Time of flight methods

Auerbach D J 1988 Velocity measurements by time-of-flight methods Atomic and Molecular Beam Methods vol 1, ed G Scoles et a/(New York Oxford University Press) pp 362-79... 

Figures Comparison of nuciear reactor and pulsed spaliation sources. For reactor sources (steady-state method), a narrow band of wavelengths is seiected with a monochromator crystal and the scattering angle (26,) Is varied to scan dspacings. Pulsed sources (time-of-flight method) use almost the entire avail-abie neutron spectrum, fix the scattering angie (26,), and simultaneousiy detect a neutron while determining its time of flight.

The combined experimental and theoretical investigation of Dixon et al. [75] applied the H-atom Rydberg time-of-flight method to measure the translational energy distribution of H atoms from the photodissociation... 

An entirely different type of transport is formed by thermal convection and conduction. Flow induced by thermal convection can be examined by the phaseencoding techniques described above [8, 44, 45] or by time-of-flight methods [28, 45]. The latter provide less quantitative but more illustrative representations of thermal convection rolls. The origin of any heat transport, namely temperature gradients and spatial temperature distributions, can also be mapped with the aid of NMR techniques. Of course, there is no direct encoding method such as those for flow parameters. However, there are a number of other parameters, for example, relaxation times, which strongly depend on the temperature so that these parameters can be calibrated correspondingly. Examples are described in Refs. [8, 46, 47], for instance. 

Luntz and co-workers have recently carried out an impressive study that follows in the spirit of the Eley-Rideal work.44 Specifically, laser-assisted recombination of N-atoms desorbing to form gas-phase N2 on Ru(0001) was investigated. Experimental measurements of state-selectively detected N2 recoiling from the surface recombination event were obtained using resonance enhanced multiphoton ionization and ion time-of-flight methods. In this way translational energy distributions of individual rovibrational states could be obtained experimentally. In addition, N2-vibrational population distributions could be derived. 

A.J. Campbell, D.D.C. Bradley, and H. Antoniadis, Dispersive electron transport in an electroluminescent polyfluorene copolymer measured by the current integration time-of-flight method, Appl. Phys. Lett., 79 2133-2135, 2001. 

Another important problem of the photoconductivity of poly-N-vinylcarbazole is the carrier transport. With the so-called time of flight method this problem is well investigated. 

Markham JPJ, Anthopoulos TD, Samuel IDW, Richards GJ, Bum PL, Im C, Bassler H (2002) Nondispersive hole transport in a spin-coated dendrimer film measured by the charge-generation-layer time-of-flight method. Appl Phys Lett 81 3266... 

It was actually shown by the time of flight method [32-34] that coulomb type traps control the drift mobility. The concentration of such traps is 1015 m 3. The real mobility (without traps) was estimated [35] to be of the order 5x10 8 m2 V 1 s 1 with a thermal activation energy of 0.28 eV. There are no correct data as yet confirming the impurity hopping model in PVC. The drift mobility is due rather to the jumps between neighbouring molecules and not shallow traps of the semiconductor. 

The increase in the mobility [200, 201] and concentration of the charge carriers [202-204] after doping was shown by time of flight methods. The drift mobility for the pure polymer had an activation energy of 0.25 eV. As a rule, the... 

Coleman, P.G., Griffith, T.C. and Heyland, G.R. (1973). A time of flight method of investigating the emission of low energy positrons from metal surfaces. Proc. Roy. Soc. Lond. A 331 561-569. 

The reaction products are ionised by either electron bombardment [98], surface ionisation [99] or photoionisation (single [100] or multiphoton [101]). The ions are then mass analysed by the use of a magnetic field, quadrupole mass filter or a time-of-flight method. The identification of reaction products is often confused by fragmentation of species in the... 

The intensity of signal transmitted to the detector is greatly improved by using time-of-flight methods instead of mechanical velocity selectors. The beam of product molecules is chopped into a sequence of short pulses and the molecules then travel a known distance before being detected. The time-of-arrival spectrum at the detector gives the velocity distribution of the products [30]. This method of velocity analysis is now widely used in studies of crossed-beam reactions [111]. 

A further improvement in transmitted intensity can be achieved in the time-of-flight method by chopping the product beam in a pseudorandom modulation sequence. In this cross-correlation method, the detected signals are then decoded to yield the time-of-flight spectrum and thence the velocity distribution [112]. 

By using a time-of-flight method to distinguish between the C, C2 and C3 components of a thermal (2550 K) carbon beam, it was possible to measure the visible emission from the CN (B2S+) product of the four-centre exchange reaction C2 + NO [614], Vibrational states up to v - 4 are populated and can be fitted by a temperature of TvIb 6900 ( 700) K. The rotational excitation of the CN (B2 +) decreases as the vibrational excitation increases (7000 > Trot > 3500 K for 0 < i> < 4). An information theory analysis of the data shows agreement of the experimental distributions with the prior forms. 

It is seen from Table 2.3 that except for the results given in the first row, the transit time is dispersive, i.e. the mobility depends on the applied field. In some cases the field dependence could be expressed as ii = cxp(B Ex/1 / kT) and temperature dependence as n = exp(—E/kT), E being in the range from 0.1 to 0.15 eV. For -6T the value 5 x 10-6 cm2 V-1 s-1 of the mobility determined by the time of flight method is a few orders of magnitude smaller than the value determined by the FE method shown later in Table 2.4. 

Table 3.6. Comparison of refinement of structure of D20 Ice VIII by neutron powder diffraction using constant wavelength and time-of-flight methods...

There are several techniques for measuring the mobility in a-Si H, most notably the time-of-flight method. All the techniques measure the average motion of the carriers over a time longer than that taken to trap a carrier in the band tail states, so that the drift mobility is always measured, rather than the free carrier mobility. The drift mobility depends on the distribution of traps and the free mobility can only be extracted if the density of states distribution is known. Chapter 3 describes how the time-of-flight experiment is used to determine the shape of the band tail through the analysis of the dispersive transport process. 

Most methods for determining the electron mobility use pulse radiolysis techniques in which the concentration of electrons is followed during or after the ionizing pulse, either by the time-of-flight method or by measurement of the change in conductivity. However, due to the inherent conductance of polar liquids, direct conductivity measurements of solvated electrons are generally difficult in these media. Therefore, the diffusion coefficient and the mobility of the solvated... 

Recently fiCF using a triple isotope mixture (H/D/T) target has drawn considerable interest [31], particularly because of resonances in the d t formation in the fit + HD collisions, predicted to be even stronger than the fit + D2 case. Our thin film target allows us to create fit collision with pure HD molecules, and time-of-flight method described here should give a direct test of the theoretical prediction. The data for the /ii- -HD collision have been collected, and the result will be reported in our future publication [32]. 

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