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 Ã2E 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 Ã 2E state. In addition, He II excitation has been used to record the entire valence shell photoelectron spectrum. © 1999 IOP Publishing Ltd.
|Number of pages
|Journal of Physics B: Atomic, Molecular and Optical Physics
|Published - 14 Jun 1999