1 Metastable levels

He(ls ) S and also from the metastable level Nr (ls2s)-1- He(ls ) in order to take care of the fraction of metastable ion in the incident beam, as well as the double-electron capture process from both ground and metastable ions. 

The method of exchange-luminescence [46, 47] is based on the phenomenon of energy transfer from the metastable levels of EEPs to the resonance levels of atoms and molecules of de-exciter. The EEP concentration in this case is evaluated by the intensity of de-exciter luminescence. This technique features sensitivity up to-10 particle/cm, but its application is limited by flow system having a high flow velocity, with which the counterdiffusion phenomenon may be neglected. Moreover, this technique permits EEP concentration to be estimated only at a fixed point of the setup, a factor that interferes much with the survey of heterogeneous processes associated with taking measurements of EEP spatial distribution. 

Because transitions between the metastable level and the ground or excited levels are so slow, we sometimes say they are forbidden. Once an electron enters the metastable level, it remains there until it can make a forbidden transition, in which case a photon is released. The time of residence for the electron in the metastable level determines the length of time that phosphorescence persists. 

In many real lasers, the upper laser level, 2 in Figure 2.6(a), is a metastable level that is, it has a long lifetime compared to the lower laser level (t > ti). If we can pump efficiently into such a longer-lived upper level, and provided that there is a lower energy level, E in Figure 2.6(a), with a short lifetime, then a population inversion is very likely to be established. 

Figure 5.61 summarizes the temperature behavior of decay time r and quantum efficiency xj of the blue luminescence from benitoite in the forms ln(r) and ln(q) as a function of reciprocal temperature 1/T. Figure 5.62.a demonstrates a suitable energy levels scheme. After excitation the metastable level 1 is populated due to nonradiative fast transition from excited level. Between levels 1 and 2 the equilibrium population is established due to nonradiative transition. The relative quantum yield of the blue emission may be described by simple Arrhenius equation ... 

One assumes that the internal conversion time from the pumping bands to the metastable level is fast in comparison to the fluorescent lifetime. This... 

For the 1 per cent europium-doped sample, a buildup and decay type of curve similar to that of Chang was found. This, of course, indicates a slow internal conversion to the 5D0 metastable level. From this rise and decay... 

Finally, it should be pointed out that Tjh represents the half width of the energy distribution. From eq. (2-7) we see that V is also the reciprocal of the lifetime of the initial nonstationary state. Of course, the width and the lifetime of the metastable level are related by the uncertainty principle. 

PENNING EFFECT. An increase in the effective ionization rate of a gas due to the presence of a small number of foreign metastable atoms. For instance, a neon atom has a metastable level at 16.6 volts and if there are a few neon atoms in a gas of argon which has an ionization potential of 15.7 volts, a collision between the neon metastable atom with an argon atom may lead to ionization of the argon. Thus, the energy which is stored in the metastable atom can be used to increase the ionization rate. Other gases where this effect is used are helium, with a metastable level at 19.8 volts, and mercury, with an ionization level at 10.4 volts. 

Fig. 11.8). The resonances associated with fission appear to cluster in bunches. Not all resonances in the compound nucleus lead to fission. We can understand this situation with the help of Figure 11.9. The normal resonances correspond to excitation of levels in the compound nucleus, which are levels in the first minimum in Figure 11.9. When one of these metastable levels exactly corresponds to a level in the second minimum, then there will be an enhanced tunneling through the fission barrier and an enhanced fission cross section. 

A hydrogen atom with its electron in a 2p orbital will decay back down to the Is orbitral in approximately 1 nanosecond, giving off a photon with A = 121 nm (determined by the energy difference between the two states). On the other hand, an electron in a 2s state is stuck (we call 2s a metastable level), since emission to the only lower state (1. s ) is forbidden by the Al = 1 selection rule. On average, it takes about 100 ms for the electron to get back down to the ground state from 2s. 

Table 1 Low-Lying Metastable Levels of Rare Gas Atoms...

Laser induced transition probabilities are often small. This is because the interaction time of the ions with the laser is short ( ns), and because the relevant matrix elements are small, or else the levels involved have large natural widths. A useful approximate expression for estimating signal strengths for an electric-dipole (El) transition from a metastable level 1) to a short lived level 2), followed by a spontaneous decay to a third level 13), when the interaction time is long compared to the lifetime of the second level is [8] ... 

Lying just 0.1 e.v. below the upper state of the resonance transition is the metastable level which can be reached by collision. 

A programme is underway to calculate multi-configuration intermediate coupling dielectronic recombination rate coefficients from the (ground plus) metastable levels of an ion to all possible final states, resolved by level, and/or bundling, appropriate for the GCR modeling. It will cover elements applicable to astrophysics and magnetic fusion viz. He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, Ca, Ti, Cr, Fe, Ni, Zn, Kr, Mo and Xe. The first... 

Here vq is the inverse value of the mean lifetime of the excited state q. For levels in which a decay by an allowed radiative transition can take place the lifetime is of the order of 10-8 s. When no radiative transitions are allowed we have metastable levels (e.g. Ar 11.5 and 11.7 eV), which only can decay by collisions. Therefore, such levels in the case of low pressure discharges may have very long lifetimes (up to 10 1 s). 

Accordingly, an over-population of the argon metastable levels would explain both the over-ionization as well as the high electron number density in the ICP. Indeed, it could be accepted that argon metastables act both as ionizing species and at the same time are easily ionized [385]. This could explain the fairly low interferences caused by easily ionized elements and the fact that ion lines are excited very effi-... 

The four 4s levels play a key role in analytical glow discharges (e.g. for Penning ionization of the sputtered atoms) and they cannot easily be depopulated by radiative decay (due to forbidden transitions for the metastable levels, and to radiation trapping for the resonant levels). Therefore some additional loss processes are incorporated for these levels, in order to describe them with more accuracy ... 

The second type of metastable level behaviour, that of most interest here, is also illustrated in Figure 1. There, represents the radial wave function for an atom bound to an inter-... 

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