Electron Scattering Cross Sections Introduction: : 1.5 Planned experiment

1.5 Planned experiment

The planned experiment is to measure the total electron scattering cross section for CH₄, NH₃, PH₃ and SiH₄ gases at intermediate electron energies using the transmission technique. These cross sections will be compared with those of other laboratories and existing theoretical models. It is important to mention here that no experiment has been performed to measure the cross section of PH₃ in other laboratories.

Electron scattering on CH₄ (Methane) was first studied at low energies by Brode⁷ in 1925. The total cross sections (TCS) of electrons colliding with CH₄ gas were measured by Barbarito et al.¹¹ for the lower energy range 0-16 eV using the Ramsauer type experimental setup. Jones¹² measured TCS with a time-of-flight electron transmission spectrometer for incident energies for 1.3 – 50 eV. The experimental errors in his work are energy dependent, giving greater error at higher energies. Lohmann and Buckman¹³ studied TCS of CH₄ gas using a low-energy time-of-flight electron spectrometer for an energy range of 0.1-20 eV. Most of the early studies of total electron scattering cross sections for CH₄ gas have been done at very lower energies.

Sueoka and Mori¹⁴ measured TCS for 1.0-400 eV electrons colliding with CH₄ using a retarding potential-time of flight (RP-TOF) method. In their work, cross sections were obtained by a normalization method rather than absolute measurements. In this work, 1.8 – 5.0% of statistical errors reported are due to the change in electron beam intensity. Dababneh et al.¹⁵ measured TCS for 1 – 500 eV electrons using a beam-transmission technique. Their work showed a 5% minimum error and a 13% maximum error. Zecca et al.¹⁶ measured TCS for CH₄ gas in the energy range 0.9 – 4000 eV using a modified Ramsauer type experimental setup. Garcia and Manero¹⁷ measured TCS for CH₄ molecules in the energy range 400–5000 eV with experimental errors of about 3% using the transmission beam technique. In a previous experiment in this laboratory, Ariyasinghe and Powers¹⁸ measured TCS for CH₄ gas in the energy range 200 – 1400 eV. They made a comparison with other experimental and theoretical work. Their comparison showed that measured cross sections are in good agreement with the empirical model proposed by Garcia and Manero¹⁹.

Total electron scattering cross sections of NH₃ gas was first studied by Sueoka, Mori and Katayama²⁰ for 1 – 400 eV using a time of flight (TOF) experimental method. In this work cross sections were obtained by a normalization method. Later, Zecca, Karwasz, and Brusa²¹ measured TCS for NH₃ gas in the 75 – 4000 eV energy range using a modified Ramsauer type experimental setup. Recently, Garcia and Manero²² measured the TCS of NH₃ molecules in the energy range 300–5000 eV using the transmission beam technique with experimental errors of about 3%.

TCS for SiH₄ gas was first measured by Sueoka, Mori and Hamada²³ using the RP-TOF experimental setup for electron energies 1 – 400 eV. Later Zecca, Karwasz, and Brusa ²¹ measured the TCS of three hydride molecules including SiH₄ for the 75 – 4000 eV energy range using the Ramsauer technique.

In their theoretical work, Jain and Baluja²⁴ calculated TCS for several diatomic and polyatomic molecular targets including CH₄, PH₃ and SiH₄ using Bethe-Born theory and the spherical complex optical potential (SCOP) method for an energy region of 10 – 5000 eV. This theoretical model predicts the cross section as a function of the number of electrons, polarizability, multipole moments, and ionization potential of the target molecule. They have discussed the limitation in this model below the 100 eV region. By using Bethe-Born theory and the SCOP method, Jain²⁵ reported theoretical cross sections over the energy range of 10 – 3000 eV for NH₃. Recently, Garcia and Manero¹⁹ proposed an empirical model to predict total cross sections of simple molecules at intermediate electron energies (energies over ~ 400 eV). This model expresses the cross section as a function of electron energy, number of electrons in the target molecule, and the polarization of the target molecule. These theoretical models and empirical models will be discussed in detail in chapter 3.