ABSTRACT
The L(a)-like S2 state (2A) of 4-(dimethylamino)benzonitrile was pumped at 267 nm in the gas phase at 130 degrees C. Nonresonant multiphoton ionization at 800 nm with mass-selective detection then probed the subsequent processes. Whereas ionization at the Franck-Condon geometry only gave rise to the parent ion, fragmentation increased on motion towards the charge-transfer (CT) state. This useful difference is ascribed to a geometry-dependent resonance in the ion. The time constants found are interpreted by ultrafast (approximately 68 fs) relaxation through a conical intersection to both the CT and the L(b)-type S1 state (1B). Then the population equilibrates between these two states within 1 ps. From there the molecule relaxes within 90 ps to a lower excited state which can only be a triplet state (T(n)) and then decomposes within 300 ps. Previous experiments either investigated only 1B --> CT relaxation-which does not take place in the gas phase or nonpolar solvents for energetic reasons--or, starting from S2 excitation, typically had insufficient time resolution (>1 ps) to detect the temporary charge transfer. Only recently temporary population of the CT state was found in a nonpolar solvent (Kwok et al., J. Phys. Chem. A. 2000, 104, 4188), a result fully consistent with our mechanism. We also show that S2 --> S1 relaxation does not occur vertically but involves an intermediate strong geometrical distortion, passing through a conical intersection.
ABSTRACT
Excitation of norbornadiene (bicyclo[2.2.1]hepta-2,5-diene) at 200 nm populates two states in parallel, the second pi(pi*) state and a Rydberg state. We monitored both populations by transient nonresonant ionization. From the pi(pi*) state the molecule relaxes in consecutive steps with time constants 5, 31 and 55 fs down to the ground-state surface, whereas the Rydberg population merges to the other path on the pi(pi*) surface within 420 fs. The relaxation steps are discussed in terms of conical intersections (CoIns) between different surfaces Information on them is inferred from known spectroscopy and, for the last CoIn, from published calculations on Dewar benzene-->prismane conversion and on ethylene photodimerization for which norbornadiene with its two nonconjugated double bonds is a model. The calculation predicts symmetry breaking for this CoIn, the two ethylenes forming a rhombus Although this distortion is hindered in norbornadiene by ring strain, this CoIn seems easily accessible as indicated by the short time (<55 fs) found for passing through it.
