Sensitive Low-Recoil VUV 1 + 1′ REMPI Detection of ND₃

In molecular beam scattering experiments, an important technique for measuring product energy and angular distributions is velocity map imaging following photoionization of one or more scattered species. For studies with cold molecular beams, the ultimate resolution of such a study is often limited by the product detection process. When state-selective ionization detection is used, excess energy from the ionization step can transfer to kinetic energy in the target molecular ion-electron pair, resulting in measurable cation recoil. With state-of-the-art molecular beam technology, velocity spreads as small as a few m/s are possible, thus a suitable product detection scheme must be not only highly sensitive, state-selective, and background-free, it must also produce significantly less cation recoil than the velocity spread of the molecular beams undergoing cold collisions. To date this has only been possible with the NO molecule, and our goal here is to extend this minimal-recoil capability to the fully deuterated ammonia molecule, ND₃. In this article a resonance enhanced multi photon ionization (REMPI) detection scheme for ND₃ is presented that imparts sufficiently low recoil energy to the ions, allowing, for the first time, high-resolution imaging of ND₃ collision products in cold molecule scattering experiments with HD. The excitation step of the 1 + 1' REMPI scheme requires vacuum ultra-violet (VUV) photons of ∼160 nm, which are generated through four-wave-mixing in Xe. We varied the wavelength of the second, ionization step between 434 and 458 nm, exciting ND₃ to a wide range of autoionizing neutral states. By velocity mapping the photoelectrons resulting from the detection scheme, it was possible to fully chart the ion recoil across this range with vibrational resolution for the final ionic states. Additionally, rotational resolution in the photoionization dynamics was achieved for selected excitation energies near one of the vibrational thresholds. Many of the peaks in the spectrum of autoionizing Rydberg states are assigned to specific Rydberg series using a simple Rydberg formula model.