Oscillator strengths and integral cross sections of the ÃA2″1← X̃1A1 excitation of ammonia studied by fast electron impact.

The vibrationally resolved generalized oscillator strengths of the first and strongest singlet excitation ÃA2″1← X̃1A1 of ammonia have been determined at an impact electron energy of 1500 eV with an energy resolution of 80 meV. The comprehensive comparison of the present results with the previous experimental and theoretical ones shows that the high-energy limit, where the first Born approximation holds, has been reached at an impact electron energy of 1500 eV in K2 < 1 a.u., while it is still not satisfied in the K2 > 1 a.u. even at 1500 eV. It is also observed that the minimum position of the generalized oscillator strength of the vibronic state shifts toward the larger squared momentum transfer with the increasing vibrational quantum number. By extrapolating the generalized oscillator strength to the zero momentum transfer, the optical oscillator strength of the ÃA2″1 state has been obtained, which gives an independent cross check to the previous results. The integral cross sections of the ÃA2″1 state have been obtained systematically from the threshold to 5000 eV with the aid of the BE-scaling method.