Three-photon resonances, molecular photonics

The value of coherent control experiments lies not only in their ability to alter the outcome of a reaction but also in the fundamental information that they provide about molecular properties. In the example of phase-sensitive control, the channel phase reveals information about couplings between continuum states that is not readily obtained by other methods. Examination of Eq. (15) reveals two possible sources of the channel phase—namely, the phase of the three-photon dipole operator and that of the continuum function, ESk). The former is complex if there exists a metastable state at an energy of (D or 2 >i, which contributes a phase to only one of the paths, as illustrated in Fig. 3b. In this case the channel phase equals the Breit-Wigner phase of the intermediate resonance (modulo n),... 

The rather weak response of 5T and 6T [174] is due to the unfavorable orientation of the long molecular axis with respect to the normal of the textured thin films. Since the long molecular axis makes an angle of 32° with respect to the normal [12,133] (section 7.4.3.1), the lowest Bu state contributes accidentally very little to the actual three-photon resonance enhancement (vide infra). A marked increase of the three-photon resonance would be observed if the long axis would lie in the substrate plane. 

The use of optical methods which probe interface electronic and vibrational resonances offers significant advantages over conventional surface spectroscopic methods in which, e.g. beams of charged particles are used as a probe, or charged particles emitted from the surface/interface after photon absorption are detected. Recently, three-wave mixing techniques such as second-harmonic generation (SHG) have become important tools to study reaction processes at interfaces. SHG is potentially surface-sensitive at nondestructive power densities, and its application is not restricted to ultrahigh vacuum (UHV) conditions.However, SHG suffers from a serious drawback, namely from its lack of molecular selectivity. As a consequence, SHG cannot be used for the identification of unknown surface-species. 

In this section we consider the role of molecular vibrations in resonant L NLO properties. Tliese properties govern the intensity of light absorption (or emission) accompanying tlie transition between two vibronic energy levels. Of interest here is one- and two-photon absorption (OPA and TPA). Applications of TPA, in particular, include three-dimensional optical data storage and photodynamic therapy. 

Fig. 11.3. Photon stimulated desorption from a surface of Ce02, as a function of photon energy, in the range of the Ad — / giant resonance. The top three curves show the desorption signal for several molecular ions containing H, while the bottom curve shows the photoabsorption cross section for comparison (after B.E. Koel et at. [602]).

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