Multiphoton ionisation spectroscopy of free radical species

Ashfold, Michael N. R.; Clement, Simon G.; Howe, Jonathan D.; Western, Colin M.

doi:10.1039/ft9938901153

Cited by 57 publications

(24 citation statements)

References 131 publications

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“…A further photon leads to ionization directly from the excited state. By introducing an anode into the flame the electrons ejected from the molecule can be detected [43]. REMPI is a good alternative to detect species that can absorb radiation but do not fluoresce and hence are poor candidates for LIF measurements.…”

Section: Techniques Employing Lasersmentioning

confidence: 99%

Laser diagnostics and minor species detection in combustion using resonant four-wave mixing

Kiefer

Ewart

2011

Progress in Energy and Combustion Science

131

112

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Laser-based methods have transformed combustion diagnostics in the past few decades. The high intensity, coherence, high spectral resolution and frequency tunability available from laser radiation has provided powerful tools for studying microscopic processes and macroscopic phenomena in combustion by linear and nonlinear optical processes. This review focuses on nonlinear optical techniques based on resonant four-wave mixing for non-intrusive measurements of minor species in combustion. The importance of minor species such as reaction intermediates is outlined together with the challenges they present for detection and measurement in the hostile environments of flames, technical combustors, and engines. The limitations of conventional optical methods for such measurements are described and the particular advantages of coherent methods using nonlinear optical techniques are discussed.The basic physics underlying four-wave wave mixing processes and theoretical models for signal calculation are then presented together with a discussion of how combustion parameters may be derived from analysis of signals generated in various four-wave mixing processes. The most important four-wave mixing processes, in this context, are then reviewed:Degenerate Four-Wave Mixing (DFWM), Coherent Anti-Stokes Raman Scattering (CARS), Laser Induced Grating Spectroscopy (LIGS) and Polarization Spectroscopy (PS). In each case the fundamental physics is outlined to explain the particular properties and diagnostic advantages of each technique. The application of the methods mentioned to molecular physics studies of combustion species is then reviewed along with their application in measurement of concentration, temperature and other combustion parameters. Related nonlinear techniques and recent extensions to the ultra-fast regime are briefly reviewed. Finally practical considerations relevant to multi-dimensional and multi-species measurements, as well as applications in technical combustion systems are discussed.Keywords: nonlinear optical spectroscopy, four-wave mixing, combustion diagnostics, minor species detection, laser-induced gratings, polarization spectroscopy IntroductionEnergy from renewable sources such as solar and wind power is the goal of much current research and technological development. Combustion, however, is and is likely to remain for the foreseeable future, the most important energy source for heat, electrical power and especially for transportation. In technical combustion systems the chemical energy of fossil fuels such as coal, crude oil and natural gas is converted into heat through oxidation with oxygen from air. However, fossil fuel resources are limited and alternatives like biofuels [1] or novel technologies such as fuel cells [2] are not yet able to meet the demand. Consequently, for a sustainable use of fossil fuels it is desirable to further improve the efficiency, maintainability and reliability of existing combustion devices. Achieving such improvements depends upon an improved understanding of t...

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Section: Techniques Employing Lasersmentioning

confidence: 99%

Laser diagnostics and minor species detection in combustion using resonant four-wave mixing

Kiefer

Ewart

2011

Progress in Energy and Combustion Science

131

112

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“…When a molecule is placed in an intense visible or UV laser field, multiphoton absorptions may occur and population of high energy states is possible. [31][32][33][34][35][36][37] Molecules placed in this field may continue to absorb photons until the molecule loses its energy by emission, ionization, decomposition or radiationless transition. 32,37 The mechanisms of multiphoton ionization and dissociation have been characterized by two different pathways: ladder climbing and ladder switching.…”

Section: Mass Spectra At Energies Corresponding Tomentioning

confidence: 99%

Wavelength Dependence of Photooxidation vs Photofragmentation of Chromocene

2001

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Photooxidation and metal-ligand photolysis reactions of bis(cyclopentadienyl)chromium, chromocene, in the range 24 390-15 630 cm -1 are studied in the gas phase by using time-of-flight mass spectroscopic detection. Photooxidation of the intact chromocene molecule unexpectedly dominates in the range 23 530-24 000 cm -1 . The relative importance of photooxidation compared to photofragmentation is strongly wavelength dependent. A prominent species at all wavelengths is the chromium ion, but in a wavelength region corresponding to the lowest energy ligand to metal charge transfer excited electronic state absorption, the strongest peak is from the chromocene ion. The excitation spectra are reported for three selected species: chromocene ion, mono-(cyclopentadienyl)chromium ion, and the chromium ion. The spectrum obtained by monitoring the metal ion contains sharp peaks that are assigned to neutral chromium atom resonances. Sharp losses of intensities in the molecular ion spectra are observed at these wavelengths. The wavelength dependencies of the photoreactions are interpreted and explained in terms of the identity of the initially populated excited electronic state and the ionization energy of the molecule. When the initially populated excited electronic state is the ligand to metal charge transfer state, the first photon causes minimal bond weakening and the second photon excites the intact chromocene above the ionization energy, resulting in efficient ionization of the parent molecule. When the initially populated state is a lower energy ligand field excited electronic state, bond weakening occurs and absorption of a second photon results in significant photodissociation producing intense fragment peaks dominated by the metal ion peak.

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“…͑The dissociation dynamics have been studied extensively and the photofragment velocity distribution and translational anisotropy have been fully determined. [32][33][34][35] Although the H atom distribution generated in the photodissociation of HBr is bimodal, due to the 20% yield of Br*, the slow velocity component makes negligible contribution to reaction due to the rapidly increasing excitation function for the HϩCO 2 reaction.͒ The most probable collision energy in the center of mass was 241 kJ mol Ϫ1 ͑2.5 eV͒ with a full width at half maximum ͑FWHM͒ of 15 kJ mol Ϫ1 ͑0.15 eV͒ for a sample of CO 2 gas at 298 K. The absorption cross section of CO 2 at 193.3 nm is negligible. 36 After a delay of 75-110 ns, short enough to allow reaction ͑1͒ to proceed under single collision conditions, the scattered OH(Ј,NЈ,⍀, f ) fragments were detected via LIF, using the ⌬Јϭ0 sequence of the A 2 ⌺ ϩ ←X 2 ⌸ band system.…”

Section: Methodsmentioning

confidence: 98%