  The fluorescence detection of ultra high energy (> 10^18 eV) cosmic rays
requires a detailed knowledge of the fluorescence light emission from nitrogen
molecules, which are excited by the cosmic ray shower particles along their
path in the atmosphere. We have made a precise measurement of the fluorescence
light spectrum excited by MeV electrons in dry air. We measured the relative
intensities of 34 fluorescence bands in the wavelength range from 284 to 429 nm
with a high resolution spectrograph. The pressure dependence of the
fluorescence spectrum was also measured from a few hPa up to atmospheric
pressure. Relative intensities and collisional quenching reference pressures
for bands due to transitions from a common upper level were found in agreement
with theoretical expectations. The presence of argon in air was found to have a
negligible effect on the fluorescence yield. We estimated that the systematic
uncertainty on the cosmic ray shower energy due to the pressure dependence of
the fluorescence spectrum is reduced to a level of 1% by the AIRFLY results
presented in this paper.
