Zero activation energy, radiation

If a substance can be decomposed by the action of radiation, it is sometimes possible to identify the minimum energy of radiation required with a bond dissociation energy. This can only be reliably done if the mechanism of the absorption of radiation and the subsequent decomposition can be established, and if the usual kinetic assumption of zero activation energy for the reverse reaction is accepted. 

Simultaneous irradiation of alumina and the exchange mixture between 50° and 150° in a flow system by Steele and Cropper (106a) showed the decay of induced activity directly. The activity fell in 3 minutes from 50 units under radiation to about 5 units without radiation and in 25 minutes to 0.5 units. The temperature dependence observed under radiation, a fall in activity with increase in temperature, was interpreted to give an approximately zero activation energy for the exchange and one of 6 kcal/mole for the decay of the induced activity. If these results at about 375°K and those at 195°K are taken at their face value, they show that the induced sites have a range of stabilities, as would be expected. 

The zero activation energy for radiation initiation, E., leads to some interesting practical consequences. The overall activation for the rate, E = E + 1/2 E - 1/2 E, where E and E are the activation energies for propagation and termination, respectively. Since E. for most catalytic initiation is about 30 and E close to zero, this leads to a value of about 22 kcal per mole compared x ith about 7 kcal per mole for radiation for v/hich E.=0. The practical advantages of this were referred to earlier. 

These equations are commonly called pyrolysis relations, in reference to the thermal (as opposed to a possibly chemical or photonic) nature of the initiating step(s) in the condensed phase decomposition process. It can be seen that while the second, simpler pyrolysis expression with constant coefficient As) preserves the important Arrhenius exponential temperature dependent term, it ignores the effect of the initial temperature, condensed phase heat release and thermal radiation parameters present in the more comprehensive zero-order pyrolysis relation. These terms To, Qc, and qr) make a significant difference when it comes to sensitivity parameter and unsteady combustion considerations. It is also important to note the factor of 2, which relates the apparent "surface" activation energy Es to the actual "bulk" activation energy Ec, Es- E /1. Failure to recognize this factor of two hindered progress in some cases as attempts were... 

It is rather difficult to separate effects of UV radiation from effects of temperature under typical experimental conditions. Systematic studies of the effects of cold UV radiation on WPC materials are not known to the author. However, observing the WPC decks in mountainous areas, where temperature is significantly lower compared to that at the bottom of mountains, I could not see any difference in the fading of composite deck boards and their plastic accessories (endcaps, etc.). It seems that fading does not depends on temperature, at least significantly. This makes sense because many photochemical reactions have their activation energy close to zero. In other words, temperature does not accelerate most of the photochemical reactions. 

Bulk polymerization of tetrafluoroethene (TFE) by radiation was studied in the gas, liquid, and solid phase over a wide range of temperatures from —196 to 90 °C by a number of methods (e.g. NMR and FTIR spectroscopy). Volkova et al. [710] studied the radiation-induced polymerization in the gas phase from 12 to 90 °C. Different activation energies were found below and above 70 °C. Enslin et al. [711] reported that the rate of polymerization in the gas phase was a zero-order function of the monomer pressure. However, the rate of polymerization was profoundly influenced by the initial monomer pressure (4.6-order dependence) and on the radiation intensity (0.36-order dependence). 

The unique feature in spontaneous Raman spectroscopy (SR) is that field 2 is not an incident field but (at room temperature and at optical frequencies) it is resonantly drawn into action from the zero-point field of the ubiquitous blackbody (bb) radiation. Its active frequency is spontaneously selected (from the infinite colours available in the blackbody) by the resonance with the Raman transition at co - 0I2 r material. The effective bb field mtensity may be obtained from its energy density per unit circular frequency, the... 

Radiometry. Radiometry is the measurement of radiant electromagnetic energy (17,18,134), considered herein to be the direct detection and spectroscopic analysis of ambient thermal emission, as distinguished from techniques in which the sample is actively probed. At any temperature above absolute zero, some molecules are in thermally populated excited levels, and transitions from these to the ground state radiate energy at characteristic frequencies. Erom Wien s displacement law, T = 2898 //m-K, the emission maximum at 300 K is near 10 fim in the mid-ir. This radiation occurs at just the energies of molecular rovibrational transitions, so thermal emission carries much the same information as an ir absorption spectmm. Detection of the emissions of remote thermal sources is the ultimate passive and noninvasive technique, requiring not even an optical probe of the sampled volume. 

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