Laser ionization detection of O($^3P_j$) atoms in the VUV; application to photodissociation of O$_2$

Abstract: Detection of nascent O($^3P_j$, $j=2,1,0$) atoms using one-photon resonant excitation to the $3s\,^3S^o_1$ state at $\sim 130$ nm followed by near-threshold ionization, i. e., 1 + 1' resonance enhanced multi-photon ionization (REMPI), has been investigated. The aim was to achieve low ion recoil, improved sensitivity, and reliable angular momentum polarization 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 1 is generated by difference frequency ($2\omega_1 - \omega_2$) VUV generation by four wave mixing in Kr gas, and the ionization step 1' uses 2$\omega_2$ at 289 nm. The presented scheme induces 9 m/s recoil of the O$^+$ 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 289 nm radiation does not saturate the 1' step. In order to test the ability of this scheme to accurately determine branching ratios, fine structure yields, and angular distributions including polarization information, it has been applied to O$_2$ photodissociation around 130 nm with subsequent O($^3P_j$) fragment detection.