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Photochemistry Volume 13
A Review of the Literature Published between July 1980 and June 1981
By D. Bryce-Smith The Royal Society of Chemistry
Copyright © 19883 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-463-5
CHAPTER 1
Part I
PHYSICAL ASPECTS OF PHOTOCHEMISTRY
1
Developments in Instrumentation and Techniques
BY A. J. ROBERTS
1 Introduction
This article is concerned with the developments in instrumentation and techniques in photochemistry and spectroscopy during the period July 1980 — June 1981. Such a wide ranging topic is impossible to review at all critically, nor is it feasible to consider every publication concerning photochemical instrumentation. Consequently, many reports concerned merely with the application of established techniques have been omitted. In this respect, it should be noted that the relative brevity of some sections (for example plasma sources, u.v.-visible spectroscopy) in no way reflects the use or application of these techniques, but merely their advanced state of development. Further it is apparent that, during the past decade, a swing away from developments in instrumentation for conventional photochemistry in favour of spectroscopy and laser photochemistry has occurred. This has been reflected in the following discussion. The author would like to thank Dr. Mike West for several helpful discussions during the preparation of this manuscript.
2 Plasma Sources
Several reports have discussed a modified version of a commercially available Grimm's glow discharge lamp for use as a hollow cathode emission source. Aluminium, copper, and graphite cathode materials were investigated. The modification of large diameter (Perkin Elmer) hollow cathode lamps for a small diameter lamp housing (Instrumentation Laboratory model 751 spectrophotometer) has been described. The selective spectral enhancement of arc discharge lamps with the addition of metal halides has been demonstrated,' and the high-energy conversion into narrow wavelengths suggests an application as CW-laser pump sources. A controlled temperature-gradient lamp has been shown to perform better (sharper emission lines and higher intensity) than an electrodeless discharge lamp for atomic absorption spectroscopy. The design of a lithium heat-pipe arc lamp for use as a laboratory source has been discussed.
Although laser sources are rapidly dominating spectral calibration in the visible and infrared regions, plasma sources are still attractive for the vacuum U.V. A deuterium discharge lamp has been developed as a radiance transfer standard between 115 and 370 nm. Although magnesium fluoride windows were used in this application, a later report, in which an argon mini-arc was utilized for standardization in the region 92– 200 nm, suggested that problems may arise due to the formation of colour centres. The errors introduced due to the polarization of the irradiance standard source are often neglected since, in many cases, the required data are unavailable. Consequently the characteristic polarization of a DXW-type filament lamp (General Electrics 1000W) has been tabulated.
Synchrotron radiation provides an attractive, completely tunable, moderately intense source for spectroscopy, and several facilities are currently available, or are in development, for such application. The use of synchrotron radiation in biophysical and biochemical research has been discussed in a collection of 28 papers." Its use in vacuum-u.v. spectroscopy and the design of suitable optical components has been considered.
3 Laser Sources
Molecular Gas Infrared Lasers. — Two brief reviews have been published on infrared laser sources and their applications. A compact high-pressure (5 atm.) CO2 laser has been designed and operated at pulse repetition rates up to 50Hz. Single-mode power densities of 300MW1-1 were achieved, although the short sealed-off lifetime of the laser limited its usefulness as a spectroscopic source. A shock tube driven CO2-Ar gas dynamic laser provided a 4 ms, 2W pulse at 18.4µm. 4W output was also obtained from a 1.2cm active length of laser medium using a conical nozzle for mixing CO2, and nitrogen. Pre-ionization of the discharge medium has been found to improve the performance of transverse electric atmospheric (TEA) CO2 lasers. Advantages were gained both from a three-fold increase in output power and better pulse-to-pulse reproducibility. A CW waveguide CO2 laser, with transverse radiofrequency pumping was found to be 8.5% efficient with up to 4.6 W output power. 3-W output was obtained from a CO waveguide laser with chilled, flowing gas. Gold electrodes have been utilized in a sealed CO laser producing 28.5 W output power. The laser was shown to have a long operational life.
A compact frequency stabilized TEA–CO2 laser was operated at a pulse repetition rate of 100 Hz with pulse energies of 80 mJ. Active frequency and amptitude stabilization was achieved for a CW CO2 laser. A bandwidth of less than 300kHz and a power fluctuation less than 3 x 10-3 were obtained. Some interest has been displayed in the possibility of short pulse production by mode-locking CO2 lasers. The introduction of a short pulse into a high-power oscillator at a time close to the lasing threshold produces injection mode-locking. This has been demonstrated for a non-dispersive 30 J TEA–CO2 laser operating on the 10 pm and 9 pm bands. Injection mode-locking has been employed together with a saturable absorber to produce reliable sub-nanosecond pulse trains from a large aperture TEA–CO2 laser. The longitudinal mode structure in a mode-locked CO2 laser has been investigated using a scanning Fabry-Perot interferometer.
A line-narrowed TEA–CO2, laser has been developed as an enhanced pump for optically pumped molecular gas laser systems such as the 16µm transition in CF4. 160 mJ output at 12.8 µm was obtained from a CO2-laser-pumped ammonia laser. The device was designed for ease of construction and should be usable with other gases also. Sub-nanosecond pulses were derived from NH3, and C2D2 lasers pumped using an injection mode-locked CO2 laser. Greater than 1 MW pulse energies were recorded. An optically pumped NSF laser provided tunable output in the range 618–658cm-1. Perchloryl fluoride (FC1O3) with CO2 laser excitation generated 34 lines between 16.3 and 17.7 µm with output powers up to 4mJ. Single-mode 100 kW output powers were found for D2O in a ring laser configuration.
Picosecond pulses tunable from 2700 to 32000 cm-1 were obtained using a travelling wave parametric process. The temporal jitter between two systems operating together could be reduced to less than 1 ps.
Solid-state Lasers. — FA (II) colour centres in alkali halides were utilized to produce tunable-i.r. laser radiation in the ranges 1.7–2µm (KBr), 2.42–2.9µm (KCl-Li), and 2.55–3.28 µm (RbC1-Li). A similar system, pumped using a Nd-YAG laser, produced 5 ns wide pulses with energies of 100 µJ. The laser was tunable over the range 3685–3697 cm-1 with 0.3-1 bandwidth.
A review of InGaAsP laser diodes, suitable for operation over the range 1.0 — 1.7 µm, has been published. 9 mW of laser output were obtained from a CdS platelet optically pumped using an Ar+ laser. The same system was also operated in a synchronously pumped mode with a mode-locked argon ion laser, producing pulses as short as 8ps with 3.2 mW average power. Considerable effort has been expended in producing short pulses by mode-locking GaAs (and related) semiconductor lasers. 28 ps wide pulses were produced at a repetition rate of 2.5 GHz by a gain-switching method. Streak-camera pulse monitoring has demonstrated that an actively mode-locked angled strip GaAlAs laser was capable of producing bandwidth-limited pulses of 16ps duration with 1 W peak power and ΔTΔv = 0.36. Amplified spontaneous emission (ASE) in GaAs without a resonant cavity, pumped using two-photon excitation from a mode-locked Nd-glass laser, produced lops wide pulses; but synchronous pumping of a similar system resulted in a 7ps pulse width. A passively mode-locked modified strip buried heterostructure GaAlAs diode laser, with an extended resonator, has been shown to produce pulses with a width of 5.1 ps, close to the transform limit. At a repetition rate of 850 MHz average powers of 5 mW were obtained. Finally, pulses less than 1 ps wide have been produced by gain-switching a GaAs laser with pumping using a synchronously pumped mode-locked dye laser.
Neodymium lasers remain a favourite source for photochemical applications providing high power outputs at 1.06 µm (and in the visible and ultraviolet region with frequency conversion). The relative performance of flashlamp-pumped NdxLa1-xP5O14 laser rods has been evaluated with x = 1.0, 0.75, and 0.20. The maximum laser efficiency was found for NdP5O14. Regenerative amplification of a 40 ps Nd–YAG laser pulse produced gigawatt output with no degradation of the temporal and spatial characteristics. 3.4 TW pulses with 100 ps width were generated using Nd–phosphate glass disc amplifiers. Many applications require short (< 100 ps) high intensity pulses and several reports have been concerned with reliable mode-locking of these lasers. For example, slow Q-switching of a passively mode-locked Nd–glass laser was found to result in a more stabilized output pulse train. Passive mode-locking was achieved using saturable absorbing dyes and a thermally compensated phosphate glass rod. Both systems produced pulses with approximately 4ps duration. An intracavity etalon incorporated into a Nd–YAG oscillator enabled passive mode-locking, forming pulses 12ps long. Active mode-locking was achieved in a Q-controlled feedback-stabilized Nd–glass laser resulting in loops pulses with energies of 1 mJ at 1.06 µm. The high temporal stability should permit synchronization with other pulsed laser systems. Injection of a single sub-nanosecond pulse into a latent ring oscillator enabled 12ps and 3ps pulses to be derived from Nd–YAG and Nd–glass lasers respectively; and 120 ps pulses were compressed to 15 ps using a saturable dye in a regenerative amplifier. A Pockels cell was utilized to vary rapidly the resonator output coupling in a Nd–YAG oscillator producing 600 ns long pulses with little loss of energy. Smooth long pulse emission (180 ps, 100 W) was also obtained from a frequency-doubled Nd–YAG laser, at a pulse repetition rate of 50 Hz. A short-cavity passively Q-switched laser was capable of generating pulses time-tunable between 2 and 17 ns. One disadvantage of Nd–YAG lasers is their low maximum pulse repetition rate. Efficient burst mode operation of a Q-switched flashlamp-pumped laser generated a 35 mJ output with a pulse repetition rate of 1 kHz. The combination of an acousto-optic and frustrated total internal reflectance modulator enabled the pulse repetition rate of a high-power Nd–YAG laser to be increased into the kHz region. Synchronization was achieved between a Q-switched mode-locked Nd–YAG laser and a mode-locked argon ion laser with less than 18 ps jitter. A Q-controlled actively mode-locked ruby laser produced 35 ps wide pulses which could be synchronized with another mode-locked laser with less than 100 ps jitter.
Dye Lasers. — Since their development in the mid-1970s CW and pulsed dye lasers have been increasingly applied in photochemistry and spectroscopy. Flashlamp- and laser-pumped systems will be considered here, with a discussion of picosecond-pulsed systems referred to a later section. Transverse excitation with an atmospheric pressure lamp has been faund to offer extended operating times when compared with conventional systems. Higher quality output has been obtained as a result of thermally isolating the dye cell from the flashlamp and ensuring a symmetrical dye flow Co-axial and pre-ionized linear flashlamps have been compared for use as excitation sources for high-power repetitive pulsed dye lasers. The efficient operation of pre-ionized flashlamps has been considered. Gas pressure and bore diameter were found to be important parameters. Excimer lasers would seem to offer a useful excitation source for near-u.v. dye lasers and a dye cell designed for this purpose has been reported. The dye solvent was found to influence the photochemical stability of such a laser pumped using 308 nm radiation from an XeCl source. 1 W output was obtained at 100 Hz.
Automatic wavelength control of a ns-pulsed dye laser has been described.' Tuning to any visible wavelength was possible with a near-transform limited-bandwidth of 450 MHz. Similar bandwidths were obtained using an intracavity etalon and a grating set at grazing incidence. Holographic gratings were found to be favourable owing to their higher efficiency. Nitrogen-laser-pumped dye lasers have also been tuned using Fizeau wedges, linewidths of 0.01 nm were obtained with tuning over a 10 nm range. A double prism arrangement was found to reduce the bandwidth of a N2-laser-pumped coumarin 500 dye laser to less than 10-3. A Michelson mode-selector was incorporated in an argon-ion-laser-pumped dye laser providing single-mode operation with 1 W output. A single longitudinal mode N2-laser-pumped dye laser, with a linewidth of 0.02 cm-1 has also been demonstrated. High-power narrow-linewidth operation has been achieved in several reports using a ring dye laser configuration. For example, a unidirectional optically stable ring laser operated with a single longitudinal mode with only a grating, at grazing incidence, for tuning. An external reference Fabry–Perot cavity in a single-mode CW ring laser provided high output powers with 150 kHz linewidths. Radiation at 292 — 305 nm with a 2MHz linewidth, 7GHz scan range, and 500 µW power was obtained with an intracavity frequency doubled, actively stabilized ring dye laser.
Picosecond Pulsed Dye Lasers. — 100 ps Pulses with lOO kW peak powers were obtained from amplified spontaneous emission in a nitrogen-laser-pumped dye laser system. Passive mode-locking of a flashlamp-pumped dye laser, with dye amplification cells pumped using nitrogen and excimer lasers, and frequency shifting by stimulated Raman scattering in Cs vapour produced pulses of 1 — 3 ps duration with 1 — 20 MW peak power in the 3.3 — 8.4 µm region. Active/passive mode-locking of a flashlamp-pumped system provided 2.5 µJ pulses with widths less than 10ps. Picosecond GaAs and GaP switches, combined with a Pockels cell enabled active mode-locking of a coumarin dye laser producing 50 ps pulses with 500 kW peak powers. Intracavity interferometers have been used both for tuning and mode-locking of CW dye lasers. With pulse amplification in nitrogen-laser-pumped dye cells 50 MW peak powers have been reported for a 2 ps pulse. 55 mJ and 15 mJ outputs were reported for rhodamine and coumarin dye lasers with three-stage dye cell amplification. A 165 mJ output was obtained in a single pulse at 589 nm using a two-stage amplification process. 33% Second harmonic generation was possible with such high powers.
Synchronously pumped CW dye lasers have become a popular source of high repetition rate ultra-short pulses. An opto-electronic feedback system has been used to increase the power stability by a factor of 5. Improved performance of a cavity-dumped synchronously pumped dye laser was reported following the addition of a saturable absorber to the laser dye solution. Bandwidth-limited pulses of 2.5 ps duration were obtained with energies in the range 4 — 8 nJ. The conditions for perfect operation have been investigation by second harmonic correlation techniques. The best pulse shape was found to correspond to perfect synchrony between dye laser and argon ion laser (obtained with precise matching of cavity lengths). A scheme for active stabilization by control of the dye laser cavity length has been proposed. The operation of a cavity-dumped synchronously pumped laser has been described. The quality of the dye laser pulses was found to depend critically upon the mode-locking of the argon ion laser. 20 mW average power was obtained in the region 710 — 770 nm with 0.7 ps pulse widths by synchronously pumping oxazine-1 with rhodamine 6G output in a tandem configuration. The interaction ot two oppositely directed pulses in a saturable absorber in a ring dye laser (colliding pulse mode-locking) has provided the shortest dye laser pulses reported to date (90 fs). The methods for measuring these short pulse durations will be discussed later.
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Excerpted from Photochemistry Volume 13 by D. Bryce-Smith. Copyright © 19883 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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