Absolute absorption and fluorescence measurements over a dynamic range of 106^6 with cavity-enhanced laser-induced fluorescence

We describe a novel experimental setup that combines the advantages of both laser-induced fluorescence and cavity ring-down techniques. The simultaneous and correlated measurement of the ring-down and fluorescence signals yields absolute absorption coefficients for the fluorescence measurement. The combined measurement is conducted with the same sample in a single, pulsed laser beam. The fluorescence measurement extends the dynamic range of a stand-alone cavity ring-down setup from typically three to at least six orders of magnitude. The presence of the cavity improves the quality of the signal, in particular the signal-to-noise ratio. The methodology, dubbed cavity-enhanced laser-induced fluorescence (CELIF), is developed and rigorously tested against the spectroscopy of 1,4-bis(phenylethynyl)benzene in a molecular beam and density measurements in a cell. We outline how the method can be utilised to determine absolute quantities: absorption cross sections, sample densities and fluorescence quantum yields.Comment: 12 pages, 6 figures, submitted to J. Chem. Phy

The dynamics of the hydrogen exchange reaction at 2.20 eV collision energy: Comparison of experimental and theoretical differential cross sections

The H+D2(v = 0, j = 0)→HD(v′, j′)+D isotopic variant of the hydrogen atom exchange reaction has been studied in a crossed molecular beam experiment at a collision energy of 2.20 eV. Kinetic energy spectra of the nascent D atoms were obtained by using the Rydberg atom time-of-flight technique. The extensive set of spectra collected has permitted the derivation of rovibrationally state-resolved differential cross sections in the center-of-mass frame for most of the internal states of the HD product molecules, allowing a direct comparison with theoretical predictions. Accurate 3D quantum mechanical calculations have been carried out on the refined version of the latest Boothroyd-Keogh-Martin-Peterson potential energy surface, yielding an excellent agreement with the experimentally determined differential cross sections. The comparison of the results from quasi-classical trajectory calculations on the same potential surface reveals some discrepancies with the measured data, but shows a good global accordance. The theoretical calculations demonstrate that, at this energy, reactive encounters are predominantly noncollinear and that collinear collisions lead mostly to nonreactive recrossing. The experimental results are satisfactorily accounted for by theoretical calculations without consideration of Geometric Phase effects. © 1999 American Institute of Physics.Financial support through the program ‘‘Acciones para la Incorporación de Doctores y Tecnólogos’’ of the Ministry of Education and Culture of Spain. We also gratefully acknowledge the computer resources ~Cray T-90! provided by the Leibniz Rechenzentrum in Munich ~Germany. Funded by the German Science Foundation ~Grant No. SCH 435/3! and the Deutscher Akademischer Austauchdienst‘‘Acciones Integradas’’. The Spanish part was financed by the DGICYT ~PB95-0918-C03! and by the ‘‘Acciones Integradas’’ Program of the Ministry of Educationand Culture.Peer Reviewe

The H+D2 reaction in the vicinity of the conical intersection

Scattering measurements performed at an energy slightly higher than that of the lowest crossing between the ground and the first electronically excited state of the H3 system are reported. The essentials of the reactive dynamics are describable in terms of a classical motion of the nuclei on the lowest adiabatic electronic potential. © 1997 American Institute of Physics.The German part of this work was supported by the German Science Foundation. The Spanish part was funded by the DGICYT of Spain under Grant PB95-0918-C02. Both parts are grateful to the German–Spanish scientific exchange program ‘‘Acciones Integradas.’’ Partial funding by the Alexander von Humboldt StiftungPeer Reviewe

Absolute fluorescence and absorption measurements over a dynamic range of 106 with cavity-enhanced laser-induced fluorescence

We present a novel spectroscopic technique that exhibits high sensitivity and a large dynamic range for the measurement of absolute absorption coefficients. We perform a simultaneous and correlated laser-induced fluorescence and cavity ring-down measurement of the same sample in a single pulsed laser beam. The combined measurement offers a large dynamic range and a lower limit of detection than either technique on its own. The methodology, dubbed cavity-enhanced laser-induced fluorescence, is developed and rigorously tested against the electronic spectroscopy of 1,4-bis(phenylethynyl)benzene in a molecular beam and density measurements in a cell. We outline how the method can be used to determine absolute quantities, such as sample densities, absorption cross sections, and fluorescence quantum yields, particularly in spatially confined samples

Energieabhängige Untersuchung der Wasserstoffaustauschreaktion

Wrede E. Energieabhängige Untersuchung der Wasserstoffaustauschreaktion. Physik. Lage: Jacobs; 1998

The H+D2 reaction in the vicinity of the conical intersection

Scattering measurements performed at an energy slightly higher than that of the lowest crossing between the ground and the first electronically excited state of the H3 system are reported. The essentials of the reactive dynamics are describable in terms of a classical motion of the nuclei on the lowest adiabatic electronic potential. © 1997 American Institute of Physics.The German part of this work was supported by the German Science Foundation. The Spanish part was funded by the DGICYT of Spain under Grant PB95-0918-C02. Both parts are grateful to the German–Spanish scientific exchange program ‘‘Acciones Integradas.’’ Partial funding by the Alexander von Humboldt StiftungPeer Reviewe

High resolution study of the H+D2 → HD+D reaction dynamics at a collision energy of 2.2 eV

The reactivity of H + D2(v = 0, j = 0) → HD(v1, j1) + D has been investigated in a high resolution crossed molecular beam experiment at a collision energy of 2.2 eV. Time-of-flight (TOF) spectra of D atoms at different laboratory (LAB) scattering angles and their total angular distribution were measured using the technique of Rydberg atom TOP spectroscopy. The resolution of this technique allows the identification of individual rovibrational states of the corresponding HD product molecule at the different LAB angles. A detailed simulation of the experimental measurements has been performed by using state resolved differential cross sections from quasiclassical trajectory (QCT) calculations on the Liu-Siegbahn-Truhlar-Horowitz (LSTH) and on the double-many-body-expansion (DMBE) potential energy surfaces (PES). A general good agreement between experiment and theory is found, although there are some discrepancies which are discussed.Financed by the German Science Foundation under Grants WE 386/19 and SCHN 435/3 and by the Deutscher Akademischer Austauschdienst (Acciones Integradas) 322-Al-e-dr. The Spanish contribution Financed by the DGCIYT under Project No. PB95-0918-C02, and the program 'Acciones Integradas Hispano-Alemanas' of the Ministry of Education of Spain (HA94-135)

New, unexpected, and dominant mechanisms in the hydrogen exchange reaction

A quasiclassical trajectory study of the state specific H+D-2(v=0, j=0)-> HD(v'=0, j'=0)+D reaction at a collision energy of 1.85 eV (total energy of 2.04 eV) found that the scattering is governed by two unexpected and dominant new mechanisms, and not by direct recoil as is generally assumed. The new mechanisms involve strong interaction with the sloping potential around the conical intersection, an area of the potential energy surface not previously considered to have much effect upon reactive scattering. Initial investigations indicate that more than 50% of reactive scattering could be the result of these new mechanisms at this collision energy. Features in the corresponding quantum mechanical results can be attributed to these new (classical) reaction mechanisms