Metastable states ionization

Lx>ng radiative lifetimes of metastable states support the high density of these particles in slightly ionized plasma, or in excited gas. Thus, according to Fugal and Pakhomov [18, 19] the density of metastable atoms of helium at pressure of the order of a few Torrs, at temperatures ranging from 4 to 300 K, is about two orders of magnitude above the density of electrons. The density of metastable atoms and molecules in... 

If the work function is smaller than the ionization potential of metastable state (see. Fig. 5.18b), then the process of resonance ionization becomes impossible and the major way of de-excitation is a direct Auger-deactivation process similar to the Penning Effect ionization a valence electron of metal moves to an unoccupied orbital of the atom ground state, and the excited electron from a higher orbital of the atom is ejected into the gaseous phase. The energy spectrum of secondary electrons is characterized by a marked maximum corresponding to the... 

Penning ionization occurs with the (trace) gas M having an ionization energy lower than the energy of the metastable state of the excited (noble gas) atoms A. The above ionization processes have also been employed to construct mass spectrometer ion sources. [21,24] However, Penning ionization sources never escaped the realm of academic research to find widespread analytical application. 

In principle, the neutral desorbed products of dissociation can be detected and mass analyzed, if ionized prior to their introduction into the mass spectrometer. However, such experiments are difficult due to low ejfective ionization efficiencies for desorbed neutrals. Nevertheless, a number of systems have been studied in the groups of Wurm et al. [45], Kimmel et al. [46,47], and Harries et al. [48], for example. In our laboratory, studies of neutral particle desorption have been concentrated on self-assembled monolayer targets at room temperature [27,28]. Under certain circumstances, neutrals desorbed in electronically excited metastable states of sufficient energy can be detected by their de-excitation at the surface of a large-area microchannel plate/detector assembly [49]. Separation of the BSD signal of metastables from UV luminescence can be effected by time of flight analysis [49] however, when the photon signal is small relative to the metastable yield, such discrimination is unnecessary and only the total yield of neutral particles (NP) needs to be measured. 

As described in Section 5.6.2, argon/helium atoms are excited to a metastable state by beta radiation from a radioactive source. The species formed is then capable of ionizing all compounds with a lower ionization potential. The products formed are then subject to an electric field (500-1100 V) and the change in current measured. 

Photons from a tunable dye laser are absorbed by a metastable state of the ion. Spontaneous emission at several wavelengths follows. The ionizing electron energy is varied over the region of interest. At each setting the laser is frequency scanned across the Doppler profile of the io.i-beam... 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

The helium ionization detector (HID) is a sensitive universal detector. In the detector, Ti3H2 or Sc3H3 is used as an ionization source of helium. Helium is ionized to the metastable state and possesses an ionization potential of 19.8 eV. As metastable helium has a higher ionization potential than most species except for neon, it will be able to transfer its excitation energy to all other atoms. As other species enter the ionization field the metastable helium will transfer its excitation energy to other species of lower ionization potential, and an increase in ionization will be measured over the standing current. 

Fig. 1. (Left) The structure of the pHe+ atomcule, where the p with large-(n, l) quantum numbers circulates in a localized orbit around the He2+ nucleus, while the electron occupies the distributed Is state. (Right) The level scheme of large-(n, l) states of the pHe+ atomcule. The solid bars indicate radiation-dominated metastable states, while the broken lines are for Auger-dominated short-lived states. The ionized pHe++ states are also shown by dotted lines. From Ref. [2]...

Fig. 1. Energy levels of the antiproton in pHe+. The p is captured by replacing one of the Is electrons, which corresponds for the p to a state with principal quantum number no JW /m, where M is the reduced mass of the atomcule, and m the electron mass. About 3% of antiprotons are captured in metastable states (black lines) at high angular momenta L n — 1, for which deexcitation by Auger transitions is much slower than radiative transitions. The lifetimes of these states is in the order of /is. The antiprotons follow predominantly cascades with constant vibration quantum number v = n — L — 1 (black arrows) until they reach an auger-dominated short-lived state. The atomcule then ionizes within < 10 ns and the pHe++ is immediately destroyed in the surrounding helium medium. The overall average lifetime of atomcules is about 3 — 4 ps...

Fig. 7. The spinner detector. The container is filled with a liquid containing the sample. Upon rotation, a metastable state develops (at left) which breaks down after an ionizing event, as is indicated by the formation of a central bubble (at right). From K. Behringer et al. [46],...

Metastable Xe (7 = 2) lies 8.28 e.v. above the ground state and is capable of ionizing anisole (7.P. 8.2 e.v.) by collision.160,161 Figure 25 shows the other xenon resonance line also has a nearby metastable state and xenon photosensitization via the metastable state can be achieved at 1295.6 A. also. A few xenon photosensitized decomposition experiments have already been discussed for the case of methane (see XIII-B, le). Figure 25 also shows the energy levels involved in krypton... 

No attempt is made here to review in detail information on metastable states of nitrogen above the (A 32+) state, except to note that a metastable state at approximately 8.5 eV which was observed by Cermak [136] may lead to associative ionization ... 

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