CAVITY RINGDOWN ABSORPTION SPECTRUM OF THE $T_1(n,\pi^{*}) \leftarrow S_0$ TRANSITION OF \\ ACROLEIN: ANALYSIS OF THE $0^0_0$ BAND ROTATIONAL CONTOUR

Abstract

Acrolein (propenal, CH$_2$=CH---CH=O) is the simplest conjugated enal molecule and serves as a prototype for investigating the photochemical properties of larger enals and enones. Acrolein has a coplanar arrangement of heavy atoms in its ground electronic state. Much of the photochemistry is mediated by the $T_1(\pi,\pi^{*})$ state, which has a CH$_2$--twisted equilibrium structure. In solution, the $T_1(\pi,\pi^{*})$ state is typically accessed via intersystem crossing from an intially prepared planar $S_1(n,\pi^{*})$ state. An intermediate in this photophysical transformation is the lowest $^3 (n,\pi^{*})$ state, a planar species with adiabatic excitation energy below $S_1$ and above $T_1(\pi,\pi^{*})$. The present work focuses on this $^3 (n,\pi^{*})$ intermediate state; it is designated $T_1(n,\pi^{*})$ as the lowest-energy triplet state of acrolein having a planar equilibrium structure. \hspace{0.2in}The $T_1(n,\pi^{*}) \leftarrow S_0$ band system, with origin near 412 nm, was first recorded in the 1970s at medium (0.5 cm$^{-1}$) resolution using a long-path absorption cell. Here we report the cavity ringdown spectrum of the $0^0_0$ band, recorded using a pulsed dye laser with 0.1 cm$^{-1}$ spectral bandwidth. The spectrum was measured under both bulk-gas (room-temperature) and jet-cooled conditions. The band contour in each spectrum was analyzed by using a computer program developed, \emph{J.~Chem.~Phys.}~\textbf{103}, 5343 (1995).} for simulating and fitting the rotational structure of singlet-triplet transitions. The assignment of several resolved sub-band heads in the room-temperature spectrum permitted approximate fitting of the inertial constants for the $T_1(n,\pi^{*})$ state. The determined values (cm$^{-1}$) are $A=1.662$, $B=0.1485$, $C=0.1363$. For the parameters $A$ and $(B+C)/2$, estimated uncertainties of $\pm 0.003$ cm$^{-1}$ and $\pm 0.0004$ cm$^{-1}$, respectively, correspond to a range of values that produce qualitatively satisfactory global agreement with the observed room-temperature contour. The fitted inertial constants were used to simulate the rotational contour of the $0^0_0$ band under jet-cooled conditions. Agreement with the observed jet-cooled spectrum was optimized by varying the homogeneous linewidth of the rovibronic transitions as well as the rotational temperature. The optimal FWHM was about 0.20 cm$^{-1}$, leading to an estimate of 25 ps for the lifetime of the $T_1(n,\pi^{*})$ state of acrolein ($v=0$) under isolated-molecule conditions.