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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 $T1(\pi ,\pi \ast )$ state, which has a CH$2$--twisted equilibrium structure. In solution, the $T1(\pi ,\pi \ast )$ state is typically accessed via intersystem crossing from an intially prepared planar $S1(n,\pi \ast )$ state. An intermediate in this photophysical transformation is the lowest $3(n,\pi \ast )$ state, a planar species with adiabatic excitation energy below $S1$ and above $T1(\pi ,\pi \ast )$. The present work focuses on this $3(n,\pi \ast )$ intermediate state; it is designated $T1(n,\pi \ast )$ as the lowest-energy triplet state of acrolein having a planar equilibrium structure. \hspace{0.2in}The $T1(n,\pi \ast )\leftarrow S0$ band system, with origin near 412 nm, was first recorded in the 1970s at medium (0.5 cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) 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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ 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 $T1(n,\pi \ast )$ state. The determined values (cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$) 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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$ and $\pm 0.0004$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, 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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, leading to an estimate of 25 ps for the lifetime of the $T1(n,\pi \ast )$ state of acrolein ($v=0$) under isolated-molecule conditions.