The electronic structure and spectroscopy of rhodium diatomic molecules
Date
2018-02-15
Authors
Fougère, Scott Gregory
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Abstract
This dissertation presents detailed spectroscopic studies of two rhodium diatomic molecular systems, rhodium monocarbide (RhC) and rhodium mononitride (RhN). The spectrum of a third rhodium diatomic species has also been recorded. All evidence suggests that rhodium monoxide (RhO) is the carrier. The rhodium-based molecules have been generated using a laser-ablation/molecular beam source.
Laser-induced fluorescence (LIF) and dispersed fluorescence (DF) techniques have been used to study the visible spectrum of rhodium monocarbide between 530 and 400 nm. Rh¹²C/Rh¹³C isotope shifts, DF and excited level lifetime measurements have assisted in classifying the observed bands into three series: the previously known C²Σ⁺ − X²Σ⁺ system and two spin subsystems of a ²Πᵢ − X²Σ⁺ system. The C − X system is inherently strong with short excited state lifetimes whereas both components of the ²Πᵢ − X²Σ⁺ system are very weak and have excited state lifetimes that are very long when compared with the C − X system. With this information, we have employed a time-filtering technique to separate effectively emission from overlapping ²Σ⁺and ²Π levels. The ²Π½ − X²Σ⁺ component was identified as the B − X system, previously identified as a ²Σ⁺ − ²Σ⁺ transition. Many new bands that belong to the ²Πᵢ − X²Σ⁺ transition have been recorded and analysed. The ²Π3/2 − X²Σ⁺ component was not observed previously except through local perturbations in the higher vibrational levels in the C state.
Rhodium mononitride has not been studied in the gas phase prior to this work. Many vibronic bands have been studied in the 700–400 nm region using LIF. Rotational analyses of the stronger bands, excited state lifetime measurements, and Rh¹⁴N/Rh¹⁵N isotope shifts have enabled identification of three electronic systems: [15.1]1 − X¹Σ⁺, [19.5]0⁺ − X¹Σ⁺, and [22.4]0⁺ − X¹Σ⁺ with (0,0) bands near 15071, 19489, and 22385 cm⁻1, respectively. Our assignment of ¹Σ⁺ as the symmetry of the ground state agrees with theoretical predictions. Dispersed fluorescence spectra revealed the presence of at least three low-lying electronic states. The three clearly identified states lie at T = 555, 1470, and 3920 cm⁻1 above the ground state. We believe some of these features are likely spin-orbit components of an expected low-lying ³Π state and another of these features may be the ¹Π state that arises from the same electron configuration.
The spectrum of what has been tentatively identified as RhO has been recorded in the 400–700 nm region using laser-induced fluorescence. The rhodium monoxide molecules have been produced in the reaction of laser-ablated rhodium metal with oxygen. Molecular orbital arguments suggest that the ground state of RhO be of ⁴Σ symmetry. The bands recorded between 550 and 640 nm appear to have a profile consistent with a ⁴Π − ⁴Σ transition. The high density of lines in several branches near the band centers, however, has prohibited detailed rotational analyses at this time.
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Keywords
Rhodium, Isotopes, Spectra