Using the CHI760E bipotentiostat, a constant potential at the OCP of approximately -0.5 V vs. Fc/Fc + was applied to the working electrode. Then, a rectangular shaped modulated potential, produced by a Waveform generator (PSG9080, Joy-IT), with a modulation frequency of 37 Hz and a variable amplitude as an offset was superimposed. Meanwhile, the working electrode is transilluminated by monochromatic light generated by an Apex2 QTH lamp (Oriel Instruments) and selected using a Cornerstone 130 (Oriel Instruments) monochromator. Above 600 nm, a 570 nm long-pass filter was introduced into the beam path to block components with a higher diffraction order. The transmitted light was detected at a DET36A Si biased detector (Thorlabs), and the signal was passed to a MFLI lock-in amplifier (Zurich Instruments). In parallel, the lock-in amplifier received the modulated potential generated by the signal generator as a reference signal and searched for a modulated component in the optical signal that oscillates at the same frequency. Considering the phase relationship between the modulated potential and the modulated component in the optical signal, it was determined whether the measured signal is an induced absorption or a bleach (see Figure and the accompanied text).