Due to its high sensitivity and selectivity, UV resonance Raman (UVRR) spectroscopy has a number of scientific and industrial applications. Deep UVRR excited within explosive absorption bands (200 – 230 nm) enables trace explosive detection at a distance due to the resonance enhancement of Raman band intensities, stronger light scattering at short wavelengths, as well as negligible florescence interference.
We are developing deep UVRR detection methodologies by investigating resonance enhancement of explosives excited in the deep UV, determining the optimal excitation wavelengths, investigating explosive UV-photochemistry, characterizing explosive UV photoproducts, and measuring UVRR spectral evolution during explosive photolysis.
We are also developing state-of-the-art UVRR instrumentation by designing and manufacturing high efficiency, high throughput standoff UVRR spectrometers, co-developing new compact solid state deep UV lasers, and designing novel deep UV optical diffracting devices.
UV Raman excitation into the ~200 nm peptide bond electronic transitions enhance peptide bond amide vibrations of the backbone. A particular band (the amide III3) reports on the Ramachandran psi angle and peptide bond hydrogen bonding. This band is Raman scattered independently by each peptide bond with insignificant coupling between adjacent peptide bonds. Isotope editing of a peptide bond (by replacing the Calpha- H with Calpha- D) allows us to determine the frequency of individual peptide bonds within a peptide or protein to yield their psi angles. Consideration of the Boltzmann equilibria allows us to determine the psi angle Gibbs free energy landscape along the psi (un)folding coordinate that connects secondary structure conformations. The psi angle coordinate is the most important reaction coordinate necessary to understand mechanism(s) of protein folding. We have also discovered an analogous correlation for the primary amide sidechain of Gln. This allows us to monitor the hydrogen bonding and structure of this sidechain.
We examine the details of peptide folding conformation dynamics with laser T-jumps where the water temperature is elevated by an 1.9 mM IR nsec laser pulse and we monitor the ~200 nm UV Raman spectrum as a function of time. These spectra show the time evolution of conformation. We will discuss the role of salts on stabilizing conformations in solution
Very high diffraction efficiencies (>80%) were observed from two-dimensional (2-D) photonic crystals made of monolayers of ∼ 490 nm diameter dielectric polystyrene spheres arranged in a 2-D hexagonal lattice on top of a liquid mercury surface. These almost close packed 2-D polystyrene particle arrays were prepared by a self-assembly spreading method that utilizes solvent evaporation from the mercury surface. Two-dimensional arrays transferred onto a dielectric glass substrate placed on top of metal mirrors show diffraction efficiencies of over 30%, which is 6- to 8-fold larger than those of the same 2-D monolayers in the absence of mirrors. A simple single particle scattering model with refraction explains the high diffraction efficiencies in terms of reflection of the high intensity forward diffraction.
Deep-ultraviolet resonance Raman spectroscopy (DUVRRS) is a potential candidate for stand-off detection of
explosives. A key challenge for stand-off sensors is to distinguish explosives, with high confidence, from a myriad of
unknown background materials that may have interfering spectral peaks. To address this, we have investigated a new
technique that simultaneously detects Raman spectra from multiple DUV excitation wavelengths. Due to complex
interplay of resonant enhancement, self-absorption and laser penetration depth, significant intensity variation is observed
between corresponding Raman bands with different excitation wavelengths. These variations with excitation wavelength
provide a unique signature that complements the traditional Raman signature to improve specificity relative to singleexcitation-
wavelength techniques. We have measured these signatures for a wide range of explosives using amplitudecalibrated
Raman spectra, obtained sequentially by tuning a frequency-doubled Argon laser to 229, 238, 244 and 248
nm. For nearly all explosives, these signatures are found to be highly specific. An algorithm is developed to quantify
the specificity of this technique. To establish the feasibility of this approach, a multi-wavelength DUV source, based on
Nd:YAG harmonics and hydrogen Raman shifting, and a compact, high throughput DUV spectrometer, capable of
simultaneous detection of Raman spectra in multiple spectral windows, are being investigated experimentally.
We examined photochemical degradation of energetic molecules upon UV resonance Raman (UVRR)
excitation of the 229 nm UVRR spectra of solid HMX, TNT and RDX. Comparisons of the UVRR spectra of these
photodegraded samples to those of different carbon samples indicate some features similar to carbon compounds with
sp2 bonding, vaguely reminiscent of graphitic carbon as well as amorphous carbon. Spinning the energetic material
samples minimizes the per molecule photon flux which decreases the photochemistry. We very roughly estimated
photochemical degradation quantum yields of <10-6.