Rotational Heating of Polyatomic Molecules and Ions

FIGURE 2.21 Mobility of Li in N2 at 7 = 295 K measurement (circles) and calculations including and ignoring the rotational excitation of N2 (in the rigid-rotor approximation). (From Rpeggen, I., Skullerud, H.R., Lpvaas, T.H., Dysthe, D.K., J. Phys. B, 35, 1707, 2002.) 

FIGURE 2.22 Rotational heating of N2 drifting in He (P = 0.5 Torr) at high P/N proven hy LIF spectroscopy. Rotational levels are marked. (From Duncan M.A., Bierbaum, V.M., Ellison, G.B., Leone, S.R., /. Chem. Phys., 79, 5448, 1983.) 

Though at high E/N the ion transport depends in a complex way on collision integrals of various orders (2.2.1), all those quantities in classic transport theory are orientationally averaged. This implies free rotation of ions regardless of E, which is true only for atomic ions or at sufficiently low E. Polyatomic ions in gas can be oriented with respect to E by two unrelated mechanisms—collisional alignment considered here and dipole alignment (2.7). 

FIGURE 2.23 An aspherical ion or molecule flying through gas experiences more collisions when in orientations associated with higher fidjr (a) than lower Ojir (b). Gas molecules hit in each case are in hlack. 

Collisional alignment was thus far studied only for small species (diatomic and triatomic). The work following pioneering experiments of Richard N. Zare at Stanford has used the velocity slip in molecular beams (e.g., He, Ne, Ar, or H2) seeded by heavier neutrals (e.g., alkali dimers, I2, CO, or Supersonic 

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