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We have studied the multiphoton dissociation dynamics of the Ẽ Ј 1 A 1 Ј Rydberg state of ammonia ͑NH 3 ͒ on a homebuilt femtosecond pump-probe system by resonance-enhanced multiphoton ionization photoelectron ͑REMPI-PE͒ spectroscopy. The highly excited Rydberg state, Ẽ Ј 1 A 1 Ј, of ammonia was accessed by two 267 nm pump photons and then ionized by a 401 nm probe pulse delayed in time. The variation of the REMPI-PE spectra of ammonia with pump-probe delay time provides valuable information on the dynamics of the excited intermediate accessed by the pump pulse. We find that the Frank-Condon preferred transition during ionization does not occur for ⌬ 1 = 0 but for ⌬ 1 = 1, which implies that the intermediate has a different geometry from the ionic ground state. Different dynamical behavior has been observed for each of the transitions ⌬ 1 =0,1,2,3, giving a full temporal description of the excited intermediate state by projection onto the eigenspace of the ionic ground state.

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