A photoabsorption, photodissociation and photoelectron spectroscopy study of NH₃and ND₃

Abstract

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 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 , and 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 à 2E 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 2A2´´ photoelectron band has been studied experimentally at a resolution of 3 meV and two vibrational progressions, each involving excitation of the 2+ 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 2+ 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 n2++4+. The 4+ 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.

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

“…The resultant photoions and photoelectrons can be experimentally manipulated with electric fields and detected with high-efficiency, possibly simultaneously (e.g., Backx et al 1976;Holland et al 1993;Edvardsson et al 1999). When all fragments are simultaneously detected, it is possible to spectroscopically examine the initial neutral species and produced ions.…”

Optical emission in the dissociation of ammonia by low energy He+ ions

“…[14,15]). More recent studies [16,17] used synchrotron radiation or other modern VUV sources. Spectroscopic control of dissociation by pumping vibrational modes with microwave photons was demonstrated in references [18][19][20].…”

Franck–Condon factors in curvilinear coordinates: the photoelectron spectrum of ammonia

“…0.12 eV (970 cm -1 ), so that the long vibrational progression has been assigned to the m 2 mode Table 1 Predicted -MP4/TZ and CC/QZ-bond lengths (r eq , Å ), valence angles \HNH (h eq , degrees) and harmonic frequencies of the two symmetric modes (m i as wavenumbers, cm -1 ) of NH 3 [39]), the so-called umbrella mode. In the high-resolution spectrum recorded by Edvardsson et al [38], the strong progression due to the symmetric bending mode is accompanied by a much weaker but well-resolved one, falling at higher energy, which was tentatively assigned to the asymmetric bending mode with one more quantum on the symmetric bending mode. As concerns neutral ammonia, a good representation of the two quasi-degenerate vibrational states is all we need for computing the photoelectron spectrum.…”

“…1 [5]. The highest intensities occur for the 7th and 8th peaks, whose intensity are nearly the same [5,38], but the origin of the band at lowest energy is still uncertain, it could be either the 0 0 0 transition (hereafter a prime will be used for the states of neutral ammonia) or a hot band. The observed vibrational spacing is ca.…”

“…Among the six normal modes of theX 2 A 00 2 cationic state, only the symmetric stretching and the symmetric bending inversion modes play a role in the photoelectron spectrum if vibronic couplings are neglected [10,38]. The stronger progression observed in the photoelectronic spectrum extends over an energy range of about 2 eV, so that a good representation of at least the lowest sixteen states associated with the inversion mode is needed.…”