Improving cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers.

The performance of sensitive spectroscopic methods in the mid-IR is often limited by fringing due to parasitic etalons and the background noise in mid-infrared detectors. In particular, the technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS), which is capable of determining the frequencies of strong rovibrational transitions of molecular ions with sub-MHz uncertainty, needs improved sensitivity in order to probe weaker transitions. In this work, we have implemented up-conversion detection with NICE-OHVMS in the 3.2 - 3.9 µm region to enable the use of faster and more sensitive detectors which cover visible wavelengths. The higher bandwidth enabled detection at optimized heterodyne frequencies, which increased the overall signal from the H3+ cation by a factor of three and was able to resolve sub-Doppler features which had previously overlapped. Also, we demonstrate the effectiveness of Brewster-plate spoilers to remove fringes due to parasitic etalons in a cavity enhanced technique. Together, these improvements reduced the instrument's noise equivalent absorption to 5.9×10-11 cm-1 Hz-1/2, which represents a factor of 34 improvement in sensitivity compared to previous implementations of NICE-OHVMS. This work will enable extended high-precision spectroscopic surveys of H3+ and other important molecular ions.

Quantitative velocity modulation spectroscopy.

Velocity Modulation Spectroscopy (VMS) is arguably the most important development in the 20th century for spectroscopic study of molecular ions. For decades, interpretation of VMS lineshapes has presented challenges due to the intrinsic covariance of fit parameters including velocity modulation amplitude, linewidth, and intensity. This limitation has stifled the growth of this technique into the quantitative realm. In this work, we show that subtle changes in the lineshape can be used to help address this complexity. This allows for determination of the linewidth, intensity relative to other transitions, velocity modulation amplitude, and electric field strength in the positive column of a glow discharge. Additionally, we explain the large homogeneous component of the linewidth that has been previously described. Using this component, the ion mobility can be determined.

Communication: High precision sub-Doppler infrared spectroscopy of the HeH⁺ ion.

The hydrohelium cation, HeH(+), serves as an important benchmark for ab initio calculations that take into account non-adiabatic, relativistic, and quantum electrodynamic effects. Such calculations are capable of predicting molecular transitions to an accuracy of ~300 MHz or less. However, in order to continue to push the boundaries on these calculations, new measurements of these transitions are required. Here we measure seven rovibrational transitions in the fundamental vibrational band to a precision of ~1 MHz using the technique of Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy. These newly measured transitions are included in a fit to the rotation-vibration term values to derive refined spectroscopic constants in the v = 0 and v = 1 vibrational states, as well as to calculate rotation-vibration energy levels with high precision.

High-precision and high-accuracy rovibrational spectroscopy of molecular ions.

We present a versatile new instrument capable of measuring rovibrational transition frequencies of molecular ions with sub-MHz accuracy and precision. A liquid-nitrogen cooled positive column discharge cell, which can produce large column densities of a wide variety of molecular ions, is probed with sub-Doppler spectroscopy enabled by a high-power optical parametric oscillator locked to a moderate finesse external cavity. Frequency modulation (heterodyne) spectroscopy is employed to reduce intensity fluctuations due to the cavity lock, and velocity modulation spectroscopy permits ion-neutral discrimination. The relatively narrow Lamb dips are precisely and accurately calibrated using an optical frequency comb. This method is completely general as it relies on the direct measurement of absorption or dispersion of rovibrational transitions. We expect that this new approach will open up many new possibilities: from providing new benchmarks for state-of-the-art ab initio calculations to supporting astronomical observations to helping assign congested spectra by combination differences. Herein, we describe the instrument in detail and demonstrate its performance by measuring ten R-branch transitions in the ν2 band of H3(+), two transitions in the ν1 band of HCO(+), and the first sub-Doppler transition of CH5(+).

Indirect rotational spectroscopy of HCO+.

Spectroscopy of the ν1 band of the astrophysically relevant ion HCO(+) is performed with an optical parametric oscillator calibrated with an optical frequency comb. The sub-MHz accuracy of this technique was confirmed by performing a combination differences analysis with the acquired rovibrational data and comparing the results to known ground-state rotational transitions. A similar combination differences analysis was performed from the same data set to calculate the previously unobserved rotational spectrum of the ν1 vibrationally excited state with precision sufficient for astronomical detection. Initial results of cavity-enhanced sub-Doppler spectroscopy are also presented and hold promise for further improving the accuracy and precision in the near future.

Predicting how polyphenol antioxidants prevent DNA damage by binding to iron.

Prevention of oxidative DNA damage due to hydroxyl radical is important for the prevention and treatment of disease. Because of their widely recognized antioxidant ability, 12 polyphenolic compounds were assayed by gel electrophoresis to directly quantify the inhibition of DNA damage by polyphenols with Fe(2+) and H2O2. All of the polyphenol compounds have IC50 values ranging from 1-59 microM and inhibit 100% of DNA damage at 50-500 microM concentrations. Gel electrophoresis results with iron(II)EDTA and UV-vis spectroscopy experiments confirm that binding of the polyphenol to iron is essential for antioxidant activity. Furthermore, antioxidant potency of polyphenol compounds correlates to the pKa of the first phenolic hydrogen, representing the first predictive model of antioxidant potency based on metal-binding. Understanding this iron-coordination mechanism for polyphenol antioxidant activity will aid in the design of more-potent antioxidants to treat and prevent diseases caused by oxidative stress, and help develop structure-activity relationships for these compounds.