Molecular dipole moment, modulation

Interaction of infrared radiation with a vibrating molecule is only possible if the electric vector of the radiation field oscillates with the same frequency as does the molecular dipole moment. A vibration is infrared active only if the molecular dipole moment is modulated by the normal vibration,... 

There are k = 3n 6 normal vibrations of a non-linear molecule with atoms. These vibrations may only absorb infrared radiation if they modulate the molecular dipole moment (which is a vector with the components fj, jj.y, and //. ) ... 

Stretching vibrations of atoms with different electronegativity modulate the molecular dipole moment, thus, they show strong infrared bands. Vibrations of bonds between equal atoms show infrared bands of very low intensity, however, they modulate the molecular polarizability and therefore show strong Raman bands. The intensity of the bands in the infrared spectrum is zero - the bands are forbidden in the infi ared spectrum - if the environment of both atoms is equivalent by symmetry. 

Infrared spectroscopy is now nearly 100 years old, Raman spectroscopy more than 60. These methods provide us with complementary images of molecular vibrations Vibrations which modulate the molecular dipole moment are visible in the infrared spectrum, while those which modulate the polarizability appear in the Raman spectrum. Other vibrations may be forbidden, silent , in both spectra. It is therefore appropriate to evaluate infrared and Raman spectra jointly. Ideally, both techniques should be available in a well-equipped analytical laboratory. However, infrared and Raman spectroscopy have developed separately. Infrared spectroscopy became the work-horse of vibrational spectroscopy in industrial analytical laboratories as well as in research institutes, whereas Raman spectroscopy up until recently was essentially restricted to academic purposes. 

The maximum modulation depth, w, = 1, is observed for molecules (such as molecule A) whose fluorescence intensity vanishes completely for certain excitation angles . A fit to the experimental data yields the phase angle and the maximum and minimum fluorescence intensities. From these, the modulation depth w, can be calculated according to Eq. (1). The orientation of the molecular dipole moment Q, is then given by ... 

Hyperfine structure due to quadrupole coupling of the nucleus (see p. 187) has been observed for the transitions J =1- 0 [1,4, 6] and J = 2 1 [7]. Stark modulation of a hyperfine structure component in the former transition was used to measure the molecular dipole moment [4]. Zeeman studies were carried out to determine magnetic constants [6]. Vibrational satellites (vi to V4 and 2v4 of V3, V4, and V2 + V4 of NFa) were observed as well [1, 3]. Rotational transitions between states with 14[Pg.200]

Mid-IR absorption and Stokes Raman deal with the same vibrations but are subject to different selection rules (and consequently the spectra differ). IR and RS provide complementary images of molecular vibrations. Vibrations which modulate the molecular dipole moment are visible in the IR spectrum, while those which modulate the polarisability appear in the Raman spectrum. Compositions that do not absorb in the IR range generally give a Raman spectrum and strong IR absorbers will produce a weak spectrum by Raman. Examples of silent Raman vibrational modes are specific point groups (e.g. C(, De, Cev, C4h, D, >3h. Den, etc.). Other vibrations may be forbidden in both spectra. Raman spectroscopy complements IR spectroscopy, particularly for the study of non-polar bonds and functional groups e.g. C=C, C—S, S—S, metal-metal bonds). 

Active control of population transfer using the control relation displayed in Eq. (5.23) has been demonstrated experimentally by Sherer et al. [18]. In this experiment gaseous I2 was irradiated with two short (femtosecond) laser pulses the first pulse transfers population from the ground-state potential-energy surface to the excited-state potential-energy surface, thereby creating an instantaneous transition dipole moment. The instantaneous transition dipole moment is modulated by the molecular vibration on the excited-state surface. At the proper instant, when the instantaneous transition dipole moment expectation value is maximized, a second pulse is applied. The direction of population transfer is then controlled by changing the phase of the second pulse relative to that of the first pulse. 

The Raman effect can be seen, from a classical point of view, as the result of the modulation due to vibrational motions in the electric field-induced oscillating dipole moment. Such a modulation has the frequency of molecular vibrations, whereas the dipole moment oscillations have the frequency of the external electric field. Thus, the dynamic aspects of Raman scattering are to be described in terms of two time scales. One is connected to the vibrational motions of the nuclei, the other to the oscillation of the radiation electric field (which gives rise to oscillations in the solute electronic density). In the presence of a solvent medium, both the mentioned time scales give rise to nonequilibrium effects in the solvent response, being much faster than the time scale of the solvent inertial response. 

A similar condition must be fulfilled for a vibration to be observed in the Raman spectrum. When a molecule is exposed to an electric field, electrons and nuclei are forced to move in opposite directions. A dipole moment is induced which is proportional to the electric field strength and to the molecular polarizability a. A molecular vibration can only be observed in the Raman spectrum if there is a modulation of the molecular polarizability by the vibration. 

The condition for a molecule to be Raman active is a change in the polarization (deformation) of the electron cloud during the interaction with the incident radiation. In case of Raman scattered radiation, the magnitude of the field vector E of the exciting radiation is modulated by the molecular vibrations. The induced dipole moment /t is... 

Raman spectroscopy (RS) is a well known technique to detect the vibrational characteristics of molecules in various media and is therefore extensively used in physics chemistry and biologyGenerally this technique is easily implemented, and does not require sample preparation. In addition RS has the advantage that it can be applied in water solutions, in contrast to IR absorption. In a classical picture RS results from the inelastic interaction between a molecular system and the electromagnetic field of a laser source." The electronic polarizability is modulated by the vibration mode associated with the motion of the molecule, at a frequency (Raman shift) which is the difference (Stokes scattering) or the sum (anti-Stokes scattering) between the laser and the molecular frequencies. The induced dipole moment can be written as ... 

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