Single molecule spectroscopy validity

Single molecule spectroscopy has begun to reveal the microscopic nature of low-temperature glasses [9, 12, 14-16]. An important question is whether the standard tunneling model of low-temperature glass developed by Anderson et al. [94] and Phillips [95] is valid or not. As far as macroscopic... 

The second factor in (2.66) describes quite generally the transition probability for all possible two-photon transitions such as Raman scattering or two-photon absorption and emission. Figure 2.30 illustrates schematically three different two-photon processes. The important point is that the same selection rules are valid for all these two-photon processes. Equation (2.66) reveals that both matrix elements D,- and Dkf must be nonzero to give a nonvanishing transition probability A,/. This means that two-photon transitions can only be observed between two states i) and I/) that are both connected to intermediate levels fe) by allowed single-photon optical transitions. Because the selection rule for single-photon transitions demands that the levels i) and A ) or A ) and /) have opposite parity, the two levels i) and I/) connected by a two-photon transition must have the same parity. In atomic two-photon spectroscopy s s or s d transitions are allowed, and in diatomic homonuclear molecules Eg Eg transitions are allowed. 

Similar conclusion was also valid for similar system, tetrapeptide Tyr-Ala-Phe-Gly [95]. Single-crystal XRD data proved that YAFG peptide very easily forms at least three crystallographic modifications, monocHnic P2j, orthorhombic P2i2i2 and tetragonal P4i2i2. Each crystal contains different amounts of water, monoclinic is mono-hydrate, orthorhombic contains 5.5 water molecules while tetragonal 4 water molecules in the asymmetric unit. SS NMR spectroscopy proved that the orthorhombic crystals are unstable... 

---