      REAL FUNCTION SIGMA(E)
C     ELECTRON COLLISIONS WITH N2
C     EXCITATION TO B^1PI_u;v=7
C     CROSS SECTION(CM^2)  :SIGMA
C     COLLISION ENERGY(KEV):E
C     T.Tabata, et al., At. Data Nucl. Data Tables.
      REAL A(12)
      DATA EMIN,EMAX,ETH /1.40E-2,4.00E-1,1.32E-2/
      DATA A /2.774E-2,9.439E-1,1.689E-2,6.113E-1,8*0.0/
      DATA SIGMA0,ER /1.E-16,1.361E-2/
      SIGMA=0.0
      IF(E.LT.EMIN .OR. E.GT.EMAX) GO TO 9999
      E1=E-ETH
      SIGMA=F2(SIGMA0,ER,E1,A(1),A(2),A(3),A(4))
 9999 RETURN
      END
C
      REAL FUNCTION F1(SIGMA0,ER,X,C1,C2)
        F1=SIGMA0*C1*(X/ER)**C2
      RETURN
      END
C
      REAL FUNCTION F2(SIGMA0,ER,X,C1,C2,C3,C4)
        F2=F1(SIGMA0,ER,X,C1,C2)
     >     /(1.+(X/C3)**(C2+C4))
      RETURN
      END
C
      REAL FUNCTION F3(SIGMA0,ER,X,C1,C2,C3,C4,C5,C6)
        F3=F1(SIGMA0,ER,X,C1,C2)
     >     /(1.+(X/C3)**(C2+C4)+(X/C5)**(C2+C6))
      RETURN
      END