Vacuum-UV negative photoion spectroscopy of gas-phase polyatomic molecules

This Review describes recent experiments to detect anions following vacuum-UV photoexcitation of gas-phase polyatomic molecules. Using synchrotron radiation in the range 10-35 eV at a resolution down to 0.02 eV, negative ions formed are detected by mass spectrometry. The molecules studied in detail include CF$_4$, SF$_6$ and CH$_4$; the CF$_3$X series where X = Cl,Br,I; the CH$_3$Y series where Y = F,Cl,Br; and SF$_5$Z where Z = CF$_3$,Cl. Spectra and raw data only are reported for other members of the CH$_x$F$_y$, CH$_x$Cl$_y$ including CCl$_4$, and CF$_x$Cl$_y$ series where ($x$+$y$) = 4; and saturated and unsaturated members of the C$_m$H$_n$ and C$_m$F$_n$ series up to m = 3. Anions detected range from atomic species such as H-, F- and Cl- through to heavier polyatomics such as SF$_5^-$, CF$_3^-$ and CH$_2$Cl$^-$. The majority of anions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written as ABC + h$v$ $\rightarrow$ D$^-$ + E$^+$ + neutral(s). In a few cases, the anion signal increases much more rapidly than a linear dependence with pressure, suggesting that anions now form via a multi-step process such as dissociative electron attachment. Cross sections for ion-pair formation can be put on to an absolute scale by calibrating the signal strength with those of F$^-$ from SF$_6$ and CF$_4$, although there are difficulties associated with the determination of H$^-$ cross sections from hydrogen-containing molecules unless this anion is dominant. Following normalisation to total vacuum-UV absorption cross sections (where data are available), quantum yields for anion production are obtained. Cross sections in the range ca. 10$^{-23}$ to 10$^{-19}$ cm$^2$ , and quantum yields in the range ca. 10$^{-6}$ to 10$^{-3}$ are reported. The Review describes the two ion-pair mechanisms of indirect and direct formation and their differing characteristics, and the properties needed for anion formation by dissociative electron attachment. From this huge quantity of data, attempts are made to rationalise the circumstances needed for favourable formation of anions, and which anions have the largest cross section for their formation. Since most anions form indirectly via predissociation of an initially-excited Rydberg state of the parent molecule by an ion-pair continuum, it appears that the dynamics of this curve crossing is the dominant process which determines which anions are formed preferentially. The thermochemistry of the different exit channels and the microscopic properties of the anion formed do not appear to be especially significant. Finally, for the reaction ABC + h$v$ $\rightarrow$ A$^-$ + BC$^+$ , the appearance energy of A$^-$ can be used to determine an upper limit to the bond dissociation energy of AB (to A + BC), or an upper limit to that of ABC$^+$ (to A + BC$^+$). Where known, the data are in excellent agreement with literature values

Vacuum-Ultraviolet negative photoion spectroscopy of SF5Cl

Using vacuum-UV radiation from a synchrotron, gas-phase negative ions are detected by mass spectrometry following photoexcitation of SF$_5$Cl. F$^-$, Cl$^-$ and SF$_5^-$are observed, and their ion yields recorded in the range 8-30 eV. F$^-$ and Cl$^-$ show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written AB + h$v$ $\rightarrow$ C$^-$ + D$^+$ (+ neutral(s)). F$^-$ is the strongest signal, and absolute cross sections are determined by calibrating the signal intensity with that of F$^-$ from SF$_6$ and CF$_4$. Resonances are observed, and assigned to transitions to Rydberg states of SF$_5$Cl. The Cl$^-$ signal is much weaker, despite the S-Cl bond being significantly weaker than the S-F bond. Appearance energies for F$^-$ and Cl$^-$ of 12.7 ± 0.2 and 10.6 ± 0.2 eV are determined. The spectra suggest that these ions form indirectly by crossing of Rydberg states of SF$_5$Cl onto an ion-pair continuum

The fate of NOx emissions due to nocturnal oxidation at high latitudes: 1-D simulations and sensitivity experiments

The fate of nitrogen oxide pollution during high-latitude winter is controlled by reactions of dinitrogen pentoxide (N2O5) and is highly affected by the competition between heterogeneous atmospheric reactions and deposition to the snowpack. MISTRA (MIcrophysical STRAtus), a 1-D photochemical model, simulated an urban pollution plume from Fairbanks, Alaska to investigate this competition of N2O5 reactions and explore sensitivity to model parameters. It was found that dry deposition of N2O5 made up a significant fraction of N2O5 loss near the snowpack, but reactions on aerosol particles dominated loss of N2O5 over the integrated atmospheric column. Sensitivity experiments found the fate of NOx emissions were most sensitive to NO emission flux, photolysis rates, and ambient temperature. The results indicate a strong sensitivity to urban area density, season and clouds, and temperature, implying a strong sensitivity of the results to urban planning and climate change. Results suggest that secondary formation of particulate (PM2.5) nitrate in the Fairbanks downtown area does not contribute significant mass to the total PM2.5 concentration, but appreciable amounts are formed downwind of downtown due to nocturnal NOx oxidation and subsequent reaction with ammonia on aerosol particles

Near-infrared and X-ray obscuration to the nucleus of the Seyfert 2 galaxy NGC 3281

We present the results of a near-infrared and X-ray study of the Seyfert 2 galaxy NGC 3281. Emission from the Seyfert nucleus is detected in both regions of the electromagnetic spectrum, allowing us to infer both the equivalent line of sight hydrogen column density, N_H = 71.0(+11.3,-12.3)e26/m^2 and the extinction due to dust, A_V = 22+/-11 magnitudes (90% confidence intervals). We infer a ratio of N_H/A_V which is an order of magnitude larger than that determined along lines of sight in the Milky Way and discuss possible interpretations. We consider the most plausible explanation to be a dense cloud in the foreground of both the X-ray and infrared emitting regions which obscures the entire X-ray source but only a fraction of the much larger infrared source.Comment: 23 pages including 9 figure

Microscopic two-nucleon overlaps and knockout reactions from 12^{12}C

The nuclear structure dependence of direct reactions that remove a pair of like or unlike nucleons from a fast $^{12}$C projectile beam are considered. Specifically, we study the differences in the two-nucleon correlations present and the predicted removal cross sections when using $p$-shell shell-model and multi-$\hbar\omega$ no-core shell-model (NCSM) descriptions of the two-nucleon overlaps for the transitions to the mass $A$=10 projectile residues. The NCSM calculations use modern chiral two-nucleon and three-nucleon (NN+3N) interactions. The $np$-removal cross sections to low-lying $T$=0, $^{10}$B final states are enhanced when using the NCSM two-nucleon amplitudes. The calculated absolute and relative partial cross sections to the low energy $^{10}$B final states show a significant sensitivity to the interactions used, suggesting that assessments of the overlap functions for these transitions and confirmations of their structure could be made using final-state-exclusive measurements of the $np$-removal cross sections and the associated momentum distributions of the forward travelling projectile-like residues.Comment: 9 pages, 7 figure

The ionization structure of the Orion nebula: Infrared line observations and models

Observations of the (O III) 52 and 88 micron lines and the (N III) 57 micron line have been made at 6 positions and the (Ne III) 36 micron line at 4 positions in the Orion Nebula to probe its ionization structure. The measurements, made with a -40" diameter beam, were spaced every 45" in a line south from and including the Trapezium. The wavelength of the (Ne III) line was measured to be 36.013 + or - 0.004 micron. Electron densities and abundance ratios of N(++)/O(++) have been calculated and compared to other radio and optical observations. Detailed one component and two component (bar plus halo) spherical models were calculated for exciting stars with effective temperatures of 37 to 40,000K and log g = 4.0 and 4.5. Both the new infrared observations and the visible line measurements of oxygen and nitrogen require T sub eff approx less than 37,000K. However, the double ionized neon requires a model with T sub eff more than or equal to 39,000K, which is more consistent with that inferred from the radio flux or spectral type. These differences in T sub eff are not due to effects of dust on the stellar radiation field, but are probably due to inaccuracies in the assumed stellar spectrum. The observed N(++)/O(++) ratio is almost twice the N(+)/O(+) ratio. The best fit models give N/H = 8.4 x 10 to the -5 power, O/H = 4.0 x 10 to the -4 power, and Ne/H = 1.3 x 10 to the -4 power. Thus neon and nitrogen are approximately solar, but oxygen is half solar in abundance. From the infrared O(++) lines it is concluded that the ionization bar results from an increase in column depth rather than from a local density enhancement

Vacuum-UV negative photoion spectroscopy of CH3F, CH3Cl and CH3Br

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH$_3$X (X = F,Cl,Br). The anions X$^-$, H$^-$, CX$^-$, CHX$^-$ and CH$_2$X$^-$ are observed, and their ion yields recorded in the range 8-35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + h$v$ $\rightarrow$ A$^-$ + B$^+$ (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F$^-$ from both SF$_6$ and CF$_4$. The cross sections for formation of X$^-$ + CH$_3$$^+$ are much greater than for formation of CH$_2$X$^-$ + H$^+$. In common with many quadrupoles, the spectra of $m$/$z$ 1 (H$^-$) anions show contributions from all anions, and only for CH$_3$Br is it possible to perform the necessary subtraction to obtain the true H$^-$ spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X$^-$ and CH$_2$X$^-$ are used to calculate upper limits to 298 K bond dissociation energies for D$^o$ (H$_3$C-X) and D$^o$ (XH$_2$C-H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F$^-$ from CH$_3$F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition

An algorithm for determining program feasibility of a multi-mode PAM commutator telemetry system Technical report no. 10

Algorithm formulation for evaluation of strapping arrangement programs for PAM multimode commutation system of Saturn telemetry syste

A proposed system for aviation noise measurement and control

January 1973Includes bibliographical references (p. 61-64)This report reviews previous work on various measures for aviation noise, and proposes a completely new system for aviation noise measurement and control compatible with real time, operational noise monitoring hardware. This new system allows new methods of control and regulation to be introduced and is designed to cover problems arising from future CTOL, RTOL, STOL, and VTOL aviation systems operating from current airports as well as new urban sites. New measures are proposed for aircraft flyover noise, airport noise exposure, and community noise impact

Towards a performative understanding of deservingness: merit, gender and the BBC pay dispute

Drawing largely on a high profile case of unequal pay at the BBC (British Broadcasting Corporation) as an illustrative example, this conceptual paper considers differences and interrelationships between merit and deservingness, where the latter captures how, through appropriate performances, merit is given recognition and value. We propose a performative understanding of deservingness that highlights its gendered and embodied dimensions. Informed by Judith Butler's account of gender performativity, we show that, while merit is conventionally conceptualized as a relatively fixed set of attributes (qualifications, skill) ‘attached' to the individual, deservingness captures how, in gendered terms, value and recognition are both claimed and conferred. As we argue, a gendered, deserving subject does not pre‐exist but is performatively constituted through embodied practices and performances of what is seen as worthy in a particular time and place