Tina S. Einfeld

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...

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...

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.

Competing dissociation channels in the photolysis of S2Cl2 at 235 nm

The photodissociation of disulfur dichloride (S2Cl2) at 235 nm has been studied by three-dimensional (3D) imaging of the chlorine product recoil in its ground state 2P3/2[Cl] and excited spin–orbit state 2P1/2[Cl*] employing the resonance enhanced multiphoton ionization and time-of-flight techniques. The photodissociation proceeds mainly along the three channels forming S2+2Cl (1), S2Cl+Cl (2), and 2 SCl (3) photoproducts where slow and fast Cl fragments are released in (1) and (2), respectively. The relative yield of channel (1) with respect to channel (2) was determined to be 1.2:1.0. The yield of Cl*, φ(Cl*)=P(Cl*)/[P(Cl)+P(Cl*)], was found to be 0.35. The obtained state-specific velocity distributions of Cl and Cl* are mainly different in the high energy range: For Cl* the two dissociation channels are almost equally present, whereas in the case of ground state Cl the contribution of dissociation channel (2) is of minor importance. The dependence of the anisotropy parameter β on the fragment recoil ve...

Photodissociation of CSCl2 at 235 nm: Kinetic energy distributions and branching ratios of Cl atoms and CSCl radicals

The photodissociation dynamics of thiophosgene (CSCl2) and the respective branching ratios of both dissociation products Cl and CSCl have been studied by 3D imaging of the photodissociation product chlorine in its ground state 2P3/2[Cl] and excited spin–orbit state 2P1/2[Cl*] employing the resonance enhanced multiphoton ionization and time-of-flight technique at a dissociation wavelength of about 235 nm. A novel technique is applied where the complete three-dimensional (3D) momentum vector of a reaction product is directly determined. The kinetic energy distribution (KED) for Cl* is observed for the first time. The obtained KEDs of Cl and Cl* are different in the low kinetic energy range due to the correlating state of the partner fragment CSCl. In the case of ground state Cl the CSCl partner radical is produced in the ground X, A, and B states with a contribution of 4±0.5%, 60±5%, and 36±3%, respectively. In the case of Cl* the corresponding CSCl is produced with a contribution of 7.5±0.5% in the groun...

Dynamics of vibrationally mediated photodissociation of CH3CFCl2

The ∼235 nm photodissociation of CH3CFCl2 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 ∼235 nm photons also tagged spin-orbit ground Cl 2P3/2 [Cl] and excited Cl 2P1/2 [Cl*] state photofragments, via (2+1) resonantly enhanced multiphoton ionization in a time-of-flight mass spectrometer. Monitoring the shapes of 35Cl and 35Cl* time-of-arrival profiles revealed their energies and angular distributions and showed broad and unstructured fragment kinetic energy distributions. Although a significant amount (∼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, β, which indicates at a fast dissociation of the parent molecule along the C–Cl dissociation coordinate. Moreover, β for Cl changes from a slightly positive value to a negative value,...

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.