Dipole molecular, rotation

The rotational g factor is the ratio of the rotational magnetic dipole moment of a molecule to its molecular rotational angular momentum [1-5]. Experimentally the rotational g factor was originally determined by measuring the rotational magnetic... 

The simplest possible physical picture of the lattice contains the electron Zeeman interaction, the axially symmetric ZFS (whose principal axis coincides with the dipole-dipole axis) and the molecular rotation. The corresponding Liouvillian is given by ... 

Strong nonresonant laser fields affect molecular rotation by exerting an angle-dependent torque on the field-induced molecular dipole [15-20]. The interaction is described by the potential... 

The polarization measurements showed that the Cl atoms and SnCl fragments had the same anisotropy factor 3 = 0.21 0.01, which, if the dissociation is fast compared with molecular rotation, suggests that there is a angle of 46.5 0.4° between the transition dipole moment and the dissociation direction. 

An interesting development in molecular rotational relaxation has been the microwave double-resonance method176-178. The technique permits the exploration of the fine detail of the processes which occur in collisions of polyatomic molecules, and results for a number of symmetric tops have been reported. For example, Oka has described experiments on NH3 in which inversion doublets for selected J values were pumped by high microwave power. Pumping disturbs the population of the inversion doublet, and also that of other doublets which are populated from the original pair by collision processes. By absorption measurements of other inversion doublets with steady state irradiation, Oka has shown that in NH3/NH3 collisions, transitions which are allowed by the electric dipole selection rules (A/ = 0, 1, + - —) are preferred. Oka s analysis indicates that relaxation is most favourable in collision with molecules having similar J values, which are termed rotational resonances (R-R transfer). For example the process... 

Absorption of microwave radiation to excite molecular rotation is allowed only if the molecule has a permanent dipole moment. This restriction is less severe than it may sound, however, because centrifugal distortion can disturb the molecular symmetry enough to allow weak absorption, especially in transitions between the higher rotational states which may appear in the far IR (c. 100cm-1). Microwave spectroscopy can provide a wealth of other molecular data, mostly of interest to physical chemists rather than inorganic chemists. Because of the ways in which molecular rotation is affected by vibration, it is possible to obtain vibrational frequencies from pure rotational spectra, often more accurately than is possible by direct vibrational spectroscopy. 

B. Protein Solutions. The dielectric properties of proteins and nucleic acids have been extensively reviewed (10, 11). Protein solutions exhibit three major dispersion ranges. One occurs at RF s and is believed to arise from molecular rotation in the applied electric field. Typical characteristic frequencies range from about 1 to 10 MHz, depending on the protein size. Dipole moments are of the order of 200-500 Debyes and low-frequency increments of dielectric permittivity vary between 1 and 10 units/g protein/100 ml of solution. The high-frequency dielectric permittivity of this dispersion is lower than that of water because of the low dielectric permittivity of the protein leading to a high-frequency decrement of the order of 1 unit/g protein/... 

R-type transition in spectroscopy. As a result of light absorption in this transition the difference A — J — J" between the quantum numbers of the angular momentum in excited (J ) and ground (J") state equals +1, and the angular momentum of the molecule increases. The transition with transition dipole moment d l at frequency u>o — fl corresponds to a diminution in the angular momentum of molecular rotation, and we have A = J — J" = — 1. Such a transition is called a P-type transition. 

Equation (45) explicitly expresses the sensitivity of the LF PAD to the molecular axis distribution. In fact, an equivalent expression is obtained by convolution of the molecular axis distribution with the vibronic transition dipole matrix elements without explicit consideration of molecular rotation [55]. The... 

Naturally, since this expression is for a property measured in the reference frame connected to the molecule, molecular rotations do not appear in this expression. The range of L in the summation Eq. (53) is 0... 2/m ix, and includes both odd and even values. In general, the MF PAD is far more anisotropic than the LF PAD, for which L = 0,2,4 in a two-photon linearly polarized pump-probe experiment in the perturbative limit. Clearly, the MF PAD contains far more detailed information than the LF PAD concerning the ionization dynamics of the molecule, as well as the structure and symmetry of the electronic state from which ionization occurs, since the partial waves that may interfere are no longer geometrically limited as they are for the LF PAD. The contributing MF ionization transition dipole components are determined by the laser polarization... 

In the exciton-photon interaction, the translational molecular motions have negligible effects owing to the small amplitude of the translation compared to the optical wavelength. In contrast, the molecular rotations may cause an important variation of the transition dipole the librations may be strongly coupled to the incident photon via its coupling to the exciton. If DX(R) is the transition dipole of an a molecule in a unit cell, the first-order expansion in the libration coordinate 8 around the u axis will give... 

As mentioned before, a nonzero electric molecular dipole moment is necessary for the observation of (dipole-allowed) rotational microwave transitions. That makes it impossible to study the rotational spectra of substances such as dinitrogen, N2. Note, however, that microwave studies were... 

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