Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

1

Developments in Instrumentation and Techniques

BY M. A. WEST

1 Introduction

Although the progressive trend is for more and more physics to enter into chemical applications, a state of affairs which has attracted comment by analytical chemists (Aiialyt. Chem., 1975, 47, 2073), photochemists must surely welcome the application of lasers and electro-optic developments to aid their research. Fields such as absorption and emission spectroscopy, chemical kinetics, and more recently, preparative chemistry, have all benefited through higher spectral resolution, selectivity, sensitivity, etc.

This two-year review (July 1974 to June 1976) discusses most of the obvious advances in instrumentation and techniques in photochemistry, photophysics, and related spectroscopy as well as referring to fringe and other developments which have potential for, or have yet to be applied to, studies on the interaction of light with matter. With such a wide subject content, it is not possible to be very critical of publications or to include all publications within the confined space of this chapter. Furthermore, although subjects have been arbitrarily separated into 10 sections, some areas could be equally well placed in several sections, for example, two-photon absorption in sections dealing with pulsed lasers, absorption, or even emission spectroscopy.

Several key developments have taken place recently in a number of relatively new techniques. Photoacoustic spectroscopy, though discovered 95 years ago, has benefited considerably by recent research which shows its considerable potential for absorption spectrometry of solids and semi-solids. Preparative photochemistry using i.r. lasers is already proving itself as a powerful technique for isotopic separations and for producing specific products. The time resolution in transient absorption measurements has now been pushed back to femtoseconds, beyond which, chemistry, as we know it, does not exist because of the uncertainty principle.

A list of recommended terms for spectroscopy was tabulated in a previous volume (Vol. 6, p. 62) and was reputably based on the S.I. system of units. Unfortunately, inconsistencies in these terms have been indicated by Mielenz, who recommends use of more logical adjectives and nouns to describe quantities and terms which are based on the transport of energy according to the laws of geometrical optics. For example, by defining absorbance as the negative logarithm to base ten of internal transmittance, it should be clear that this refers to the transmittance of an absorbing material exclusive of losses at boundary surfaces and effects of interreflection between them. Any instrument used for the measurement of spectra should simply be called a spectrometer. The word spectrophotometer, though commonly used, is a misnomer since a photometer is an instrument that measures luminous flux. Since the adjective 'luminous' implies the integral effect of visual radiation as perceived by the human eye, the spectral analysis of luminous flux has no physical meaning. It is certainly more accurate and logical to use the term absorption spectrometer and in the same way the confusion over spectrofluorimeters and spectrofluorometers would be eliminated by the term fluorescence spectrometer. One suggestion unlikely to find acceptance by photochemists, however, is replacement of the firmly established quantum yield by radiant yield or photon yield.

2 Plasma Sources

The low-pressure mercury lamp so commonly used for photochemistry has been studied recently and the intensity of the 253 nm line examined as a function of Hg pressure, tube radius, and operating current. The intensity rises to a peak at about 7 mTorr pressure and falls at higher Hg pressures and, at constant pressure, increases linearly with current. A useful review emphasizing the chemical developments of inorganic phosphors discusses their applications in changing the output wavelength of an Hg lamp. Instabilities in the output of an HPK mercury lamp have been overcome by operation from an optically stabilized supply resulting in a drift of 0.1% h-1 over a 30 h period.

The amount of obnoxious and hazardous ozone generated by xenon short arc lamps is reduced considerably by passing the normal cooling air through a baffled aluminium chamber containing iron oxide. This 'filter' decomposes the ozone to oxygen with high efficiency, but only after a warm-up time of 30 — 40 min. A comparison of Xe–Hg, D2 arc, and H2 hollow-cathode lamps has been made in an evaluation of a suitable source for background correction in atomic absorption spectrometry.' At shorter wavelengths, a new type of source generating the line radiation of the rare gas ions achieves an enhanced ion flux by incorporating a charged particle arrangement. Intense line spectra are obtained from the He, Ne, and Ar ions, affording a convenient windowless source of He(II) (30.4 and 25.6 nm) and Ne(II) (46 nm) suitable for photoelectron spectroscopy. A microwave-discharge U.V. light source has been reported to yield...