An overview is given of recent developments in laser diagnostic methods for the quantitative measurement of trace species concentrations and, in conjunction, of temperature in combustion systems. After a short introduction illustrating some experiments from the pre-laser era, the article presents typical applications and discusses advantages and limitations of laser techniques including laser absorption, linear, saturated, predissociative and multi-photon-excited laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), electronically resonant coherent anti-Stokes Raman scattering (resonance CARS), degenerate four-wave mixing (DFWM) and amplified spontaneous emission (ASE). Recent trends including two-dimensional imaging, multi-species detection and high-pressure applications will also be discussed. Throughout the article, an attempt is made to present typical results from a large portion of the relevant technical literature. A concluding section gives a short summary of the current status and comments on the perspectives for further research.

Laser Techniques for the Quantitative Detection of Reactive Intermediates

The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.

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Gas temperature determination from rotational lines in non-equilibrium plasmas: a review

Laser techniques for the quantitative detection of reactive intermediates in combustion systems

Controlling chemical reactions with light rests on the idea of exciting a vibration that becomes the reaction coordinate in subsequent chemistry. Vibrational excitation techniques such as infrared or stimulated Raman excitation of fundamental vibrations or vibrational overtone excitation of higher levels permit the preferential cleavage of a bond in a photodissociation or bimolecular reaction. The key to bond-selected chemistry is the initial preparation of a suitable vibrational state followed, in the case of bond-selected photodissociation, by electronic excitation or, in the case of bond-selected bimolecular reaction, by collision with a reactive atom. Such experiments demonstrate bond-selected chemistry, permit detailed comparison to theory, and reveal general principles of vibrational state control of chemical reactions.

Bond-Selected Chemistry: Vibrational State Control of Photodissociation and Bimolecular Reaction

The last few years have seen a surge in interest (both theoretical and experimental) in the photochemistry of heteroaromatic molecules (e.g. azoles, phenols), which has served to highlight the importance of dissociative excited states formed by electron promotion to σ* molecular orbitals. Such excited states—which, for brevity, are termed πσ* states in this Perspective article—may be populated by direct photo-excitation (though the transition cross-sections are intrinsically small), or indirectly, by non-adiabatic coupling from an optically ‘bright’ excited state (e.g. an excited state resulting from π* ← π excitation). The analogous πσ* excited states in prototypical hydride molecules like H2O and NH3 have long been recognised. They have served as test-beds for developing concepts like Rydbergisation, conical intersections (CIs) between potential energy surfaces, and for investigating the ways in which non-adiabatic couplings at such CIs influence the eventual photofragmentation dynamics. This Perspective article seeks to highlight the continuity of behaviour revealed by the earlier small molecule studies and by the more recent studies of heteroaromatic systems, and to illustrate the photochemical importance of πσ* excited states in many broad families of molecules. Furthermore, the dynamical influence of such excited states is not restricted to closed shell species; the Article concludes with a brief consideration of the consequences of populating σ* orbitals in free radical species, in molecular cations, and in dissociative electron attachment processes.

πσ* excited states in molecular photochemistry

The photodissociation of water in the first absorption band, H{sub 2}O(X) + {Dirac_h}{omega} {yields} H{sub 2}O(A{sup 1}B{sub 1}) {yields} H({sup 2}S) + OH({sup 2}II), is a prototype of fast and direct bond rupture in an excited electronic state. It has been investigated from several perspectives-absorption spectrum, final state distributions of the products, dissociation of vibrationally excited states, isotope effects, and emission spectroscopy. The availability of a calculated potential energy surface for the A state, including all three internal degrees of freedom, allows comparison of all experimental data with the results of rigorous quantum mechanical calculations without any fitting parameters or simplifying model assumptions. As the result of the confluence of ab initio electronic structure theory, dynamical theory, and experiment, water is probably the best studied and best understood polyatomic photodissociation system. In this article we review the joint experimental and theoretical advances which make water a unique system for studying molecular dynamics in excited electronic states. We focus our attention especially on the interrelation between the various perspectives and the correlation with the characteristic features of the upper-state potential energy surface. 80 refs., 14 figs.

Photodissociation of water in the first absorption band: A prototype for dissociation on a repulsive potential energy surface

We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the A state and a calculated SCF dipole moment function for the X→A transition. The dynamical calculations are performed within the infinite‐order‐sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the A state potential energy surface and details o...

Photodissociation dynamics of H2O and D2O in the first absorption band: A complete abinitio treatment

We report a detailed investigation of rotationally inelastic effects in the photodissociation of water in the first absorption band using a recently calculated ab initio potential energy surface of the A 1B1 state. Although the excited state potential anisotropy is large it has only very weak influence on the rotational state distributions which thus simply reflect the angular behavior of the ground state wave function of the parent molecule. The reason is that both potential energy surfaces have roughly the same equilibrium angle. Strong inelastic effects are observed, however, for dissociation out of excited bent states because the corresponding ground state wave function extends over a considerably wider angular range and thus the more anisotropic regions of the excited state potential are probed. Calculations are performed on three levels of accuracy for the continuum wave function: close‐coupling, coupled‐states (CS), and infinite‐order‐sudden (IOS) approximation. The ground state wave function is treated numerically exactly. The CS approximation is found to be extremely reliable for those cases studied in this work. The accuracy of the IOS approximation depends very much on the region of orientation angle probed by the ground state and therefore a general conclusion is not possible. Finally, OH rotational state distributions obtained from the dissociation of water and from H–OH scattering at equivalent translational energies are compared and found to be extremely different.

Rotational state distributions in the photolysis of water: Influence of the potential anisotropy

The C→A emission spectra for H2O and D2O are measured and calculated The theoretical model is based on an exact treatment of the dissociation dynamics of the A state using a calculated potential energy surface Agreement with the measurements is excellent The spectra extend from λ∼380 nm up to λ∼600 nm with maxima around 425 (H2O) and 440 nm (D2O)

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V. Engel

Papers

Photodissociation of water in the first absorption band: A prototype for dissociation on a repulsive potential energy surface

Photodissociation dynamics of H2O and D2O in the first absorption band: A complete abinitio treatment

Rotational state distributions in the photolysis of water: Influence of the potential anisotropy

The C̃→Ã emission in water: Theory and experiment