Surface potential of liquid microjet investigated using extreme ultraviolet photoelectron spectroscopy.

Photoelectron spectroscopy of a liquid microjet requires careful energy calibration against electrokinetic charging of the microjet. For minimizing the error from this calibration procedure, Kurahashi et al. previously suggested optimization of an electrolyte concentration in aqueous solutions [Kurahashi et al., J. Chem. Phys. 140, 174506 (2014)]. More recently, Olivieri et al. proposed an alternative method of applying a variable external voltage on the liquid microjet [Olivieri et al., Phys. Chem. Chem. Phys. 18, 29506 (2016)]. In this study, we examined these two methods of calibration using extreme ultraviolet photoelectron spectroscopy with a magnetic bottle time-of-flight photoelectron spectrometer. We confirmed that the latter method flattens the vacuum level potential around the microjet, similar to the former method, while we found that the applied voltage energy-shifts the entire spectrum. Thus, careful energy recalibration is indispensable after the application of an external voltage for accurate measurements. It is also pointed out that electric conductivity of liquid on the order of 1 mS/cm is required for stable application of an external voltage. Therefore, both methods need a similar concentration of an electrolyte. Using the calibration method proposed by Olivieri et al., Perry et al. have recently revised the vertical ionization energy of liquid water to be 11.67(15) eV [Perry et al., J. Phys. Chem. Lett. 11, 1789 (2020)], which is 0.4 eV higher than the previously estimated value. While the source of this discrepancy is still unclear, we estimate that their calibration method possibly leaves uncertainty on the order of 0.1 eV.