F.C. Pontes

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.

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.

Ionic desorption in valence- and core- excited polymers: poly(vinyl chloride) and poly(vinylidene chloride)

Photon stimulated ion desorption (PSID) studies have been performed in poly(vinyl chloride) (PVC) and poly(vinylidene chloride) (PVDC) using synchrotron radiation, covering from valence to core electron (Cl 2p and C 1s) energy ranges. Data acquisition was performed at the TGM beam line from the Brazilian Synchrotron Light Source (LNLS), operating in a multi-bunch mode and using a time-of-flight mass spectrometer (TOF-MS). A new pulsed system developed uses as a trigger for the TOF-MS experiments the pulsed extraction high voltage applied to the sample. Ionic desorption from PVC and PVDC shows strong selectivity in the formation of chlorine ions around the Cl 2p-edge while very similar fragmentation patterns are observed for the other energies studied.

UM ESPECTRÔMETRO DE MASSAS DE TEMPO-DE-VÔO PARA O ESTUDO DA IONIZAÇÃO DE AMOSTRAS GASOSAS POR ELÉTRONS RÁPIDOS (0,5 - 1,0 KEV)

A TIME-OF-FLIGHT MASS SPECTROMETER FOR HIGH ENERGY (0.5 ‐ 1.0 KEV) ELECTRON IONIZATION OF GASES. A simple and inexpensive time-of-flight mass spectrometer, dedicated to the study of gas-phase ionization processes induced by high energy electrons (0.5 ‐ 3.0 keV), is described. The spectrometer design is based on the Wiley-McLaren principle, with a total length of about 18 cm. As a demonstration of the performance of the apparatus, mass spectra for Ar, CH 4, CO2, and SF6, obtained at 1 keV electron energy, are presented.

Application of a multivariate normal distribution methodology to the dissociation of doubly ionized molecules: The DMDS (CH3‐SS‐CH3) case

RATIONALE The ion-ion-coincidence mass spectroscopy technique brings useful information about the fragmentation dynamics of doubly and multiply charged ionic species. We advocate the use of a matrix-parameter methodology in order to represent and interpret the entire ion-ion spectra associated with the ionic dissociation of doubly charged molecules. This method makes it possible, among other things, to infer fragmentation processes and to extract information about overlapped ion-ion coincidences. This important piece of information is difficult to obtain from other previously described methodologies. METHODS A Wiley-McLaren time-of-flight mass spectrometer was used to discriminate the positively charged fragment ions resulting from the sample ionization by a pulsed 800 eV electron beam. We exemplify the application of this methodology by analyzing the fragmentation and ionic dissociation of the dimethyl disulfide (DMDS) molecule as induced by fast electrons. The doubly charged dissociation was analyzed using the Multivariate Normal Distribution. RESULTS The ion-ion spectrum of the DMDS molecule was obtained at an incident electron energy of 800 eV and was matrix represented using the Multivariate Distribution theory. The proposed methodology allows us to distinguish information among [CHn SHn ]+ /[CH3 ]+ (n = 1-3) fragment ions in the ion-ion coincidence spectra using ion-ion coincidence data. Using the momenta balance methodology for the inferred parameters, a secondary decay mechanism is proposed for the [CHS]+ ion formation. As an additional check on the methodology, previously published data on the SiF4 molecule was re-analyzed with the present methodology and the results were shown to be statistically equivalent. CONCLUSIONS The use of a Multivariate Normal Distribution allows for the representation of the whole ion-ion mass spectrum of doubly or multiply ionized molecules as a combination of parameters and the extraction of information among overlapped data. We have successfully applied this methodology to the analysis of the fragmentation of the DMDS molecule. Copyright

Improving sensitivity of direct-form switched-capacitor filters by coinciding pole-zero pairs

The aim here is to study the inclusion of mutually cancelling pole-zero pairs in the transfer functions of switched-capacitor filters to reduce their sensitivity to capacitance ratios. Combined with an optimum allocation strategy of poles and zeros for the original transfer function, the proposed technique proved effective in comparison with designs developed from classical approximations, such as Butterworth, Chebyshev and elliptic. Spice simulations carried out with a fully differential prototype filter designed in a 0.35 mum CMOS technology are shown to validate the theory. As a result of the extra circuitry to implement a coincident pole-zero pair, the capacitance spread decreased 6-fold and the frequency response deviation decreased by a factor of 2.6 in the passband, whereas the power consumption increased only 13.6%, from 30.2 to 34.3 mW.