Knowledge Bank

The infrared absorption of molecules subsequent to vibrational overtone excitation reveals the presence (or absence) or vibrational state mixing at the excited level and provides information about the nature of the dark states that mix with the zeroth-order bright state. We have used an optical-infrared double resonance method to measure the infrared spectrum of the vibrationally excited HOOH, $HONO2$, and $(CH3)3COOH.$ A pulsed dye laser first excites a $4\leftarrow 0$ OH stretch overtone transition. An infrared pulse from a Nd:YAG pumped optical parametric oscillator then excites the molecule to a state above its dissociation threshold. The infrared spectrum is generated by monitoring the OH predissociation product via laser induced fluorescence as the infrared frequency is scanned. The $4vOH$ level of HOOH is at most only very slightly mixed, and this fact allows us to perform sequential local mode-local mode double resonance excitation of the two equivalent OH oscillators. In contrast, the $4vOH$ level of $HONO2$ and $(CH3)3COOH$ are extensively mixed. In these two molecules, the zeroth-order dark states appear to mix with the bright state in proportion to their relative density at the $4vOH$ energy, regardless of their vibrational character.