TEMPERATURE PROGRAMMING FOR ISOTHERMAL DESORPTION RATE MEASUREMENTS

We describe a digital feedforward/feedback controller for the implementation of rapid changes in the temperature of a small, resistively heated metal adsorbent in ultrahigh vacuum. The feedforward loop is based on a second-order model of the system's thermal response, modified by inclusion of a time delay to account for the characteristic time for heat conduction in the adsorbent, and the feedback loop includes proportional and derivative actions. These features allow the temperature of a nickel single crystal, 10 mm diameter and 1 mm thick, to be increased 80 K within approximately 4 s without overshoot, after which desorption rates can be measured at constant surface temperature with a mass spectrometer. The performance of the temperature controller and the advantages of isothermal data collection and analysis are demonstrated with CO desorption from Ni(110), for which the functional dependence of the rate on surface coverage at constant temperature identifies clearly the participation of a mobile precursor state in the desorption process.