Sensitive Low-Recoil VUV 1+1$'$ REMPI Detection of ND$_3$

Abstract

In molecular beam scattering experiments, an important technique for measuring product energy and angular distributions is velocity map imaging following photoionization. For studies with cold molecular beams, the resolution is often limited by the product detection process. When state-selective ionization detection is used, excess photon energy can transfer to kinetic energy in the molecular ion-electron pair, resulting in measurable cation recoil. With advanced molecular beam technology, velocity spreads as small as a few m/s are possible. Thus, a suitable product detection scheme must be not only sensitive, state-selective, and background-free, it must also produce less cation recoil than the velocity spread of the molecular beams. To date this has only been possible with the NO molecule. Our goal here is to extend this low-recoil capability to fully deuterated ammonia, ND$_3$. We present a resonance-enhanced multi-photon ionization (REMPI) detection scheme for ND$_3$, that imparts sufficiently low ion recoil, allowing, for the first time, high-resolution imaging of ND$_3$ products in a cold ND$_3$-HD scattering experiment. The first 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 ND$_3$ to a wide range of autoionizing neutral states. By velocity mapping the resulting photoelectrons, 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.