Threshold wavelength

It will be necessary to provide quantitative criteria for studies of photoelectrochemical phenomena. These phenomena are a function of photon energy and appear only at energies above a particular threshold energy hv (or below a particular threshold wavelength, of the light). The threshold energy is an important characteristic of any given phenomenon. 

Fig. 5.65 Dependence of the solar conversion efficiency (CE) on the threshold wavelength (Ag) for a quantum converter at AM 1.2. Curve 1 Fraction of the total solar power convertible by an ideal equilibrium converter with no thermodynamic and kinetic losses. Curve 2 As 1 but the inherent thermodynamic losses (detailed balance and entropy production) are considered. Continuous line Efficiency of a regenerative photovoltaic cell, where the thermodynamic and kinetic losses are considered. The values of Ag for some semiconductors are also shown (according to J. R. Bolton et al.)... 

TABLE 4.4 Threshold Wavelength and Cross Section for Photoionization... 

The threshold wavelength for the appearance of F+ in (iii) has an energy equal to the ionization energy plus the enthalpy of fluorine. However, because some F+ is generated in (ii) via an excited F2 state, one needs to subtract this quantity of F+ (measured by observing the F-ion) in order to obtain the true threshold at 652.5 A [equivalent to D0(F2) of 157.6 1.0 kJ],... 

The reaction should be capable of operating over a wide bandwidth of the visible and ultraviolet portions of the solar spectrum with a threshold wavelength well into the red or near infrared. 

Figure 15.4 Differential reflectograms of TNT on various surfaces. Individual curves have been staggered for clarity. Note The lower and upper threshold wavelengths of the 400 nm shoulder are essentially alike.

Table 4.2 summarizes the threshold wavelengths for production of ground-state.oxygen atoms, 0(3P), as well as electronically excited O( D) and O( S) atoms. Dissociation of 02 in the 175- to 242-nrn region to produce atoms is particularly important in the stratosphere because it is the only significant source of 03 via reactions (1) and (2) (see Chapter 12) ... 

TABLE 4-2 Threshold Wavelengths for the Production of Ground-State or Electronically Excited Oxygen Atoms from 02 Photolysis"... 

Table 4.10 gives the calculated wavelengths below which it is energetically possible to produce the fragments in each of the electronic states shown if there is no contribution from internal energy of the molecule. The threshold wavelength for production of ground-state NO and O atoms is 397.8 nm. 

The threshold wavelength of incident photons to produce the electronically excited Na(2P) is 2440 A. The quenching cross sections by H2. HC1, C02, and H20 for the sodium D lines have been measured as a function of exciting wavelength above 500"C. The quenching cross sections by HCI and C 02 decrease with an increase of relative velocities between the excited Na atoms and quenching molecules [Hanson (441), Earl et al. (332, 333)]. 

The threshold wavelength below which the dissociation of C02 into COIX1 I + O( P) is energetically possible. 

Table Vl-3a. Threshold Wavelengths (A) below Which Indicated Reactions are Energetically Possible in the Photolysis of N20...

Table VI-5 shows that the dissociation process, N02 - NO + O( D) takes place energetically below 2439 A. Usclman and Lee (985) have measured the production of O( >) as a function of incident wavelength near 2439 A. They have found that the contribution of rotational energy to dissociation is insignificant near the second threshold in contrast to the case near the first threshold at 3980 A where the contribution of rotational energy is substantial. They attribute the lack of rotational contribution to the presence of large rotational barriers at high J values in the excited state (987). The quantum yield of O( D) production increases to a plateau of about 0.5 0.1 towards shorter wavelengths, indicating that at least two processes, (VI-59) and (V1-60). occur concurrently below the second threshold wavelength.

A in the Mars atmosphere (200 K) are approximately one half those at room temperature. Table VI 1 shows the threshold wavelengths below which indicated reactions are energetically possible. 

VI -3d and VI 3b gives threshold wavelengths below which indicated reactions are energetically possible. 

An excellent review on 03 photochemistry up to 1971 has been given recently by Schiff (857). Ozone dissociates by absorption of light of wavelengths below 9000 A. Table VI-7 gives the threshold wavelengths below... 

Lin and DeMore (637) have irradiated mixtures of 03 and isobutane with monochromatic light of wavelengths from 2750 to 3340 A at —40 C. The bandwidth was 16 A. The relative quantum yields of OCD) production were obtained from the yield of isobutyl alcohol, a product of the reaction 0( D) + isobutane. The results are shown in Fig. VI -13. The quantum yields are constant below 3000 A and show a sharp cutoffat 3080 A, the thermochemical threshold wavelength for the production of O( D) + 02( A). 

The width of the bands suggests that each of the excited states is strongly predissociated. Most of the recent work has been aimed at determining the branching ratio for the various possible primary processes. These primary processes are listed in Table 1 with their threshold wavelengths (124). 

Figure 2 Plot of energy loss per mole of photons due to the quantized nature of light ( toss) against wavelength for Ag = 800 or 400 nm. Energy from all light of wavelength longer than the threshold wavelength is also lost...

Figure 3. Plot showing the variation of solar energy conversion efficiency with threshold wavelengths at sea level. The solid vertical lines represent the energies required for decomposition of water using one or two photons per molecule of water respectively (4 photons would be at 2000 nm). The dashed vertical lines represent computed values for these threshold wavelengths taking into account unavoidable thermodynamic losses (adapted from ref. 8)...

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