C. Maul

Photo induced three body decay

The photo induced three body decay : ABC hnu A B C, where a molecule ABC decays into three fragments A, B and C upon irradiation, is reviewed. Various experimental and theoretical techniques for the investigation of this reaction and their application to a wide range of molecular species are discussed. Emphasis is laid on the distinction between concerted and stepwise processes, consisting of one single or two consecutive kinetic events, respectively. The concerted fragmentation scheme is further classified as being of either synchronous or asynchronous character, depending on whether or not the bond breaking processes take place in unison. The three body decays of acetone, azomethane and s-tetrazine are discussed in detail as prototypes for these mechanisms. A novel kinematic analysis approach, based on the evaluation of fragment kinetic energy distributions, is presented and applied to the ultraviolet photodissociation of phosgene. Competing pathways are found to be operative, dominated by the asynchron...

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.

SPIN SELECTIVITY IN THE ULTRAVIOLET PHOTODISSOCIATION OF PHOSGENE

The ultraviolet photodissociation of phosgene in its first absorption band 1A2←1A1 was investigated by resonance enhanced multiphoton ionization and time of flight techniques. Nascent atomic chlorine fragments were observed and their state specific kinetic energy distributions were determined. Of the chlorine atoms 15% are produced in the excited 2P1/2 spin–orbit state with a mean kinetic energy of 3200 cm−1 compared to a value of 1500 cm−1 for the mean kinetic energy of the ground state chlorine atoms. The analysis of the kinetic energy spectra yielded evidence for a concerted three‐body decay. The formation of intermediate COCl is of minor importance in the dissociation process, the formation of a stable final COCl product can be excluded. Competing pathways on the upper potential energy surface are discussed. A significant excitation of the carbon monoxide CO fragments is predicted.

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...

Photodissociation dynamics of HN3. The N3 fragment internal energy distribution

Abstract The H-atom velocity distribution of the photofragmentation of HN 3 at 243 nm has been characterized using resonant enhanced multiphonon ionisation (REMPI) in combination with time-of-flight (TOF) measurements. The internal energy distribution of the N 3 fragment has been determined from the experimental results. The structures in the N 3 internal energy distribution were assigned to the excitation of the N 3 symmetric stretching mode. The relative population of the (000):(100):(200):(300):(400) modes was found to be 0.09:0.19:0.28:0.29:0.15. The means of the vibrational and rotational energies are 〈 E vib 〉 = 2970 cm −1 and 〈 E rot 〉 = 720 cm −1 . The excitation of the symmetric stretching mode indicates the dominating influence of the NN 2 H, N 2 NH, and N 3 H coordinates of the upper potential energy surface (PES) on the N 3 internal energy distribution.

Photodissociation dynamics of phosgene: New observations by applying a three-dimensional imaging technique

The photodissociation dynamics of COCl2 has been studied by monitoring ground Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) fragments by applying a novel technique where the three-dimensional momentum vector of a single reaction product is directly determined. The photodissociation at 235 nm produces exclusively three fragments: COCl2+hν→CO+2Cl. The kinetic energy distributions of Cl and Cl* are bimodal and exhibit a different behavior for the different spin–orbit states. Our attention was turned to the dependence of the anisotropy parameter β on the fragment velocity which was observed for the first time. For both spin–orbit states the anisotropy parameter differs clearly for slow and fast chlorine atoms, where a pronounced change from the value ∼0.7 to zero at about 20 kJ/mol is observed. Slow chlorine atoms are released isotropically and predominantly in the ground state Cl whereas fast chlorine atoms have an anisotropy parameter close to the theoretically limiting value and are distributed between groun...

Photodissociation dynamics of OClO: O(3PJ) state and energy distributions

The photodissociation dynamics of OClO in the near ultraviolet (UV) was investigated in a state specific and energy selective manner. At a dissociation wavelength of 308 nm, OClO(X 2B1) was excited to the OClO(A 2A2(18,0,0)) state, from which it decays into ClO(X 2Π3/2,1/2) and O(3P2,1,0). The nascent oxygen fragments were detected spin selectively by resonant enhanced multiphoton ionization and time of flight measurements (REMPI-TOF). Based on the measurements and the conservation of energy and linear momentum, the internal energy of the ClO partner fragment was obtained. On average, more than 60% of the available energy is transferred into internal excitation of the ClO radical. Nearly the whole internal energy is vibrational energy with vibrational levels populated up to the energetic limit. Besides, the internal energy depends on the oxygen spin-orbit state because the fraction of highly excited ClO fragments increases with increasing total angular momentum J. The bimodal behavior of the fragment ene...

Evidence for the onset of three-body decay in photodissociation of vibrationally excited CHFCl2

Excitation of C–H stretch overtones of CHFCl2 followed by ∼235 nm photodissociation was applied to investigate the effect of internal parent excitation on the dynamics of two- and three-body photofragmentation. The ∼235 nm photons also tagged ground Cl 2P3/2 [Cl] and spin–orbit excited Cl 2P1/2 [Cl*] state photofragments, via (2+1) resonantly enhanced multiphoton ionization in a time-of-flight mass spectrometer, and monitored their time-of-arrival profiles. These profiles revealed the product velocities and angular distributions of 35Cl and 35Cl* and suggest the contribution of three-body decay in photodissociation of CHFCl2 pre-excited with five quanta of C–H stretch. This is the first evidence for three-body decay in photodissociation of vibrationally excited molecules.

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.