Information depth in EELS of an organic solid

We have determined the depth to which 5 eV electrons in an EELS experiment can give information about the impact excitation of the C-H molecular vibrations in an organic film. We find this information depth to be no more than 0.4 nm. We studied long alkoxy chains, synthesized with progressively more CF2 groups at the outer ends, thereby shielding the underlying C-H vibrators. The molecules were packed and aligned normal to a flat substrate using the Langmuir-Blodgel technique. It was found that two CF2 groups reduced the vibrational excitation of the C-H by a factor of about 1 e. These observations yield a remarkably small information depth with a simple theoretical analysis. © 1994.