Femtosecond resonance-enhanced multiphoton-ionization photoelectron spectrum of ammonia

Abstract

We have studied the multiphoton dissociation dynamics of the ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}}^{\ensuremath{'}}\phantom{\rule{0.2em}{0ex}}^{1}A_{1}^{{}^{\ensuremath{'}}}$ Rydberg state of ammonia $({\mathrm{NH}}_{3})$ on a homebuilt femtosecond pump-probe system by resonance-enhanced multiphoton ionization photoelectron (REMPI-PE) spectroscopy. The highly excited Rydberg state, ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{E}}^{\ensuremath{'}1}\phantom{\rule{0.2em}{0ex}}{A}_{1}^{\ensuremath{'}}$, of ammonia was accessed by two $267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ pump photons and then ionized by a $401\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ 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 $\mathrm{\ensuremath{\Delta}}{\ensuremath{\upsilon}}_{1}=0$ but for $\mathrm{\ensuremath{\Delta}}{\ensuremath{\upsilon}}_{1}=1$, 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 $\mathrm{\ensuremath{\Delta}}{\ensuremath{\upsilon}}_{1}=0,1,2,3$, giving a full temporal description of the excited intermediate state by projection onto the eigenspace of the ionic ground state.