Photons, from molecular dipoles

In Sect. 2.4 we saw that the first-order Doppler effect can be exactly canceled for a two-photon transition if the two photons hcoi = fuo2 have opposite wave vectors, i.e., k = —k2. A combination of Doppler-free two-photon absorption and the Ramsey method therefore avoids the phase dependence (p Vx) on the transverse velocity component. In the first interaction zone the molecular dipoles are excited with the transition amplitude a and precess with their eigenfrequency (jl> 2 = ( 2 — E )/h. If the two photons come from oppositely traveling waves with frequency u), the detuning... 

On the basis of the dressed photon theory and the important role of electric dipole-forbidden molecular vibrational excitations, recent theoretical studies have proposed a simple model to describe atom or atom-cluster desorption due to the dressed photons from a nanometric particle deposited on a substrate [49]. Assuming an anharmonic potential for each atomic binding, an effective atom-nanodot potential was evaluated to determine the desorption energy and the stabilized dot size. The model shows that electric dipole-forbidden molecular vibrational excitations play an important role in the phonon-assisted process, which could potentially lead to a novel fabrication method, in addition to controlling the size and position of nanostructures [51]. 

The sketch of the experimental set-up is shown in Figure 1. A Q-switched Nd-YAG laser, operating at 1.06 ixm and a pulse repetition 2-12.5 Hz was used to provide the fundamental (pump) beam. The peak power was 200-300 kW. The beam was focused with a 43 cm lens so that the power density on the sample placed in a thermostate was about 100-200 MW-cm. " For investigation the field-induced SHG, short pulses (tp = 20 fxs) of high voltage Up = 4kV) were provided by an electrical generator. The pulse duration was chosen from the condition Trelaxation time for dipolar (Debye) polarization, and T is the director reorientation time. Under such a condition, molecular dipoles are oriented by the field but the Fredericks transition does not take place. The sensitivity of our set-up was about 30 photons of the optical second harmonic per single laser pulse. The cell temperature was stabilized with an accuracy of 0.1° K. 

In molecular crystals, there are two levels of bonding intra—within the molecules, and inter—between the molecules. The former is usually covalent or ionic, while the latter results from photons being exchanged between molecules (or atoms) rather than electrons, as in the case of covalent bonds. The hardnesses of these crystals is determined by the latter. The first quantum mechanical theory of these forces was developed by London so they are known as London forces (they are also called Van der Waals, dispersion, or dipole-dipole forces). 

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