ABSTRACT
Density functional and ab initio calculations, along with photodissociation spectroscopy and ion imaging of MnO+ from 21,300 to 33,900 cm-1, are used to probe the photodissociation dynamics and bond strength of the manganese oxide cation (MnO+). These studies confirm the theoretical ground state (5Π) and determine the spin-orbit constant ( A' = 14 cm-1) of the dominant optically accessible excited state (5Π) in the region. Photodissociation via this excited 5Π state results in ground state Mn+ (7S) + O (3P) products. At energies above 30,000 cm-1, the Mn+ (5S) + O (3P) channel is energetically accessible and becomes the preferred dissociation pathway. The bond dissociation energy ( D0 = 242 ± 5 kJ/mol) of MnO+ is measured from several images of each photofragmentation channel and compared to theory, resolving a disagreement in previous measurements. MRCI+Q calculations are much more successful in predicting the observed spectrum than TD-DFT or EOM-CCSD calculations.
ABSTRACT
A combination of photodissociation spectroscopy, ion imaging, and high-level theory is employed to refine the bond strength of the aluminum dimer cation (Al2+) and elucidate the electronic structure and photodissociation dynamics between 38 500 and 42 000 cm-1. Above 40 400 cm-1, structured photodissociation is observed from an extremely anharmonic excited state, which calculations identify as the double minimum G 2Σ+u state. The photodissociation spectrum of the G 2Σ+u â X 2Σ+g transition in Al2+ gives an average vibrational spacing of 170 cm-1 for the G 2Σ+u state and ν0 = 172 cm-1 for the ground state. Photofragment images of G 2Σ+u â X 2Σ+g transitions indicate that once the Al (4P) + Al+ (1S) product channel is energetically accessible, it dominates the lower energy, spin-allowed pathways despite being spin-forbidden. This is explained by a proposed competition between radiative and non-radiative decay pathways from the G 2Σ+u state. The photofragment images also yield D0 (Al+-Al) = 136.6 ± 1.8 kJ/mol, the most precise measurement to date, highlighting the improved resolution achieved from imaging at near-threshold energies. Additionally, combining D0 (Al+-Al) with IE (Al) and IE (Al2) gives an improved neutral D0 (Al-Al) = 136.9 ± 1.8 kJ/mol.
ABSTRACT
We present the details of a fast ion velocity map imaging mass spectrometer that is capable of imaging the photofragments of trap-cooled (≥7 K) ions produced in a versatile ion source. The new instrument has been used to study the predissociation of N2O+ produced by electric discharge and the direct dissociation of Al2+ formed by laser ablation. The instrument's resolution is currently limited by the diameter of the collimating iris to a value of Δv/v = 7.6%. Photofragment images of N2O+ show that when the predissociative state is changed from 2Σ+(200) to 2Σ+(300) the dominant product channel shifts from a spin-forbidden ground state, N (4S) + NO+(v = 5), to a spin-allowed pathway, N*(2D) + NO+. The first photofragment images of Al2+ confirm the existence of a directly dissociative parallel transition (2Σ+u â 2Σ+g) that yields products with a large amount of kinetic energy. D0 of ground state Al2+ (2Σ+g) measured from these images is 138 ± 5 kJ/mol, which is consistent with the published literature.
