Fulgides for Memories and Switches.

The chemistry of the actinide elements

Introduction Photoacoustic (sometimes called optoacoustic) and photothermal methods measure the heat emitted by radiationless processes deactivating excited species. These methods are complementary to other photophysical and photochemical methods and most of them have the advantage of being applicable also to non-transparent and to highly scattering media. The basis of photoacoustic spectroscopy (PAS) and its applications have been extensively reviewed (Brisset, 1983; Pao, 1977; Rosencwaig, 1980; Tam, 1983) as well as the uses in biology (Cahen et al., 1980; Moore, 1983). Selected examples of the applications of PAS to biology have been presented by Balasubramanian and Rao (1981, 1986). I also refer the reader to communications on the subject presented at the Third International Conference on Photoacoustic and Photothermal Spectroscopy, Paris, France, 1983 (Biichner et al., 1983; Carpentier et al., 1983; Giese and Kolmel, 1983; Jabben et al . , 1983; Kirkbright et al. , 1983; Moore et al . , 1983; Poulet and Chambron, 1983; Renard and Delmelle, 1983; Veeranjaneyulu and Das, 1983; Xiao etal., 1983) and to those presented at the Fourth lnternational Topical Meeting on Photoacoustic, Thermal, and related Sciences, Ville d’Esterel, Quebec, Canada, 1985 (Boucher and Leblanc, 1985; Buschmann and Prehn, 1985; Canaani and Malkin, 1985; Canaani et al., 1985a and c; Carpentier et al. , 1985a; Desormeaux and Leblanc, 1985; Larue e ta l . , 1985; Malkin, 1985; Nicolaus et a[ . , 1985; Pessoa et al . , 1985; Upadhyaya, 1985). This review briefly summarizes the reports on PAS applied to biological samples, especially those published in the last two years. It focuses mainly on the question of the time profile of the heat emission after electronic excitation. In general, instrumentation problems and analytical applications will not be dealt with in depth, apart from some novel contributions. Since the photoacoustic spectrum is essentially the action spectrum of the heat emitted by the sample after the absorption of light (Moore, 1983), the data can be used to determine the energy content of the heat emitting species (vide infra). The knowledge of the heat-time profile permits the eventual determination of lifetime and energy content of short-lived species formed in the course of a reaction, as well as the quantum yield of their production. The techniques also provide information on the thermoelastic properties of the medium. The latter aspect, however, will also be omitted from this review, Two modes can be used for the excitation of the sample. One of them utilizes an intensity-modulated continuous wave (conventional or laser) source, and phase sensitive detection. For the detection of the heat release different transducers may be used. PAS detects the thermally induced pressure wave by means of a gas coupled microphone. Photorefractive techniques detect the induced change of refractive index above or within the sample. If the exciting source is a laser beam, this sensing can be accomplished by observing the changes which the thermal lens induces upon the focus of the excitation beam (thermal-lensing or thermal-blooming) (Fang and Swofford, 1983; Whinnery, 1974). A probe laser beam can also be used to detect the refractive index changes. The general principles of the photorefractive techniques and their applications to absorption measurements have been reviewed by Fang and Swofford (1983). Boccara et al. (1980) and Jackson et al. (1981) have described the collinear and transverse arrangement of the excitation and probe beams and have compared the acoustic and beam deflection detection techniques. An IR detector has been used to detect the heat deposited in the sample (Nordal and Kanstad, 1979). This alternative has been called photothermal detection. In the other mode a pulse of light (normally a pulsed laser) excites the sample. The heat emitted is detected either with a piezoelectric transducer via the acoustic pulse generated (Pate1 and Tam, 1981), with a probe laser beam (Fuke et al . , 1983; Mori et al., 1982; Rossbroich et al . , 1985), or with an infrared detector (Tam, 1985).

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-1097.1986.tb05645.x

Photo acoustic and photothermal methods applied to the study of radiationless deactivation processes in biological systems and in substances of biological interest

Behaviour of U(VI) solids under conditions of natural aquatic systems

Physical applications of photoacoustic spectroscopy

The absorption spectra of thirteen sintered trivalent lanthanide oxides (La to Lu) and six actinide dioxides (Th to Cm) have been measured by photoacoustic spectroscopy. Though both series of elements have in common successive filling of the 4f and 5f shells, respectively, the absorption spectra of the two classes of oxides are very different. While absorption spectra of the thirteen trivalent lanthanide oxides show narrow weak absorption bands and lines in the uv, visible, and near-IR wavelength ranges due to internal 4fn → 4fn transitions between the numerous energy levels of the Xe 4fn configuration, the absorption spectra of the actinide oxides display very intense and broad absorption bands extending from the ultraviolet to the visible and sometimes to the infrared. In the absorption spectra of the lanthanide oxides of higher valencies, such as Tb4O7 or Pr6O11, the sharp absorption lines cannot be detected, as the much stronger 4fn → 4fn−15d transitions overlap the weak 4fn → 4fn transitions. The conclusion is that the 5fn electrons in the actinide oxides are not as well localized as the 4fn electrons of the lanthanide sesquioxides. The 5f electrons probably participate in bonding, e.g., in electron transfer processes from a molecular oxygen orbital to a partly filled or empty 5f electron state of the oxidizing central atom which gives rise to intensive broad absorption bands in the ultraviolet and visible.

A Comparative Study of the Photoacoustic Spectra of Lanthanide and Actinide Oxides

A photoacoustic study of photochromic reactions in the crystalline and adsorbed states

Photoacoustic spectroscopy has been used to measure directly the absorption spectra of powdered actinide oxides (thorium, uranium, neptunium, plutonium, americium and curium). The photoacoustic spectra in the UV, visible and near-IR region were measured with a single beam instrument supplemented by a small glove box which contained the sample cell and the detector. In contrast with the photoacoustic spectra of the lanthanides, which show sharp, narrow absorption bands, the absorption spectra of the actinide oxides reveal quite different spectroscopic properties. All the spectra are characterized by intense and broad absorption bands, especially in the visible and UV ranges. The narrow, weak absorption bands, which are attributed to electron transitions within the 5f shells, cannot be observed in uranium, americium and curium dioxides. In neptunium and plutonium dioxide as well as in americium sesquioxide they are observable, but they are overlapped by broader absorption bands. Most of the 5f-electron levels are significantly hybridized and it seems that covalent metal-oxygen ligand mixing is an important factor. The intense, broad absorption bands in the UV and visible regions may be explained by electron transfer processes between 5f-electron states and oxygen molecular orbitals.

G. Heinrich

Papers

A Comparative Study of the Photoacoustic Spectra of Lanthanide and Actinide Oxides

A photoacoustic study of photochromic reactions in the crystalline and adsorbed states

Photoacoustic spectroscopy of actinide oxides