Foil/material that is invisible to electrons but not to infra-red I'm wondering if there is a material, probably existing in a foil or sheet form, that is absolutely invisible to electrons but not to infra red. This material should not have any interaction with electrons, they should just pas through it. Infra-red, however, must be absorbed.
Is there such a material?
 A: Electrons, like photons, behave like waves and their wavelength depends on their momentum: $\lambda = \frac{h}{p}$ (de Broglie wavelength). 
We could set up a scattering experiment where the target is your special material, and try throwing different particles at it. Whether the particles are photons, electrons, or even molecules, there should exist an incident momentum for each (remember momentum is directly related to wavelength) such that the particles interact with the crystal lattice of the target (i.e. its atoms). 
In short, your material might exist, but it only fulfills the condition if you know the momenta of the incident electrons beforehand, and pick the composition accordingly to avoid interactions. 
Note: Scattering is not just a matter of de Broglie wavelength since electrons also carry charge and you can be sure that this adds complexity when figuring out how strongly they interact with a lattice. 
A: 
Is there a material that is absolutely invisible to electrons?

No. All materials interact with electrons, just to different extents. This is the principle behind the backscatter detectors found in an SEM: 

Higher atomic number elements backscatter more electrons, making them appear brighter on the image.

Image source: AZO Materials "Electron Beam Versus Optical Imaging of Particles, Microscopy Technique Comparisons By Aspex" 
