Sub-Doppler excitation spectroscopy in a collimated cold supersonic beam
of 5% NO seeded in argon has been performed with a spectral resolution down
to 10 MHz for four vibronic bands within the perturbed B ?X A system of
NO . The rotational temperature of about 15 K achieved by adiabatic cooling
allows the separation of closely lying vibronic bands which overlap in absorption
spectra taken at room temperature and which therefore could not be iden$-
fied from the broad band excitation measurements