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Photoionization quantum efficiency

Figure 2 Variation of the quantum efficiencies of photodissociation of photoionization and of nonradiative transition to the ground state, r] r = l — — t]i, in liquid water as a function of... Figure 2 Variation of the quantum efficiencies of photodissociation of photoionization and of nonradiative transition to the ground state, r] r = l — — t]i, in liquid water as a function of...
The extinction coefficients for the T-T neutral aminyl, and cation radical absorptions of 97 were used to calculate the quantum efficiencies for N—H cleavage and photoionization. The results indicate that in cyclohexane, the efficiency of cleavage is ca. 90%. Thus, roughly 90% of those upper triplet states that do not relax to T undergo cleavage. In acetonitrile on the other hand, the efficiencies for neutral and cation radical production are 0.53 and 0.45, respectively. In other words, of the upper triplets that do not regenerate Tv half decay to neutral radical and the other half to cation radical. It should be noted that the actual proportion of direct cleavage events may be smaller than indicated from the efficiencies because one of the cation radical decay routes is deprotonation to form the neutral radical. [Pg.272]

Mahan (56) found a value of 1.2 0.1 by mass spectrometric analysis for CO with NO photoionization being used to monitor lamp intensity. Warneck (59) found a value of 1.0 with mass spectrometric analysis for CO and lamp intensity measurements by O2 actinometry with an assumed value of 2.0 for the quantum efficiency of O3 formation. Recently Slanger et al. (70) have measured the rate of CO production in the CO2 photolysis at different wavelengths and expressed the results relative to the rate at 147 nm. Their results are 0.76 0.11 at 121.5 nm,... [Pg.27]

The photoconductor, as shown in Fig. 7, depends upon the creation of holes or electrons in a uniform bulk semiconductor material, and the responsivity, temporal response, and wavelength cutoff are unique to the individual semiconductor. An intrinsic photoconductor utilizes across-the-gap photoionization or hole-electron pair creation. An extrinsic photoconductor depends upon the ionization of impurities in the material and in this case only one carrier, either hole or electron, is active. The same is true for a quantum-well photoconductor, in which electrons or holes can be photoexcited from a small potential well in the narrower band-gap regions of the semiconductor. The quantum efficiency for the structure in the figure is determined by the absorption coefficient, o, and may be written 2isrj = (l — / )[ — where R is the reflection coefficient at the top surface. Carriers produced by the radiation, P, flow in the electric field and contribute to this current flow for a time, r, the recombination time. The value of the current is... [Pg.220]

While the distinction of the electron trajectories as being either direct or indirect and the observation of quantum mechanical interferences among the trajectories can be understood in terms of the DC electric field strength and the photoelectron kinetic energy with respect to the saddlepoint in the Coulomb + DC electric field potential, this is not the only quantity that characterizes the photoelectrons that are emitted. Above the saddlepoint in the Coulomb + DC potential there exists a continuation of the Stark manifold. This Stark manifold manifests itself in the excitation spectrum of the atom, which shows pronounced peaks in the photoionization efficiency as a func-... [Pg.48]

From Eq. (9) it is clear that correct determination of QY depends on accurate knowledge of the overall detection efficiency/, which is generally about 0.5. It can be determined in two ways. One is to compare the measured ratio C2+/C+ with the calculated ratio for an atomic rare gas such as Xe at a wavelength where the true relative production ratio A2+/N+ is known from photoionization mass spectrometric measurement of the ion yields. This method has been used in deriving the Xe data of Fig. 15. As expected, the quantum yield in the atomic case is unity (100%) within experimental error this must be generally true because deactivation of superexcited states by light emission is very rare. Thus the second and quicker method is simply to measure the apparent QY for an atomic gas and determine / accordingly. [Pg.132]

From the comparison of the photoionization-efficiency curves for H2, HeH and NeH", they were able to show that HeH and NeH" ions are produced by vibrationally excited H2 ions, the vibrational threshold being L = 3 and v = 2 for HeH and NeH formation, respectively, at zero kinetic energy. These thresholds are somewhat different from those obtained in the electron impact studies [161, 162] v = h and 2, respectively). Above the vibrational threshold, the reaction cross-section increases with vibrational quantum number and there is no kinetic energy threshold, in contrast with the result of Friedman and his co-workers who observe small kinetic energy thresholds for these vibrational states. [Pg.363]

OCSe at 172nm forms Se( 5 ) with a quantum yield of 0.63. Substantial quantities of free electrons are formed [by photoionization of Se( 5)] and these quench the S state extremely rapidly (at a rate constant of 1.2 X 10 cm molecule s ) this undesirable effect can be controlled by addition of which itself undergoes an efficient electron-attachment... [Pg.164]

Fig. 9.8 (a) Energy-level diagram of Rh atom, and laser transitions used for selective ionization of this atom. Three dye-laser pulses raise 10% of Rh atoms to a Rydberg state with an effective principal quantum number of n fs 15 20 ns later, the highly excited atoms are ionized with an efficiency close to unity by means of an electric field pulse, (b) The first data on rhodium concentrations at the K/T boundary (in the Sumbar-SM-4 section, Turkmenia) obtained by the ultrasensitive laser photoionization spectroscopy technique. The maximum Rh concentration in the sample studied was 24.2 ng g . The Rh/Ir ratio was 0.34 0.06, which is close to the cosmic ratio of these elements. (Prom Bekov et al. 1988.)... [Pg.170]


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See also in sourсe #XX -- [ Pg.83 , Pg.109 ]




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