Multichannel Quantum Defect Theory (MQDT) Analysis of the (2 + 1‘) Mass Analyzed Threshold Ionization (MATI) Spectroscopy of NH₃

Abstract

MATI spectra of NH 3 have been recorded using two-color (2 + 1′) multiphoton excitation via the B ˜and C ˜′ states and have been simulated using multichannel quantum defect theory. New vibrational bands in the MATI spectra are reported for intermediate levels up to ν 2 ) 6 in the B ˜state. The spectra obtained should in principle be identical to ZEKE spectra recorded under the same resolution. A good agreement between experiment and theory is obtained by using quantum defects determined previously from optical spectra and optimization of unknown parameters. The effects of pπ-dδ mixing in the core region are included to account for "forbidden" transitions, and the optimization of quantum defect parameters is used to quantify the strength of this mixing. Molecular symmetry group arguments are used to define which channels must be included in the quantum defect matrix. The paper demonstrates that in favorable cases a complete theory of ZEKE/ MATI line intensities is achievable including the effects of bound-state couplings. † Part of the special issue "Edward M. Schlag Festschrift".

A two-color infrared-vacuum ultraviolet laser pulsed field ionization photoelectron study of NH3

“…30,31 However, the observed ⌬K = 0 with l = odd transitions do not follow this atomiclike picture that assumes a predominant 2p z character for the nonbonding orbital. The observation of ⌬K = odd as well as l = odd transitions has been rationalized by Niu and White 14 and Dickinson et al 12,13 that l spoiling, i.e., p-d␦ mixing, in the process of photoionization of NH 3 is substantial because the photoelectron goes through the pyramidal potential at short range, resulting in both l = even and odd transitions. This interpretation could be problematic because the nuclear wave function in the ground state of NH 3 is located equally in both potential wells associated with the umbrella motion.…”

“…Ammonia is also recognized as an astrophysical molecule, which has been found to exist in primitive planetary atmospheres, comets, and interstellar space. [1][2][3][4] Partly for this reason, NH 3 has been the subject of many previous photoionization and photoelectron measurements, [5][6][7][8][9][10][11][12][13][14][15] which have provided valuable energetic and spectroscopic information for NH 3 and its cation NH 3 + . From an experimental point of view, the relatively large rotational constants ͑Ϸ10 cm −1 ͒ of both NH 3 and NH 3 + represent an advantage, 11,16,17 making it easier to obtain fully rovibrationally resolved transitions in photoionization and photoelectron studies.…”

“…Electronic mail: cyng@chem.ucdavis.edu bands of NH 3 + have also been reported by Softley and coworkers by ͑2+1Ј͒ resonance-enhanced multiphoton excitation via the NH 3 ͑B and C Ј͒ intermediate states. 12,13 A major effort of the latter study was devoted to the interpretation of rotational line strengths observed in the PFI-PI spectra using the multichannel quantum-defect theory.…”

Ab initio potential energy surface, electric-dipole moment, polarizability tensor, and theoretical rovibrational spectra in the electronic ground state of

“…m 2 þ m 3 [1][2][3]. At low resolution, the 14 NH þ 3 ion has been studied by photoelectron and photoionization spectroscopy [4][5][6][7][8][9][10][11][12][13][14][15]. Further, the infrared spectrum of NH þ 3 , trapped in a solid neon matrix, has been recorded [16].…”

Vibrational Autoionization in Polyatomic Molecules

“…The future clearly lies in more rigorous calculations based on quantum chemical input data and fully developed MQDT analyses. Several significant MQDT studies of the photoionization dynamics of other polyatomic molecules, including H 2 O and NH 3 (82)(83)(84)(85), have already been performed. Such studies have allowed the assignment of structure in the near-threshold photoionization spectra and in pulsed-field ionization spectra.…”