K.-H. Gericke

Imaging chemical reactions – 3D velocity mapping

Visualising a collision between an atom or a molecule or a photodissociation (half-collision) of a molecule on a single particle and single quantum level is like watching the collision of billiard balls on a pool table: Molecular beams or monoenergetic photodissociation products provide the colliding reactants at controlled velocity before the reaction products velocity is imaged directly with an elaborate camera system, where one should keep in mind that velocity is, in general, a three-dimensional (3D) vectorial property which combines scattering angles and speed. If the processes under study have no cylindrical symmetry, then only this 3D product velocity vector contains the full information of the elementary process under study.

Three-dimensional imaging technique for direct observation of the complete velocity distribution of state-selected photodissociation products

We report an experimental technique provided to study the full three-dimensional velocity distribution of state-selected products of a chemical process. Time-of-flight mass spectroscopy and resonance enhanced multiphoton ionization combined with a position sensitive detector (delay-line anode) are employed. The technique has a space resolution of 0.4 mm, a time resolution better than 1 ns, and it provides the possibility to detect several products with a minimal difference between arrival times of 17 ns. One major achievement of the new technique is the possibility to determine the full three-dimensional momentum vectors of a chemical reaction product. This is especially valuable for cases where no symmetry is considered in the process. Second, the high sensitivity of the method allowing to observe single ions enables us to study physical and chemical processes at extremely low densities. Three methods for measuring the temperature of a molecular beam with the technique are demonstrated. A novel result of...

Joint product state distribution of coincidently generated photofragment pairs

The joint product state distribution of coincident fragment pairs formed in the same elementary photodissociation process has been determined. This correlation between quantum state populations of two molecular products has been measured by high resolution Doppler spectroscopy in conjunction with level‐specific detection of the ejected photofragments. One product molecule, formed in a specific quantum state, is excited by laser induced fluorescence and the wing of the corresponding Doppler profile is analyzed to determine the product state distribution of the partner fragment. In the photodissociation of jet‐cooled hydrogen peroxide at 193 nm two OH partner radicals are formed with comparable angular momenta. For a specific rotation of one hydroxyl radical the product state distribution of the partner fragment is centered at about the same rotation with a width of only a few rotational quanta. The orbital angular momentum is of the order of 1 ℏ and the impact parameter is extremely small. Experimentally o...

Three-dimensional velocity map imaging: Setup and resolution improvement compared to three-dimensional ion imaging

For many years the three-dimensional (3D) ion imaging technique has not benefited from the introduction of ion optics into the field of imaging in molecular dynamics. Thus, a lower resolution of kinetic energy as in comparable techniques making use of inhomogeneous electric fields was inevitable. This was basically due to the fact that a homogeneous electric field was needed in order to obtain the velocity component in the direction of the time of flight spectrometer axis. In our approach we superimpose an Einzel lens field with the homogeneous field. We use a simulation based technique to account for the distortion of the ion cloud caused by the inhomogeneous field. In order to demonstrate the gain in kinetic energy resolution compared to conventional 3D Ion Imaging, we use the spatial distribution of H(+) ions emerging from the photodissociation of HCl following the two photon excitation to the V (1)Sigma(+) state. So far a figure of merit of approximately four has been achieved, which means in absolute numbers Delta v/v = 0.022 compared to 0.086 at v approximately = 17,000 m/s. However, this is not a theoretical limit of the technique, but due to our rather short TOF spectrometer (15 cm). The photodissociation of HBr near 243 nm has been used to recognize and eliminate systematic deviations between the simulation and the experimentally observed distribution. The technique has also proven to be essential for the precise measurement of translationally cold distributions.

Measurement of the differential cross section of the photoinitiated reactive collision of O(D1)+D2 using only one molecular beam: A study by three dimensional velocity mapping

In order to measure the state selective double differential cross section of a reactive collision, the preparation of the reactants with defined initial velocities and quantum states in number densities high enough to achieve an acceptable count rate is most important. At the same time, secondary collisions have to be prevented in order to ensure that the nascent products are not thermalized. Usually, the best way to control the initial conditions is to use crossed molecular beams, but the number density decreases quadratically with the distance from the nozzle orifice which can be a problem, especially if a molecular product with a large number of populated states is to be analyzed state specifically by REMPI spectroscopy. In this contribution we would like to present a method for measuring the quantum state selective differential cross section of a photoinitiated reaction that combines the advantages of the PHOTOLOC technique (high reactant densities) and the parallel beams technique used by the groups of Kitsopoulos, Orr-Ewing, and Suits (defined relative velocity of the reactants). Moreover, an algorithm based on a Bayesian backward reconstruction developed by W. H. Richardson [J. Opt. Soc. Am. 62, 55 (1972)] has been derived. Both, one reactant and the precursor of the other reactant, are present in the same molecular beam and the center of mass velocity is selected by shifting the dissociation and the detection laser in time and space. Like in comparable methods, this produces a bias in the measured velocity distribution due to the fact that the reaction takes place in the whole volume surrounding the laser beams. This has been also reported by Toomes et al. in the case of the parallel beams technique and presents a general problem of probing reaction products by REMPI spectroscopy. To account for this, we develop a general approach that can be easily adapted to other conditions. The bias is removed in addition to deconvolution from the spread in reactant velocities. Using the benchmark system O((1)D)+D(2) with N(2)O as the precursor, we demonstrate that the technique is also applicable in a very general sense (i.e., also with a large spread in reactant velocities, products much faster than reactants) and therefore can be used also if such unfortunate conditions cannot be avoided. Since the resulting distribution of velocities in the laboratory frame is not cylindrically symmetric, three dimensional velocity mapping is the method of choice for the detection of the ionized products. For the reconstruction, the distance between the two laser beams is an important parameter. We have measured this distance using the photodissociation of HBr at 193 nm, detecting the H atoms near 243 nm. The collision energy resulting from the 193 nm photodissociation of N(2)O is 5.2+/-1.9 kcal/mol. Our results show a preference for backward scattered D atoms with the OH partner fragment in the high vibrational states (v=4-6), in accord with previously published results claiming the growing importance of a linear abstraction mechanism for collision energies higher than 2.4 kcal/mol.

Direct observation of the three-dimensional velocity distributions of Cl(2 P 3/2,1/2) atoms in the photodissociation of selected chlorides

72 The recent breakthrough in the study of molecular dynamics is due to the advent of a technique for imaging the spatial distributions of photodissociation or chemical reaction products [1] and, especially, of an improved version of this method referred to as velocity map imaging (1997) [2]. In this technique, the products are ionized by means of resonance-enhanced multiphoton ionization (REMPI) and the nascent ions are accelerated by an electric field and detected as scintillations on a phosphorescent screen. Recently, we suggested a new version of the imaging technique [3], an alternative to velocity map imaging. The major difference between this method and the imaging technique is that the phosphorescent screen is exchanged for a position-sensitive delay-line detector (DLD) [4] with high time resolution. The DLD is a ceramic frame on which two wires are wound parallel to the X and Y axes, which are delay lines. A charge appearing somewhere on the delay line is propagated in both directions toward the line ends, where the signal arrival time is recorded. Each ion gives rise to two pairs of times ( X 1 , X 2 ) and ( Y 1 , Y 2 ) for the delay lines along the X and Y axes, respectively. The coordinates X and Y of this event in time units can be calculated as ( X 1 – X 2 , Y 1 – Y 2 ), and the time of the event,

Competition between two- and three-body decay of Cl2O

Abstract The photodissociation dynamics of Cl 2 O at 235nm and 207nm are investigated. The chlorine atoms generated via dissociation into the radical and three-body decay channel are detected state specifically in a (2 + 1)-REMPI process. At 235nm the Cl 2 O molecule was excited to the 1 2 B 1 state and the decay is dominated by the radical dissociation Cl 2 O+hν→ClO+Cl. At 207nm an excitation into the 2 1 A 1 state takes place, which dissociates predominantly into the three-body channel Cl 2 O+hν→2Cl+O. The three-body decay is characterized based on the fragment kinetic energy distributions at 235nm and 207nm. The shape of the energy distributions points to an asynchronous concerted decay mechanism.