Sticking Probability and Reactivity of Hyperthermal O2 Molecules Impinging on CO Pre-covered Pd(100): Effect of Rotational States with K > 1

Collisional alignment in the supersonic expansion of oxygen seeded in lighter carriers allows to produce rotationally aligned beams which were used to study the sticking probability, S, and the reactivity, Pco2 , of helicoptering and cartwheeling O2 in the lowest rotational state impinging normally onto Pd(100). Molecules with different rotational alignment are thereby chosen by mechanically selecting either the slow or the fast speed tail of the beam velocity distribution. Here we show for O2 reacting with CO pre-covered Pd(100) that even a moderate increase in the nozzle temperature, TN, with respect to room temperature implies significant changes in both S and Pco2. Since TN affects mainly the rotational state distribution in the formed beam, we conclude that molecules in rotational states with K C 3 behave very differently from those with K = 1. Data are presented for both small and intermediate CO pre-coverage.