modulated, has been deployed as the radiation source in a high- resolution sub-Doppler (Lamb-dip) absorption spectrometer. The presented experimental findings have shown that this device, which effectively functions as a USB powered/controlled W-band source, has sufficient output power ( ∼ 2 mW peak) to perform spectral-hole burning saturation experiments and a phase-noise floor low enough to determine spectral line positions with a precision of 1 part in 10⁹ and accuracy within the error of measurements made with traditional millimeter-wave sources.D. J. Nemchick et al., "Sub-Doppler Spectroscopy With a CMOS Transmitter," IEEE Trans. THz Sci. Technol., vol. 8, no. 1, pp. 121-126, 2018.hese findings highlight the promise of exploiting CMOS architectures for use in gas specific, low-power, and potentially low-cost in situ sensors.

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Footnotes:

D. J. Nemchick et al., "Sub-Doppler Spectroscopy With a CMOS Transmitter," IEEE Trans. THz Sci. Technol., vol. 8, no. 1, pp. 121-126, 2018.T

The 8-26.5 GHz conformer specific rotational spectrum of allyl chloride was recorded in a room temperature spectrometer using the strong field coherence breaking (SFCB) technique.Hernandez-Castillo, A.O., Abeysekera, C., Hays, B.M., Zwier, T.S. “Broadband Multi-Resonant Strong Field Coherence Breaking as a Tool for Single Isomer Microwave Spectroscopy.” J. Chem. Phys. 145, 114203 (2016).llyl chloride, which has cis- and skew- conformers as well as ³⁵Cl and ³⁷Cl isotopologues, was chosen as the initial molecule for testing this method in the room temperature chirped pulse waveguide setup.Reinhold, B., Finneran, I.A., Shipman, S.T. “Room temperature chirped-pulse Fourier transform microwave spectroscopy of anisole.” J. Mol. Spec. 270, 89 (2011).his data was compared to results from the SFCB technique performed at Purdue University using a jet expansion at 1-2K. The application of this and other methods for the simplification of room temperature spectra will be discussed.

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Footnotes:

Hernandez-Castillo, A.O., Abeysekera, C., Hays, B.M., Zwier, T.S. “Broadband Multi-Resonant Strong Field Coherence Breaking as a Tool for Single Isomer Microwave Spectroscopy.” J. Chem. Phys. 145, 114203 (2016).A Reinhold, B., Finneran, I.A., Shipman, S.T. “Room temperature chirped-pulse Fourier transform microwave spectroscopy of anisole.” J. Mol. Spec. 270, 89 (2011).T

Chirped pulse Fourier transform spectroscopy (CP-FTMW) has become a widely used technique for the detection of molecular rotational spectra owing to its broad frequency coverage. Traditional CP-FTMW set ups involve top-quality broadband arbitrary waveform generators (AWG), high-power amplifiers, and digitizers, which are expensive due to their specifications. One method to lower costs with only a mild sacrifice of efficiency is to divide the total bandwidth into smaller sections and step from section to section with a tunable local oscillator; these so-called “segmented” CP-FTMW spectrometers have much lower costs by decreasing the required amplifier power and digitizer bandwidth. Inspired by the work of Finneran et al. (Rev. Sci. Inst. 84, 2013, 083104), our group has designed a 6-18 GHz segmented CP-FTMW broadband spectrometer that also replaces the AWG with a direct digital synthesizer (DDS), further lowering the spectrometer cost. To our knowledge, this is the first instrument in which a DDS has been coupled with the segmented approach to achieve a tunable intermediate frequency (IF). Design, cost analysis, progress, and performance will be discussed in this talk.