A photoabsorption, photodissociation and photoelectron spectroscopy study of NH₃ and ND₃

The absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of ammonia and deuterated ammonia have been measured from the ionization threshold to 25 eV using a double ion chamber and monochromated synchrotron radiation. The photoabsorption spectrum displays extensive vibrational progressions associated with Rydberg series converging onto excited vibrational levels of the X~ ²A₂? state. New structure has been observed for ND₃ in the 10.0-11.3 eV range, and vibrational progressions due to transitions into the ?, F~ and G~ Rydberg states have been recorded with improved resolution. Features have been observed, for the first time, in the photoabsorption spectra of NH₃ and ND₃ due to Rydberg series converging onto the òE ionization threshold, and interpretations for some of these features have been proposed based upon the corresponding photoelectron spectra. The He I excited NH₃⁺ (1a₂?)^-1 X~ ²A₂? photoelectron band has been studied experimentally at a resolution of 3 meV and two vibrational progressions, each involving excitation of the v₂⁺ mode, have been observed. The vibrational lines in the main progression show a complex structure associated with rotational excitations. This structure changes gradually in a way that can be explained by the variation of the H-N-H bond angle with the v₂⁺ mode. The effective bond angle has been found to be 120° for v₂⁺ = 0, and similar to that of the neutral ground state near v²⁺ = 6. The second progression, of weak lines, has been interpreted tentatively as being due to nv₂⁺ + v₄⁺. The v₄⁺ mode is doubly degenerate and the excitation of a single quantum has been explained by vibronic coupling with the à ²E state. In addition, He II excitation has been used to record the entire valence shell photoelectron spectrum. © 1999 IOP Publishing Ltd.