We report the first observation of ion recoil induced by the ionization process for a molecule. The applicability of the method has been demonstrated by determining the b2 and b4 anisotropy parameters of the angular distributions of NO þ ions in the resonanceenhanced two-photon ionization of NO(XP1=2, v 1⁄4 0; J ) via the A2Rþ(v 1⁄4 0, N ) state at 226 nm. An energy resolution in the range of 10 leV is demonstrated. The method might prove to be particularly useful for studying complex ionization-fragmentation processes as they occur for example for superexcited molecules. 2004 Elsevier B.V. All rights reserved.

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 ground and excited state Cl. These observations can be explained by a sequential decay where the first Cl fragment is released in a fast process characterized by the nonvanishing positive β parameter and a lifetime of ⩽210 fs, whereas the second Cl fragment is released after a period which is long on a rotational time scale. A significant contribution of a symmetric mechanism can be excluded.

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 the present work is the study of angular distribution of NO ions due to electron recoil in the ionization of NO(A 2Σ+). Finally the advantages of the method are examined by studying the speed distributions of Cl atoms in the photolysis of Cl2 at 355 nm.

The photodissociation dynamics of Cl2O 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 Cl2O molecule was excited to the 12B1 state and the decay is dominated by the radical dissociation Cl2O+hν→ClO+Cl. At 207nm an excitation into the 21A1 state takes place, which dissociates predominantly into the three-body channel Cl2O+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.

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 Cl35 and Cl*35 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.

The similar to 235 nm photodissociation of CH3CFCF2 pre-excited to three, four, and five quanta of C-H methyl stretches was studied to investigate the effect of internal parent excitation on the dynamics of two- and three-body photofragmentation. The similar to 235 nm photons also tagged spin-orbit ground (ClP3/2)-P-2 [Cl] and excited Cl(2)p(1/2) [Cl*] state photofragments, via (2+1) resonantly enhanced multiphoton ionization in a time-of-flight mass spectrometer. Monitoring the shapes of Cl-35 and Cl-35* time-of-arrival profiles revealed their energies and angular distributions and showed broad and unstructured fragment kinetic energy distributions. Although a significant amount (similar to 50%) of the available energy is transferred into internal energy of the CH3CFCl fragment, the spatial Cl distribution is characterized by a nonvanishing anisotropy parameter, beta, which indicates at a fast dissociation of the parent molecule along the C-Cl dissociation coordinate. Moreover, beta for Cl changes from a slightly positive value to a negative value, while that for Cl* increases when the pre-excitation is increased from three to five quanta of C-H methyl stretches. This is attributed to the promotion of one of the nonbonding electrons located on the Cl atoms to the sigma* antibonding C-Cl orbital and involvement of several upper states with different symmetry properties. (C) 2001 American Institute of Physics.