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 significant photon fluxes at 26.9 and 40.81 eV.

Mention will be made in other sections of the use of light from a synchrotron, but it is worth noting here a collection of papers dealing with this intense plasma source and its applications.

3 Laser Sources

Before reporting developments in laser sources, it is appropriate to comment on safety codes regarding eye protection. Although there is little doubt that nearly every laser system radiates a beam which is hazardous to the eye, current safety codes in this country and elsewhere need to be revised regularly in view of developments in laser sources. Minimum permissible exposures depend on laser wavelength, exposure time, and peak power and, for many lasers, are estimated and certainly not based on ophthalmic measurements of thresholds for retinal or corneal lesions. Although some current safety codes have been criticized for being confusing, too conservative, and unrealistic (Laser Report, 1976, 12, 6, 7), there has been a report that standards for the near-u.v. may be inadequate (Laser Focus, 1976, 12(1), 41) since the corneal-damage threshold for the N2 laser for 10 ns pulses is only 10 µJ cm-2. Even more disturbing is recent evidence showing that the eye is 800 times more susceptible to damage from blue light than from radiation in the near-i.r. Both laser users and developers must be aware of realistic safety requirements, particularly in view of present and planned legislation on safety.

The following sections outline some of the numerous publications on lasers with a reporting bias towards high-energy U.V. and tunable sources of all wavelengths which are being, or can be, used in photochemistry and spectroscopy.

CW Lasers. — There are few U.V. lasers known with adequate CW output power, and frequency doubling of visible lasers is not normally very efficient. Intra-cavity SHG, with temperature-tuned KDP or ADP crystals in a folded argon ion laser cavity, produces an output power of 300 mW at 257.25 nm. The important design criteria for this 32% power conversion efficiency are: (i) temperature tuning of the SHG crystal to better than [+ or -] 0.02 °C; (ii) cutting the crystals at the Brewster angle; and (iii) producing a 50 µm beam waist in the crystal. A lower cost and potentially useful laser for photochemistry is a CW CuII laser obtained by exciting a neon discharge in a copper hollow cathode. Lines at 248.6, 250.6, 259.1, and 259.9 nm at a power output of between 7 and 210 mW have been reported. The He–Cd laser, which usually emits at 325 and 441 nm, can produce simultaneous emission on five wavelengths in the red, green, and blue which can be mixed to give a 'white light' 1aser. CW laser oscillations on 23 transitions of CU(II) between 450 and 799 nm were obtained by exciting He–Ar, He–Ne, or He–Xe discharge in a hollow copper cathode. A high output power (0.5 W) and a bandwidth of 0.004 nm have been reported for a rhodamine 6G (Rh6G) laser pumped by an argon ion laser. Removal of unwanted background fluorescence from this type of laser within 0.5 nm of the exciting Ar+ line at 488 nm line has been achieved using an external diffraction grating. Two astigmatic and coma-free prism ring dye lasers have been described for the jet-stream CW system.

CW laser action at 546 nm from an Hg laser has been obtained in one case in a sealed-off system, suggesting use as a low-power (3 mW) green laser. DOTC and hexacyanine-3 cyanine dyes pumped by a 1.5 W krypton laser produce laser emission covering the range 754 — 888 nm. A compact external cavity for use with Group III and IV compound semiconductor injection lasers incorporates a grating which allows tuning from 860 to 910 nm.

Among i.r. lasers reported are those obtained by non-linear mixing of emission from Nd-YAG and Rh6G lasers in LiIO3 (range 1.28 — 1.62 µm), a spin-flip Raman laser for the range 1905 — 1850 cm-1 which was calibrated by absorption spectroscopy of COS, NO, DBr, and H2O using acousto-optic detection, and chemical lasers of HF and DF. In one case, F atoms produced in a mixture of SF6, and He by microwave-discharge apparatus produced a laser with a CW output power of 4 W between 2.5 and 2.9 µm. Laser gain profiles (at 10.8 µm) were measured in a low-pressure Na-catalysed N2O–CO transverse flow chemical laser under a variety of flow conditions.

Pulsed Gas Lasers. — The search for new U.V. lasers that are highly efficient has been particularly stimulated by requirements of isotope separation and laser-induced thermonuclear fusion. Electron-beam pumping of high-pressure noble gases is well known to be efficient, and recent studies with xenon and xenon–He–Ar mixtures revealed a continuously tunable source over 5 nm at 172 nm. Investigations of laser systems using collisional energy transfer to create population inversions between electronic states of acceptor molecules have concentrated on electron-beam pumping of gas mixtures, e.g. Xe–O2, Ar–N2. An intense band emission at 340 — 344 nm from Ar–I2, mixtures has been attributed to emission from molecular iodine with an overall fluorescence yield of 13 [+ or -] 4%. Since Velazco and Setser suggested that the diatomic noble-gas halides were possible laser systems, the following have been observed to lase following electron-beam excitation: XeBr at 282nm; KrF at 249 nm; XeCl at 308 nm; XeF at 351 and 353 nm; and ArF at 193 nm. These systems are of great interest as a new class of powerful tunable U.V. lasers. For example, using an axial electron-beam excitation scheme to excite a mixture of Ar, Kr, and F, 108 J of laser energy corresponding to a peak power of 1.9 GW was obtained from KrF and 1.6 GW from ArF. Electron-beam pumping is not essential since transverse electrical excitation (similar to that used in the N2, laser) of mixtures of He or Ne, Xe, and NF, at pressures between 300 and 1000 Torr produced strong laser emission at 351 and 353 nm (attributed to XeF) with an energy of 7 mJ (compared with 2 mJ from nitrogen under the same conditions).

Laser action on the U.V. bands of I2 at 342 nm and bromine at 292 following electron-beam irradiation has been reported with an experimental arrangement similar to that used for the rare gas halide lasers.

The nitrogen laser (at 337 nm) must be the most common laser used in photochemical laboratories. The literature on these devices up to 1974 has been reviewed, and a detailed analysis of their dynamic behaviour and circuit theory and design presented. In the last paper, the usual flat-plate design is modified to spiralled striplines rolled around the cavity. In this way, a reproducible power of 1.2 MW was obtained at a charging voltage of only 12 kV. Other models constructed include a double parallel-plate design similar to that of Basting and Steyer (Vol. 4, p. 88) with a third electrode in the cavity for preionization, giving an output power of > 3 MW and pulse energy of > 20 mJ, a low divergence (0.2 x 0.3 mR) laser of maximum intensity 5 MW mrad-2, a MW system from a simple 25 cm device, and a low-threshold coaxial arrangement using a Nanolite pulser of maximum power 140 kW. A stabilization technique employing a corona-type discharge prior to pulsing a Blumlein circuit has been employed, and calculations made on laser intensity and linewidth. Construction of an N2, laser operating at 1 atm pressure and producing 335 µJ in a 1 ns pulse and a segmented flat-plate Blumlein circuit generating 400 ps pulses at a peak power of 1 MW have also been reported. Addition of SF6 to a nitrogen laser has been found to produce a considerable increase in output power up to 100%.

Electron-beam pumping of Ar-N, mixtures results in laser emission at 357.7 nm with a much higher efficiency (0.08 — 0.4%) than nitrogen alone. A compact Ar–N2, excitation transfer laser emits 40 ns pulses at a repetition rate of 1 kHz with a peak power up to 300 kW. In this case, a 12-stage Marx bank generator drives the cathode directly with an input voltage of 540 kV.

Travelling-wave excitation of high-pressure nitrogen can produce single pulses from the second positive band of N2, with the duration decreasing from 300 ps at 1 atm to 50 ps at 6 atm. Mixtures of argon and iodine-donor compounds (HI, CF3I, or CH3I) can be electron-beam pumped to produce lasing from iodine at 301 nm, at average output powers up to 25 MW.

Further investigations of the copper laser (reported in Vol. 6, p. 69) have shown that this could have potential as a high-energy visible laser. Quasi-continuous pulsed laser output at 510.6 and 578.2 nm has been reported from 600 °C copper iodide discharges at repetition rates near 8 kHz and up to 30 kHz with copper chloride. At slightly longer wavelengths, laser oscillations have been observed on the green bands of XeO and KrO excimers pumped by an electron beam at around 550 nm with peak powers up to 100 kW. A multiple wavelength laser could be obtained in a single laser tube by using metals known to lase individually. Copper and gold as laser materials, for example, produce a total power of 17 mW at repetition rates up to 1.7 kHz with simultaneous emission at 510.6, 578.2, and 627.8 nm. A discharge-heated lead vapour laser with emission at 406.2 and 405.7 nm has been reported.

High-power photochemical iodine lasers (emission at 1.315 µm) have the potential of providing the short and powerful pulses which are necessary for laser fusion. In order to reach maximum inversion quickly, it is necessary to pump CF3I or C3F7I molecules with a flash lamp or light from a laser-produced plasma, or in one case, to increase the number density of the iodide by shock compression. The high-gain characteristics of these lasers may result in premature super-radiant emission along an amplifier chain unless the various amplifier stages are optically isolated from each other. This was accomplished by a single saturable absorber consisting of an electric discharge passed through a cell containing CF3I gas or iodine vapour. Q-switching and mode-locking have been achieved with this laser, resulting in the latter case in 160 ps pulses.

There have been numerous reports of carbon dioxide lasers, lasing at 10.6 µm, with details of high-power TEA lasers, a chemical waveguide laser with energy from the exothermic chain reaction between D2, and F2 initiated by flash photolysis, and laser amplification by stimulated emission of CO2, by transfer from products of the oxidation of alkaline-earth metal vapours in N2O. Q-switching with aromatic halogenated hydrocarbons and rapid modulation by operating a thin film Pb1-xSnxTe optical shutter have also been described. Conversion of a Coherent Radiation CO2, laser to create a CO laser results in laser emission at 5.4 — 5.6 µm with a power of 1 W. Pumping CH3F gas with a 200 MW TEA CO2, laser produced far-i.r. laser pulses (at 496 µm) with powers > 1 MW. Laser action at 11.5 and 12.2 µm was observed in electron-beam stabilized electric discharges with He-Co-CS2, and He-Ne2-CS2, mixtures.

Dye Lasers. — The welcome development of dye lasers offering higher output powers, shorter pulse durations, and higher repetition rates have been accompanied by numerous studies of fluorescent dyes. There has been a growing interest in studies of both the photophysical and photochemical properties of these dyes in attempts to achieve conditions of high output power and minimum photochemical degradation.