Metastable atoms collision processes

Farago, P. S. Free-electron physics. Baltimore Penguin Books, Inc. 1970, pp. 269 Feldman, D., Novick, R. Observations on the (Is2s2p) Ps/2 metastable state in lithium, in Atomic collision processes, McDowell, M. R. C. (ed.) Amsterdam North Holland Publishing Company 1964, pp. 201-210 Fox, R. E., Curran, R, K. J. Chem. Phys. 34, 1595 (1961)... 

Table C2.13.1 Collision processes of electrons and heavy particles in non-thennal plasmas. The asterisk denotes short-lived excited particles, the superscript m denotes long-lived metastable excited atoms or molecules.

Crossed-molecular-beam studies of differential scattering of metastable noble-gas atoms with ground-state noble-gas atoms or simple molecules is the major topic of this chapter. These studies have been carried out recently at several laboratories with the common goal of finding both real and imaginary parts of the interaction potentials to further our understanding of the dynamics of collision processes involving metastable noble-gas atoms. 

Main fragmentations occurring both in the ion source, and in metastable and collision-induced dissociation (CID) experiments involve the losses of a hydrogen atom, HCN, and MeCN. These are competitive processes, whose abundances depend on the position of the nitrogen in the pyridine ring. Stable isotope labeling showed that the loss of HCN occurs from the five-membered ring (Scheme 1). 

Since each vibrational state within a harmonic oscillator is at the same temperature, only 3N-6 vibrational temperatures and one bath temperature T are required to describe an N atom metastable polyatomic. For CH3F this corresponds to 9 vibrational temperatures and T however, degenerate or nearly degenerate modes are generally tightly coupled by kinetic collision process such as (e.g.)... 

If the electronic excitation energy of a metastable atom A exceeds the ionization potential of another atom B, their collision can lead to an act of ionization, the so-called Penning ionization. The Penning ionization usually proceeds through the intermediate formation of an unstable excited quasi molecule in the state of auto-ionization cross sections of the process can be very high. Cross sections for the Peniung ionization of N2, CO2, Xe, and Ar by metastable helium atoms He(2 S) with an excitation energy of 19.8 eV reach gas-kinetic... 

A last comment will be on metastable species. Atoms and nmlecufes are sometimes excited into sudi electronk states from which a radiative transition is not allowed by quantum mechanical rules. Atoms and molecules in these so-called metastable states may have very long lifetimes and consideraUy influence the overall discharge chemistry. A metastable atom (molecule) can transfer its energy throu a collision with another particle and, if this is of lower ionization potential, the result may be an ionization or dissociative ionization event Such processes are known as Praning ionization (nrocesses and may be in rtant under cotain conditions in plasma chemistry. 

One of the simplest and most accurate techniques for investigating the collision processes involving metastable atoms is the pulsed afterglow method. In this technique a sample of metastable atoms is created in a cylindrical or spherical cell by means of a pulsed discharge and the decay of the metastable density is observed following the termination of the excitation pulse. By measuring the decay rate as a function of gas pressure it is often possible to obtain enough information to determine the individual contributions to the observed decay rate made by each of the processes listed above. 

Conclusion. We have had space here to consider only a few of the collision processes involving metastable atoms. 

The collision processes involving metastable atoms and molecules are considered in great detail by... 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

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