Photofragments velocity

In addition to the previously mentioned disadvantages, all of these methods have another drawback in the large molecule photofragment velocity measurements. For example, in the studies of UV photon photodissociation of polyatomic molecules, like alkene and aromatic molecules, molecules excited by the UV photons quickly become highly vibrationally excited in the ground electronic state through fast internal conversion, and dissociation occurs in the ground electronic state. 

Analysis of photofragment velocities (speed and angle) has become the mainstay of modern photodissociation studies [14,15], Simple energy and momentum conservation relations are the basis of these studies. 

J. Wei, A. Kuczman, J. Riedel, F. Renth and F. Temps, Photofragment velocity map imaging of H atom elimination in the first excited state of pyrrole, Phys. Chem. Chem. Phys., 5 (2003) 315-320. 

Figure 22.5 shows a pictorial representation of the so-called two-vector correlation, both in photodissociation (half-collisions) and in atom-exchange reactions (full collisions). The important point to consider is that photodissociation is an anisotropic process in which the polarization of the electric field Sp of the photolysis laser defines a direction with respect to which the vector describing both products and parent molecule can be correlated. As a consequence, one can measure and analyse the correlation between the parent transition dipole moment fi and the recoil photofragment velocity vector, i.e. the v correlation. Thus, the angular distribution of the photofragments I 6) can be described in the form (Zare, 1972)... 

Ion-imaging techniques have mostly been used for the measurement of photofragment velocity distributions (speed and angle) from unimolecular dissociation processes. The technique is very powerful for several reasons ... 

Parker D, Eppink A. (1997) Photoelectron and photofragment velocity map imaging of state-selected molecular oxygen dissociation/ionization dynamics. J. Chem. Phys. 107 2357-2362. 

Figure A3.5.2. The Ar photofragment energy spectmm for the dissociation of fiions at 752.5 mn. The upper scale gives the kinetic energy release in the centre-of-mass reference frame, both parallel and antiparallel to the ion beam velocity vector in the laboratory.

Syage, J. A., Photofragment Imaging by Sections for Measuring State-Resolved Angle-Velocity Differential Cross Sections, J. Chem. Phys., 105, 1007-1022 (1996b). 

When a pulsed molecular beam is used with a pulsed laser beam, the velocity of the photofragments can be obtained from the delay time between... 

About 10 years ago, the ion-imaging method was developed for the measurement of the velocity of a quantum-state-ionized photoproduct [15,23]. This technique relies on using a position-sensitive ion detector at the end of a TOF mass spectrometer to measure the recoil of photofragments from the centerline of the spectrometer. This technique has a large multiplex advantage over other approaches. 

The oldest known vector correlation is the angular distribution of the photofragments, i.e., the relation between the recoil velocity v and the polarization vector E,... 

Briihlmarm, U., Dubs, M., and Huber, J.R. (1987). Photodissociation of methylnitrite State distributions, recoil velocity distribution, and alignment effects of the NO(X2n) photofragment, J. Chem. Phys. 86, 1249-1257. 

---