Rotational-state distribution and ortho-para conversion of molecularly chemisorbed H on Pd(210)

Ueta, Hirokazu   ; Sasakawa, Yuya*; Ivanov, D.*; Ono, Satoshi*; Ogura, Shohei*; Fukutani, Katsuyuki

Imperfections of the surface, such as steps, are known to affect adsorption and reaction processes. While hydrogen typically chemisorbs dissociatively on a flat metal surface, it is known that hydrogen can molecularly chemisorb on stepped surfaces, such as Ni(510) and Pd(210), in which the potential is expected to be highly anisotropic, unlike the case of physisorption. The anisotropic potential would modify the rotational state of H and promote ortho-para (o-p) conversion. For the system of H/Pd(210), the conversion time of ~1 sec was predicted based on the measurement on rotational-state-selective thermal desorption of H (J=0 and J=1). The predicted value is indeed much shorter than those in previously reported physisorption systems. In this contribution, to further probe the rotational state and conversion of molecularly chemisorbed H, a combination of a pulsed molecular beam, photo-stimulated desorption and resonance-enhanced multiphoton ionization techniques is used. We will present the rotational state distribution of photo-desorbed H, and the o-p conversion process of molecularly chemisorbed H will be discussed.