Femtosecond resonance-enhanced multiphoton-ionization photoelectron spectrum of ammonia

Abstract

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.

Observation of ultrafast NH3 (Ã) state relaxation dynamics using a combination of time-resolved photoelectron spectroscopy and photoproduct detection

“…This is in-line with earlier findings from fs REMPI-PES experiments on ionization from the NH 3 (A ˜) state, 14 but differs from the observation of a Dn 1 = 1 rule observed after excitation to, and ionization from, the higher lying NH 3 (E ˜) state. 30 Photoelectron peaks ii and iv are assigned to the umbrella progression in the cation, where only even 2 m quanta are present for ionization from n 0 2 ¼ 2 and 4 levels, and only odd 2 m quanta are present for ionization from n 0 2 ¼ 3 and 5 levels. This even/odd progression is evident in previous studies of 2-photon fs ionization of NH 3 (A ˜).…”

“…Measured decay times are in-line with expectations from rotationally resolved linewidth measurements (B35 fs) 10 and the time-resolved photoelectron/photoion coincidence (PEPICO) experiments (B40 fs) 41 on NH 3 (A ˜). Whilst, TRIY 29 and TRPES 30 measurements have identified a lifetime of B60 fs for the NH 3 (A ˜) state, following nonadiabatic coupling from the initially populated NH 3 ð ẼÞ n 0 2 ¼ 2 state (which occurs on the order of ps).…”

“…Direct measurement of ammonia excited state lifetimes, via fs pump-probe spectroscopy, has been employed less widely. Time-resolved ion yield (TRIY) 29 and time-resolved photoelectron spectroscopy (TRPES) 30 studies of the high-lying NH 3 (E ˜) state have reported a bi-exponential population decay with lifetimes of 1 ps and 60 fs, assigned to NH 3 (E ˜)/ NH 3 (A ˜) and NH 3 (A ˜)/NH 3 (X ˜) transfer, respectively. More recently, the time-resolved velocity map ion imaging (TR-VMI) work by Wells et al 19 has measured the H-atom KE distribution and emission timescales after NH 3 ð ÃÞ n 0 2 ¼ 4 excitation.…”

Ultrafast dynamics of aniline in the 294-234 nm excitation range: The role of the πσ* state

“…This state resembles the well studied nσ * lowest excitation of ammonia and alkyl amines, which induces a pyramidalization of the amino group and has quasi-dissociative character along the N-H bond. The photochemistry of the nσ * state in ammonia and simple alkyl amines has been the subject of a number of theoretical [6][7][8][9][10] and experimental studies in the time [11][12][13][14] and frequency domain. [15][16][17][18][19] Particularly in the last decade, the relevance of similar character πσ * states in the photophysics of a a) Author to whom correspondence should be addressed.…”

Effect of laser spectral bandwidth on coherent control of resonance-enhanced multiphoton-ionization photoelectron spectroscopy

“…Recently, resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) has shown to be an effective method to study the energy-level structure of the excited or Rydberg state and the photoionization or photodissociation processes. [1][2][3][4][5][6][7][8] Femtosecond laser pulse was considered as an ideal excitation source because of its ultrahigh laser intensity and ultrashort pulse duration. However, the femtosecond-induced REMPI-PS suffers from poor spectral resolution due to the large spectral width of the femtosecond laser pulse, and this greatly limits its further applications and developments.…”