The physics of colour vision

Investigation of human colour vision relies primarily upon psycho-physical methods which require active observer participation in the experiments. The principal results of such experiments are stated and discussed in relation to the functional organisation of human colour vision. Generally, objective techniques, such as electro-physiology, are employed in the study of colour vision responses in non-human vertebrate species. The structural and functional organisation of vertebrate colour vision as revealed by these objective methods is described and the relationship between objective and psycho-physical results examined. Other sections of the article are concerned with defective colour vision and with parametric variations in normal colour vision. It is concluded that a reasonably consistent scheme for the organization of human colour vision emerges from the various types of experimental investigation, and that this scheme is adequate for the interpretation of a variety of colour vision phenomena.     

RADIATION ACTIVATION OF CARCINOGENS AND THE ROLE OF OH AND O2-

Abstract— Radiation-induced covalent binding of labelled carcinogens to DNA has been investigated under a variety of conditions using ultrafiltration or millipore filtration of TCA precipitable complexes. High yields of carcinogen binding at high DNA concentrations are also observed for a variety of small molecules and are not carcinogen-specific. At high carcinogen concentrations, radiation-induced unstable electrophilic carcinogenic species are produced, and undergo free-radical reactions which simulate cellular redox reactions involved in metabolic carcinogen activation, leading to the formation of covalently bound carcinogen adducts to DNA as a potential target macromolecule. The yields of carcinogen-DNA adducts increase linearly with dose and depend upon carcinogen concentration. The results of scavenger studies indicate that the oxidising species O₂^- and OH are the principal activating species. Rate constants for the selective radiation-induced oxidation reactions of various chemical carcinogens with superoxide have been measured by a competition kinetic method using pulse radiolysis. The relatively long-lived superoxide radical reacts with carcinogens at a rate which is two orders of magnitude slower than the diffusion-controlled rate for the hydroxyl radical, thus allowing a measure of O₂^- specificity in the presence of competing reactants within the cell.     

Simplicity Beneath Complexity: Counting Molecular Electrons Reveals Transients and Kinetics of Photodissociation Reactions

Time‐resolved pump–probe gas‐phase X‐ray scattering signals, extrapolated to zero momentum transfer, provide a measure of the number of electrons in a system, an effect that arises from the coherent addition of elastic scattering from the electrons. This allows to identify reactive transients and determine the chemical reaction kinetics without the need for extensive scattering simulations or complicated inversion of scattering data. We examine the photodissociation reaction of trimethylamine and identify two reaction paths upon excitation to the 3p state at 200 nm: a fast dissociation path out of the 3p state to the dimethyl amine radical (16.6±1.2 %) and a slower dissociation via internal conversion to the 3s state (83.4±1.2 %). The time constants for the two reactions are 640±130 fs and 74±6 ps, respectively. Additionally, it is found that the transient dimethyl amine radical has a N?C bond length of 1.45±0.02 Å and a C?N?C bond angle of 118°±4°.     

Thermal effects observable during quasi-phasematched second harmonic generation in periodically poled crystal fibres

The impact of transient and steady-state thermal effects on the conversion efficiency of quasi-phasematched second harmonic generation in periodically poled LiNbO₃ fibres is discussed. The response time and wavelength variation of the quasi-phasematching condition depend on the thermal conductivity of the fibre, its enclosure and the thermal coefficient of the crystal’s refractive index.     

Difference frequency generation with a synchronously mode-locked cw dye laser system

The generation of continuous trains of tunable infra-red pulses by the nonlinear mixing of the wave-lengths available from a self-contained synchronously mode-locked cw dye laser is described. The factors determining the optimization of the process are identified and their roles demonstrated.     

Coherent interaction of picosecond pulses in a fluorescent medium and the timing of pulse trains for two-wavelength excite and probe measurements

Coherent interaction between two cw, picosecond pulse trains of the same wavelength absorbed by a fluorescent material and the effect on the emission has been studied. As the path difference between the two beams is varied, the infra-red emission from colour centres in KCl: Na or KCl: Li fluctuates in phase with the transmitted intensity of the weaker of the two beams. The effect may be applied to the accurate relative timing of cw pulse trains of different wavelengths for use in two-wavelength, excite and probe measurements. A small fraction of the pump pulse train is superimposed on the probe beam and passes along precisely the same optical path. Coherent interaction between the two pulse trains at the pump frequency causes modulation of the transmitted probe beam intensity when pump and probe pulses are accurately synchronised.     

Generation of picosecond optical phase conjugate pulses using cw mode-locked lasers

Low power picosecond optical phase conjugation has been demonstrated using cw mode-locked argon-ion and dye lasers via 4-wave mixing in ruby. Streak camera and power measurements show that the generated pulses are identical to the input pulses.     

The analysis of excite and probe measurements of relaxation times with fluctuating pulse duration

The effect of fluctuations in the pulse duration of synchronously pumped modelocked pulse trains on excite and probe measurements is discussed. Relaxation times comparable with the pulse durations can be measured even when large pulse-to-pulse fluctuations in duration exist. The pump and probe pulse durations are assumed to be correlated. When the probe pulses are the second harmonic of the pump, or vice versa, the third harmonic must also be generated to permit deconvolution of experimental excite and probe data. When the pump and probe pulses have the same time dependence, the excite and probe curves consist of the desired response function convolved with the time-averaged second harmonic autocorrelation function which is easily measured. Deconvolution yields the relaxation time but fluctuations in pulse duration increase the root-mean-square voltage fluctuation at the output of the detector system and limit the accuracy with which the relaxation time can be calculated.