Ultrafast vectorial and scalar dynamics of ionic clusters: Azobenzene solvated by oxygen

摘要

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 > 1 a quasi-O-4 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 (< 100 fs) then decay. The transient species are assigned as charge-transfer complexes: A center dot O-2(-) for A(-)O(2) and A center dot O-4(-)center dot(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 n) 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. (c) 2006 American Institute of Physics.