Channel competition in dissociation of ammonia clusters (n ≤ 5) induced by femtosecond laser fields

We investigated the ionization and dissociation processes of ammonia clusters ranging from dimer to pentamer induced by 800-nm femtosecond laser fields. Time-of-flight (TOF) mass spectra of the ammonia clusters were recorded over a range of laser intensities from 2.1 × 10¹² W/cm² to 5.6 × 10¹² W/cm². The protonated ion signals dominate the spectra, which is consistent with the stability of the geometric structures. The ionization and dissociation channels of ammonia clusters are discussed. The competition and switching among observed dissociation channels are revealed by analyzing the variations in the relative ionic yields of specific protonated and unprotonated clusters under different laser intensities. These results indicate that the ionization of the neutral multiple-ammonia units, produced through the dissociation of cluster ions, may start to contribute, as well as the additional processes to consume protonated ions and/or produce unprotonated ions induced by the femtosecond laser fields when the laser intensity is above ∼ 4 × 10¹² W/cm². These findings provide deeper insights into the ionization and dissociation dynamics in multi-photon ionization experiments involving ammonia clusters.