Deuterium, thermal desorption from

Laser-induced thermal desorption experiments indicate that the desorption of deuterium from a Si (111) surface has Arrhenius parameters of 1015 2 s-1 and 247 kJ mol-1. Calculate the length of time required to desorb deuterium from a surface initially covered with 0.8 ML to a coverage of 0.2 ML at 720 K. 

K. Ashida, K. Ichimura, M. Matsuyama, K. Watanabe, Thermal desorption of hydrogen, deuterium and tritium from pyrolytic graphite, J. Nucl. Mater. 128-129 (1984) 792... 

H. Atsumi, S. Yamanaka, P. Son, M. Miyake, Thermal desorption of deuterium and helium from ion irradiated graphite, J. Nucl. Mater. 133-134 (1985) 268... 

Figure 25.2 Thermal desorption spectrum of hydrogen deu-teride (HD, mass 3) from a Pd(210) surface that had received a simultaneous exposure of hydrogen, Hj, and deuterium, Dj, at 40 K. The ft states represent atomically adsorbed hydrogen (deuterium), while the y states are due to the molecular species. Apparently, practically no isotopic scrambling occurs in the y states (absence of HD), while the exchange is complete in the atomic f states. After Schmidt et al. [9,10].

The slower rate (by a factor of 1.8) of decomposition of deuterogermane shows then that deuterium desorbs more slowly (by the same factor) than hydrogen from a germanium surface. This factor is consistent with normal zero-point energy differences for hydrogen isotop>es. The existence of a kinetic isotope effect for desorption is what is expected from a normal activated thermal-bond breaking and does not support the idea that the activation energy corresponds to some barrier which electrons have to surmount, possibly caused by a barrier layer due to adsorption surface states (5). 

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