Eur. Phys. J. D 19, 193-209 (2002)
https://doi.org/10.1140/epjd/e20020071

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters^*

Size dependent photoelectron spectroscopy in a three color femtosecond pump-control-probe experiment

¹ Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Str. 2A, 12489 Berlin-Adlershof, Germany
² Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

Corresponding authors: ^a radloff@mbi-berlin.de - ^b hertel@mbi-berlin.de

Received: 22 September 2001
Revised: 31 January 2002
Published online: 15 May 2002

Abstract

Neutral ammonia clusters (NH₃)_m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH₃)NH₄(3s)NH₂ with the subunit NH₄ in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH₃)NH₄(3p )NH₂ states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m>2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation.