Laser ionisation detection of O(3 P j) atoms in the VUV; application to photodissociation of O2

Abstract

Detection of O((Formula presented.)) atoms using one-photon resonant excitation to the (Formula presented.) state at (Formula presented.)  nm followed by near-threshold ionisation, i.e. 1 + 1' resonance-enhanced multi-photon ionisation, has been investigated. The aim was to achieve low ion recoil, improved sensitivity, and reliable angular momentum polarisation information, with an as simple as possible laser setup. An efficient 1 + 1' scheme has been found where the VUV light for the first step is generated by difference frequency ((Formula presented.)) VUV generation, and the ionisation step 1' uses (Formula presented.) around 289 nm. The presented scheme induces 9 m/s recoil of the O (Formula presented.) ion using a two-dye laser system, and zero recoil should be possible by generating 302 nm radiation with a third dye laser. While this approach is much more sensitive than a previous 1 + 1' scheme using 212.6 nm for the 1' step, we found that the relatively intense radiation of the (Formula presented.) beam does not saturate the 1' step. To test the ability of this scheme to accurately determine branching ratios, fine structure yields, and angular distributions including polarisation information, it has been applied to O (Formula presented.) photodissociation around 130 nm with subsequent O((Formula presented.)) fragment detection.