Daniel E. Weibel

Ionic desorption in poly(methyl methacrylate) induced by fast electrons

Abstract Electron stimulated ion desorption (ESID) studies have been performed in poly(methyl methacrylate) (PMMA) using a time-of-flight mass spectrometer and a pulsed electron beam source with pulse duration of 0.18 μs (FWHM) and frequency of 3 kHz. The incident electron energy was 350–1200 eV. Several ionic fragments have been observed, H + being the main ionic species desorbed. Partial ion yields have been determined for the main fragments as a function of the electron energy. They show a maximum at 500–600 eV. Ionic desorption from PMMA is tentatively discussed in terms of the Auger stimulated ion desorption (ASID) mechanism. This is, to our knowledge, the first description of ESID on polymers.

Quantum yield dependence on the wavelength in the gas phase photolysis of perfluorosuccinic anhydride

Abstract The photolysis of perfluorosuccinic anhydride vapour was studied in the pressure range 3.0 – 70.2 Torr using light with wavelengths of 254, 232 and 220 nm. The products were CO, CO 2 and C 2 F 4 . At higher pressures the quantum yield was 0.50 at all wavelengths, but at the zero pressure limit, the quantum yield at 254 and 232 nm was 0.74 and at 220 nm it was unity. The data are interpreted in terms of a variation either in the vibrational level of the lowest excited singlet or in the electronic transition.

Quantum yield in the gas phase photolysis of perfluoroacetyl chloride: a comparison with related compounds

Abstract The photolysis of perfluoroacetyl chloride vapour was studied in the pressure range 4.6–59.9 Torr and with addition of inert gas up to 535.2 Torr using light of 254 and 280 nm. The quantum yield for the decomposition of CF 3 COCl taken as Φ [CF 3 Cl+2C 2 F 6 ] = 0.98±0.13 was not affected by the total pressure, the intensity and the wavelength of the light within the studied range of conditions. In the presence of c-C 6 H 12 a clear hydrogen abstraction reaction took place indicating the presence of CF 3 radicals. An upper limit for the rate of hydrogen abstraction of log k H (cm 3 mol −1 s −1 ) = 7.9 at 298 K was obtained. Light emission was not observed over the range 330–600 nm. Mechanisms for the decomposition of perfluoroacyl halides are discussed.

New and simple mass calibration procedure for time-of-flight studies of electron stimulated desorption of ions from solid samples

A technique that provides a simple mass scale calibration for linear time-of-flight (TOF) measurements is presented. This procedure has been used in the calibration of the mass scale in experiments of electron stimulated desorption of ions from solid surfaces. It is based on the admission of one or more calibration rare gases inside the ultrahigh vacuum chamber while a TOF spectrum of desorbing ions from the surface is simultaneously recorded. Peaks related to desorbing ions as well as to the atomic gases can consequently be observed in the same mass spectrum, thus allowing an exact mass scale calibration. The present calibration procedure is especially suitable in the low mass range (<200 amu) and can be easily adapted for photon stimulated desorption experiments.

Irradiação de polímeros com feixe de elétrons: caracterização de íons positivos através da técnica de tempo-de-vôo

With the aim of studying the interaction of fast electrons with solid surfaces we have developed an experimental set-up based on electron stimulated desorption (ESD) coupled to time-of-flight (TOF) mass spectrometry. Poly(methyl methacrylate) and poly(vynil chloride) samples have been irradiated by a pulsed electron beam of 1.2 keV and 0.18 µs FWHM. The results show that H+ is the main ionic species to desorb after electron bombardment. In addition, other ionic fragments were also observed and assigned. These results show the potentiality of this technique in the study of ESD of polymers.

EFFECT OF HCl ADDITION AND TEMPERATURE ON THE HETEROGENEOUS CHEMISTRY AND PHOTOCHEMISTRY OF ClONO 2 ADSORBED ON ICE CRYSTALS

The heterogeneous chemistry and photochemistry of chlorine nitrate adsorbed on HCl-doped crystals was studied mainly at 181 K and at 190 K. Under our experimental conditions the main gaseous products found at 181 K were Cl2O and Cl2, while at 190 K, Cl2O was mainly observed. At both temperatures a net enhancement in the rate of gaseous product formation was observed when light of wavelength longer than 350 nm entered the reactor. The obtained results in conditions where the ClONO2 concentration is higher than HCl concentration, and both are high enough to maintain saturation of the surface, showed that the rate of gaseous products formation at 181 K depends on the reactive species adsorbed while at 190 K, it is independence of them. The implications of these findings to the polar stratospheric chemistry are briefly discussed.

Quantum yield of perfluoroglutaryl dichloride and 4-chloroperfluorobutanoyl chloride in gas phase : possible use as actinometers

Abstract The photolysis of perfluoroglutaryl dichloride and the photolysis of 4-chloroperfluorobutanoyl chloride vapour were studied in the wavelength range 232–270 nm and 240–270 nm respectively. The quantum yield for the first reaction was measured as φ CO = 1.02 ± 0.06 and φ Cl(CF 2 ) 3 COCl = 1.05 ± 0.07, and for the second reaction as φ CO = 1.03 ± 0.06 and φ Cl(CF 2 ) 3 Cl = 1.00 ± 0.07. The quantum yields for both reactions were not affected by the total pressure, the photon flow of the irradiation and the wavelengths of the light in the conditions studied. The perfluoroglutaryl dichloride and 4-chloroperfluorobutanoyl chloride are recommended as conveniently reliable gas phase actinometers.

Photochemical and thermal decomposition of perfluoroglutaryl dichloride in the gas phase

Abstract The photolysis of perfluoroglutaryl dichloride was investigated in the gas phase with the light of the full arc of a high pressure mercury lamp. At high intensities and conversions the main products are CO and ClCF2CF2CF2Cl, and the ratio CO Cl(CF 2 ) 3 Cl is 2. However, in runs with attenuated light and very short times, this ratio reaches higher values because less Cl(CF2)3Cl is formed. In the first step the stable intermediate compound ClCO(CF2)3Cl is formed, producing Cl(CF2)3Cl by absorption of a second photon. The thermal decomposition studied in the temperature range 611–713 K produces CO, CF2Cl2, C2F4Cl2, C2F4, COCl2 and a very small amount of c-C3F6. A mechanism is suggested to explain these results.

Low-Cost Temperature Control from 160 K to Ambient Temperature Using Liquid Nitrogen Evaporation

A low-cost temperature controller that allows variation from 110 K to room temperature of liquid, gas, or solid samples is described. The desired stabilized temperature is obtained within a few minutes by controlling the current through an electrical resistor immersed in a liquid nitrogen reservoir. The stability of the temperature ranges from less than ± 0.5 K at temperatures higher than 230 K to ± 1.1 K at 160 K. The maximum consumption of liquid nitrogen for cooling a glass photochemical reactor was less than 0.60 liters per hour at temperatures lower than 160 K, about 0.4 liters per hour at 160 K to 190 K, and decreases considerably over 200 K. A linear dependence of the temperature with a flow of evaporated gas was found (240–285 K). This device can be used easily in experiments that need low temperature regulation, involving gas–surface interactions, adsorption–desorption processes, heterogeneous atmospheric chemistry, thermodynamics of gases, and so forth.