Photodissociation dynamics of small aromatic molecules studied by multimass ion imaging

The photodissociation dynamics of various aromatic molecules, studied using multimass ion imaging techniques, is reviewed. The experimental data reveals new isomerization and dissociation mechanisms. Our investigation of benzene, pyridine, and pyrimidine finds that H-atom elimination thresholds remain the same for the three molecules. We also notice that ring-opening dissociation thresholds decrease rapidly with the increase of the number of nitrogen atoms in the aromatic ring. Hydrogen atom elimination is the sole dissociation channel for benzene at 193 nm. Along with H-atom elimination, we observe five distinct ring-opening dissociation channels for pyridine at 193 nm. No dissociation channels were observed for benzene and pyridine at 248 nm. Ring-opening dissociation channels are the major channels for pyrimidine, which dissociates at 193 nm and also at 248 nm. A six-membered to seven-membered ring isomerization was observed for photodissociation processes involving toluene, m-xylene, aniline, 4-methylpyridine, alpha-fluorotoluene, and 4-fluorotoluene, indicating a general isomerization mechanism for all such aromatic molecules. What is significant, is that during the isomerization, atoms (i.e., carbon, nitrogen, fluorine, and hydrogen) belonging to respective alkyl or amino groups are involved in an exchange with atoms within the aromatic ring. This type of isomerization is not observed in other aromatic isomerization mechanisms. For small tyrosine chromophores, such as phenol, 4-methylphenol, and 4-ethylphenol, H-atom elimination from a repulsive excited state plays a key role. However, dissociation is quenched in large chromophores like 4-(2-aminoethyl)-phenol. Our work demonstrates the capability and high sensitivity of multimass ion imaging techniques in the study of aromatic compounds.