Experimental Study On Collision Processes:Noble Gas And SF₆ With Negative Ions

The study on negative ions is an important component of atomic physics. The structure of a negative ion is qualitatively different from that of an atom or positive ion. This difference can be traced to the nature of the force that binds the outermost electron. the excess electron in negative ions is bound in a short-range potential. So the negative ions provide excellent opportunities for studies of atomic structure, collision dynamics and interactions in systems. Negative ions play an important role in a number of areas. The studies of negative ions are very important in electron correlation theory, and are also of great importance in plasma physics, ionosphere physics, astrophysics and flame chemistry.Using Time of flight mass spectrometry and coincidence measurement tech-nique, the relative ionization and dissociation cross sections of gas under impact of negative ions are studied in present work. We have carried out the following work:(1) An experimental system has been set up to study the relative ionization and dissociation of gas by the impact of anions. The system bases on existed the experimental apparatus to study the collision of anions and gas. The new experimental system includes many devices and two user programs. The hard-ware devices contain the beam pipe, time-of-flight mass spectrometer, position sensitive detector and anode detector with electronic and data acquisition system, gas injection system, electrostatic deflection system. The user programs contain the data acquiring program written with using the C++language and the CAPI offered by instrument companies. We’ve also modified the go4 data processing program to suit our requirement. The work on experimental system is published in Nuclear Instruments and Methods in Physics Research B.(2) Experimental study of ionization of Ar, Kr and Xe by impact of H-. The experimental study on multiple ionizations of atoms in collision with ions is a considerable interest field in atomic physics. Many studies on collisions between atoms and ions have been carried out. Single and double ionization cross sections in collisions with such simple structure projectiles, such as electrons, photons or bare ions, have been well described. However, studies of the ionization of atoms or molecules under the impact of negative ions are very scarce. So we have studied the ionization of Ar, Kr and Xe by impact of H- using the experimental setup in (1). The results show that the cross-section ratios, taken between double and single ionization of Ar, Kr and X in the channels of single and double electron loss of H-, become larger and larger with the incident energy increasing, and that the cross-section ratios will cross at some certain energy for the two different electron loss channels. The crossing energy for Xe is about 23keV. The work about the ionization of Ar, Kr and Xe by impact of H-is published in European Physics Letter.(3)Relative dissociation fractions of SF6 under 15-30 keV H- and C- nega-tive ions impact. Sulfur hexafluoride is a man-made molecule and used in many technological areas, such as high-voltage transmission and distribution equipment; plasma etching of silicon and GaAs-based semiconductors; thermal and sound in-sulation. It is also a potent greenhouse gas. Because sulfur hexafluoride has unique structure and much applied interest, it has attracted the attention of many experi-mental, theoretical and computational physicists. The ionization and dissociation of sulfur hexafluoride has been studied by some groups. But the studies mostly fo-cus on its interactions with electrons, photons, and positive ion. So far, there is no data on the cross sections of ionization and dissociation of SF6 for negative ions im-pact. In this work,the relative dissociation fractions for the production of the frag-ment ions and ion pairs of SF6 are studied for H- and C- impact in energy range from 15 keV to 30 keV. The recoil ions (SF4+,SF3+,SF2+,SF+,S+,F+,SF42+,SF22+) and ion pairs (SF3+＋F+, SF2+＋F+, SF+＋F+, S+＋F+and F+＋F+) are detected and identified in coincidence with scattered projectiles in two charge states (q=0 and q=+1) by using the experimental system in (1). We also analyze the coinci-dent time-of-flight spectra of two fragment-ions resulting from double ionization of SF6 by H- and C- impact and describe the major dissociation pathways of SF62+ for H- and C- impact in the energy range from 15 keV to 30 keV. The work on relative dissociation fractions of SF6 under 15-30keV H-and C- negative ions impact is published in Physical Review A.