Secondary Electron Imaging of Liquid Water Films in the Environmental Scanning Electron Microscope
High gas pressure scanning electron microscopy is now routinely possible with new microscopes operating between 0.1 and 20 torr gas pressure. Since the specimen chamber is separated from the electron optical column by pressure-limiting apertures, high gas pressures present at the level of the specimen do not affect the high vacuum in the column. The use of a single PLA underneath the last probe-forming lens allows maintainance of ∼0.1 torr. Twenty torr can be stabilized with two PLA, forming two differentially pumped pressure zones which may be incorporated into the last probe-forming lens.One of the most important new features of high gas pressure microscopy is the possibility to alter the type of gas and its pressure over a large range. High gas pressures (>1 torr) are required for charge neutralization on rugged insulators and for gas amplification of the SE signal. Additionally, using water vapors, liquid water can be stabilized. However, the aqueous environment is only one example among many possibilities.The secondary electron imaging of liquid water is a fascinating new aspect of scanning electron microscopy. At saturated water vapor pressures, water is stable indefinitely. Increasing or decreasing the vapor pressure allows condensation or evaporation of water and provides means to generate water films. As can be seer from Fig. 1, the stabilizing pressure for water at 20°C is 17.5 torr. Such a pressure would require a very short working distance for SE imaing. Therefore, it is preferable to reduce the specimen temperature and to establish vapor saturation at lower pressures. However, liquid water can only be stabilized at pressures greater than ∼4.5 torr.