CrN, CuB, and AuB: A Tale of Two Dissociation Limits

Abstract

Predissociation thresholds corresponding to dissociation at ground state separated atom limits (SALs) have been recorded in this group for more than 100 d-and fblock metal-containing molecules. The metal atom electronic degeneracies in these molecules generate a dense manifold of electronic states that allow high-lying vibronic levels to couple to pathways leading to dissociation. However, CrN, CuB, and AuB fail to dissociate at their ground SAL. Instead, the molecules remain bound at energies that far surpass their bond dissociation energies (BDEs), and their bonds break only when excited at or above an excited SAL. Sharp predissociation thresholds at excited SALs nevertheless allowed BDEs to be derived: D 0 (CrN): 3.941( 22) eV; D 0 (CuB): 2.26(15) eV; D 0 (Au 11 B): 3.724(3) eV. A previous measurement of D 0 (AlCr) is re-evaluated as dissociating to a higher energy limit, giving a revised value of D 0 (AlCr) = 1.32(2) eV. A discussion of this physical behavior is presented.

High-Resolution Photoelectron Spectroscopy of the Ground and First Excited Electronic States of MgXe+

“…The PFI-ZEKE-PE spectrum exhibits a regular vibrational progression of the X + ground state of MgXe + . Vibrational bands with v + ≥ 3 show a weak doublepeak structure on the low-frequency side of the main peak, which we attribute to the 25 MgXe and 26 MgXe isotopomers. The isotopic shifts depend on the reduced mass and primarily reflect the contributions from the Mg isotopes because the shifts induced by the different Xe isotopes are not observable at the resolution of our measurements.…”

“…(b) Measured and (c) calculated overview PFI-ZEKE-PE spectra of the X + 2 Σ + (v + ) ←a 3 Π 0 (v″ = 0) ionizing transition of MgXe. The inset shows the measured isotopic shifts of the vibrational levels of 25,26 Mg 132 Xe + (red and blue dots) determined from the PFI-ZEKE-PE spectrum. The dotted and solid lines display the isotopic shifts calculated assuming that the lowest observed level corresponds to v + = 0 and v + = 1, respectively.…”

“…28−32 In summary, the experimental setup consists of a laser-ablation source of Mg in a pulsed supersonic expansion of a He/Xe gas mixture (10:1), generating a rotationally cold molecular beam containing MgXe in the a 3 Π 0 metastable state. Various isotopomers are formed according to the abundances of the natural isotopes of Mg ( 24 Mg: 79.0%; 25 33 After passing a skimmer with a 3 mm diameter orifice located ∼26 cm downstream from the nozzle, the molecular beam enters the photoexcitation/photoionization region consisting of a resistively coupled cylindrical electrode stack within a double-layer magnetic shield, where it is crossed at right angles by one or two lasers. Pulsed electric fields are applied across the electrode stack and used to extract the electrons or ions generated by photoionization perpendicular to the propagation axes of the molecular beam and excitation lasers.…”

“…The observation of the onset of a dissociation continuum is the most direct way, pioneered in H 2 by Herzberg and Jungen . In the case of metal-containing diatomic molecules, it was extensively used by Morse and co-workers (see refs – and references therein for recent examples). Here, we exploit the observation of the onset of the spin–orbit interaction-induced predissociation of the B + state of MgXe + to determine the position of the Mg + (3p) 2 P 1/2 + Xe 1 S 0 threshold, from which the other dissociation thresholds can be derived using thermochemical cycles.…”

Observation of Continuum State Dissociation Enables the Determination of N2 Bond Dissociation Energy to Spectroscopic Accuracy

“…Compared with the method presented in the recent study, where predissociation of resonant bound rotational levels close enough to the corresponding dissociation limit is required, the method demonstrated in the present study is more applicable for measuring the BDEs of diatomic molecules with much higher accuracies. Continuum states are readily present in the vicinity of a dissociation threshold, and furthermore, multiple dissociation thresholds are usually available for a diatomic molecule, which guarantees that the method demonstrated here should always be applicable for measuring the BDEs of diatomic molecules with high accuracies.…”