Femtosecond resonance-enhanced multiphoton-ionization photoelectron spectrum of ammonia

We have studied the multiphoton dissociation dynamics of the E-' (1)A(1)(') Rydberg state of ammonia (NH3) on a homebuilt femtosecond pump-probe system by resonance-enhanced multiphoton ionization photoelectron (REMPI-PE) spectroscopy. The highly excited Rydberg state, E-'1 A(1)('), 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 Delta upsilon(1)=0 but for Delta upsilon(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 Delta upsilon(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.