NONLINEAR OPTICAL SPECTROSCOPY OF MALONALDEHYDE: AN INVESTIGATION OF PROTON-TRANSFER DYNAMICS IN THE FIRST EXCITED SINGLET STATE

Abstract

Malonaldehyde, the most stable isomer of 3-hydroxy-2-propenal, is a prototypical system for the investigation of hydrogen tunneling dynamics. While numerous experimental and theoretical studies of the $\tilde{X}^{1} A_{1}$ ground electronic state have demonstrated conclusively that the gas-phase molecule exists almost completely as the chelated enol tautomer (cf. figure below) with a finite barrier serving to hinder proton transfer, very little work has been reported on the corresponding electronically-excited potential surfaces. The absorption-based nonlinear optical technique of Degenerate Four-Wave Mixing (DFWM) has been used to probe the exceptionally weak $\tilde{A}^{1}B_{1} - \tilde{X}^{1}A_{1} (\pi^{\ast} \leftarrow n)$ transition of malonaldehyde at $-28500 cm^{-1}$. Our DFWM spectra, recorded under bulk-gas conditions (e.g., 400 mTorr sample pressure; $\sim 5cm$ pathlength) with$<1 cm^{-1}$ resolution, indicate a pronounced vibronic progression of $-200cm-1$ with all members exhibiting a distinct structural doubling of $-16 cm^{-1}$ magnitude. These results will be discussed in relation to recent theoretical calculations regarding hydrogen migration processes in the $^{1}\pi^{\ast} \leftarrow^{1}n$ excited state.