Ionization lifetime

Jencks has discussed how the gradation from the 8fjl to the 8n2 mechanism is related to the stability and lifetime of the carbocation intermediate, as illustrated in Fig. 5.6. In the 8n 1 mechanism, the carbocation intermediate has a relatively long lifetime and is equilibrated with solvent prior to capture by a nucleophile. The reaction is clearly a stepwise one, and the energy minimxun in which the caibocation mtermediate resides is significant. As the stability of the carbocation decreases, its lifetime becomes shorter. The barrier to capture by a nucleophile becomes less and eventually disappears. This is described as the imcoupled mechanism. Ionization proceeds without nucleophilic... 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

When water undergoes self-ionization, a range of cationic species are formed, the simplest of which is the hydronium ion, HjO (Clever, 1963). This ion has been detected experimentally by a range of techniques including mass spectrometry (Cunningham, Payzant Kebarle, 1972), as have ions of the type H+ (HaO) with values of n up to 8. Monte-Carlo calculations show that HjO ions exist in hydrated clusters surrounded by three or four water molecules in the hydration shell (Kochanski, 1985). These ions have only a short lifetime, since the proton is highly mobile and may be readily transferred from one water molecule to another. The time taken for such a transfer is typically of the order of 10 s provided that the receiving molecule of water is correctly oriented. 

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... 

The delay time between the pump and the probe laser pulses is usually very short in these experiments. The delay time is less than 5 ns when the pump and the probe laser pulses are the same, and the delay time is as long as several hundred nanoseconds when the pump and the probe laser pulses are from two different sources. The short delay time ensures that the fragments flying with different velocities are equally sampled before they leave the detection region. Since the delay time is much shorter than the lifetime of the excited molecules (.A ), most of these molecules do not dissociate into fragments when the probe laser pulse arrives. As a result, the probe laser can easily cause dissociative ionization of the vibrationally excited molecules due to their large internal energy. 

The decay of benzene from the S2 state under collision-free condition has also been studied. J. P. Reilly and co-worker studied the nanosecond UV laser induced multiphoton ionization/fragmentation processes. The rate equation model was used for the simulation and the lifetime of the second excited singlet state was estimated to be 20 ps.19 More recently the... 

The physical nature of the ZEKE states has been the subject of intense experimental and theoretical investigation in the past several years. In the well-studied case of NO,14,21 we know from the 3 cm-1 red shift of the ZEKE-PFI threshold band relative to the true adiabatic ionization potential (extrapolated from highly accurate measurements of Rydberg series) that the ZEKE states have principal quantum number n 200 and lifetime of 2 (is or longer. Recent work has found ZEKE states with lifetimes as long as 20 ps.22... 

Figure 6. Possible intensity profiles for two mechanisms for the formation of ion clusters (a) Ionization through AID mechanism, (b) Ionization through ADI mechanism. The signal would persist for long times due to the lifetime of the NH4 in the cluster, and its ensuing ionization, (c) Ionization through both AID and ADI mechanisms. Taken with permission from ref. 65.

Consider the dynamics of ionization of clusters through the C state of an ammonia molecule, where it should be noted that it is also possible that the excitation leads to some population of the B states due to the broad spectral bandwidth of the femtosecond laser pulses. The measurements indicate lifetimes... 

Figure 11. C2H4 ion yield as a function of time in femtoseconds for a pump-photoionization probe experiment. Heavy line Predicted ion yield using the AIMS data and assuming an ionization threshold of 3.5eV. Dashed line Exponential fit to the AIMS ion yield predicting an excited state lifetime of 35 fs. Gray shaded area Reported ion yield [152] obtained using an exponential fit to the experimental data predicting an excited state lifetime of 30 15 fs. (Figure adapted from Ref. 214.)...

In a metal, there are excited states for electrons that lie below the ionization energy. This can be conceived as an electron in a "conduction band" and a "hole" that interact so that the combination is neutral but not of lowest energy. Such an excited state is called an exciton. Excitons may move by diffusion of the electron-hole pair or by transfer of a molecular exciton to another molecule. Reversion of the exciton to a lower energy state may be slow enough for the lifetime to be longer that of lattice relaxation processes. 

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