Third harmonic interference effects in the mpi spectrum of acetylene

We report a study of the 3 + 1 MPI spectrum of acetylene in the 74450-90100 cm¹ energy range. Many Rydberg and some valence states are observed, most of which are stable on the time scale of the experiment Only in a region with high density of valence states is there evidence for non-radiative decay. We also report non-linear optical processes in C₂H₂, particularly third-harmonic generation (THG). At pressures above a few Torr, THG effectively quenches the MPI signal for all states of the correct symmetry. This observation explains the absence of most Rydberg structure in a previous 3+1 MPI study of C₂H₂, and demonstrates the need to work at very low densities.