Thermoinduced and photoinduced reactions of NO on Si(111)7×7. II. Effects of potassium coadsorption

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Abstract
The adsorption, thermoreactions, and photoreactions of NO coadsorbed with potassium on Si(111)7×7 at 90 K have been investigated using work-function measurements, high-resolution electron energy loss spectroscopy, and mass spectrometry. A minimum in the work function of Si(111)7×7 at 90 K vs the potassium exposure is observed and it is suggested that higher K exposures passed the work-function minimum result in the formation of K multilayers, which upon thermal heating desorb at 315 K. Submonolayers of potassium introduce a new adsorption configuration of NO on Si(111)7×7 at 90 K. This new NO species exhibits a weak intramolecular bonding and competes with NO adsorbed in other configurations. During thermal heating, this NO species gradually dissociates, thus contributing to N2 recombinative desorption, and at relatively high K coverages leads to desorption of N2 and N2O at 555 K via N2O synthesis from NO. The surface after thermal heating to >555 K is depleted of molecular species and covered with atomic N and O. Preferential surface oxidation occurs in the presence of potassium. Under photon irradiation (300–900 nm), desorption of N2, NO, and N2O is observed. The photodesorption intensities decrease monotonically as the K coverage increases. The experiments provide further evidence that the dominant contribution to the three photodesorbed species comes from molecularly adsorbed NO and that N2O is synthesized under photon irradiation.