Mirror removal technique

In addition to the chemical activation non-equilibrium systems, the thermally induced decomposition of hydrocarbons and hydrocarbon radicals has also been widely encountered. The earliest hydrocarbon reactions to be studied were the thermal unimolecular decompositions of alkanes10 and alkyl radicals11 in which mirror removal techniques were used to demonstrate the actual presence of the radicals. These thermal reaction systems tend to be complex and, despite continued investigation, 12-13 many are not fully understood. 

Experimental confirmation of free radical participation in chemical reactions was first obtained by Paneth and Hofeditz by the chemical method of mirror removal. This most familiar technique has been used subsequently in many studies of free radicals. The apparatus consisted of a Sow tube operated at pressures of about 1-2 torr. In the original experiments of Paneth and Hofeditz the reactant was lead tetramethyl, and the carrier gas was specially purified hydrogen. A lead mirror was deposited in the downstream part of the tube by local application of heat. If now the tube was heated upstream from the mirror, the first mirror gradually disappeared, suggesting that methyl radicals, produced pyrolytically from lead tetramethyl, removed the first mirror by the reaction... 

There are two main ways in which the mirror technique has been applied. In the first, the concentration of radicals is determined by following the rate of mirror removal by one of a number of methods. These include determination of mirror opacity, change in radioactivity from a radioactive mirror, measurement of electrical conductivity, and so on. In the other method, the products formed by the removal of the mirrors are characterized, thus giving an indication of both the nature and concentration of the radicals concerned. Thus Paneth and Hofeditz ... 

In this 0-switching technique, one of the cavity mirrors is effectively removed during pumping and then suddenly replaced. The build-up time of the giant pulse is determined by the switching speed and the initial gain of the pumped laser. 

This technique was quickly adopted by others and it was soon found by F.O. Rice and co-workers that the pyrolysis of many organic compounds at 800 to 1000°C removed metallic mirrors, implicating the formation of free radicals. The cleavage of larger free radicals into smaller radicals and olefins under these conditions, was also proposed (equation 22), as well as chain reactions in which radicals abstract hydrogen from alkanes. Reactions of alkyl halides with metal atoms in the gas phase were also found by M. Polanyi and co-workers to yield alkyl radicals (equation 23). 

Paneth technique. Method demonstrating the existence of free radicals (e.g., methyl) or atoms, which is based on the removal of a metallic mirror by a stream of gas containing the radicals. The reaction products can be collected and assayed. 

For many applications, there may be some advantage in employing phase modulation 54,85) instead of the usual amphtude modulation. In the latter technique the path of the radiation from the source to the detector is blocked and opened periodically by a chopper (cf. Fig. 20 and Section 4.3). For phase modulation, the chopper is removed from the spectrometer and the fixed mirror of the Michelson interferometer is moved back and forth about its mean position with a certain frequency. In contrast to the interference modulation (see Section 4.2), the amplitude of the mirror motion is small, being a quarter of the wavelength of the light. For the analogue Fourier transform or interference modulation, the amplitude of the mirror has to have many wavelengths in order to achieve a reasonable resolution... 

---