K. "Onda

T." Kato

National Institute for Fusion Science (NIFS-Repository)

§8. Theoretical Study of Elementary Coil ision Processes Related to Vibrationally Excited Molecules: A Case Study of Isotope Effect on Dynamics of a Collinear Collision of He with H; (v) Gnda, K. (Sci. Univ. of Tokyo), Kato, T. In gas divertor plasmas, it is estimated that gas temperature is in the range of several e V and num-ber density of particles is in the order of 1021 rv 1022 m - 3 . Hydrogen atoms in these relatively high density regions are easily recombined into molecular form on divertor plates. Newly formed hydrogen molecules can be in rotationally and vibrationally excited states, and are predominantly abundant in number density over any other molecules. Since hydrogen molecules and its molecular ions have no permanent electric dipole moment, radiative lifetime of rotationally and vibra-tionally excited states is known to be 105 rv 106 s. Hydrogen molecules with different mass_ combinations are considered to be a heteronuclear molecule having very small permanent electric dipole moment in the order of 10-(4 rv5)D (1 debye(D) = 10- 18e.s.u.). AI-thouah radiative lifetime of them becomes shorter, it b is highly probable that H2 molecules and its molec-ular ions in vibrationally excited states collide with other atoms, molecules, or their ions in the divertor chamber . The Schrodinger equation 1 describing collision pro-cesses between an atom and diatomic molecule has been numerically solved with quantum mechanically sufficient accuracy. We have applied our method to a collinear collision of He with Hi (Vi) and specifically aimed at investigating chemical reaction and dissocia-tion of the target molecule in this collision system by employing the reliable potential energy surface pro-vided by Joseph and Sathyamurthy. The energy de-pendence of the dissociation and atom exchange re-action probabilities has been studied at the total en-ergy up to 10 eV, and interesting numerical results have been obtained.2 The dissociation energy of this collision system is 2.787 e V measured from the low-est minimum of the diatomic well. In this report, the mass of a 1H atom is 1.008 u (1 u being 1 uni-versal atomic mass unit = one twelfth the mass of a 12C atom) and that of He is 4.003 u as the normal mass, respectively. The normal mass of He is em-ployed throughout this report. In order to investigate isotope effect on dynamics of this collinear He + Hi collision, we have studied He + HD+(Vi)(O ::; Vi ::; 20), HT+(Vi)(O ::; Vi ::; 21), DH+(Vi)(O ::; Vi ::; 20), and TH+(Vi)(O ::; Vi ::; 21) collisions at the total energy from 4 to 10 eV by considering all of the vibrational bound states as an initial state. We mention that a collinear collision of A + BC is different from that of A + CB. The former arrangement produces AB as a reaction product, and the latter does AC. Representative results of isotope effect are briefly mentioned on He + HD+ collision. The non-reactive vibrational transitions dominate over the atom ex-change reaction ~nd dissociation processes for 0 ::; Vi ::; 3 and 12 ::; Vi ::; 19 at 4 ::; E tot ::; 10 e V. Atom exchange reactions become the major pro-cesses for 4 ::; Vi ::; 11 at the total energy 4 ::; E tot ::; G eV, and the magnitude of these probabilities is about 0.4 - 0.6. The reaction probability appreciably de-creases as the increase of the total energy at 4 ::; E < 6 eV for V < 14 but becomes almost inde-tot _ t _ , pendent of E tot for Vi 2:: 16. The dissociation probability becomes larger as the total energy is increased from 4 to 10 eV, but dissocia-tion in this collision system is more or less suppressed, and the values of these probabilities are less than 0.3 except for the highest vibrational state Vi = 20. The internal energy stored in the vibrational mode is effective to enhance the dissociation process for 3 ::; Vi ::; 12 at 4 ::; Etot ::; 10 e V. Both the reac-tion and dissociation probabilities are comparable in maanitude to each other for the vibrational states of b 12 ::; Vi ::; 19 at the total energy of 4 ::; Etot ::; 6 e V, and are undulatory as a function of Etot . These results3 ,4 are appreciably different from those of the collinear He + Hi (Vi) collisions. Different characteristics are observed for this collision system with different mass combinations. Thus, it is impor-tant to emphasize that the detailed studies for differ-ent isotopes are necessary to understand the present collinear collision dynamics as no simple interpolation or extrapolation seems work. References 1. K.Sakimoto and K.Onda, J.Chem.Phys. 100 (1994) 1171. 2. K. Onda and K . Sakimoto, J .Chem.Phys. 111 (1999) 988 . 3. K . Onda, in press. 4. K . Onda, in preparation . 361 

Theoretical Study of Elementary Collision Processes Related to Vibrationally Excited Molecules: A Case Study of Isotope Effect on Dynamics of a Collinear Collision of He with H2+(v)

Theoretical Study of Elementary Collision Processes Related to Vibrationally Excited Molecules: A Case Study of Isotope Effect on Dynamics of a Collinear Collision of He with H2+(v)

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