Angular momentum polarisation in the O(¹D) products of O₂ photolysis via the B state

The first fully allowed spectroscopic transition in O₂ is the transition comprising the well-known Schumann–Runge bands and continuum. We report a velocity-map imaging study in which the O(¹ D) angular momentum polarisation and O(³ P) spin–orbit branching ratios arising from this process have been measured. We show that direct 157 nm excitation into the Schumann–Runge continuum leads to extremely strong angular momentum polarisation in the O(¹D) product. Comparison with previous studies indicates that this is a general feature of dissociation via the B state. The fine structure branching ratios in the co-fragment O(³P _J=2,1,0) were measured to be 88.5 ± 1.6 : 9.7 ± 1.4 : 1.9 ± 0.4. Based on a consideration of the Massey parameter for the system, the data were modelled using theoretical calculations based on adiabatic and diabatic models of the dissociation. While both models were able to describe some aspects of the dissociation accurately, neither was able to predict both the fine structure branching ratios of the O(³P) products and the O(¹D₂) alignment. We have also investigated O(¹D₂) alignment arising from 203.8 and 205.5 nm photodissociation via the state of O₂ vibrationally excited to v=6–11. As in the 157 nm photodissciation of vibrationally ground state O₂, strong polarisation of the O(¹D₂) photofragments is observed.

Keywords

• photodissociation,
• molecular oxygen,
• O(1D),
• polarisation,
• Schumann–Runge