Observing molecular vibrations

Ruhman S, Joly A G and Nelson K A 1987 Time-resolved observations of coherent molecular vibrational motion and the general occurrence of impulsive stimulated scattering J. Chem. Phys. 86 6563-5... 

We find it convenient to reverse the historical ordering and to stait with (neatly) exact nonrelativistic vibration-rotation Hamiltonians for triatomic molecules. From the point of view of molecular spectroscopy, the optimal Hamiltonian is that which maximally decouples from each other vibrational and rotational motions (as well different vibrational modes from one another). It is obtained by employing a molecule-bound frame that takes over the rotations of the complete molecule as much as possible. Ideally, the only remaining motion observable in this system would be displacements of the nuclei with respect to one another, that is, molecular vibrations. It is well known, however, that such a program can be realized only approximately by introducing the Eckart conditions [38]. 

Eig. 4. Transmission profile for a siUca-based glass fiber. Region A represents electronic transitions B, the transmission window and C, molecular vibrations. Point LL is the lowest loss observed in an optical fiber. Absorption profiles for (-) OH and ( ) Fe are also shown. See text. 

SFG [4.309, 4.310] uses visible and infrared lasers for generation of their sum frequency. Tuning the infrared laser in a certain spectral range enables monitoring of molecular vibrations of adsorbed molecules with surface selectivity. SFG includes the capabilities of SHG and can, in addition, be used to identify molecules and their structure on the surface by analyzing the vibration modes. It has been used to observe surfactants at liquid surfaces and interfaces and the ordering of interfacial... 

Fujiyoshi, S., Ishibashi, T. and Onishi, H. (2006) Molecular vibrations at a liquid-liquid interface observed by fourth-order Raman spectroscopy. J. Phys. Chem. B, 110, 9571-9578. 

The last step in the calculation of the frequencies of molecular vibrations, as observed in the infrared spectra, is carried out by combining Eqs. (54) and (55). The vibrational energy of a polyatomic molecule is then given in this, the harmonic approximation, by... 

When the surface was covered with trimethyl acetate (TMA), which has no resonance with the pump and probe wavelengths, no molecular vibration was observed. When adsorbates were replaced by p-nitrobenzoate (pNB), which has two-photon resonance, the SH intensity showed a modulation at a fifth frequency assigned as a molecular vibration [76]. The study thus provided a first successful application of TRSHG to organic adsorbates. 

In ultraviolet and visible region, electronic transition of atoms and molecules are observed. This is why it is called electronic spectroscopy. In infrared region the absorption of radiation by an organic compound causes molecular vibrations and so it is called vibrational spectroscopy. 

Symmetry-forbidden transitions. A transition can be forbidden for symmetry reasons. Detailed considerations of symmetry using group theory, and its consequences on transition probabilities, are beyond the scope of this book. It is important to note that a symmetry-forbidden transition can nevertheless be observed because the molecular vibrations cause some departure from perfect symmetry (vibronic coupling). The molar absorption coefficients of these transitions are very small and the corresponding absorption bands exhibit well-defined vibronic bands. This is the case with most n —> n transitions in solvents that cannot form hydrogen bonds (e 100-1000 L mol-1 cm-1). 

The synthesis of [2.2]paracyclophane (2) and its identification by X-ray structural analysis were first reported in a short communication by Brown and Farthing 8> in 1949. A more detailed report on its molecular structure followed in 19534>. Further investigations by Lonsdale et al.5> at two different temperatures (93 and 291 °K) provided additional information about the thermal expansion and molecular vibrations in the crystal. A recent X-ray structural analysis > confirms and supplements Lonsdale s observations. 

A distinctive feature of the O2 and S2 luminescence spectra in minerals is a quasi-linear vibrational structure of the broad luminescence band (Tarash-chan 1978). The O2 and S2 molecular ions are isoelectronic. From the molecular orbital diagram describing their electron structure the emission transition Eg- n l2 is determined. When observing luminescence spectra at 77 K, a fine structure associated with the frequency of intra-molecular vibrations of O2 and S2 is detected. This frequency depends on the type of the molecular ion, on inter-nuclear distance and upon the particular position of the molecular ion in the structures. For S2 the maximum of the emission band lies within the range of 600-700 nm with a mean vibration frequency of 500-600 cm , while for O2 the respective maximum is 450-550 nm with a frequency in the 800-1,200 cm range. 

---