Ultrafast vectorial and scalar dynamics of ionic clusters: Azobenzene solvated by oxygen
Journal of Chemical Physics
The ultrafast dynamics of clusters of trans-azobenzene anion ͑A − ͒ solvated by oxygen molecules was investigated using femtosecond time-resolved photoelectron spectroscopy. The time scale for stripping off all oxygen molecules from A − was determined by monitoring in real time the transient of the A − rise, following an 800 nm excitation of A − ͑O 2 ͒ n , where n =1-4. A careful analysis of the time-dependent photoelectron spectra strongly suggests that for n Ͼ 1 a quasi-O 4 core is formed and
... core is formed and that the dissociation occurs by a bond cleavage between A − and conglomerated ͑O 2 ͒ n rather than a stepwise evaporation of O 2 . With time and energy resolutions, we were able to capture the photoelectron signatures of transient species which instantaneously rise ͑Ͻ100 fs͒ then decay. The transient species are assigned as charge-transfer complexes: A ·O 2 − for A − O 2 and A ·O 4 − · ͑O 2 ͒ n−2 for A − ͑O 2 ͒ n , where n =2-4. Subsequent to an ultrafast electron recombination, A − rises with two distinct time scales: a subpicosecond component reflecting a direct bond rupture of the A − -͑O 2 ͒ n nuclear coordinate and a slower component ͑1.6-36 ps, increasing with n͒ attributed to an indirect channel exhibiting a quasistatistical behavior. The photodetachment transients exhibit a change in the transition dipole direction as a function of time delay. Rotational dephasing occurs on a time scale of 2 -3 ps, with a change in the sign of the transient anisotropy between A − O 2 and the larger clusters. This behavior is a key indicator of an evolving cluster structure and is successfully modeled by calculations based on the structures and inertial motion of the parent clusters.