Peter Andresen

Laser-induced fluorescence with tunable excimer lasers as a possible method for instantaneous temperature field measurements at high pressures: checks with an atmospheric flame

A new method for instantaneous temperature field measurements based on LIF studies of OH, O(2), and H(2)O in an open atmospheric flame with a tunable excimer laser is suggested. In this method the crucial problem of quenching at higher pressures is almost completely eliminated by excitation to a fast predissociating state. The various possible excitation and fluorescence processes that can be induced in the narrow tuning range of the KrF laser are characterized experimentally by excitation and dispersion spectra for the three molecules OH, O(2), and H(2)O. Of particular importance is the large power of the KrF laser, which allows efficient excitation of even weak transitions. The fast predissociation of these molecules in connection with the powerful excitation laser suggests that instantaneous temperature field measurements should be possible at higher pressures.

Nuclear and electron dynamics in the photodissociation of water

The photodissociation of water in its first absorption band is studied by photolyzing H2O at 157 nm with an excimer laser. This dissociation proceeds directly to produce the electronic ground states of H and OH. Both nascent internal state distributions and alignment of the product OH (2Π) are probed by laser induced fluorescence. This is done with both warm (300 K) and cold (∼10 K) water. About 88% of the excess energy is translation, 10% vibration, about 2% rotation. The first three vibrational levels 0, 1, 2 have population ratios 1:1:0.15, respectively. The rotational distributions depend strongly upon the H2O temperature and are very different for the upper and lower energy components of the Λ doublets, which are measured via Q and P, R lines, respectively. For Q lines, the distributions can be described by rotational temperatures which are 930 K for warm and 475 K for cold water, a surprising difference. For P,R lines strong deviations from Boltzmann behavior are found for cold H2O. The spin distrib...

Fluorescence imaging inside an internal combustion engine using tunable excimer lasers

Tunable excimer lasers are used to obtain 2-D images of molecular (and some state-specific) density distributions inside a cylinder of a modified four-cylinder in-line engine that has optical access. Natural fluorescence (i.e., without a laser) is used for some OH pictures, normal laser-induced fluorescence (LIF) for those of NO and of the isooctane fuel, and laser-induced predissociative fluorescence (LIPF) for other OH pictures and for those of O(2). Relevant spectroscopy is done to find the laser and fluorescence frequencies needed to measure isolated species. LIPF works well at high pressures, is state specific, and is ideally suited to follow turbulent processes. No similar measurements in engines have been previously reported. Pictures are taken in succeeding engine cycles. Their sequence is either at a particular point of the engines cycle to show cyclic fluctuations, or at succeeding portions of the cycle to illustrate the progress of the gasdynamics or of the combustion.

State to state photodissociation of H2O in the first absorption band

The photodissociation of H2O in the first absorption band is studied from single rotational states of vibrationally excited water. A tunable IR laser is used to prepare single rotational states in the asymmetric stretch mode. The subsequent photodissociation at 193 nm favors product formation from these single prepared states. The formation of the OH product in different rotational, Λ‐doublet, and spin states is analyzed for a series of initial rotational states of H2O. This is the first direct photodissociation studied on a state to state level. The product state distributions depend sensitively upon the prepared state in the parent molecule H2O and exhibit pronounced quantum structure. The experimental results are understood almost quantitatively in terms of theory. The photodissociation of water turns out to be a limiting case of a dissociation which is governed by transfer of parent motion to products. The experiment leads to a highly improved understanding for the selective population of Λ‐doublet st...

Resolution of interference effects in the rotational excitation of NO (N = O) by Ar

Pulsed nozzle sources are used to analyze the rotational excitation of NO by Ar. Integral cross sections are determined and compared with the results of a coupled‐state calculation. (AIP)

Electronic fine structure transitions and rotational excitation in NO rare gas collisions

The excitation of rotationally cold NO in the ( j=1/2,2Π1/2) state by collisions with different rare gases is studied for multiplet conserving and multiplet changing transitions. In the crossed beam experiment we use jet cooling to prepare the cold NO and LIF to measure the state distribution of the collisionally excited NO in the scattering center. The measured integral state to state cross sections are compared to theory. The calculations are performed in the CS approximation and based on recent advances in treating open shell molecules. For multiplet conserving transitions the agreement between experiment and theory is good. Although for the multiplet changing collisions the general structure is in qualitative agreement, the strength of the Ω=1/2→3/2 transitions is underestimated in the theory.

Analysis of chemical dynamics via Λ doubling: Directed lobes in product molecules and transition states

Many molecular processes, such as chemical reactions, inelastic collisions, photodissociation, and surface scattering, yield selective populations of Λ‐doublet states. The interpretation of such experiments has been difficult, so that little quantitative information could be extracted. Based on the results of our experiment on the photodissociation of H2O, and upon calculations presented here, this situation becomes much better. We discuss processes which yield product 2Π diatomics XY in which the Λ doublets are a manifestation of two different orientations of an unpaired pπ electron orbital. Chemical dynamics which produce such selective populations indicate a stereochemical effect: for example, the unpaired pπ orbital in the XY may point in the direction of the previous bond between XY and the transition complex, and, in an energetic breakup, would have a spatial relationship to J, the total angular momentum of XY. We describe here: (a) the directions of the orbitals at high J, (b) the effect of J, and ...

Strong propensity rules in the photodissociation of a single rotational quantum state of vibrationally excited H2O

It is pointed out that the photodissociation of H2O in the first absorption band is an ideal process for studies which can lead to an understanding of the basic principles of simple fragmentation phenomena. With modern quantum methods, this fragmentation can be studied on an ab initio basis, and theoretical predictions can be compared with experimental results. The present investigation is concerned with the preparation of a single rotational state in an excited vibrational state, taking into account a procedure based on the use of a tunable IR laser. The conducted experiment demonstrates that photodissociation of single, selectable states is possible. New insight is obtained with respect to fine details of simple fragmentation processes. 10 references.

Bimodal velocity distributions after ultraviolet-laser-induced desorption of NO from oxide surfaces. Experiments and results of model calculations

After UV‐laser‐induced desorption we observe bimodal velocity distributions independent of internal vibrational excitation [up to v=2 (4%)] applying resonance‐enhanced multiphoton ionization techniques. Both contributing desorption channels are of nonthermal origin. We introduce a model where the two desorption channels are correlated with the rupture of the molecule surface bond of the librating molecule either on the way toward or away from the surface. We have performed trajectory calculations to simulate the desorption processes. The calculated momentum distributions of the desorbing molecules show either one or two maxima, depending on lifetime, in agreement with experimental results. The vibrational distribution of the desorbing molecules can be reproduced by assuming transition into a state that is characterized by an altered N–O bond length as it is found, for example, in NO−. The model calculations both for velocity distributions and vibrational excitations result in similar lifetimes of the exci...

Spatially Resolved Raman Scattering for Multi-Species and Temperature Analysis in Technically Applied Combustion Systems: Spray Flame and Four-Cylinder In-Line Engine

Spatially resolved Raman scattering is used to measure the single shot stoichiometry before ignition inside a realistic internal combustion engine with high single shot precision of l%–4% (depending on the extent of spatial averaging). The high precision results from the simultaneous detection of fuel and N2 (O2), which yields stoichiometry via a relative measurement. The cycle-to-cycle fluctuations of stoichiometry are clearly resolved. The feasibility of averaged spatially resolved simultaneous multi-species detection is demonstrated in a commercial oil-burning furnace as well. The limited precision that is usually obtained in Raman scattering by interfering emissions is highly improved using the fact that the interfering emission is unpolarized whereas Raman scattering is highly polarized. Therefore, Raman measurements provided good signal-to-noise ratios in the spray flame even in the area where fuel droplets occur and during combustion in the engine. The optical multichannel analyzer yields one-dimensional spatial resolution, and offers the capability to easily combine Raman scattering with Rayleigh scattering and laser-induced fluorescence detection of minority species.