Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 4(3), 81-85, March (2014) Res. J. Chem. Sci. International Science Congress Association 81 Pulse Radiolysis Studies of Collisional Deexcitation of Ne() by NKhadka Deba Bahadur Central Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu, NEPAL Available online at: www.isca.in, www.isca.me Received 8th February 2014, revised 7th March 2014, accepted 17th March 2014Abstract The temperature dependence of the rate constants for the deexcitation of Ne() by N2 has been measured in the temperature range from 136 K to 294 K using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy and thus the collisional energy dependence of the deexcitation cross sections is obtained. The deexcitation cross sections are in the range of 6.1- 9.3 Å for Ne() and almost constant or increase slightly with increasing the collisional energy. The deexcitation cross sections are compared with those various reported methods. The results of Ne() by N has been also compared with the results of Ne() by N. The comparison shows that more experimental and theoretical research works should be needed to clarify the deexcitation mechanism. Keywords: Dewar vessel, pulse radiolysis method, metastable atoms, 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. The rate constants or the cross sections of these processes have been measured by various techniques such as flowing afterglow, beam, and pulse radiolysis methods. There have been several repots for the measurements of the rate constants or cross sections for the deexcitation of excited helium atoms in comparison with the excited neon atoms1-11. A number of theoretical investigations of these processes have been made12-16and the present author has also been reported the collisional energy dependence of the cross sections for deexcitation of the resonance and metastable states8-10. Recently several research works have also been published17-20. In the present investigation, the cross sections for the deexcitation of Ne() by N2 has been measured as a function of the mean collisional energy in the range of 17.5-37.9 meV or in the temperature range from 136 K to 294 K using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. The measured deexcitation cross sections are in the range of 6.1- 9.3 Å for Ne() and almost constant or increase slightly with increasing the collisional energy.The results of Ne() by N has been also compared with the results of Ne() by N. The comparison shows that more experimental and theoretical research works should be needed to clarify the deexcitation mechanism. Material and Methods Sample gases used were all research grade. The Ne (99.99%), (99.9999%), and SF6 (99.8%) and mixture (SF/Ne = 0.000747), where the values in the parentheses are the stated purity. The SF6 was used after freeze pumping purification under 77 K. A pulse radiolysis method, which has the advantage of measuring absolute deexcitation cross sections, is employed in this experiment. The experimental apparatus for the measurement of time-resolved optical absorption by a pulse-radiolysis method is schematically shown in figure 1. The excitation source is a single nanosecond electron beam pulse (the maximum electron energy: 600 keV, a peak current: 7 kA) from a Febetron 706. The optical detection system is composed of an Ushio 450 W xenon flash lamp, a JASCO CT-100 1 m grating monochromator and a Hamamatsu Photonics R-928 photomultiplier tube. The signal is stored in a transient digital memory, which is connected with a microcomputer. The time resolution of the signal detection system is 10 ns which are mainly due to the time width of each resolved channel of transient digital memory. The sample cell is a 65 mm long, 30 mm internal diameter cylinder of Pyrex glass or quartz which has two optical windows perpendicular to the direction of the excitation electron beam. The front end of the cylinder is sealed with 80 µm thick aluminum foils which is the target window for the excitation beam by Arraldite cement and the near end is closed. The cell, which is set in a copper holder for homogeneous cooling, is put in a Dewar vessel for the measurements at low temperatures. The Dewar vessel is 500 mm tall and 90 mm internal diameter and has a 20 mm internal diameter aperture on the side for the excitation beam, which is sealed with polymer film from both inside and outside of the vessel for thermal insulation. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 81-85, March (2014) Res. J. Chem. Sci. International Science Congress Association 82 Figure-1 Experimental apparatus Temperature control is carried out by the rate of cold N gas flow to Dewar vessel. The flow rate of cold N gas is regulated by electric current through a heater in a liquid N2 container and the current is controlled by a copper-constantan thermocouple attached to the cell holder and by a thermo controller and a thyristor (Oyo-Denshi U-1326-2A and U-1111). The temperature of the cell is monitored at two points on the outside of it. Accuracy of temperature control is within ±2 deg. The time- resolved optical absorption of [Ne() : 1s 2p] at 633.44 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. 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 –1 Ne* Ne (in pure Ne), (1) kSF6Ne* + SF6 Products, (2) 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 N. The value of kSF6 was obtained previously21. 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. [N] plots in figure 2 at constant [SF]. Figure-2 Kinetic plots for deexcitation rates tt-1 versus number densities of N2 for Ne() In figure 3 a typical decay curve is shown for Ne() by N. 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 N, respectively. Figure – 3 Decay curve for Ne()-N system. Ne (200 Torr), N (0.265 Torr) and SF (0.133 Torr). = 633.44 nm; (1s ®® 2p) Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 81-85, March (2014) Res. J. Chem. Sci. International Science Congress Association 83 The obtained experimental cross sections for the deexcitation of Ne() by N2 as a function of the mean collisional energy are shown in figure 4. The deexcitation cross sections are in the range of 6.1- 9.3 Å for Ne() and almost constant or increase slightly with increasing the collisional energy. 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 N using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. Since electronic energy of the lowest excited Ne() (16.62 eV) exceeds the ionization potentials of the N2 (15.58 eV). Therefore the possibility of several ionization channels, in addition to Penning ionization and associative ionization can also occur. The reported results of West et al. show that the possible ion products for Ne(2,0) by N are NeN which were determined to be in a 94.1:5.9 at a collisional energy around 45 meV22. Similar branching ratio might be expected for the deexcitation of Ne() by N. The present cross sections at the mean collisional energy corresponding room temperature are in agreement with those by Yokoyama and Hatano using the pulse radiolysis method23. There have been limited data for the deexcitation of Ne() by 2 compared to those deexcitation of Ne() by rare gas atoms. The obtained deexcitation cross sections of Ne() by N2 together with the reported data are shown in figure 5. The comparison shows that the collisional energy dependence of the cross sections has the same general behavior. The present cross sections are in good agreement within the experimental errors with those obtained by West et al., Yokoyama and Hatano and Broom et al. at room temperature22-24. The cross sections obtained by Baudon et al., van den Berg et al. and Aguilar et al. are larger than the present cross sections for Ne() by N25-27. The present cross sections are velocity averaged, so I average the compiled results of velocity selected cross sections in Baudon et al. ( in figure 5) with a Maxwellian velocity distribution to allow a more direct comparison25. However, such velocity averaged cross sections in figure 5 still have different cross sections as the unaveraged ones. Since the data for the metastable states have been obtained with quite different experimental methods, systematic experimental error could be involved in either or both sets of experiments. On the other hand, there is no results for Ne() by N2 in the investigated energy range by various methods. Thus it would be useful to compare the results to the cross sections for the same experimental method and collisional energy but so far there have not been determined. The reported deexcitation cross sections are in the range of 9.7- 16.3 Å for Ne() and the values are slightly larger than those of Ne() by N. The behavior of the collisional energy dependence are similar both of the metastable and resonant states.Experimental and theoretical results on the wide collisional energy dependence of the cross sections for deexcitation of the Ne() by N2 should be investigated by various research groups and methods. Figure-4 The experimental cross sections for deexcitation of Ne() by N Figure-5 A comparison of the cross sections for deexcitation of Ne() and Ne(2,0) by N with those different experiments. : represent the present results Conclusion In the present research work, the cross sections for the deexcitation of Ne() by N2 has been measured as a function of the mean collisional energy in the range of 17.5-37.9 meV or in the temperature range from 136 K to 294 K using a pulse radiolysis method as combined with time-resolved optical absorption spectroscopy. The measured deexcitation cross sections are in the range of 6.1- 9.3 Å for Ne() and almost Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 81-85, March (2014) Res. J. Chem. Sci. International Science Congress Association 84 constant or increase slightly with increasing the collisional energy.The results of Ne() by N has been also compared with the results of Ne() by N. A better understanding of the deexcitation mechanisms for the Ne() by N2 can be achieved by considering information on different observations such as crossed beam results and flowing afterglow results in a wide collisional energy. Theoretical investigations such as an ab initio calculation of the optical potentials are greatly needed for further understanding. Acknowledgements The author is thankful to Ministry of Education, Science, Sports, and Culture, Japan for financial support. References 1.Siska P.E., Molecular Beam Studies of Penning Ionization, Reviews of Modern Physics, 65(2), 337-412 (1993) 2.Hotop H. and Niehaus A., Reactions of Excited Atoms and Molecules with Atoms and Molecules, Z. 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