15min:

NIKOLAUS C. HLAVACEK, MICHAEL O. MCANALLY AND STEPHEN DRUCKER, Department of Chemistry, University of Wisconsin-Eau Claire, Eau Claire, WI 54702.

\hspace0.2inAcrolein (propenal, CH₂=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 T₁( , ^*) state, which has a CH₂--twisted equilibrium structure. In solution, the T₁( , ^*) state is typically accessed via intersystem crossing from an intially prepared planar S₁(n, ^*) state. An intermediate in this photophysical transformation is the lowest ³ (n, ^*) state, a planar species with adiabatic excitation energy below S₁ and above T₁( , ^*). The present work focuses on this ³ (n, ^*) intermediate state; it is designated T₁(n, ^*) as the lowest-energy triplet state of acrolein having a planar equilibrium structure.

\hspace0.2inThe T₁(n, ^*) S₀ 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 0⁰₀ 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\footnoteR.~H.~Judge et al. , J.~Chem.~Phys. ~\textbf103, 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 T₁(n, ^*) 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 0⁰₀ 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 T₁(n, ^*) state of acrolein (v=0) under isolated-molecule conditions.