Beam velocity

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.

The fragment recoil velocity resolution depends on the divergence of the molecular beam, molecular beam velocity distribution in the direction of the molecular beam axis, and the distance of fragments expanded in the velocity axis of the two-dimensional detector. If the divergence of the molecular beam is small and the fragment recoil velocity is much larger than the velocity difference of parent molecules, the recoil velocity resolution can be simply expressed as AV/V = s/L, where L is the length of expansion of... 

Another method is to measure the disappearance rate of the excited parent molecules, that is, the intensity changes of the disk-like images at various delay times (therefore, at various photolysis laser positions) along the molecular beam. This is very useful when the dissociation rate is slow and the method described above cannot be applied. This measurement requires a small molecular beam velocity distribution and a large variable distance between the crossing points of the pump and probe laser beams with the molecular beam. The small velocity distribution can be obtained through adiabatic expansion, and the available distances between the pump and probe laser beams depend on the design of the chamber. For variable distances from 0 to 10 cm in our system and AV/V = 10% molecular beam velocity distribution, dissociation rates as slow as 3 x 103 s 1 under collisionless condition can be measured. 

Using a forward-convolution program131 with instrumental and experimental parameter inputs (aperture sizes, flight distances, beam velocities, etc.), along with two center-of-mass (CM) input functions (the translational energy release distribution, P(E), and the CM angular distribution, T(0)), TOF spectra and lab angular distributions were calculated and compared... 

Fig. 6. Variation of the center-of-mass velocity with beam crossing angle, 7, for the reaction 0(3P) + C2H2 with the 0(3P) and C2H2 beam velocities of 2798ms-1 and 827ms 1, respectively. The Newton diagrams for 7 = 45°, 90°, and 135° are shown (see text).

In practice, series expansions of the stopping cross section in powers of Zi are only useful at high beam velocities where neither the Bloch nor... 

Similar results obtained with lysozyme (a protein of molecular weight 14,600) in a 75% ethanol-25 % water solution are shown in Figure 5. The vertical bars indicate the repelling voltage required to stop singly charged monomers, dimers, and trimers of lysozyme traveling at the calculated beam velocity of 754 meters/sec. Sharp dropoffs are seen to... 

These data provide strong evidence that singly charged macroions are being observed, and that the ions are traveling near the calculated beam velocity with a relatively narrow velocity distribution. Thus this technique can be used for low-resolution (approximately 5-10%) M/z analyses. 

Scattering of Noble-Gas Metastable Atoms in Molecular Beams velocity. The total reaction cross section is... 

If the laser frequencies uq,uq are fixed (as for a C02 laser for example), the beam velocity must be changed between resonances. However, if uq and uq can be chosen so that resonances occur at similar beam velocities, a considerable reduction in sensitivity to the absolute beam velocity is still obtained. In this case one can write... 

In an initial experiment [97], spectroscopy was performed with the output beam of the unmodified laser counter-propagating to the ion beam. The beam velocity was measured using nuclear resonance and time-of-fiight techniques. The weak, hyperfine-induced 21S o — 23Po resonance was then observed [98], see fig. 9. This enabled the J = 0 — 1 fine structure splitting to be obtained by using suitable laser lines to take account of most of the frequency difference, and then measuring the small interval in beam velocity between the resonances. 

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