Ultrafast Photodissociation Dynamics of the F State of Sulfur Dioxide by Femtosecond Time-Resolved Pump-Probe Method

A femtosecond pump-probe method is employed to study the dissociation dynamics of sulfur dioxide. SO₂ molecules are excited to the F state by absorbing two photons of 267 nm femtosecond laser pulses, and ionized by 400 nm laser pulses at different delay times between the two lasers. Transients of both parent ions (SO₂⁺) and the fragment ions (SO⁺, S⁺ and O⁺) are observed. The SO₂⁺ transient can be well fitted to a biexponential decay comprising a fast and a slow component of 280 fs and 2.97 ps lifetimes, respectively. The SO⁺ transient consists of two growth components of 270 fs and 2.50 ps. The results clearly show that the F state of SO₂ dissociates along an S−O bond. The transients of S⁺ and O⁺, however, have different behavior, which consist of a fast growth and a long decay component. A possible mechanism of the fragment formation is discussed to understand the dissociation dynamics of the F state of SO₂.