Field-free alignment

Figure 5.3. The classical picture the energies of dipoles varies continuously from parallel alignment with the applied magnetic field (-m-B) to antiparallel (+m-B). On the right is shown the distribution of molecules that results and the lower mean energy of the ensemble relative to the field-free environment.

J. G. Underwood, B. J. Sussman, and A. Stolow. Field-free three dimensional molecular axis alignment. Phys. Rev. Lett., 94(14) 143002 (2005). 

A typical application is the use of the (2 + 1) REMPI scheme for measuring the (v,./) distribution of H2 produced in associative desorption from a surface. When the laser is tuned to a spectroscopic transition between individual quantum states in the X -> E electronic band, resonant two-photon absorption populates the E state and this is subsequently ionized by absorption of another photon. The ion current is proportional to the number in the specific (v,./) quantum state in the ground electronic state that is involved in the spectroscopic transition. Tuning the laser to another spectroscopic feature probes another (v, J) state. Therefore, recording the ion current as the laser is scanned over the electronic band maps out the population distribution of H2(v, J) produced in the associative desorption. Ef of the (v, J) state can also often be simultaneously measured using field - free ion TOF or laser pump - probe TOF detection techniques. The (2 +1) REMPI scheme for detecting H2 is almost independent of the rotational alignment and orientation f(M) of molecules so that only relative populations of the internal states... 

Two polarization mechanisms are possible. If the molecules possess a permanent electric dipole moment pbp rm, each molecule can align its moment with the field direction by reorientation, producing a macroscopic dipole moment. Even if perm = 0 in the field-free limit, each molecule can achieve a field-dependent dipole moment pind by induction. The induced dipole moment is proportional to field strength, pind = a , where a is the electric polarizability of the molecule. In both cases, work must be performed on the system to achieve the macroscopic polarization. Molecules with large permanent dipole moments correspond to high k. 

Fig. 2.2. Schematic diagram of a hybrid-aligned nematic cell. The field-free director (shown by the continuous curved line) has a splay-bend curvature distortion in the xz plane. A DC field applied along the y axis rotates the polarization and the director (shown by the curved dashed line) acquires a 4>(z) profile. (Reproduced from Dozov et al. with the permission of EDP Sciences, http //publications.edpsciences.org.)...

Daems D, Guerin S, Hertz E, Jauslin HR, Lavorel B, Faucher O (2005) Field-free two-direction alignement alternation of linear molecules by elliptic laser pulses. Phys Rev Lett... 

Polar liquids and gases possess permanent dipoles which can be aligned by an applied electric field. The aligning energy provided by the field is in competition with thermal motion which tends to randomize the orientation of the dipoles. Such materials are called paraelectrics. When the field is removed, the dipoles relax with a time constant t, which is on the order of 10 s for liquids such as water where the dipoles are free to rotate. These dipoles can couple with microwave radiation to cause heating of the material, which is the principle on which microwave ovens operate. 

B. Schematic representation of the channel aligned with the free energy profile above and indicating the direction of the field. Reproduced with permission from Ref. 45)... 

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