Threshold photoelectron photoion coincidence spectroscopy of trichloroethene and tetrachloroethene

The threshold photoelectron, the threshold photoelectron photoion coincidence and ion breakdown spectra of trichloroethene and tetrachloroethene have been recorded from 9 – 22 eV. Comparisons with the equivalent data for the three dichloroethene molecules and theoretical calculations highlight the nature of the orbitals involved during photoionisation in this energy range. The ground electronic state of C$_2$HCl$_3^+$ (C$_2$Cl$_4^+$) is bound, with excited valence states dissociating to C$_2$HCl$_2^+$ (C$_2$Cl$_3^+$) and C$_2$HCl$^+$ (C$_2$Cl$_2^+$). Appearance energies suggest that C$_2$HCl$^+$ forms from C$_2$HCl$_3^+$ by loss of two chlorine atoms, whereas C$_2$Cl$_2^+$ forms from C$_2$Cl$_4^+$ by loss of a Cl$_2$ molecule. The translational kinetic energy release into C$_2$HCl$_2^+$ (C$_2$Cl$_3^+$) + Cl is determined as a function of energy. In both cases, the fraction of the available energy released into translational energy of the two products decreases as the photon energy increases

Photo-induced enhanced Raman spectroscopy (PIERS): Sensing atomic-defects, explosives and biomolecules

Enhanced Raman relies heavily on finding ideal hot-spot regions which enable significant enhancement factors. In addition, the termed “chemical enhancement” aspect of SERS is often neglected due to its relatively low enhancement factors, in comparison to those of electromagnetic (EM) nature. Using a metal-semiconductor hybrid system, with the addition of induced surface oxygen vacancy defects, both EM and chemical enhancement pathways can be utilized on cheap reusable surfaces. Two metal-oxide semiconductor thin films, WO3 and TiO2, were used as a platform for investigating size dependent effects of Au nanoparticles (NPs) for SERS (surface enhanced Raman spectroscopy) and PIERS (photo-induced enhanced Raman spectroscopy – UV pre-irradiation for additional chemical enhancement) detection applications. A set concentration of spherical Au NPs (5, 50, 100 and 150 nm in diameter) was drop-cast on preirradiated metal-oxide substrates. Using 4-mercaptobenzoic acid (MBA) as a Raman reporter molecule, a significant dependence on the size of nanoparticle was found. The greatest surface coverage and ideal distribution of AuNPs was found for the 50 nm particles during SERS tests, resulting in a high probability of finding an ideal hot-spot region. However, more significantly a strong dependence on nanoparticle size was also found for PIERS measurements – completely independent of AuNP distribution and orientation affects – where 50 nm particles were also found to generate the largest PIERS enhancement. The position of the analyte molecule with respect to the metal-semiconductor interface and position of generated oxygen vacancies within the hot-spot regions was presented as an explanation for this result

Selected ion flow tube study of the reactions between gas phase cations and CHCl2F, CHClF2 and CH2ClF

The branching ratios and rate coefficients have been measured at 298 K for the reactions between CHCl$_2$F, CHClF$_2$ and CH$_2$ClF and the following cations (with recombination energies in the range 6.3 - 21.6 eV); H$_3$O$^+$, SF$_x$$^+$ (x = 1 - 5), CF$_y$$^+$ (y = 1  3), NO$^+$, NO$_2$$^+$, O$_2$$^+$, Xe$^+$, N$_2$O$^+$, O$^+$, CO$_2$$^+$, Kr$^+$, CO$^+$, N$^+$, N$_2$$^+$, Ar$^+$, F$^+$ and Ne$^+$. The majority of the reactions proceed at the calculated collisional rate, but the reagent ions SF$_3$$^+$, NO$^+$, NO$_2$$^+$ and SF$_2$$^+$ do not react. Surprisingly, although all of the observed product channels are calculated to be endothermic, H$_3$O$^+$ does react with CHCl$_2$F. On thermochemical grounds, Xe$^+$ appears to react with these molecules only when it is in its higher-energy $^2$P$_{1/2}$ spin-orbit state. In general, most of the reactions form products by dissociative charge transfer, but some of the reactions of CH$_2$ClF with the lower-energy cations produce the parent cation in significant abundance. The branching ratios produced in this study and by threshold photoelectron-photoion coincidence spectroscopy (preceding paper) agree reasonably well over the energy range 11 - 22 eV. In about one fifth of the large number of reactions studied the branching ratios are in excellent agreement and appreciable energy resonance between an excited state and the ground state of the ionized neutral exists, suggesting that these reactions proceed exclusively by a long-range charge transfer mechanism. Upper limits for the enthalpy of formation at 298 K of SF$_4$Cl (-637 kJ mol$^{-1}$), SClF (-28 kJ mol$^{-1}$) and SHF (-7 kJ mol$^{-1}$) are determined

Enhancing hybrid metal-semiconductor systems beyond SERS with PIERS (photo-induced enhanced Raman scattering) for trace analyte detection

Hybrid metal-semiconductor systems are promising substrates for field Raman analysis due to their ability to use both electromagnetic and chemical enhancement pathways for surface enhanced Raman spectroscopy (SERS). Photo-induced Raman spectroscopy (PIERS) has previously been shown to be a promising method utilizing an additional enhancement route through photo-inducing atomic surface oxygen vacancies in photocatalytic metal-oxide semiconductors. The photoinduced vacancies can form vibronic coupling resonances, known as charge transfers, with analyte molecules, enhancing the signal beyond conventional SERS enhancements. However, conventional UV sources most often used for excitation of the PIERS substrate are impractical in combination with portable Raman systems for field analysis. In this work we show how a small UVC LED, centered at 255 nm, can replicate the same results previously reported with the benefit of allowing greater in-situ real time measurements under constant UV exposure. The UV LED source can be controlled more easily and safely, making it a practical UV source for field PIERS analysis