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Hi all,

I'm reading a book on electron-molecule interactions and I'm puzzled by a quantity (well, by the difference in definition of two quantities). The book is, for your information, "fundamental electron interactions with plasma processing gases" by L. G. Christophorou.

One considers the elastic cross sections of electron-molecule interactions for a given molecule in a given state (say, ground state), with the differential cross section:

[tex] dN_s = \sigma_{e,diff}(\epsilon,\theta) N_e N_t d\Omega [/tex]

where a beam of N_e electrons per cm^2 per second strike a target gas containing N_t molecules, and dN_s is the number of electrons which are elastically scattered per second in the solid angle dOmega under an angle theta.

(Formula 2.1 in the book).

He then defines the total elastic scattering cross section (eq. 2.2):

[tex] \sigma_{e,t}(\epsilon) = \int_0^{2 \pi} \int_0^{\pi}\sigma_{e,diff}(\epsilon,\theta) \sin \theta d\theta d\phi [/tex]

which is of course clear,

but he also defines:

[tex] \sigma_{m}(\epsilon) = \int_0^{2 \pi} \int_0^{\pi}\sigma_{e,diff}(\epsilon,\theta)(1-\cos \theta) \sin \theta d\theta d\phi [/tex]

in equation (2.3) and calls it the "momentum transfer cross section".

Anybody an idea what that stands for ?

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# What is momentum transfer cross section ?

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