Core-to-Rydberg band shift and broadening of hydrogen bonded ammonia clusters studied with nitrogen K-edge excitation spectroscopy

Abstract

Nitrogen 1s (N ls) core-to-Rydberg excitation spectra of hydrogen-bonded clusters of ammonia (AM) have been studied in the small cluster regime of beam conditions with time-of-flight (TOF) fragment-mass spectroscopy. By monitoring partial-ion-yield spectra of cluster-origin products, cluster specific excitation spectra could be recorded. Comparison of the cluster band with monomer band revealed that the first resonance bands of clusters corresponding to N 1s → 3sa 13pe of AM monomer are considerably broadened. The changes of the experimental core-to-Rydberg transitions FWHM (N 1s → 3sa 13pe) ∼0.20∼0.50 eV compare well with the x ray absorption spectra of the clusters generated by using density functional theory (DFT) calculation. The broadening of the core-to-Rydberg bands in small clusters is interpreted as being primarily due to the splitting of non-equivalent core-hole N 1s states caused by both electrostatic core-hole and hydrogen-bonding (H 3NH-NH 2) interactions upon dimerization. Under Cs dimer configuration, core-electron binding energy of H-N (H-donor) is significantly decreased by the intermolecular core-hole interaction and causes notable redshifts of core-excitation energies, whereas that of lone-pair nitrogen (H-acceptor) is slightly increased and results in appreciable blueshifts in the core-excitation bands. The result of the hydrogen-bonding interaction strongly appears in the n- orbital correlation, destabilizing H-N donor Rydberg states in the direction opposite to the core-hole interaction, when excited N atom with H-N donor configuration strongly possesses the Rydberg component of anti-bonding (N-H) character. Contributions of other cyclic H-bonded clusters (AM) n with n 3 to the spectral changes of the N 1s → 3sa 13pe bands are also examined. © 2012 American Institute of Physics.