CHAPTER 1
Part I
PHYSICAL ASPECTS OF PHOTOCHEMISTRY
1
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 other laboratory-scale source and could be of picosecond durations. Strong quasi-uniform continua covering the wavelength region 4 — 200 nm were produced by focusing the output of a Q-switched ruby laser (1 J) on to the rare earth metals suggesting use as a reliable background source for absorption spectroscopy in the vacuum-u.v. and soft X-ray region.
The successful application of a commercially available radiometric voltage-to-frequency converter has resulted in the reduction of light source fluctuations with tungsten-halogen and xenon arc sources. Modulated d.c. currents through compact arc lamps (such as xenon and krypton) have been shown to induce arc instabilities at discrete frequencies which are sufficiently severe to extinguish the lamp. An arc lamp pulsing unit incorporating thyristor switching has been described which provides pulses variable from 10 to 990 µs. The emission spectra of two identically-labelled types of fluorescent blacklight lamp (low pressure mercury lamps with u.v. phosphor coatings) have been reported to be significantly different with emission peaks at 350 and 365 nm. The medium pressure mercury lamp has been shown to be an excellent source of far-i.r. radiation, with an effective blackbody temperature of 3000 K and a stabilized d.c. supply for a 125 W mercury lamp has been shown to eliminate frequency modulation in a far-i.r. spectrometer. Intense visible emission from Ca and Mg hydrides (620 — 640 and 470 — 610 nm, respectively) in an r.f.-excited heat pipe oven was produced with electrical efficiencies as high as 5 % suggesting possible use as an efficient narrow band light source.
3 Laser Sources
It is again appropriate to comment on safety codes regarding eye protection against laser radiation. There are two principal guides which are recommended and which provide information on maximum permissible levels (MPEs) under a variety of laser exposure conditions. The American National Standard published in 1976 contains a collection of tables and figures for a variety of experimental conditions. The earlier published (1972) British Standards guide also gives corneal MPEs for common lasers under a narrower range of operating conditions. Although there is a mass of accumulated data on threshold values for laser damage, it must be stressed that only a few measurements of threshold values for the human eye have been determined and since the publication of both guides, reports have appeared of much lower threshold values for certain of the newer types of lasers, especially mode-locked and u.v. lasers. Picosecond pulses in particular have been shown to be potentially hazardous and the revised British standard (to be published in 1978) requires that no greater than 10-9 J cm-2 be incident on the cornea for a time duration of some picoseconds. A comprehensive bibliography of laser radiation hazards in biological systems has been published and a useful collection of articles related to laser measurements for a regulatory standard includes information on types of detectors, calibration, and measurements of beam properties.
CW Lasers. — Sources of coherent u.v. radiation, emitting more than 1 W CW power are attractive for many applications such as spectroscopy, dye-laser pumping, photochemistry, and isotope separation. At present, tunable u.v. is best obtained from pulsed sources (although recent work with intracavity SHG in dye lasers is very encouraging) but CW operation offers better amplitude stability and beam quality and is usually not accompanied by electromagnetic interference. At present noble gas ion lasers still represent the highest average power sources in the u.v. region even though excimer lasers of the noble gas monohalide type hold great promise for the future. CW laser action of up to 16 W from the 351.1 and 363.8 nm lines of ArIII 6.7 W from the 351 and 356 nm lines of KrIII and 1.8 W from the 375 and 378 nm lines of XeIII has been observed in purely wall-confined noble gas discharges with a report that the maximum output is limited only by the rapid optical degradation of the u.v. cavity mirrors. Higher powers from a highly ionized, low pressure discharge of up to 61 W from the combined 351 and 364 Ar lines have been obtained with an axial homogeneous magnetic field of 20 G.
Twenty new laser transitions have been measured from a helium discharge in a gold-plated hollow cathode spanning the region 253 — 763 nm. In general, lasers of this type require lower threshold currents (<4 A) than the noble gas ion lasers. A multiline output of 125 mW has been demonstrated in the 250 — 290 nm region. CW laser transitions in Ne-Ag and He-Ag mixtures extended to 224.3 nm, with the strongest output at 318.1 nm of more than 350 mW. A preliminary report has been made of CW laser action in NeII producing 6 W at 332.4 nm. New u.v. ion laser transitions in F2, Cl2, I2, Br2, and S2 look promising for future CW operation with three new transitions found in Br2 below 280 nm. The strongest line at 236.2 nm produced a peak power of 60 W. Generation of tunable CW u.v. radiation was reported earlier (Vol. 4, p. 87; Vol. 5, p. 81) using intracavity SHG of a CW rhodamine 6G (Rh6G) dye laser but over a limited tuning range (285 — 315 nm). A convenient method has now been developed for generating broadly tunable u.v. from 285 to 400 nm by SHG and sum frequency mixing. Two pump lasers (5 W Ar and 5 W Kr), a single dye laser (either a Rh6G or Oxazine 1) and three non-linear crystals (ADA, ADP, and RDP) were used. The output of the dye laser was either used directly in extracavity SHG or combined with the Kr laser by means of a dichroic mirror. The maximum power at 313 nm was reported to be 0.75 mW with a linewidth of 0.016 nm and powers in excess of 5 µW over the range 290 to 390 nm. Over 30 mW of CW radiation in the 285 — 315 nm range was produced by doubling the CW Rh6G laser with either ADA or ADP in the Cavity. The conversion efficiency was shown to be limited by the onset of thermally-induced phase mismatching. Peak u.v. output was increased to 85 mW if the radiation from the laser was chopped. U.v. laser lines having characteristics close to those presented in the visible by CW tunable single mode dye lasers were produced by superposition of both a CW Rh6G and pulsed (doubled Nd: YAG) laser. The recorded linewidth of less than 20 MHz was shown to be suitable for a high resolution study of rubidium Rydberg states. Laser action at five lines between 358 (maximum power 7 mW) and 747 nm (1 mW) has been observed by exciting neon buffer gas in an aluminium hollow cathode. A similar arrangement with a silver cathode produced 18 lines from 408.6 to 585.2 nm.
A long-life He-Cd laser tube (greater than 1000 h) has been described in which sputter protection of the cathode was achieved by a cadmium film coverage. The additions of small amounts of iodine to this laser discharge was shown to increase the power of the 441.6 nm line by 30 — 40%. The He-Cd laser was shown to exhibit self mode-locking at 325 nm and retinal damage levels with this laser (0.36 J cm-2) seem anomalously low when contrasted with similar wavelengths for Ar and Kr lasers where retinal damage is not reproducibly incurred at corneal thresholds of 67 J cm-2. It is not known if this sensitivity is related to the mode-locked nature of the laser.
Laser oscillation on transitions of Cd2+ in He-CdI2, He-CdCl2, and He-CdBr2 and similar transitions in Zn2+ compounds have been reported over the range 442 — 888 and 491 — 759 nm, respectively. The hollow cathode He-Cd laser operating on the red (636, 635.5 nm), green (537.8, 533.7 nm), and blue (441.6 nm) offers the possibility of obtaining efficient blue, green, or white light oscillation from a single laser tube. A hollow cathode discharge with internal anodes has been shown to operate at significantly higher discharge voltages than conventional hollow cathode discharges allowing use of lower discharge currents and thereby increasing the output of noble gas mixtures. CW laser oscillation was reported at 531.4 and 486.3 nm from XeII.
Up to 52 W of dye laser power was obtained from a Rh6G jet stream dye laser pumped by all the lines of a high power Ar laser (175 W). No thermo-optical problems were reported with water as the solvent with a viscosity-raising additive (polyvinyl alcohol). (Other dye laser studies are reported later.) Other CW lasers in the visible include molecular iodine pumped by the 514 nm Ar line (wavelength range 583 — 1338nm at a power conversion efficiency of 8%), a practical sealed-off He-I+ laser (emission at 541, 568, 613, and 658 nm with a total power of 20 — 30 mW) and quasi-CW operation of three laser lines (479.7, 521.0, and 650.1 nm with respective output powers of 25, 5, and 40 W) in a pure mercury low pressure discharge. The first visible laser having nuclear energy as the only source of excitation was reported to be He-Hg (lasing at 615 nm) pumped by a1n (10B, a) Li nuclear reactor.
In the near-i.r., ArIII was shown to lase at 692 and 747 nm with a slot cathode discharge and CuII produced laser oscillations at 740 and 790 nm in a hollow anode-cathode discharge. Up to 300 mW of true CW laser power was reported from ruby at 77 K pumped by the 514 nm Ar laser line.
Alkali halide host crystals containing F-centres, which have wide fluorescence bandwidths, have been optically pumped to produce tunable emission in the 0.8 — 3 µm band. The advantage of these lasers is the long fluorescence lifetimes, the coincidence of the pump bands with Nd: YAG and GaAs laser wavelengths, and the strong refractory nature of the host crystals.
CW laser action at room temperature between 1.06 and 1.12 µm has been obtained from double-heterojunction structures of Inx Ga1 - x/InyGa1 - yP and other developments in semiconductor lasers has been reported in a collection of 34 papers. Laser emission has even been found with a single crystal Nd: YAG fibre end pumped by a single high radiance LED.
Chemical lasers operate on a population inversion produced directly or indirectly in the course of an exothermic chemical reaction and since their discovery (in 1964), numerous studies have been made largely on the high power HF and DF, CO and I* lasers. Although currently reported devices are i.r. emitters, the lowest reported wavelength is close to 800 nm and it is likely that very high power lasers operating in the visible region will be produced, probably from metal-oxide systems. A purely chemical HC1 laser in which Cl atoms were produced by a branched chain reaction between NO and ClO2 produced a multiline power of 4 W and a chemical efficiency of 6.6%. The first example of CW laser action on a transition between two distinct electronic states which was pumped only by a chemical reaction and requiring no external sources of power was achieved on the 2P3/2 [right arrow] 2P3/2 transition of the iodine atom by energy transfer from the 1Δg, metastable state of O2. The excited oxygen was generated chemically by flowing chlorine gas through a basic solution of 90% H2O2 and the effluent mixed with molecular iodine at the entrance to a longitudinal flow laser cavity.
A detailed description of a liquid N2-cooled CW CO laser was given in a significant publication describing monitoring techniques for CO produced by the u.v. photolysis of formaldehyde. A CO chemical laser pumped by the O + CS reaction was shown to produce nearly 1 W of power oscillating on the 1 [right arrow] 0 vibrational band.
There are numerous examples of pulsed CO2 lasers and their applications elsewhere in this Report (see Sections 6 and 8) but specific papers on CW CO2 lasers have included reports of 36 new lines in the 9 — 11 µm region, an optically-pumped gas-dynamic laser operating on the 14 and 16 µm transitions of CO2, glow discharge stabilization of transverse flow lasers, a description of the high repetition rate photoionized TEA laser techniques to CW CO2 lasers which may also be suitable for discharge-pumped noble gas halide lasers, and a single CO2 laser amplifier producing 40 W of CW power suitable for pumping far-i.r. lasers.
Carbon dioxide laser pumping has been used to produce CW far-i.r. emission from HCHO and DCDO(195 — 949 m) and NH3. Other lasers operating in the far-i.r. include HCN (at 337 µm), DCN (190 and 195 µm), and an efficient waveguide laser (771 — 1222 µm). Far-i.r. laser lines in optically-pumped MeCHF2 were first discovered by use of the opto-acoustic effect.
