Enhanced Sensitivity to the Time Variation of the Fine-Structure Constant and mₚ/mₑ in Diatomic Molecules: A Closer Examination of Silicon Monobromide

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Recently it was pointed out that transition frequencies in certain diatomic molecules have an enhanced sensitivity to variations in the fine-structure constant α and the proton-to-electron mass ratio mₚ/mₑ due to a near cancellation between the fine-structure and vibrational interval in a ground electronic multiplet [V. V. Flambaum and M. G. Kozlov, Phys. Rev. Lett. 99, 150801 (2007)]. One such molecule possessing this favorable quality is silicon monobromide. Here we take a closer examination of SiBr as a candidate for detecting variations in α and mₚ/mₑ. We analyze the rovibronic spectrum by employing the most accurate experimental data available in the literature and perform ab initio calculations to determine the precise dependence of the spectrum on variations in α. Furthermore, we calculate the natural linewidths of the rovibronic levels, which place a fundamental limit on the accuracy to which variations may be determined.