The energy straggling of 2.0~5.4 MeV carbon ions in aluminum films evaporated onto K4 glass substrates was measured with the Au marked RBS method. The surface roughness with zero was obtained from experiment. It supplied a method for determination of the surface roughness with zero. The present result was compared with the Bohr theory and the results of published experimental work It is shown that the Bohr theory is not appropriate to description of the energy straggling of low energy ...