Drift mobility table

Where b is Planck s constant and m and are the effective masses of the electron and hole which may be larger or smaller than the rest mass of the electron. The effective mass reflects the strength of the interaction between the electron or hole and the periodic lattice and potentials within the crystal stmcture. In an ideal covalent semiconductor, electrons in the conduction band and holes in the valence band may be considered as quasi-free particles. The carriers have high drift mobilities in the range of 10 to 10 cm /(V-s) at room temperature. As shown in Table 4, this is the case for both metallic oxides and covalent semiconductors at room temperature. 

The drift mobility of electrons in nonpolar liquids ranges from high values such as that for liquid xenon of 2000 cm /Vs to low values like that for tetradecane of 0.02 cm /Vs. It has often been suggested that the mobility is high for symmetrical molecules and low for straight chain molecules like -alkanes. Inspection of Table 2 shows that liquids with symmetrical molecules are indeed at the top of the list. However, other less symmetrical molecules like A-trimethylsilylmethane and 2,2,4,4-tetramethylpentane also show high drift mobility. A more important factor may be the existence of many methyl groups in the molecule. In any case, for liquids for which 10 cm /Vs, the electron is considered to be quasi-free. This is supported by the Hall mobility studies, as discussed below. 

When apolar electron acceptors (p = 0) were examined the hole drift mobility increased with increasing electron affinity (EA) as can be seen from Fig. 4 and Table 1. This trend applies to all compounds listed in Table 1 However, compounds having EA exceeding 1.4 eV turned out to be chemically unstable at the high electric fields afforded for these measurements. TCNQ is a typical compound behaving in that way. 

Table 7.6 Hole drift mobilities of amorphous molecular materials [a].

Numerous studies of charge transport in amorphous molecular materials have shown that hole drift mobilities of amorphous molecular materials vary widely from 10 6 to 10 2 cm2 NT1 s 1 at an electric field of 1.0 X 105 V cm-1 at room temperature, greatly depending upon their molecular structures. Table 7.6 lists hole drift mobilities of some amorphous molecular materials that function as holetransporting materials in OLEDs. 

Table 21,1, Geometry ratio Ud of the fast-flying, metastable clusters with an assumed cylindrical shape calculated from the velocity difference of the two isomers compared with results obtained from quantum chemical calculations and drift mobility measurements.

TABLE 5.4 Drift Mobility and Life Time of Carriers in Se. 

By using higher injection levels and the theories (Many and Rakavy (1962) Helfrich and Mark (1962)) which take account of the resulting space charges, Rossiter and Warfield (1971) were able to extend the drift velocity measurements to 78 K. As shown in Table 5.4 the observed activation energy for holes is 0.095 eV, considerably smaller than those of other workers. They find, moreover, a break in the T-dependence of for holes at 150 K. Below this temperature the activation energy is only 0.0093 eV. In addition, they report a small field dependence of the hole drift mobility for fields in excess of 10 V/cm. This had not been seen by the earlier groups. 

Table 4. Ionization Potential and Hole Drift Mobility of Polysilanes...

The following table presents values for the carrier concentration n calculated from lattice constants assuming that n is equal to the number of rare earth ions per cm of the compound, carrier concentration nn calculated from the Hall effect (for M " Se by the formula for the one-band model), effective mass mVmo of current carriers estimated from the position of the Fermi level and n, drift mobility and Hall mobility i (n and nn both in cm and hh both in cm --s" ) ... 

Consider, for example, a dilute aqueous solution of KC1, in which a field of 1 millivolt/cm is maintained. From the mobilities given in Table 3 we calculate that, when, for example, -is second has elapsed, the average drift in either direction for the K+ and the Cl- ions will have been less than (0.0007 X 10 3)/25 cm, that is to say, less than 3 X 10- cm (which is the diameter of one water molecule). Clearly, this distance is nothing but an average drift of the ions for during the 5 5 second, the ions in their (almost) random motion will, of course, have moved in all directions. As mentioned above, periods of molecular vibration usually lie between 10"1 - and 10- 5 sec and in 3V second each ion may have shifted its position many thousand times. Owing to the presence of the applied field the motion of the ions will not be quite random as a result of their drift the solution will appear to carry a steady current. 

Table 7.46 shows the LC-FTIR interface detection limits. Detection limits approaching those for GC-FHR light-pipe interfaces have been reported for flow-cell HPLC-FTIR when IR-transparent mobile phases are employed. For both the moving-belt and thermospray LC-MS couplings the detection limits are in the ng range. Selective evaporation consisting of fraction collection followed by DRIFT identification achieves a detection limit of 100 ng. 

TABLE 10.3 Saturation Drift Velocity in High-Mobility Liquids... 

Mobility and leachability of diflubenzuron in soils is low, and residues are usually not detectable after 7 days. In water, half-time persistence (Tb 1/2) is usually less than 8 days and lowest at elevated temperatures, alkaline pH, and high sediment loadings (Fischer and Hall 1992) (Table 17.2). Increased concentrations of diflubenzuron in soils and waters are associated with increased application frequency, flooding of treated supratidal areas, wind drift, and excessive rainfall (Cunningham 1986). 

If the solvents are immiscible, the LC system will fail. If the pump will be delivering an eluent that is not soluble with the previous mobile phase or if the new mobile phase consists of two immiscible solvents, the net result is to have tiny slugs of different solvents traveling through the HPLC. Typical indications of this problem are (1) erratic flow rate, (2) noisy baseline, and/ or (3) baseline drift. To insure that these problems are not caused by a mismatch of solvents, refer to Table 6-4 for the miscibility numbers (M) and their use. The discussion on determining solvent miscibility using miscibility numbers is adapted from reference 20. 

Insufficient conditioning of the column with mobile phase will result in retention time drift (see Figure 10.6). If the amount of time allowed for the conditioning process is too short, there will be a shift in retention time over the course of a large number of samples as the column continues to equilibrate. The column equilibration time will be dependent on the dimensions of the column. It is usual to condition the column with between 10 and 20 column volumes of mobile phase (see Table 10.1). 

Most of the early published investigations on models of ion mobility were made by physicists in relatively simple systems, mainly those in which monoatomic ions drifted through an inert monoatomic or diatomic neutral gas. - - - This is evident in Table 1 given in Appendix 1 of Reference 4. As this chapter is concerned with polyatomic ions drifting through polar or polarizable gases, especially air, there are not many detailed experimental and theoretical studies that can be cited as relevant. ... 

The validity of the models described can be tested by comparing experimentally measured reduced mobilities of several ions in the linear IMS with the predicted coefficients calculated according to the three models. The main features of interest were the correlations of mass with mobility and temperature with mobility another interesting feature is the effect of the drift gas on mobility coefficients (the last two are discussed in Chapter 11). Six parameters are needed in the modeling a, r, z, polarizability, reduced mass, and temperature. The last three arise from direct physical measurements, while the other parameters (fl, r, z) are optimized by a fitting procedure to minimize the deviation between calculated and measured mobility constants. The values of T and were calculated from a, r, and z, and the dimensionless collision cross section (1 was taken from Table 1 in Reference 9. In practice, a discrete value of a was chosen, and initial values for and z were estimated. The parameters Tq and z were then optimized to obtain a good fit with experimental data points by minimizing the squared sum of deviations between theory and experiment. Special attention... 

Currently, there are four commercial companies offering handheld and portable gas phase time-of-flight ion mobility spectrometers, with drift tubes shorter than 5 cm Smiths Detection, Bruker-Daltonics, GE-Interlogix, and G.A.S. Gesellschaft fiir analytische Sensorsysteme mbH. Table 1 presents their respective instruments and their major applications. These instruments generally use an applied electric field of around 250 Vcm , and they operate at either ambient or elevated temperature and at atmospheric pressure. The samples are ionized by radioactive nickel-63 source mostly but also by corona discharge and photoionization source. 

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