The photooxidation of deoxygenated solutions of tyrosine in water at pH 8.5–12.6 has been studied at room temperature using e.p.r. spectroscopy. By means of e.p.r., four radicals were detected during irradiation with the full spectrum of a Hg–Xe arc lamp. These were identified as the tyrosyl radical, [Formula: see text] and two radicals derived from dopa. A tentative mechanism suggested for [Formula: see text] formation involves the reaction of tyrosine with the hydrated electron, which has been ejected from another tyrosine molecule in one of its excited electronic states. A mechanism similar to the one proposed for the photooxidation of phenol has been suggested for the formation of the radicals derived from dopa. Support for this mechanism is provided by the observation of proton nuclear spin polarization during irradiation.

Conventional S20 multialkali photocathodes have a wide wavelength coverage from < 200 to > 850 nm, but their high transparency and the surface work function result in low quantum efficiencies at longer wavelengths. Theoretical modelling of the photon and excited electron interactions that define the cathode performance provides a realistic prediction of the measured response. The theory emphasizes that the basic light absorption is strongly sensitive to the cathode thickness, wavelength, polarization and incident angle. Parameters can be selected which predict that even at long wavelengths (e.g. 900 nm), absorption may be increased from ∼1% to ∼100%. Cathode topographies can be designed to exploit these responses and offer increased absorption at the longer wavelengths. Alternative designs, which include waveguiding of light within the cathode window, or in structured surfaces, can similarly lead to almost total absorption of the incident light by increasing the number of interactions. These concepts of optimal incidence and waveguiding have been both theoretically modelled and demonstrated in newly fabricated cathode designs. The methods have variously reached quantum efficiencies in excess of 50% at wavelengths in the range from 200 to > 750 nm under different operational conditions. The improvement factors relative to normal incidence on planar cathodes increase for longer wavelengths, and examples of 20–50 times by ∼900 nm were noted. Whilst the absolute S20 efficiency values at long wavelengths are still small, the improvements offer a usable sensitivity even beyond 1 µm, as demonstrated by spectroscopy data up to at least 1140 nm.

The absorption and fluorescence spectra of biphenyl-h10 and biphenyl-d10 neat crystals, and of lightly and heavily doped isotopically mixed crystals have been studied at 4.2 °K. The first singlet excited state of biphenyl is assigned as 1B3g (33128.2 cm−1 in biphenyl-h10, 33270.4 cm−1 in biphenyl-d10) and transitions to it are found to be consistent with a magnetic dipole mechanism by polarization measurements and calculations. The bulk of the intensity in the B3g1⇄Ag(X) transitions is electric dipole arising from Herzberg-Teller coupling through a mode b2u (9) at 626 cm−1 (h10) and ≈ 593 cm−1 (d10). The next excited state is assigned as 1B2u (M-polarized) and the splitting of the 1B3g and 1B2u states of 746.8 ± 3 cm−1 provides the inter-ring resonance interaction corresponding to the splitting of the 1B2u (D6h) state of benzene through excitation exchange interactions and second-order shifts. The isotopic mixed crystal spectra provide evidence that the width of the biphenyl crystal exciton band (from 1B3g) is less than 4 cm−1. The observation of doublets in the 1B2u excited states of biphenyl-d10 and doublets in the absorption and emission of biphenyl-h10 in biphenyl-d10 suggest that the crystal structure of d10 is changed, at 4.2°K, from the 300°K structure having two centrosymmetric molecules per unit cell. For the first time for organic crystals we report the phonon-induced spectrum: These spectra are electric dipole dipole sidebands of the magnetic zero-phonon transitions.

The single-pulse photoacoustic technique was used to measure cross sections for IR multiphoton absorption. A discussion of this technique and the applicable pressure range is given. The source of the signals from the microphone was identified. The first peak is proportional to the energy absorbed by the gas in the focal region, while the subsequently arriving signals correspond to a mixture of the signal from the entrance and exit volumes and the ringings of the microphone and the gas cell.

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