Photochemical energy threshold

Like thermal reactions, photochemical reactions have energy thresholds below which the reaction is not energetically feasible. The energy equivalence (E, in kcal einstein ) of the wavelength or frequency of radiation can be determined from ... 

Photodetectors exhibit well-defined, cutoff wavelength thresholds, the positions of which are determined by the magnitudes of the band gap activation energy, E, or impurity-activation energy, E. The cutoff wavelength, corresponds to a photochemical activation energy, E, where. 

The rate of dye decomposition should be negligible at exposure energies up to several thousand times the energy absorbed during the read process and increase rapidly thereafter. Dye compositions which exhibit this type of threshold behavior must have relatively high photochemical and thermal stability and decompose by a free radical process above the melting point of the substrate.199... 

Here, L total is the depth of the etched hole per pulse and is assumed to be the sum of photochemical and photothermal contributions, Tphoto and Thermal, respectively 0Ceff is the effective photon absorption coefficient of the medium and can vary with laser emission characteristics, e g., photon density Fis the incident laser fluence Fth is the medium s threshold fluence A and F are the effective frequency factor with units of pm/pulse and the effective activation energy with units of J/cm2, respectively, for the zeroth-order thermal rate constant F0, comparable in magnitude to Fth, is important only at low fluences.64 Equation (5) is obtained after assuming that the polymer temperature T in the laser-exposed region of mass mp and the thermal rate constant k are given, respectively, as... 

Direct conversion systems are threshold devices, that is, there is a minimum photon energy which can initiate the photochemical reaction. This is called the band-gap ei rgy Eg with a corresponding band-gap wavelength Xg. Eg usually corresponds to the 0-0 transition to the lowest excited singlet state of the absorber (see Fig. 1). Hence, it is important to know what fraction of the incident solar power is available at the band-gap energy. 

The photolysis of 03 produces molecular oxygen and atomic oxygen, either or both of which may be in electronically excited states, depending on the excitation energy. Table 4.5 shows the wavelength threshold below which each combination of products may be formed. For example, the primary photochemical process... 

Photochemical. Determination of the threshold wave length for the photolytic dissociation of a metal-alkyl bond yields an upper limit for the corresponding thermal bond dissociation energy (5, 19), but the assumption that the photochemical threshold approximates the bond dissociation energy does not appear to be warranted. 

The third possibility is to design photochromic materials in which the photochemical bleaching reaction of the colored form has an activity energy barrier that could act as a threshold. Picosecond laser photolysis studies on furyl fulgides12 demonstrated that the transformation from the excited state of the colored form to the S o potential surface of furyl fulgides has an activity energy barrier. 

For many applications and for the understanding of the photochemical reactivity of molecules of environmental significance it is necessary to compare the homo-lytic bond dissociation enthalpies and the corresponding photon energies THw (threshold wavelengths) in relation to the absorption onset of these molecules (Bolton and Cater, 1994). D%s is identical with the bond strength of a de-... 

The most recent threshold determination for the radical process (I) utilizes the.Meinel band chemiluminescence of the O(- P) + HCO - OH + CO reaction (20). The basic photochemical questions have been and remain the determination of the true precursors for the two product channels and the energy (or excitation wavelength)-dependent quantum yields for each channel. 

Figure 25 also shows the threshold energies for both the thermal and the photochemical reduction processes. In general, a high selectivity can be achieved in photochemical reactions due to the highly selective nature of light absorption, which allows the injection of energy into particular bonds or molecules. 

The ablation threshold is the irradiance value at which the material starts to separate from the sample. As a rule, the material separates through vaporization to a depth where the energy supplied by the laser beam equals the energy of vaporization. The material can separate in other forms including solid or liquid fragments and products of the photochemical decomposition of the sample. Because the energy deposited over the sample must exceed its latent heat of vaporization (//, J/lcg), the threshold fluence J/m-) is given by... 

Effects of Radiation. The threshold for decompn of NaN, by electron bombardment is at 11j65 0.2 volt-electrons (Ref 52). Groocock Tompkins (Ref 160) studied the decompn with 100 and 200 v-electrons at RT and found theoretical calcns agreed with exptl results. Muller Brous (Ref 52) found that photochemical decompn occurred at wave lengths below 405 mft at a rate directly proportional to the UV intensity. No relation between energies necessary for electronic and photochemical decompn was apparent. 

Irradiation of methyl bromide, again on a silver(lll) surface, follows the same pattern. The principal photochemical event is the fission of the C—Br bond with the bromide being retained on the surface and with expulsion of the methyl fragment. The study reports that the threshold energy for the C—Br bond fission was higher for multilayer systems... 

---