Suppression of Spontaneous Emission in the Optical Pumping of Molecules: Pump–Dump–Sweep–Probe

Optical pumping experiments on molecules are often complicated by spontaneous emission, which competes with stimulated absorption and emission processes. One well-known example is stimulated emission pumping (SEP), an optical pumping method relying on a Λ-transition via an excited electronic state to produce highly vibrationally excited molecules in their ground electronic state. Here, spontaneous emission populates a host of untargeted vibrational states in the optical pumping. In this Letter, we report a novel approach to suppressing spontaneous emission and show its application by preparing NO molecules in X 2 Π(ν = 16) via the transient A 2 Σ + (ν = 2) state. Subsequent to nanosecond time scale optical pumping, we depopulate the upper state in the Λ-transition by selective resonant excitation to rapidly dissociating states. We demonstrate reduction in the spontaneous emission from the A 2 Σ + (ν = 2) state by more than 90%. Because this method employs pulsed lasers, it is easily implemented in the ultraviolet.