Figure 1.

An experimental ZEKE (pulsed-field ionization) spectrum plotted vs. the frequency ω₁ of the second photon (see Fig. 2) for NO with three different configurations of weak (300 mV/cm) DC fields as shown. (a) The field F_ℓ present only for 10 ns during excitation, indicated by an asterisk. (b) The field F_ℓ is present during excitation and is switched off for several microseconds and switched on again. (c) The field F_ℓ is present during excitation and is switched off and then switched on again. While the field F_ℓ is off, the field F_m is switched on for 10 ns. See ref. 26 for more details about such experiments.