https://doi.org/10.1351/goldbook.08339
When signal averaging in a single-pulse nuclear magnetic resonance spectroscopy experiment, flip angle giving the best signal-to-noise ratio for a given combination of spin-lattice relaxation time and repetition rate.
Note: Named after Richard Ernst. It is employed to allow the maximum signal to noise to be generated in a fixed amount of time, \(\cos (\theta_{\rm{E}}) = e^{-(t_{\rm{d}} + t_{\rm{a}})/T_{1}}\), where \(t_{\rm{d}}\) is the interpulse delay, \(t_{\rm{a}}\) is the acquisition time, and \(T_{1}\) is longitudinal relaxation time.