Translational energy release

Translational energy release for decompositions occurring within ion sources is an area of inquiry with a long history [311] and one in 

With the various experimental techniques, the actual measurement concerns product ions after they have been extracted from the source. That is to say, the decomposition occurs in a source before acceleration, but what is actually measured is translational energy after the product ion has been accelerated. To be still more precise, it is, in most cases, the distribution of the component of velocity along the axis of the mass spectrometer (i.e. in the direction in which the ions were accelerated out of the source) which is, in effect, measured. The measured quantity is, therefore, distinct from the translational energy distribution of the product ion (called the laboratory distribution ) as it was upon its formation in the source (i.e. before acceleration). The measured distribution needs to be analysed to obtain the laboratory distribution. Working with means or averages is much simpler, but there are possible pitfalls (see the discussion of the time-of-flight technique below). 

The translational energy distribution of the product ion in the source (laboratory distribution) is still not what is needed. What is required is the distribution of energy released in the centre-of-mass framework. The conversion of the translational energy distribution in the laboratory framework to the energy release distribution in the centre-of-mass is not a simple exercise [723]. If mean or average energies are considered, however, an expression for conversion from laboratory to centre-of-mass coordinates can be written down [311]. 

Pulsing techniques are used to achieve the necessary field-free conditions [310, 683]. 

One further point is worth bearing in mind. The precision in the measurement of translational energies of ions formed in a source is generally much less than that in analogous measurements on metastable ions (Sect. 3.2.3). This is because the amplification effect (Sect. 3.2.3) on moving from centre-of-mass to laboratory framework is insignificant with decompositions in a source because the reactant ions are moving relatively slowly. The laboratory distributions measured for source reactions often span less than leV, whereas those for metastable ions may span tens of electron volts and more. 

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In addition to CO(v = 0—2,7) populations, Houston and Kable recorded CO Doppler profiles to measure the translational energy release, and the vector correlation between the recoil velocity vector and the angular momentum vector of CO. Together, these data paint a compelling picture that two pathways to CH4 + CO are operative. The rotationally hot CO population (85% of total CO)... 

Because the pathway to H + HCO on So is barrierless (with a loose TS), whereas the pathway on Ti has an exit barrier (tight TS), the dissociation dynamics of the two pathways can be expected to differ markedly. Measuring the translational energy release and the product state distributions of the HCO fragment are therefore appropriate experimental techniques for exploring this competition. 

Because dissociation on So is barrierless, the product state distributions should be well approximated by statistical theories, especially when the excess energy is small, as in the Valachovic study. Product state distributions arising from the So pathway should be characterized by small translational energy release, but significant rovibrational excitation of HCO. This signature is demonstrated in the top panel of Fig. 17, which shows a HRTOF spectrum with... 

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... 

The translational energy release distributions shown in Fig. 12 are similar in shape for both products, with the major difference being how much of the available energy is channelled into product translation. For YH2, this amount is 28%, while for YOCH3 it is considerably larger (40%). The CM angular distributions are strikingly different for these two product... 

Fig. 30. Contour plot of photoelectron-photodissociation coincidence spectrum as a distribution of photoelectron intensity (dark shade = low, light shade = high) against the electron binding energy and relative translational energy of the photofragments. Also shown on the left and at the bottom are the partially averaged distributions for the translational energy release and the electron binding energy, respectively.

Infra-red chemiluminescence has been used to measure the vibrational state distributions of CO2 formed by reaction on Pt and Pd surfaces [45-50]. While the detection sensitivity of infrared chemiluminescence does not approach that of LIF or REMPI, it is an attractive way to probe molecules such as CO2 where there is substantial vibrational excitation and REMPI schemes are not available. In some cases Doppler measurement of the translational energy release can be achieved, giving direct information on the translational energy release of vibrationally excited C02 [45]. Recently infrared diode laser spectroscopy has been used to detect vibrationally excited CO2 from CO oxidation over... 

Figure 26 Translational energy released into N2 formed by reaction of NO and H2 at a Pd(l 1 0) surface held at 600 K [127]. The solid line is a fit to a thermal translational energy release, giving T = 425 K.

Phase space theory has been able to reproduce experimental distributions of translational energy releases closely for a number of decompositions which do not have energy barriers to the reverse reactions [165, 485] (see Sect. 8). Phase space theory does focus attention on a very late stage of reaction since the degrees of freedom of the loose transition state can be identified as vibrations, rotations and translations. 

Translational energy release in the decomposition of metastable ions... 

Consider a single-valued translational energy release, T, in the decomposition of a metastable ion M+ in a field-free region to give a product ion m+. The spread in translational energy (AeV) of m+ along the original direction of motion can be expressed as [186]... 

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