Ionization quantum yields

In Figure 3.57 we show almost the full identity of the results obtained with IET and UT. They demonstrate the well pronounced maximum appearing when i > is shorter than the characteristic time of the subsequent geminate recombination [22]. In the contact approximation the results are qualitatively the same but the ionization quantum yield / is half as much as in distant theories (see Table III). This was expected because at such a short To a significant fraction of ions are produced during the initial static ionization that is missed in the contact approximation. 

Neither the maximum nor the descending branches of the upper curves, representing geminate recombination, are reproduced in the Markovian theory. It predicts the monotonous ion accumulation and still further decrease in the ionization quantum yield /. This is because the Markovian theory does not account for either static or subsequent nonstationary electron transfer. When ionization is under diffusional control, both these are faster than the final (Markovian) transfer. EM is a bit better in this respect. As a non-Markovian theory, it accounts at least for static ionization and qualitatively reproduces the maximum in the charge accumulation kinetics. However, the subsequent geminate recombination develops exponentially in EM because the kinematics of ion separation is oversimplified in this model. It roughly contradicts an actual diffusional separation of ions, characterized by numerous recontacts and the power dependence of long-time separation kinetics studied in a number of works [20,21,187],... 

Casanovas, J., Guelfucci, J. R, and Terrisol, M., Determination of the ionization quantum yield and thermalization distance of photoelectrons created by VUV photoionization in pure liquid alkanes and tetramethylsilane, Radiat. Phys. Chem., 32, 361, 1988. 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

At present, most fluorescence sensors or assays are based on intensity measurements, i.e., intensity-based sensing, in which the intensity of the probe molecules change in response to the analyte of interest. Intensity-based methods are initially the easiest to implement because many fluorescent probes change intensity in response to analytes. These intensity changes can be due to changes in extinction coefficient due to probe ionization, changes in quantum yield of the probe on analyte binding, or due... 

The above show that both the quantum yield and fluorescence lifetime can be modified by any factor that affects the relative contributions of the nonradiative (k) and radiative (F) decay processes. As described in Section 2.2, these factors include environmental factors such as solvent polarity, ionization and... 

Figure 3 The photoionization quantum yields (rji) of CH4 as a function of the incident photon energy measured via the double ionization chamber and synchrotron radiation as mentioned in Section 2.1. The bandpass was 0.1 nm, which corresponds to the energy width of 32 meV at the incident photon energy of 20 eV. The vertical ionization potentials of the ionic states involved are indicated by the vertical bars [11] along with the first adiabatic ionization potential by the arrow [17]. (From Ref. [7]. Reprinted with permission from Elsevier Science.)...

In this section, we present an overview of the photoabsorption cross section (o ) and the photoionization quantum yields (rh) for normal alkanes, C H2 +2 ( = 1 ), as a function of the incident photon energy in the vacuum ultraviolet range, and of the number of carbon atoms in the alkane molecule, because normal alkanes are typical polyatomic molecules of chemical interest. In Fig. 5, the vertical ionization potentials of the valence electrons, which interact with the vacuum ultraviolet photons, in each of these alkane molecules are indicated to show how the outer- and inner-valence orbitals associated with carbon 2p and 2s orbitals, respectively, locate in energy [7]. 

In Fig. 7 [7], we compared the photoionization quantum yields (t/,) of CH4, C2H6, and C3H8, which were measured by our group using the double ionization chamber and synchrotron radiation, as described in Section 2.1. The photon energies are considered in two ranges, as follows, in terms of the behavior of the t/, curves as a function of the incident photon energy [7] ... 

Passing through the threshold energy for ionization of liquid alkanes, the quantum yield of ionization, [Pg.368]

Most of the energy associated with an incident x-ray or y-ray is absorbed by ejected electrons. These secondary electrons are ejected with sufficient energy to cause further ionizations or excitations. The consequences of excitations may not represent permanent change, as the molecule may just return to the ground state by emission or may dissipate the excess energy by radiationless decay. In the gas phase, excitations often lead to molecular dissociations. In condensed matter, new relaxation pathways combined with the cage effect greatly curtail permanent dissociation. Specifically in DNA, it is known that the quantum yields for fluorescence are very small and relaxation is very fast [6]. For these reasons, the present emphasis will be on the effects of ionizations. 

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