Characterisation of a new VUV beamline at the Daresbury SRS using a dispersed fluorescence apparatus incorporating CCD detection

The design and performance of a new normal incidence monochromator at the Daresbury Synchrotron Radiation Source, optimised for experiments requiring high flux of vacuum-UV radiation, are described. The re-developed beamline 3.1, based on the Wadsworth design of monochromator, is the source of tunable vacuum-UV photons in the range 4 – 31 eV, providing over two orders of magnitude more flux than the vacuum-UV, Seya monochromator in its previous manifestation. The undispersed and dispersed fluorescence spectra resulting from photoexcitation of N$_2$, CO$_2$, CF$_4$ and C$_6$F$_6$ are presented. Emitting species observed were N$_2^+$ B$^2\Sigma_u^+$ - X$^2\Sigma_g^+$, CO$_2^+$ A$^2\Pi_u$ - X$^2\Pi_g$ and B$^2\Sigma_u^+$ - X$^2\Pi_g$, CF$_4$$^+$ C$^2$T$_2$ - X$^2$T$_1$ and C$^2$T$_2$ - A$^2$T$_2$, CF$_3$* $^2$A$^’_2$ - $^2$A$^”_2$, and C$_6$F$_6^+$ B$^2$A$_{2u}$ - X$^2$E$_{1g}$. A CCD multi-channel detector has significantly reduced the time period needed to record dispersed fluorescence spectra with a comparable signal-to-noise ratio

VUV photon induced fluorescence study of SF5CF3

The interaction of SF$_5$CF$_3$ with vacuum-UV radiation has been investigated by photon induced fluorescence spectroscopy. Total fluorescence yield and dispersed fluorescence spectra of SF$_5$CF$_3$ were recorded in the 200-1000 nm fluorescence window. In all cases, the fluorescence spectra resemble those of CF$_3$X (X=H, F, Cl, and Br) molecules. At photon energies below 20 eV, the emission is attributed to the excited CF$_3$ and CF$_2$ fragments. The threshold for the CF$_3$ emission is 10.2 ± 0.2 eV, giving an upper-limit estimate for the SF$_5$-CF$_3$ bond dissociation energy of 3.9 ± 0.3 eV. The excitation functions of the CF3 and CF2 emissions were measured in the photon energy range 13.6 – 27.0 eV. The resonant structures observed in SF$_5$CF$_3$ are attributed to electronic transitions from valence to Rydberg orbitals, following similar assignments in CF$_3$X molecules. The photoabsorption spectrum of SF$_5$CF$_3$ shows features at the same energies, indicating a strong contribution from Rydberg excitations

Gas phase vacuum-ultraviolet (vuv) spectroscopy of small halogenated polyatomic molecules

The work carried out in this thesis is concerned with photoexcitation of small and halogenated molecules in the gas phase with vacuum-ultraviolet (VUV) radiation. For all the experiments described in this thesis, the source of radiation was from tuneable VUV radiation from a synchrotron source. The majority of the experiments used the UK national source located at Daresbury Cheshire. Fluorescence spectra were taken using the newly commissioned Wadsworth monochromator on beamline 3.1 of the Daresbury synchrotron radiation source. Dispersed florescence spectra were taken using a newly developed fluorescence apparatus incorporating a multi-channel CCD detection system. One of the objectives of this thesis is to compare the sensitivity and resolution of this new system with that of an apparatus used at the BESSY 1 synchrotron source in the 1990s. Many of the molecules studied here were chosen because previous data recorded at BESSY 1 were available for this comparison. Molecules studied include the MCl$_4$ series (M = C, Si and Ge) where both the VUV photoabsorption and fluorescence spectra were recorded. A new analysis of the absorption spectra is presented. Emissions are observed in MCl$_2$, MCl and the parent ion of SiCl$_4^+$ and GeCl$_4^+$. The CF$_3$X series (X = H, F, Cl, Br, I and SF$_5$) where the fluorescence spectra and some photoabsorption spectra were recorded. Emissions are observed in CF, CF$_2$, CF$_3$ and parent ion CF$_4^+$, CF$_3$H$^+$. Undispersed and dispersed fluorescence spectra of BX$_3$ (X = F, Cl and Br) were also recorded. Emission is now observed in BX$_2$, BX and parent ion BX$_3^+$. Finally the fluorescence spectra of PX$_3$ (X = F, Cl and Br) were recorded and are presented here. Emissions are observed in PX$_2$, PX and parent ion PX$_3^+$

Selected ion flow tube cation-molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic C5F8

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron and threshold photoelectron photoion coincidence spectra of cyclic-C$_4$F$_8$ in the range 11–25 eV have been recorded. The parent ion is observed very weakly at threshold, 11.60 eV, and is most likely to have cyclic geometry. Ion yield curves and branching ratios have been determined for five fragments. Above threshold, the first ion observed is C$_3$F$_5^+$, at slightly higher energy C$_2$F$_4^+$, then successively CF$_3^+$, CF$_2^+$ and CF$^+$ are formed. The dominant ions are C$_3$F$_5^+$ and C$_2$F$_4^+$, with the data suggesting the presence of a barrier in the exit channel to production of C$_3$F$_5^+$ whilst no barrier to production of C$_2$F$_4^+$. In complementary experiments, the product branching ratios and rate coefficients have been measured in a selected ion flow tube at 298 K for the bimolecular reactions of cyclic-C$_4$F$_8$ with a large number of atomic and small molecular cations. Below the energy where charge transfer becomes energetically allowed, only one of the ions, CF$_2^+$, reacts. Above this energy, all but one of the remaining ions reacts. Experimental rate coefficients are consistently greater than the collisional values calculated from modified average dipole orientation theory. The inclusion of an additional ion-quadrupole interaction has allowed better agreement to be achieved. With the exception of N$^+$, a comparison of the fragment ion branching ratios from the TPEPICO and SIFT data suggest that long-range charge transfer is the dominate mechanism for reactions of ions with recombination energy between 12.9 and 15.8 eV. For all other ions, either short-range charge transfer or a chemical reaction, involving cleavage and making of new bond(s), is the dominant mechanism