Improved methods and calibration technique in gas-phase electron-diffraction experiments: Use of nitrogen as a wavelength standard

Improved methods are described for extracting electron-scattering data from films used in gas-phase electron-diffraction (GED) experiments. These involve use of a precision transmission scanner in conjunction with powerful, public-domain image-processing software (ImageJ). Procedures for finding an accurate center of the diffraction ring pattern are given and the software permits flexible determination of the intensity average of each radial circle or any arc thereof. In addition, nitrogen gas is proposed as the ideal calibration molecule because virtually all molecules are in the ground vibrational state at all temperatures normally encountered in GED experiments. For this state the rotational constant is very accurately known, as are several small corrections needed to obtain an appropriate calibration value for the GED experiment. The result from theory is ra(N≡N) = 1.10087 Å with a standard deviation (σ) of 0.00035 Å from fitting experiments.