Measurements of angular distribution

It is obvious that the reaction products will show, in addition to the velocity distribution, a distribution of scattering angle which is characteristic of the specific mechanism involved. Thus the full understanding of the reaction dynamics requires simultaneous measurements of the angular and velocity distributions. 

Before such measurements were fully developed, the measurements of the angular distribution alone were carried out by Turner et al. [97] in a crossed beam experiment. They studied the differential cross-sections of the reaction + D2 N2D + D over the laboratory energy range 

using a movable quadrupole mass filter. They found peaking of the secondary ions in the forward direction and suggested the formation of an intermediate complex by comparing the observed angular distributions with the calculated ones. However, it was not possible to deduce any decisive information on the dynamics of the reaction from this kind of measurement, in contrast to the measurements of the velocity distribution alone described above. 

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It is essential to remember the assumption that K2 = 1. If the actual angular distribution of NO + ions in the laboratory system is widespread, the true cross-section could be larger than the values of Figure 1 by a factor of 2 or 3. Determining the angular distribution of NO+ ions from this reaction is a most important area for future work. Measurement of angular distributions from ion reactions has just begun (1, 22). 

There have been a large number of measurements of angular distributions in desorption which show sharply peaked distributions and these have recently been reviewed by Kislyuk [44]. In some cases the products of reaction on fcc(l 1 0) surfaces are found to be peaked at an angle to the surface normal along the [001] azimuth (Fig. 9) notably for N2 produced by NO [77, 78] or N20 [79] decomposition, C02 formed by CO oxidation [80, 81] and CO formed by C + O recombination [82]. Sharply peaked distributions indicate a repulsive energy release which lies at an angle to the surface normal [83]. This occurs either because reaction takes place on (111) facets on the reconstructed (1x2) missing row surface (e.g., CO formation on Pt(l 1 0)-(l x 2) surface [82]) or, as in the case of N20 decomposition, because the symmetry of the transition state creates a repulsion which is directed away from the surface normal [84, 85]. 

The second kind of cavity which has been used is Fabry-Perot cavity [10]. These cavities are completely tunable and have the advantage that they are open, allowing far better access to the atoms than the closed cavity shown in Fig. 1. The improved access was critical for measurements of angular distributions of the electrons ejected in microwave ionization [11]. Their cylindrical symmetry is useful for measurements involving circularly polarized microwaves, but a closed cylindrically symmetric cavity is equally good [12]. 

Time-of fli t spectroscopy, along with measurement of angular distributions in photofragmentation, was achieved in a spectacular (and expensive) technique developed by Wilson and his co-workers and by Diesen et al. A schematic diagram of Wilson s experimental system is shown in Figure 4 instead of the... 

The quantities /, T, n and perhaps also L and S for well defined nuclear states thus impose selection rules on nuclear reactions. These quantities and the associated nucleon configuration may in many cases be determined by measurements of angular distributions or transition probabilities, particulary in resonance reactions. Many examples of this procedure are discussed in Part D. 

Light-heavy-light reactions are also of experimental significance. For example, molecular beam measurements of angular distributions [2], and kinetic experiments yielding rate constants [3], have been carried out on the D+BrH- -DBr+H reaction. Furthermore, there has been much recent interest in the competition between non-reactive and reactive energy transfer in collisions such as H+C1H[4]. Vibrational quenching reactions like... 

In conventional photoelectron spectroscopy, angular distributions are known to provide valuable insight into the underlying dynamics. A few groups have, moreover, recently reported measurements of angular distributions in pump-probe photoelectron spectra [332, 406]. Althorpe and Sei-deman have also examined the angular distributions of photoelectrons in the pump-probe ionization for a rigid diatomic molecule NO [8]. 

Orth R, Dunbar RC, Riggin M (1977) Measurement of angular-distribution and entugy of ionic fragments from photodissociation of molecular-ions. Chem Phys 19 279-288... 

Two teclmiques exist for measuring the angular distribution of products. In the crossed-beam setup, the... 

Keil and co-workers (Dhamiasena et al [16]) have combined the crossed-beam teclmique with a state-selective detection teclmique to measure the angular distribution of HF products, in specific vibration-rotation states, from the F + Fl2 reaction. Individual states are detected by vibrational excitation with an infrared laser and detection of the deposited energy with a bolometer [30]. 

Auger Electron Diffraction, AED, is an exact analogy to XPD, providing basically the same information. Instead of measuring the angular distribution of the ejected photoelectrons one uses the Auger electrons (Chapter 5). 

Indeed, by using soft El ionization, we have been able to unambiguously detect products from all five reaction pathways (2a)-(2e), determine their branching ratio and characterize their dynamics.34 Here we discuss some of the results that we have obtained on this reaction, which well exemplify the power of soft El ionization. First of all, from measurements of the El efficiency curves at various to/e ratios (15, 42, and 43), we have found that the parent ion at m/e = 43 (CH2CHO+, corresponding to one of the main reaction channels, the vinoxy radical,) is not stable, so measurements of angular and TOF distributions were carried out at m/e = 42. Incidentally, from the El ionization efficiency curve at m/e = 42 we have obtained some direct information on the IE of the vinoxy radical, for which no such information was available till now. The IE should be <11 eV. 

If it is desired to measure the angular distribution parameter / , the experimental set-up of Fig. 1.17 can be used. A rotation of the sector-analyser around the photon beam direction keeps = 90°, but changes the angle or, equivalently, 4>. This set-up has the advantage that the analyser always views the same source volume, independent of the angle 4>. The angle-dependent intensity 7exp of detected electrons, equ. (1.53), then reduces to... 

Figure 9.6 Experimental setup for measuring the angular distribution of the scattered light at different temperatures and externally applied electric fields. L is a He-Ne-laser, A/2 a half-wave retarder plate, P a Glan-Thomson prism, BS a beam splitter, PDl and PD2 are photodiodes and HV the high voltage amplifier. The sbn sample with 0.66 mol% Cerium is placed on a stack of Peltier-elements to control the temperature.

We see on Fig. 7a that only NO desorbs from the support. Moreover the rise time of the NO desorbing pulse is equal to the rise time of the impinging pulse (20 ms). Thus, we can answer some of the previous questions NO does not dissociate nor react with the MgO and the interaction time is smaller than 20 ms (at RT). Therefore we can expect that NO is either reflected from the substrate or physisorbed. To distinguish between these two possibilities we proceed to a second experiment. The continuous NO beam is directed toward the sample in a given direction and we measure the angular distribution of scattered NO (in the incidence plane). We see on Fig. 7b that the angular distribution is peaked in the specular direction with a large lobe that means that at least part of the NO is quasi-elastically reflected... 

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