Organic peroxy radicals: Kinetics, spectroscopy and tropospheric chemistry

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

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

The observation of coherent dynamics ensuing from the excitation of molecular systems by femtosecond laser pulses is at the heart of femtochemistry. The time-dependent evolution of coherent superpositions of quantum states, the physical basis for the observation of coherent dynamics and their manipulation are of central importance to coherent control of physicochemical processes. In the early days of femtochemistry, there was significant skepticism regarding the type of information that could be learned from spectroscopic experiments using very short pulses. There were arguments that one could infer the dynamics from frequency-resolved experiments and that the femtosecond experiments did not offer new information. It was also assumed that experiments with very short pulses would smear the available spectroscopic information because of their broad bandwidths. Approximately two decades after the initial experiments, it has become clear that femtosecond experiments have opened an extremely valuable window into the dynamic behavior of atomic and molecular systems that is influencing how we think about physics, chemistry, and biology. In particular, we highlight the fact that some of the highest resolution spectroscopic measurements being carried out employ femtosecond laser pulses. These experiments, specifically those taking advantage of rotational coherence, provide resolution that rivals microwave spectroscopy. The reason spectroscopic information is not lost in femtochemistry experiments is coherence, a property that can be manipulated to control physicochemical processes by a number of different approaches, which will be reviewed here. This review presents a summary of some of the most salient contributions to the field of coherent laser control from an experimentalist’s perspective. While a number of theoretical papers have made key contributions to the field, it is from the experimental successes, as well as failures, that we can best learn how to implement new strategies and develop future applications. Coherent laser control, in the context of this review, encompasses experiments in which the coherent properties of the laser and/or the molecule are required for controlling a particular physicochemical process. We distinguish for each of the experiments between coherence in the laser field(s), * To whom correspondence should be addressed. Phone (517) 3559715 (ext-315). Fax (517) 353-1793. E-mail dantus@msu.edu. 1813 Chem. Rev. 2004, 104, 1813−1859

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Experimental coherent laser control of physicochemical processes.

Collisional Quenching of CH(A), OH(A), and NO(A) in Low Pressure Hydrocarbon Flames

We have measured the threshold photoelectron spectra of HCl and DCl with varying resolution (3–40 meV) using tunable synchrotron radiation and employing the penetrating-field electron-spectroscopic technique over a wide photon energy range (up to 34 eV) that includes the inner-valence ionization region in the molecules. The observed spectra display features produced by both direct ionization and autoionization processes. Autoionization is found to play a particularly important role in the outer-valence ionization region of these molecules. For example, in the formation of the v+=0 band of the X  ( 2 Π 3/2 ) subsystem, in the populating of high vibrational levels of the X  ( 2 Π i ) system in the Franck–Condon gap region between the X  ( 2 Π i ) and A  ( 2 Σ + ) states, and in the predissociated levels of the A  ( 2 Σ + ) state. The production of extended vibrational structure in the X  ( 2 Π i ) system is attributed to resonance autoionization of 4sσ, 5sσ and 6sσ 1 Σ + Rydberg states. These and other threshold photoelectron results suggest a propensity for threshold electron production from sσ Rydberg states. Photoionization of HCl and DCl in the inner-valence ionization region between 25 and 30 eV yields nine vibrationally resolved band systems that include the `main line' 3sσ−1 band system and four sets of pairs of vibrationally very similar satellite ion systems. Absolute vibrational numbering in these systems was greatly facilitated by having the isotopomer threshold photoelectron spectra.

Threshold photoelectron spectroscopy of HCl and DCl

Time-resolved kinetic studies of electronically excited CH radicals II. Quenching efficiencies for CH(A 2Δ)

A detailed study of the vacuum ultraviolet fluorescence excitation spectrum of I2 in the region 110–210 nm is reported. Both absorption and fluorescence excitation spectra were recorded simultaneously using continuously tunable synchrotron radiation. At long wavelengths (176–210 nm) the fluorescence excitation spectrum is dominated by the D(0+u) ion‐pair state. Perturbations between the D(0+u) state and isoenergetic levels of the c6 Rydberg state lead to pronounced resonances (dips) in the fluorescence excitation spectrum. Weaker absorption to the F(0+u) and F’(0+u) ion‐pair states is identified at 170 and 150 nm, respectively. Below 149 nm electronically excited iodine atoms (I 6s 4P5/2 and I 6s 2P3/2) are formed by predissociation of high‐lying Rydberg and ion‐pair states.

Vacuum ultraviolet fluorescence excitation spectrum of I2

Resonance fluorescence study of electronically excited sulphur atoms: reactions of S(31D2)

Both one‐ and two‐color zero kinetic energy‐pulsed field ionization (ZEKE‐PFI) spectra of the first electronically excited state of I+2 (A 2Π3/2,u) as well as a new electronic state, the a 4Σ−u state, have been recorded for the first time In the one‐color (two photon) experiment, transitions to the quartet state are formally spin‐forbidden and this is reflected in the weak transition intensity observed compared with the A 2Π3/2,u state However, in the two‐color (1+2′) experiment, which used the valence B 3Π0+u state as an intermediate, transitions into both A 2Π3/2,u and a 4Σ−u states are fully allowed and appear in the spectrum with comparable intensity The a 4Σ−u state appears in the one‐color experiment by virtue of spin–orbit coupling with excited electronic states for which direct ionization from the neutral ground state is fully allowed Values for ωexe of 046±001 cm−1 for the A 2Π3/2,u state and 038±002 cm−1 for the a 4Σ−u state were derived, together with lower limits for ωe of 138±2 and 

Zero kinetic energy pulsed field ionization (ZEKE‐PFI) spectroscopy of electronically and vibrationally excited states of I+2: The A 2Π3/2,u state and a new electronic state, the a 4Σ−u state

R.J. Donovan

Papers

Threshold photoelectron spectroscopy of HCl and DCl

Time-resolved kinetic studies of electronically excited CH radicals II. Quenching efficiencies for CH(A 2Δ)

Vacuum ultraviolet fluorescence excitation spectrum of I2

Resonance fluorescence study of electronically excited sulphur atoms: reactions of S(31D2)

Zero kinetic energy pulsed field ionization (ZEKE‐PFI) spectroscopy of electronically and vibrationally excited states of I+2: The A 2Π3/2,u state and a new electronic state, the a 4Σ−u state