Swarm experiments

Crompton, R.W. Hegerberg, R. Skullerud, H.R. In Proceedings of the International Seminar on Swarm Experiments in Atomic Collision Research, Tokyo Ogawa, I., Ed. 1979 18 pp. 

Most of the work in solving the Boltzmann equation for electrons has been for the relatively simple conditions of electron swarm experiments. In these experiments, electrons are released from a cathode in low concentrations and drift under the influence of a uniform applied electric field in a low-pressure gas towards an anode at which the electrons are collected. If... 

A typical swarm experiment, to measure Dj/n in this case, is shown in fig. 2.4. Electrons from a suitable source enter a diffusion chamber through a small hole (typically 1 mm diameter) and drift and diffuse in a uniform electric field to a collector consisting of a central disk Ai and surrounding annulus A2. The ratio of the currents received by Ai and A2 is measured and Dj/n found for the particular value of E/N and T from a solution of the diffusion equation with appropriate boundary conditions. 

Fig. 2.4. Schematic diagram of a swarm experiment to measure the ratio Dt/h-Gi is an electrode containing a small central hole and C is a collector containing a central disk At and a surrounding annulus A2.

Qan ratio of unity. This was one of the first determinations of a Qan ratio for electron molecule reactions. The ECD values of the excited-state electron affinity, Qlm, and A i and E agree with the swarm values. The low-temperature response of the ECD can be calculated from parameters measured in swarm experiments. 

The electron beam and electron swarm experiments [13] can also be used to determine attachment rate constants. However, these are determined as a function of energy and can then be extrapolated to thermal energy. Other techniques used to... 

The values of thermal rate constants for rare gases from swarm experiments (Smith and Adams, 1980 Smith et al, 1980 Lindinger and Smith, 1983, Lindinger, 1983) fit the reaction window concept quite well (Fig. 11), with the highest values reaching 10 cm s if there is a final state of the products available for which the crossing occurs at about 4 A. For reactions deviating from that, the rate constants drop steeply. The measured rate constant for the state-to-state reaction... 

The effects of energy on ion-molecule rate processes have been investigated by a variety of methods. The influence of reactant translational energy, as studied by SIFDT (selected ion-flow drift-tube) techniques in swarm experiments, by ICR, and by beam and other single-collision techniques, is reviewed in other parts of this chapter or of this book. In this section, we will concentrate specifically on the influence of reactant internal energy on ion-molecule reactions. There are basically two sources of data that address this problem ... 

In swarm experiments, an important step forward was the development of a new method in which the SIFT techniques were combined with laser-induced-fluorescence (LIF) detection for monitoring the ion vibrational states (Kato et al., 1993). In this way, both the vibrational states of the reactant ion and the vibrational states of some reaction products could be detected, and the influence of vibrational energy on reaction rates of thermal ions (where the translational-to-vibrational energy transfer is negligible) could be studied. The method was primarily used to study reactions of Nj(t) = 0 to 4) with Ar (which will be discussed separately), N2 and O2 (Kato et al., 1993), Kr (Kato et al., 1996), H2 (de Grouw et al., 1995), and HCl (Krishnamurthy et al., 1997). In the reaction... 

Linuma, K. Takebe, M. Satoh, Y Seto, K. Design of a continuous guard ring and its application to swarm experiments. Rev. Sci. Instrum. 1982, 53, 845-850. 

Both the mobility and the swarm experiments showed that the electron energy distribution in nitrogen is not thermal, even at E/N < 0.1 Td however, when N2 was replaced by CO2 as the drift gas, the rate constant was determined as 2.38 ( 0.15) X 10 cm s and this value was independent of E/N at values up to 1.6 Td, the limit of experimentation. Furthermore, the value of the rate constant in nitrogen as /A->0, extrapolated to the same value [49],... 

In a previous study [1], NF has been found to form with low or no kinetic energy in a dissociative capture process at zero electron energy, possibly by the process NF3 + e- NF 2F. Its resonance maximum was at 2.8 eV. The cluster ion NF3-F was identified in electron swarm experiments with a mixture of 0.04% NF3 in N2 using a drift tube mass spectrometer system [13]. 

Borghesani, A. F., Bruschi, L., Santini, M., and Torzo, G., Simple photoelectronic source for swarm experiments in high-density gases. Rev. Sci. Instrum., 57, 2234,1986. 

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