A new optical method of measuring electron impact excitation cross section of atoms: Cross section of the metastable 6s6p 3P0 level of Hg

► We introduce a new optical method. ► We measure electron impact excitation cross sections of mercury atom in plasma. ► The method is universally applicable to any other atoms in different situations. ► This is the first application of cavity ringdown spectroscopy to the cross section measurements. ► The data also contains the first cavity ringdown spectroscopy of mercury at 405 nm.