1
A double-ion chamber for electron collisions C. L. F. de Held *† 1 , M. G. P. Homem * 2 * Departamento de Qu´ ımica, Universidade Federal de S˜ ao Carlos, S˜ ao Carlos, Brazil † Instituto Federal de S˜ ao Paulo, S˜ ao Carlos, Brazil Synopsis In this work we present a double-ion chamber spectrometer for the directly measurement of the grand- total and the total ionization cross sections for electron collisions with molecules in gas phase in the energy range from the ionization threshold to 1000 eV. Electron collisions with molecules provide an interesting approach to control chemical reac- tions. In this sense, gas-phase cross section for several electron-molecule interaction channels are important to shed light on the underlying physics of such electron-driven reactions. Elec- tron impact ionization cross sections are one of the most important information for better under- standing of the chemistry interactions. The dis- sociation pattern following electron collisions are subjects of studies currently in the Laborat´ orio de Colis˜ oes Eletrˆ onicas e Fotˆ onicas (LCEF) of the Chemistry Department - UFSCar. These studies are being performed using a time-of-flight (TOF) mass spectrometer. Usually the partial and the total ionization cross section are ob- tained using the relative flow technique (RFT)[1]. However, the application of the RFT is time con- suming and the development of new strategies for obtaining the cross sections is desirable. In this work, we present the development of a new spec- trometer based on the double-ion chamber tech- nique [2, 3] for the measurement of the grand- total cross section (σ t ) and the total ionization cross section (σ i ) for the impact of electrons on gas phase molecules. This technique was orig- inally developed for measurements of photoab- sorption and photoionization cross sections and is being adapted for studies of electronic collisions with molecules, allowing to obtain σ i directly in absolute scale. In the double-ion chamber tech- nique, the total cross section is given by [3]: σ t (E)= 1 nl ln i 1 i 2 , (1) where E is the electron energy, i 1 and i 2 are the ion currents measured by two ion-collectors of same length (l), and n is the molecular density. The ionization efficiency η(E), defined as the ratio between the σ i and σ t , is given by [3]: η(E)= σ i σ t = i 1 2 e(i 1 - i 2 )I 0 (E) , (2) where e is the electron charge and I 0 (E) is the intensity of the electron beam. The present spec- trometer will operate from the ionization thresh- old up to 1000 eV. Figure 1. Electron spectrometer based on the double-ion chamber technique currently in devel- opment in the LCEF. Figure 1 shows a picture of the spectrometer in development. This system is currently in com- missioning stage and the first results will be pre- sented in the Workshop. References [1] S. K. Srivastava et al. 1975 J. Chem. Phys. 63 [2] J. A. R. Samson 1964 J. Opt. Soc. Am. 54 [3] J. A. R. Samson and L. Yin 1989 J. Opt. Soc. Am. 64 1 E-mail: [email protected] 2 E-mail: [email protected]
