Velocity map image

Figure 13. Modified Velocity Map Imaging spectrometer showing the double einzel lens, L, Li, and 5-eV kinetic energy initially transverse trajectories from an extended source volume with Vjgp = 3000 V, Vext = 0.695 x Vjep, and Vl = Vli = 1000 V. Taken with permission from Ref. [102]. Copyright (c) 2005, American Institute of Physics.

Fig. 7. Total kinetic energy release derived from velocity map images of 0(3P2) and D(2S) fragment atoms following photodissociation of OD at 226 and 243 nm, respectively. The initial vibrational state of OD is determined from energy balance with TKER = hv + E(vib)oD — Do(OD). The bar graphs show the calculated photodissociation yields for OD X2Il(v) at a vibrational temperature of 1700 K. (From Radenovic et al.97)...

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. 

A recent and probably more accurate value from velocity map imaging investigations is = 14798(1) cm [OOWre]. 

Photochemistry of gas-phase molecules, including species of atmospheric importance the molecular photodissociation dynamics are traced by photofragment ion (velocity map) imaging. 

The information contained in the energy-dependent photoelectron angular distributions Pk 0k, tpr) [Eq. (3.93)] may be conveniently visualized as velocity map images. Examples will be given in Sec. 5.4. Comparison of such images across different delay times tpr reveal the time evolution of the system. 

When the system undergoes fast (or somewhat sudden) evolution, such d Tiamics reflected in the velocity map image can be more sensitively extracted in terms of its time derivatives. Time derivatives of the image can be computed from Pk 0k, tpi) using finite differences using a small time shift r,... 

Note that the time-derivative is to be taken with respect to the change of the time-delay between the pump and probe lasers, tpr, and not with respect to t. An example will be shown in Secs. 5.4.2.4 and 5.4.3.3, illustrating that time-derivative photoelectron velocity map imaging can be quite helpful in uncovering and/or identifying useful characteristics of wavepacket motion, which may otherwise be overlooked in the original images. 

The time-derivative velocity map images (Sec. 3.3.11) were also computed from the obtained Pfc(0 tpr) through finite differences. A small time shift of T = 1.0, 0.4, and 0.2 fs were tried and all gave the same results within the resolution of the figures shown. 

Fig. 5.26 (a) Computed femtosecond time-resolved photoelectron velocity map images... 

Fig. 5.30 (a) Femtosecond time-resolved photoelectron velocity map images without... 

Fig. 5.31 Time evolution of (a) velocity map images and (b) their time derivatives for the controlled NO2 dynamics. (Reprinted with permission from Y. Arasaki et al., J. Phys. B 45, 194006 (2012)).

Besides angular momentum and the total kinetic energy release, PST can also be used to evaluate the distribution and average of the purely translational part of the KER. This quantity is important because it can be experimentally measured by velocity map imaging. The spectrum of translational kinetic energy is sensitive to the interaction between the products, but also to their shape. In addition, its connection with the internal temperature of the products make it a valuable thermodynamic indicator from which phase transitions can be probed. 

Arthur Suits is a professor of chemistry at Wayne State University where he and his group study the many roles office radicals—atoms or molecules with unpaired electrons—in chemical processes. One of the techniques developed in his lab, DC Shce Velocity Map Imaging, uses a video camera to record images of ions formed by photo ionization after (for one example) a dissociation reaction occurs. For instance, Suits has used laser photodissociation (Section 7.4) to break deuterium bromide ( HBr or DBr) into deuterium and bromine radicals. The photodissociation occurs at a fixed photon energy... 

While velocity mapped imaging gives a direct picture of momentum space, there are some serious issues using this technique with cold molecules. The two most important are the electron recoil during the ionization step and charge repulsion from producing multiple ions per laser shot. While... 

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. 

Parker D, Eppink A. (1997) Velocity map imaging of ions and electrons using electrostatic lenses Apphcation in photoelectron and photofragment ion imaging of molecular oxygen. Rev. Sci. Instr. 68 3477-3484. 

---