Ken-ichi Kowari

Electron degradation and yields on initial products. II. Subexcitation electrons in molecular nitrogen

Subexcitation electrons lose their kinetic energy through vibrational excitation, rotational excitation, and elastic collisions in molecular gases. Initial yields of vibrationally and rotationally excited states of nitrogen molecules are calculated by using the Spencer–Fano equation (SFE) and its simplification, the continuous‐slowing‐down approximation (CSDA), both in time‐independent and time‐dependent representations. One focus of the present study is a close comparison of the CSDA with the rigorous treatment of the SFE in the subexcitation domain. The present result reveals for the first time distinct energy regions in which either vibrational excitation or rotational excitation dominates. This recognition explains the different time dependence of the yields of vibrational and rotational excitation.

Electron degradation and yields of initial products. III. Dissociative attachment in carbon dioxide

We demonstrate the importance of subexcitation electrons in CO2 (with energies below 6 eV) by studying the yield of negative‐ion formation in the dissociative attachment process e−+CO2→CO+O−. We evaluate the electron degradation spectrum and the time dependence of the degradation process within the continuous‐slowing‐down approximation. Slowing down by vibrational and other excitation collisions and the O− production are competing processes. This explains why the O− yield is larger for subexcitation electrons with energies above 3.8 eV, which avoid the large energy loss by electronic excitation and can still pass through the resonance at about 4 eV. The attachment at 8 eV with a much larger resonance‐like cross section contributes only about 30% to the total O− yield in the degradation process.

Electron degradation and yields of initial products. IV. Subexcitation electrons in molecular oxygen

Electron slowing‐down processes in molecular oxygen gas in the subexcitation domain (below the ionization threshold) are studied by using the Spencer–Fano (SF) equation and its simplification, the continuous‐slowing‐down approximation (CSDA), both in time‐dependent and time‐independent representations. Compared to the previously studied cases of N2 and CO2, O2 has the special features in its inelastic cross sections of (i) strong delta‐function‐like peaks in the vibrational excitation cross section below 1.3 eV and (ii) very low energy thresholds of electronic excitation channels. These features provide a stringent test for the CSDA. Indeed, our results clearly show for the first time that the CSDA fails even qualitatively to reproduce the electron degradation spectrum given by the exact SF method over the whole energy regime studied.

Energy spectra of subexcitation electrons

Abstract Subexcitation electrons are those electrons that are produced in irradiated matter and have kinetic energies ( T ) lower than the first electronic excitation threshold of the major component. The abundant subexcitation electrons play an important role in many contexts such as the excitation of minor-component molecules. For quantitative assessment of effects of the subexcitation electrons, their energy spectra must be known. Often they are assumed to be of the Platzman form: A+B /( T+I ) 3 , where I is the ionization threshold. We point out that this form is justified for He and H 2 , but not in general. For Ar we have carried out an accurate calculation and have obtained a spectrum not representable by the Platzman form.

Time-dependent aspects of electron degradation—IV. Subexcitation electrons in nitrogen and carbon dioxide

Abstract We discuss here the temporal behavior of subexcitation electrons and the yields of products due to these subexcitation electrons. Our examples concern cases in which resonance scattering of electrons occurs, such as vibrational and rotational excitation in N 2 or negative-ion formation in CO 2 . One focus of the present work is a test of the continuous-slowing-down approximation (CSDA), which we compare to the recently developed full solution of the time-dependent Spencer-Fano theory.