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

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 → 3sa₁/3pe of AM monomer are considerably broadened. The changes of the experimental core-to-Rydberg transitions ΔFWHM (N 1s → 3sa₁/3pe) = ∼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 (H₃N···H–NH₂) 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−σ* 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 σ* (N−H) character. Contributions of other cyclic H-bonded clusters (AM)_n with n ≥ 3 to the spectral changes of the N 1s → 3sa₁/3pe bands are also examined.