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of the secondary electrons with gas molecules produces positive Ions near the sample surface. The ions are com- bined with excess electronic charge on the surface. This leads to a neutralisation of the charging surface. |
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| | In variable pressure (VP) mode, the gaseous pressure in the specimen chamber is raised whilst the GEMINI® column vacuum is maintained at a high level.
With a higher gas pressure (2-133 Pa) in the vicinity of the specimen, electrons leaving the specimen collide with gas molecules causing ionisation to occur. Ions created as a consequence of the gaseous collisions are responsible for neutralising any charge build up in insulating specimens.
In general, the more insulating the specimen the greater the gas pressure required. Another advantage of the elevated pressure is the capability to investigate moist specimens. Together with a Peltier cooling stage water can be retained in moist samples to avoid artefacts caused by dehydration.
VP mode enables imaging of specimens in their natural state without the need for time consuming and expensive preparation techniques. |