Part I

PHYSICAL ASPECTS OF PHOTOCHEMISTRY

Developments in Instrumentation and Techniques

BY M. A. WEST

1 Introduction

Instrumentation and techniques in photochemistry have probably advanced more in the last two years than over any other comparable period. Developments published during the period July 1976 to June 1978 are covered in this Report and papers cited are designed to be representative of the mass of literature covering photochemistry and related subjects, though not comprehensive. It has often been said that advances in photochemical techniques would have been impossible without parallel advances in electro-optics and electronics. The developments in lasers, without doubt the physical photochemists most useful and applicable excitation source, have enabled unprecedented advances to be made in many areas. For example, in high resolution spectroscopy, i.r. diode lasers now offer spectral resolution of 10-4cm-1; fast pulsed techniques now employ tunable picosecond dye lasers for both transient absorption and emission measurements; and selective photochemistry enables high power tunable lasers to be employed effectively for isotope separation and preparative photochemistry.

The reader is referred to two particular issues of Physics Today which include introductory review articles in i.r.-laser-induced-unimolecular reactions (by N. Bloembergen and E. Yablonovitch), rare-gas halide lasers (J. J. Ewing), and sub-picosecond spectroscopy (E. P. Ippen and C. V. Shank); also physics and photochemistry (V. S. Letokhov), high-resolution spectroscopy of atoms, molecules (T. W. Hansch), and coherent Raman spectroscopy (M. D. Levenson). A compilation of papers reviewing the state-of-the-art of lasers in chemistry included sections on laser Raman and other scattering, pollution and combustion, atomic and molecular spectroscopy, isotope separation and selective excitation, fast pulsed techniques, and developments in lasers and laser techniques.

Although little information appears in the open literature on laser separation of uranium isotopes, there is no doubt that this photochemical process will have a significant impact on fuel processing costs if it can be made economically viable. Equally important in the energy context, of course, is the separation of deuterium for production of heavy water, and there are many laboratory examples (see Laser Focus, July 1978, p. 22) of laser-induced enrichment of this particular isotope.

Apart from advances in laser applications, the reader will notice that great strides have been made in photoacoustic spectroscopy and the applications of vidicon detectors: cf. Vols. 6 and 8. The thermal lens effect offers a new way of determining weak absorption and the quantum yield of fluorescence. Several methods have also appeared for high sensitivity atomic detection at levels of 1 atom cm-3.

2 Plasma Sources

Despite the advances in reducing the wavelength of lasers in the vacuum-u.v., electric discharge and microwave-powered lamps still offer the most useful continuous sources for photochemical applications. For example, a miniature cold-cathode discharge source using flowing neon with emission lines at 73.6 and 74.4 nm maintained its flux to ca. [+ or -] 2% with continuous use over 6 months. A commercially available d.c.-excited cold-cathode design of housing provided Lyman-α radiation of high spectral purity by using uranium hydride in a sealed tube as the source of molecular hydrogen. A flowing hollow-cathode lamp produced intense ion resonance radiation (from Ca, Ba, Zn, Mg, Sr, Yb, or Eu) at ca. 1 mW into 4π/25 steradians. Sulphur hexafluoride at 8 K has been photolysed with a microwave-excited argon discharge in a windowless vacuum-u.v. photolysis lamp with principal lines at 105 and 107 nm. The emission spectra of a similarly excited bromine lamp (used for the gas-phase photolysis of pent-1-ene) and a krypton supersonic jet excited by an electron beam have also been published.

Experiments have shown that the coaxial orifice discharge tube is an effective windowless resonance lamp for argon and neon photolysis of matrix-isolated molecules such as CCl4. A current-regulated power supply for cold cathode discharge lamps has maintained a current deviation of < [+ or -]0.08% with hydrogen.

A vacuum-u.v. and soft X-ray radiation source has been proposed, which was based on spontaneous anti-Stokes scattering from an atomic population stored in a metastable level. The brightness of this source was predicted to be higher than any...