This paper reviews the theory and applications of photoacoustic (also called optoacoustic) methods belonging to the more general area of photothermal measurement techniques. The theory covers excitation of gaseous or condensed samples with modulated continuous light beams or pulsed light beams. The applications of photoacoustic methods include spectroscopy, monitoring deexcitation processes, probing physical properties of materials, and generating mechanical motions. Several other related photothermal methods, as well as particle-acoustics and wave-acoustics methods are also described. This review complements an earlier and narrower review [Rev. Mod. Phys. 53, 517 (1981)] that is mainly concerned with sensitive detection by pulsed photoacoustic spectroscopy in condensed matter.

Applications of photoacoustic sensing techniques

An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn–Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step. For an accurate description of the hydrogen bonding in the liquid, it was necessary to extend the exchange functional with a term that depends on the gradient of the electron density. A further important technical detail is that supersoft pseudopotentials were used to treat the valence orbitals of the oxygen atoms in a plane wave expansion. The structural and dynamical properties of the liquid were found to be in good agreement with experiment. The ab initio molecular dynamics also yields information on the electronic structure. The electronic feature of special interest is the lowest unoccupied molecular orbital (LUMO) of the liquid which is the state occupied by a thermalized excess electron in the conductive state. The main result of calculating the liquid LUMO is that it is a delocalized state distributed over interstitial space between the molecules with a significant admixture of the σ* orbitals of the individual water molecules.

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‘‘Ab initio’’ liquid water

A theoretical model for electron‐impact ionization cross sections, which has been developed primarily for atoms and atomic ions, is applied to neutral molecules. The new model combines the binary‐encounter theory and the Bethe theory for electron‐impact ionization, and uses minimal theoretical data for the ground state of the target molecule, which are readily available from public‐domain molecular structure codes such as GAMESS. The theory is called the binary‐encounter Bethe (BEB) model, and does not, in principle, involve any adjustable parameters. Applications to 19 molecules, including H2, NO, CH2, C6H6, and SF6, are presented, demonstrating that the BEB model provides total ionization cross sections by electron impact from threshold to several keV with an average accuracy of 15% or better at the cross section peak, except for SiF3. The BEB model can be applied to stable molecules as well as to transient radicals.

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New model for electron-impact ionization cross sections of molecules

Gold surfaces modified with C3−C18-alkanethiols (CH3(CH2)X-1SH; HXSH; x = 3, 8, 12, 16, 18) and C16-alkanethiols, fluorinated at the outer 1, 2, 4, and 10 methylene positions (CF3(CF2)Y-1(CH2)XSH; FyHxSH; y = 1, x = 15; y = 2, x = 14; y = 4, x = 12; y = 10, x = 6) were characterized by He(I) UV-photoelectron spectroscopy (UPS). (Detailed X-ray photoelectron spectroscopic characterization of the partially fluorinated thin films is given in the Supporting Information). Long incubation times of the gold surface with the alkanethiol solutions lead to compact monolayer films for all of the alkanethiols, as indicated by the exponential decrease in emission intensity versus alkyl chain length for both the gold Fermi edge (UPS data), and by a parallel decrease in Au(4f) photoemission intensity using X-ray photoelectron spectroscopy. Changes in the effective work function of these surfaces due to the presence of significant interfacial dipoles are observed (i) as alkyl chain length is increased, and (ii) as the fr...

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Interface Dipoles Arising from Self-Assembled Monolayers on Gold: UV−Photoemission Studies of Alkanethiols and Partially Fluorinated Alkanethiols

Rate constants have been measured by the flowing afterglow technique at 300 °K for the quenching of Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2) by a large number of small molecules. For the same reagent, the magnitudes of the cross‐sections usually increase in the series Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2). The Ar(3P2) and Ar(3P0) data are compared to results in the literature for these states and to data for Ar(3P1) and Ar(1P1). The set of thermal quenching cross sections are used to test the correlations between the magnitudes of the cross sections and properties of the reagents as predicted by the orbiting, absorbing‐sphere, golden rule, and curve‐crossing mechanisms for quenching. The best correlation is between the cross sections and the C6 coefficient. The analysis supports the proposition that the orbiting‐controlled, curve‐crossing model is the general mechanism governing the magnitude of the thermal cross sections for quenching of the metastable states. This model explains the very large quenching cross sections of F2 and OF2 (relative to other molecules composed of first row elements) because covalent–ionic curve crossing occurs outside the conventional orbiting radius. The validity of the simple van der Waals dispersion forces as being the dominant entrance channel interaction between the excited state rare gas atoms and the reagents is discussed.

Rate constants and quenching mechanisms for the metastable states of argon, krypton, and xenon

Handbook of He(I) photoelectron spectra of fundamental organic molecules : K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, 1981, Japan Scientific Societies Press, Tokyo, ISBN 4-7622-0263-X and Halsted Press, New York, 268 pp., US $44.95, ISBN 0-470-27200-7.

Fluorination effects on the inner-shell spectra of unsaturated molecules

Excited electronic states of the simple alcohols

The electron impact spectra of the fluoromethanes

It is proposed that Rydberg and valence σ* conjugate orbitals have separate existences and can be seen in the same spectrum if the σ* MO can be disentangled from the Rydberg manifold. Because the energy of the σ* MO is a consequence of the σ–σ* split resulting from bond formation, the spectra of molecules having weak bonds should show low‐lying transitions to σ* in addition to the conjugate Rydberg bands. Inelastic electron scattering spectra in the x‐ray region (270–730 eV) of molecules having bond strengths in the 20–50 kcal/mol regime clearly show well‐isolated transitions to low‐lying σ* MOs, and in some cases the simultaneous presence of virtual σ* and Rydberg conjugate orbitals. The general characteristics of excitations from C 1s, O 1s, and F 1s inner orbitals to σ* MOs are listed and illustrated by the x‐ray spectra of several compounds in which the weak bond involves the O–O or O–F linkage. Quantitative inner‐shell optical oscillator strengths derived from the energy loss spectra are reported for...

M.B. Robin

Papers

Handbook of He(I) photoelectron spectra of fundamental organic molecules : K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki and S. Iwata, 1981, Japan Scientific Societies Press, Tokyo, ISBN 4-7622-0263-X and Halsted Press, New York, 268 pp., US $44.95, ISBN 0-470-27200-7.

Fluorination effects on the inner-shell spectra of unsaturated molecules

Excited electronic states of the simple alcohols

The electron impact spectra of the fluoromethanes

Inner-shell excitations in weak-bond molecules