Project

Discipline

High-resolution spectroscopy; Combustion; Radicals; Discharge; Four-wave mixing; Molecular beam; Nonlinear spectroscopy

Lead
Spektroskopische Charakterisierung von RadikalenIn diesem Forschungsgesuch werden spektroskopische Experimente an transienten Molekülen beschrieben, die einerseits für die Verbrennungsdiagnostik von grosser Bedeutung sind und deren thermodynamischen Eigenschaften andererseits aufgrund der Zustandssumme näher bestimmt werden können.
Lay summary
Inhalt und Ziel des Forschungsprojektes Das übergeordnete Ziel ist die Kraftstoff-Physik und -Chemie besser zu verstehen, um die Effizienz von Verbrennungsprozessen zu steigern und die Entstehung von Schadstoffen (NOx, Russ etc) zu reduzieren, bzw. zu verhindern. Reaktionsmechanismen von kleinen transienten Molekülen/Radikalen spielen in diesem Zusammenhang eine ausschlaggebende Rolle. Eine spektroskopische Bestimmung der quantenchemischen und -physikalischen Eigenschaften gibt Aufschluss über die Stabilität und Dynamik dieser Teilchen und ihr Verhalten unter hohem Druck und Temperatur. Die gemessenen Daten werden in Modellen der Verbrennung und für die Diagnostik von technischen Prozessen benötigt. Zusätzlich sind kleine, aus wenigen Atomen bestehende Radikale auch auf anderen Gebieten, wie in der Atmosphärenchemie und Astronomie ausschlaggebend und gewonnene Informationen können wesentlich zum Verständnis von Fragen wie etwa dem antropogenen Einfluss auf die Luft oder der Chemie des Weltalls beitragen. Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojektes Weil immer noch ca. 80% des Energiebedarfs weltweit mit fossilen Energieträgern gedeckt wird, ist ein effizienter und schadstoffarmer Umgang mit den schwindenden Ressourcen von grosser Bedeutung. Zum Verständnis von Verbrennungsprozessen sind kleine Radikale, bestehend aus wenigen Atomen ausschlaggebend. Weil diese Radikale sehr reaktiv und kurzlebig sind, ist die Bestimmung der thermodynamischen Eigenschaften mit konventionellen Methoden schwierig, weshalb wesentliche Daten fehlen.

Direct link to Lay Summary

Last update: 26.03.2013

Number	Title	Start	Funding scheme

This continuation proposal describes non-linear spectroscopic experiments on combustion relevant transient species that are required for laser diagnostics in technical combustion processes and to determine thermodynamic functions via the partition function and statistical mechanical relationships. In contrast to stable molecules, direct measurements of thermodynamic properties of radicals are generally impossible due to the high reactivity and transient character of these molecules.Substantial advances have been achieved in the framework of the preceding proposal that has been initiated at the beginning of 2010. Our expertise and continuing improvements of the four-wave mixing spectroscopy and the generation of radicals have resulted in several publications within this project. Even though the non-linear spectroscopic method is quadratically dependent on the number density, we have shown that high signal-to-noise ratios are achievable for optical-optical double-resonance measurements in the low density environment of the molecular beam. Among other works, we have exploited the high sensitivity and selectivity of the method to perform deperturbation studies of the Swan band transitions of the carbon dimer. This electronic system is widely used to detect the molecule in combustion studies to test and characterize chemical mechanisms. In addition, the deperturbation unraveled a new high-spin state and solved major issues of a longstanding problem of the carbon dimer spectroscopy.Further results have been obtained on the carbon trimer and on the dissociation of hydrogen peroxide. Both molecules are important combustion intermediates.C3 is highly abundant in fuel-rich flames and can react with unsaturated hydrocarbons to form hydrogen-deficient free radicals that can be important in the formation of soot particles. In addition, recent calculations suggest stable cyclic tricarbon configurations exhibiting low lying potential energy surfaces that might be relevant in the combustion chemistry. The computation emphasize the importance of triplet states in this respect. These triplet states, however, are in general not accessible by optical transitions from a singlet ground state and, therefore, little information is available. We have applied, for the first time, perturbation assisted optical-optical double resonance (PFOODR) spectroscopy on a polyatomic molecule to obtain information on the triplet manifold of the carbon trimer. To finalize the subject and to assign the observed transitions to the triplet states, further analysis of the spectra is required and ab initio structure calculations have to be performed.The dissociation of hydrogen peroxide into two OH radicals has been identified as one of the key reactions in the autoignition process leading to engine knock. We have applied recently polarization resolved degenerate four-wave mixing for the first time to quantify nascent anisotropic distributions in a photodissociation process. It is shown that the non-linear method is advantageous to earlier LIF studies because the anisotropy parameters are obtained from a background free method. The theoretical treatment of the four-wave mixing process to obtain information on the photolysis pathway, namely, the correlations between the transition dipole moment and the orientations of the recoil velocities and the angular momenta of the products is complex and further work is required to analyze the intensities and spectral shapes that have been observed. Furthermore, experiments are planned to implement the modified radical source at the VUV beamline of the Swiss Light Source. Photoelectron-photoion coincidence (PEPICO) measurements on mass selected radicals emerging from this source would yield accurate appearance potentials of hydrocarbons of the general formula, CxHy.