Dipole Alignment in Vacuum

Electric held seeks to orient dipoles along E, with the torque t and dipole energy proportional to the dipole moment p. If x is directed along E, the angle between E and p is and 

Unlike collisional alignment (2.5) that requires gas by definition, the dipole alignment may occur in its absence. Efforts to align polar molecules in crossed-beam 

FIGURE 2.26 Stroboscopic picture of dipole vectors in various alignment regimes. As eitjA drops, the motion evolves from free rotation (a) to hindered rotation (b), loose pendulum (c), and tight pendulum (d). 

The values ofp for ions are close to those for similar neutrals, and E 1 MV/cm needed to align dipoles with p 4 D at T= 300 K is precluded in gases by the electrical breakdown (1.3.3). Nor r 4 K required to align such dipoles at = 20 kV/cm (incidentally, a value close to the breakdown limit in N2 or air at STP) is possible because of gas liquefaction. So aligning small ions in IMS does not appear feasible. However, macroions often have p 100 D and even 10 10 D 

The mobilities of aligned ions are still governed by orientationally averaged cross sections, but Hdk for different orientations should be averaged with their weights W  

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