We find the entire form of the amplitude of two fermion strings (with
different chirality), a massless scalar field and one closed string
Ramond-Ramond (RR) in IIA superstring theory which is different from its IIB
one. We make use of a very particular gauge fixing and explore several new
couplings in IIA. All infinite u- channel scalar poles and t,s- channel
fermion poles are also constructed. We find new form of higher derivative
corrections to two fermion two scalar couplings and show that the first simple
(s+t+u)− channel scalar pole for p+2=n case can be obtained by having new
higher derivative corrections to SYM couplings at third order of α′. We
find that the general structure and the coefficients of higher derivative
corrections to two fermion two scalar couplings are completely different from
the derived α′ higher derivative corrections of type IIB.Comment: 29 pages, no figure,Latex file,published version in EPJ