The lowest-energy triplet state of ammonia obtained by electron energy loss spectroscopy

Revisiting the photoabsorption spectrum of NH3 in the 5.4-10.8 eV energy region

We present a comprehensive revisited experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectrum of ammonia, NH₃, covering for the first time the full 5.4-10.8 eV energy-range, with absolute cross sections determined. The calculations on the vertical excitation energies and oscillator strengths were performed using the equation-of-motion coupled cluster method restricted to single and double excitation levels and used to help reanalyze the observed Rydberg structures in the photoabsorption spectrum. The VUV spectrum reveals several new features that are not previously reported in the literature, with particular reference to the vibrational progressions of the (D̃¹E^'←X̃¹A₁ ^'), the (F̃¹E^'←X̃¹A₁ ^'), and the (G̃¹A₂ ^″←X̃¹A₁ ^') absorption bands. In addition, new Rydberg members have been identified in nda₁ ^'←1a₂ ^″D̃^''1A₂ ^″←X̃¹A₁ ^', where n > 3 has not been reported before as well as in nde^″←1a₂ ^″F̃¹E^'←X̃¹A₁ ^' and in nsa₁ ^'←1a₂ ^″G̃¹A₂ ^″←X̃¹A₁ ^'. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of ammonia in the Earth's atmosphere (0-50 km).

Oscillator strengths and integral cross sections of the ÃA2″1← X̃1A1 excitation of ammonia studied by fast electron impact

The vibrationally resolved generalized oscillator strengths of the first and strongest singlet excitation ÃA2″1← X̃¹A₁ of ammonia have been determined at an impact electron energy of 1500 eV with an energy resolution of 80 meV. The comprehensive comparison of the present results with the previous experimental and theoretical ones shows that the high-energy limit, where the first Born approximation holds, has been reached at an impact electron energy of 1500 eV in K² < 1 a.u., while it is still not satisfied in the K² > 1 a.u. even at 1500 eV. It is also observed that the minimum position of the generalized oscillator strength of the vibronic state shifts toward the larger squared momentum transfer with the increasing vibrational quantum number. By extrapolating the generalized oscillator strength to the zero momentum transfer, the optical oscillator strength of the ÃA2″1 state has been obtained, which gives an independent cross check to the previous results. The integral cross sections of the ÃA2″1 state have been obtained systematically from the threshold to 5000 eV with the aid of the BE-scaling method.

Ab initio-based double many-body expansion potential energy surface for the first excited triplet state of the ammonia molecule

A global single-sheeted double many-body expansion potential energy surface is reported for the first excited triplet state of NH(3). It employs an approximate cluster expansion of the molecular potential that utilizes previously reported functions of the same family for the triatomic fragments. Four-body energy terms have been calibrated from extensive accurate ab initio data so as to reproduce the main features of the title system. A new switching function formalism has been reported to approximate the true multisheeted nature of NH(3)((3)A(2) ('')) potential energy surface, thus allowing the correct behavior at the NH(2)((2)A(")) + H((2)S) and NH(2)((4)A(")) + H((2)S) dissociation limits. The resulting fully six-dimensional potential energy function reproduces the correct symmetry under the permutation of identical atoms, and predicts the correct behavior at all dissociation channels while providing a realistic representation at all interatomic separations. The major attributes of the NH(3) double many-body expansion potential energy surface have also been characterized, and found to be in good agreement, both with the calculated ones from the raw ab initio energies and the theoretical results available in the literature.