Infrared laser-induced desorption of NO and CO from alumina substrates

We present results of laser-induced desorption experiments on layers of CO and NO physisorbed at low temperatures (5–40 K) on fire-polished alumina substrates. The radiation source is a linetunable CO laser capable of operating on over 400 lines in the 1500–1900-cm−1 region with a cw output power on the strong lines of 0.3–0.5 W. The experiments are carried out in a bakable UHV chamber with coverages of about a monolayer. The desorbed molecules are detected with a quadrupole mass analyzer operating in a time-of-flight mode. The desorption yields from both molecules show a broad, nonresonant (wavelength-independent) behavior. Additionally, NO shows a strongly resonant peak at 1843 cm−1 that we attribute to the direct excitation of the NO stretch mode. Effective temperatures for the desorbed molecules are obtained by fitting the time-of-flight data to a Boltzmann distribution. These vary from approximately the substrate temperature to 10–20 K higher, depending on the laser fluence and the substrate temperature.