Time-of-flight distribution

Finally, the photofragment time-of-flight distribution for CD2CDO photodissociation at 31,980 cm-1 is shown in Fig. 4. This will be analyzed in detail in the next section, but for now it suffices to point out that the wings in the distribution are from D atoms, indicating that photodissociation channel (2) to D + CD2CO is indeed occurring. 

Figure 7. Time-of-flight distributions of metastable helium beams. Singlet distributions are generally narrower than triplet ones. The 540-meV distribution was obtained with plasma-jet source (see Section III.A.6).

In these experiments, there was no double-resonance tagging, so all of the HI moieties were available for photodissociation. What is observed experimentally is the resulting atomic hydrogen time-of-flight distribution which is obtained by using the HRTOF technique (Ashfold et al. 1992 Schneider et al. 1990 Wen et al. 1994). This method provides excellent resolution and S/N compared to other TOF methods. The dominant features (solid curves) are due to HI monomer. However, upon magnification, features are seen that are due to... 

Lifetimes of reactant ions in the range 1—100ps can be determined in PIPECO studies from asymmetric time-of-flight distributions. Ions with these lifetimes decompose during acceleration [908] and hence the translational energy of these fragment ions so formed is less than that of fragment ions formed before acceleration. 

The only reservation about these determinations of lifetimes by PIPECO might concern their accuracy. The analysis of the asymmetric time-of-flight distribution constitutes, in essence, an analysis of peak... 

Fig. 8. Representative time-of-flight distributions for reactively scattered OH m/z = 17) following reaction of four different incident oxygen-atom beams, Ei) = 21 (A), 47 (B), 297 (C), and 504 (D) kJ mol, with a squalane surface. The angle of incidence for all four beams was 60° and the angle of detection was 45°. The distributions have been deconvolved into hyperthermal (shorter flight times) and thermal (longer flight times) components.

Fig. 10. Representative time-of-flight distributions for inelastically scattered oxygen atoms (rnfz = 16) following impact with a squalane surface. The incident atomic-oxygen beam had an average energy of 504 kJ mol and impinged on the surface at an incident angle of 60°. The scattered O atoms were detected at three final angles 70°, 45°, and 10°. Two populations of scattered products are identified, those with thermal (dashed line) and hyperthermal (solid line) energies.

Fig. 21. Time-of-flight distributions of CO2 exiting a continuously oxidized Kapton surface following exposure to pulses of five hyperthermal argon beams whose average translational energies aie shown. The distributions are normalized with respect to the respective incident beam intensity.

Fig. 8. The time-of-flight distributions of neutrons scattered at 0 = 90° from samples of polypropylene possessing decreasing tacticity, and from a sample of quenched highly crystalline pol3 propylene. The sample densities were pp-1 = 0.9068, pp-2 = 0.866, pp-3 = 0.8882, pp-4 = 8710, and pp-5 = 0.8SS4...

Fig. 10. The time-of-flight distributions of neutrons scattered from Nylon-6 at 65°. The abscissa corresponds to the number of 28 /4- ec, time-of-flight channels. A scale at the top of the figures shows the energy gain in mev while the arrows give the energies of the observed peaks in cm h The elastic peak occurs at Channel 165...

To see more fringes we have to increase the coherence length and therefore decrease the velocity spread. For this purpose we employ a mechanical velocity selector, as shown after the oven in Fig. 1. It consists of four slotted disks that rotate around a common axis. The first disk chops the fullerene beam. Only those molecules are transmitted which traverse the distance from one disk to the next in the same time that the disks rotate from one open slot to the next. Although two disks would suffice for this purpose, the additional disks decrease the velocity spread even further and help to eliminate velocity sidebands. By varying the rotation frequency of the selector, the desired velocity class of the transmitted molecules can be adjusted. To measure the time of flight distribution we chopped the fullerene beam with the chopper right behind the source (see Fig. 1). The selection is of course accompanied by a significant loss in count rate, but we can still retain about 7% of the unselected molecules. 

Fig. 1. Time-of-flight distributions of photofragments obtained in the photodissociation of CI2 molecules deposited thin and thick on an Si wafer cooled to 100 K. Laser wavelength is 193 nm. Flight length is 16 cm. Drift time in the mass filter is 26 ps for C1+.

The time of flight distribution for H+ products produced from three different vibrational levels v is shown in Fig. 11. The time-of-flight is inversely proportional to the velocity component of the products along the initial relative velocity. For each value of v a different bimodal distribution is obtained. We attribute the presence of two peaks to electron exchange between the H2+ and H2 prior to dissociation. Assuming the charge transfer mixing is complete, one expects two H+... 

Fig. 11. The time-of-flight distributions of H+ ions produced in collisions of H2+(v) + H2 at Ecm = 6eV for v = 1, 4, and 7 [15]. Also shown are the slow H2 ions produced by charge transfer (CD- The large central peak of unreacted H2 ions is omitted here. The TOF s of the H+ ions have been converted to laboratory energies the scale is shown in the Figure...

Figure 5.22 The product time of flight distribution for the dissociation of aniline ions at various photon energies ranging from 12.68 to 13.59 eV. The solid lines through the experimental points are calculated TOP distributions in which the indicated dissociation rates were assumed. Taken with permission from Baer (1986), and Baer and Carney (1982).

Increases in both scattering and absorption decrease the output intensity. However, increased scattering increases the pulse width while increased absorption tends to decrease it [512]. Therefore, the shape of the time-of-flight distribution of the photons can be used to distinguish between scattering and absorption. Qualitatively, early photons are mainly influenced by scattering, whereas later photons are increasingly influenced by absorption as well. 

In diffuse reflection experiments, the depth of scattering and absorption changes in the tissue can be derived from time-resolved data [481]. The first and second moments of the time-of-flight distributions are especially sensitive to changes in deep tissue layers [325, 328]. 

---