Precise determination of adiabatic ionisation energies of large polyatomic molecules: <i>p</i>-diaminobenzene

We present a procedure, based on established principles, to accurately determine the ionisation energies of medium-sized large polyatomic molecules in supersonic beams. The method combines single-photon excitation from electronic ground state using tunable vacuum-ultraviolet laser radiation, delayed pulsed-field high Rydberg states with sequences pulsed electric fields, and modelling lineshapes calculations field-ionisation dynamics rotational contours photoelectron bands. is illustrated by measurement adiabatic energy p-diaminobenzene (p-DB), for which we obtain value 54,617.47(15) cm−1, represents more than 20-fold accuracy improvement over previous determinations.