Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 4(4), 54-57, April (2014) Res. J. Chem. Sci. International Science Congress Association 54 Measurement of Deexcitation Cross Sections of Ne() by CH Using a Pulse Radiolysis MethodKhadka Deba BahadurCentral Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu, NEPALAvailable online at: www.isca.in, www.isca.me Received 13th February 2014, revised 8th March 2014, accepted 14th April 2014Abstract The cross sections for the deexcitation of Ne() by CH4 have been measured as a function of the mean collisional energy in the range of 17.2-36.8 meV or in the temperature range from 134 K to 285 K using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. The deexcitation cross sections are in the range of 10- 24 Å for Ne() by CH. The behavior of the collisional energy dependence cross sections are increase slightly with increasing the collisional energy. The results are interpreted in terms of an electron –exchange interaction. Keywords: Pulse radiolysis method, resonance atoms, collisional energy, electron-exchange interaction and deexcitation cross sections. Introduction Collisional deexcitation of excited rare gas atoms by atoms and molecules is of great importance in both fundamental and applied sciences, which provides the essential features of chemical reactions, in particular, those including electronic energy transfer1-4. The collisional deexcitation is a key also to understand fundamental processes in the interaction of ionizing radiation with matter and the phenomena in ionized gases such as reactive plasmas and upper atmosphere4-7. Collisional deexcitation processes of excited neon atoms have not been extensively studied in comparison with those of excited helium atoms1-13. On the other hand, there are limited data for resonance atoms. A major part of the deexcitation of the lowest excited neon and helium atoms by atoms and molecules, M, is ascribed to the collisional ionization of M1-7. Previous studies of the collisional deexcitation of excited rare gas atoms have shown that a pulse radiolysis method combined with the time-resolved optical emission and absorption spectroscopy has a clear advantage over the others methods such as flowing afterglow and beam methods to obtain absolute rate constants or the cross sections for the deexcitation of excited rare gas atoms not only in metastable states but also in resonant states2, 3-6. There have been several research works published 14-22 and the present author has also been reported the collisional energy dependence of the cross sections for deexcitation of the resonance and metastable states8-11. In this paper, the cross sections for the deexcitation of Ne() by CH4 have been measured as a function of the mean collisional energy in the range of 17.2-36.8 meV or in the temperature range from 134 K to 285 K using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. Material and Methods For measuring the deexcitation cross sections a pulse radiolysis method is employed. The experimental apparatus and experimental procedure have been described in detail previously3-6,7-11,23. Briefly, after an irradiation by an intense electron beam pulse onto a mixture of Ne with a trace amount of SF and M (M = CH) in a gas cell, the time- resolved optical absorption of [Ne() : 1s 2p at 607.43 nm was measured, thereby, the time dependent variation of the density of Ne() was obtained. Artifacts such as collisional mixing and cascade optical emission followed by recombination, which are due to thermal electrons, are almost completely removed by the addition of SF as a thermal electron scavenger. The temperature of a sample gas, which corresponds to a mean collisional energy, is controlled by introducing cold N2 vapor onto the gas cell. The cell was kept temperature range from room temperature to 134 K within the error limit of 2 degree. Sample gases used were research grade. Pure Ne (99.99%), and mixture of Ne-CH (CH/Ne = 0.0201) and (SF/Ne = 0.000747), where the values in the parentheses are the stated purity. Results and Discussion The obtained time dependent density signals of Ne*, where Ne* is Ne (), the deexcitation rate constants, and thus cross sections, are obtained. In the deexcitation of Ne* in the present condition of a Ne-SF-M system, the following reactions are exclusively dominant3-6, 8-11, 23 –1 Ne* Ne (in pure Ne), (1) kSF6Ne* + SF6 Products, (2) Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 54-57, April (2014) Res. J. Chem. Sci. International Science Congress Association 55 kNe* +M Products, (3) where 0 is the effective lifetime of Ne* in pure Ne, kSF6 and kare the deexcitation rate constants of Ne* by SF6 and CH. The value of kSF6 was obtained previously24. The total deexcitation rate of Ne(), -1, at room temperature is given by -1 = –1 + kSF6[SF] + k[M], (4) where [SF] and [M] are the number densities of SF6 and M, respectively. The value of k is given by the slope of -1 vs. [CH] plots in figure 1 at constant [SF]. Figure-1 Kinetic plots for deexcitation rates tt-1 versus number densities of CH4 for Ne() In figure 2 a typical decay curve is shown for Ne() by CH. A total deexcitation rate constant, k, at each temperature, T, is converted into a velocity averaged cross section, , at a mean collisional energy, E = (3/2) kT, following, = k /(8kT/pm1/2 (5) where k is the Boltzmann constant, T is the absolute temperature and is the reduced mass of Ne and CH, respectively. To the best of my knowledge, this is the first measurement of the temperature dependence rate constants or the collisional energy dependence of the deexcitation cross sections of Ne() by CH using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. The obtained experimental cross sections for the deexcitation of Ne() by CH4 as a function of the mean collisional energy are shown in figure 3. The deexcitation cross sections are in the range of 10- 24 Å for Ne( ) by CH. The behavior of the collisional energy dependence cross sections are increase slightly with increasing the collisional energy.The experimental cross sections and their collisional energy dependence are compared with the Watanabe and Katsuura (WK) theoretical ones in figure 4. The absolute values of the experimental cross sections are in the same order of magnitude as the theoretical cross sections WK). Strictly speaking, however, the experimental cross sections are smaller for CH4 than those by WK. The energy dependence of the present cross sections is not correctly explained by WK. Previously it was concluded that the WK formula does not reproduced well, even qualitatively, the deexcitation cross sections of Ne() in collisions with rare gas atoms due the rectilinear-trajectory approximation and the assumption of the only dominating dipole-dipole interaction. I pointed out the importance of bent trajectories and an electron exchange interaction. The difference between WK and present experimental cross sections for Ne() by CH can be explained in the same manner. Figure-2 Decay curve for Ne()-CH system. Ne (200 Torr), CH(0.17 Torr) and SF (0.18 Torr). = 607.43 nm; (1s ®® 2p) The collisional energy dependence of the cross sections for the deexcitation of Ne() by CH is shown in figure 4 for comparison by plotting the experimental results together with both the results of optical model calculation and the results using the WK formula (WK). The absolute values of the cross sections are in good agreement with the results of optical model calculation and explained reasonably well. Furthermore, there is obtained significant difference between the calculated cross sections with and without electron exchange interaction, namely, between diex and di, respectively, which shows predominant contribution of the electron exchange interaction to the deexcitation cross sections. There is also observed small difference between the calculated cross sections of diex and di. This is due to the contribution from the dipole-dipole interaction. The comparison between the obtained experimental results with theoretical ones confirms that the deexcitation for Ne() by CH4 is dominated by collisional ionization induced by an electron-exchange interaction. Further experimental Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 54-57, April (2014) Res. J. Chem. Sci. International Science Congress Association 56 measurements of the cross section in an adequately wide collisional energy and theoretical investigations such as an ab initio calculation of the optical potentials are greatly needed for further understanding. Figure-3 The experimental cross sections for the deexcitation of Ne() by CH4 Figure-4 The experimental cross sections for the deexcitation of Ne() by : CH. Theoretical total Penning ionization cross sections for Ne() by an optical-model calculation of — — — , ssdi; - - - - -, ssex ; ————, ssdiex; considering the dipole-dipole interaction, the electron-exchange interaction, and both, respectively, and a semi-classical cross sections, — - –– - —, ssWK; are shown together Conclusion In this paper, the cross sections for the deexcitation of Ne( ) by CH4 have been measured as a function of the mean collisional energy in the range of 17.2-36.8 meV or in the temperature range from 134 K to 285 K using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. The deexcitation cross sections are in the range of 10- 24 Å for Ne() by CH. The behavior of the collisional energy dependence cross sections are increase slightly with increasing the collisional energy. The results are interpreted in terms of an electron–exchange interaction. For further understanding of the deexcitation mechanisms for the Ne() by CH cross sections in an adequately wide collisional energy and theoretical investigations such as an ab initio calculation of the optical potentials are greatly needed. Acknowledgement The author is thankful to Hatano Laboratory, Department of Chemistry, Tokyo Institute of Technology, Tokyo, Japan for providing the research facilities for conducting the research. The author is thankful to Ministry of Education, Science, Sports, and Culture, Japan for financial support. References 1.Hotop H. and Niehaus A., Reactions of Excited Atoms and Molecules with Atoms and Molecules, Z. Physik, 228, 68-88 (1969) 2.Siska P.E., Molecular Beam Studies of Penning Ionization, Reviews of Modern Physics, 65(2), 337-412 (1993) 3.Hatano Y., Electronic and atomic collision processes in ionized gases as studied by the pulse radiolysis method. In: Proceedings of 19th Int. Conf. on Phenomena in Ionized Gases. (ed.) Zigman, V.J. Univ. 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