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Low temperature infrared spectra of pure and matrix isolated methylammonium chloride have been obtained from 4000 to 300 cm −1 . The β and γ crystalline phases of this compound were studied with infrared polarized light as oriented thin films, and as powders in the Raman spectral range 4000 to about 50 cm −1 . Many exciton components of each infrared band do not have coinciding Raman counterparts, thus allowing the choice of centrosymmetric structures for both crystal phases. This fact and the number of observed components are consistent with a unit cell symmetry D 2h for the β, and C 2h for the γ phase, respectively, with no CH 3 NH + 3 ions on any symmetry element of the lattice. At room and low temperature the spectra of the solid solution in CsCl conform to at least a threefold symmetry of the methylammonium ion. Moreover, the extreme broadness of the bands in the room temperature spectrum suggests that at this temperature the ions achieve large motions around any molecular axis. The motions are strongly hindered at low temperature where, however, a relatively easy reorientation around the C-N axis of the ion survives, as suggested by the relative broadness of v 12 . The frequency of the v 6 mode in the spectrum of the matrix has been estimated from the frequency of the combination v 6 + v 9 .

Two-photon absorption spectroscopy is proving a powerful method to probe one-photon forbidden electronic transition in molecules. Two-photon absorption occurs by a non-linear process in which two photons simultaneously interact with a molecule to induce a transition. The energy of the resultant excited state equals the sum of the energies of the incident photons. What is actually measured in such an experiment is the excited fluorescence. Three main requirements should however be fulfilled for a two-photon absorption-excited fluorescence experiment to be successfully performed. First the excitation frequency must be chosen to equal one half that of an allowed transition. Second, the sample must not absorb the excitation frequency by a one-photon process and, third, a reasonable fraction of the resultant fluorescence should occur in transparent regions of the sample. Highlights of the method are: bulk excitation is achieved for solutions prepared with optically dense solvents, one-photon forbidden electric dipole transitions of even parity, e.g. , g → g or u → u, can be excited; the experimental set up can be easily assembled. This note reports on the two-photon spectra of 2,2′-bipyridine in solutions of organic non-polar solvents and in the solid state. Excitation was obtained by a nitrogen pumped dye laser with tuning ranges 5200–5900 A, (cumarine 153), and 5650–6100 A, (rhodamine 6G). The excited fluorescence, isolated by a filter with a 3300–4000 A transparent window, was detected with a 1P28 photomultiplier tube. The anode current was fed to a Box-car averager for signal processing. The spectra were registered continuously by scanning the spectrum of the dye with a step motor driven holographic grating. The excited fluorescence was check to ensure a quadratic dependence on laser power. The normalized spectra of different solutions (cyclohexane, carbon tetrachloride, benzene, almost saturated) do not differ noticeably. Bands at 33613, 34246 and 34662 cm −1 were detected, whose assignment seems correctly be cast as 1 A g (OO), OO + 615 (a g ) cm −1 and OO + 995 (a g ) cm −1 , respectively. The 615 and 995 cm −1 lines are Raman active vibrational modes of trans 2,2′-bipyridine [1]. The calculated value of the 1 A g (OO) transition is 35945 cm −1 [2]. The assignment of other features, e.g. , 35087, 36036 and 36670 cm −1 seems ambiguous being, possibly, either 1 B u (OO) (magnetic dipole transition), 1 B u + 1000 (b u ) cm −1 , 1 B u + 1500 (b u ) cm −1 or 1 A g + 1500 (a g ) cm −1 , 1 A g + overtone 1 , 1 A g + overtone 2 , respectively. In fact, a 1 B u transition has been calculated at 34977 cm −1 [2]. All the frequencies were observed in solution and in the crystal without major differences. The decay time for fluorescence both in solution and in the crystal is of the order of few nanoseconds which would indicate a strong interaction with the environment, either solvent molecules or other molecules in the crystal.

The gas phase structures of anisole dimer in the ground and first singlet electronic excited states have been characterized by a combined experimental and computational study. The dimer, formed in a molecular beam, has been studied by resonance-enhanced multiphoton ionization and high-resolution laser-induced fluorescence techniques. The assignment of the rotational fine structure of the S(1)-- S(0) electronic transition origin has provided important structural information on the parallel orientation of aromatic rings of anisole moieties. By comparison with the DFT/TD-DFT computational results, it has been possible to infer the detailed equilibrium structure of the complex. The analysis of the equilibrium structure and interaction energy confirms that the anisole dimer is stabilized by dispersive interaction in the gas phase. This is, to the best of our knowledge, the first detailed work (reporting both theoretical and high-resolution experimental data) on an isolated cluster in the pi-stacking configuration.