The near-IR phosphorescence of singlet delta oxygen, O2(a1Ap), has provided a wealth of information since it was first observed in solution-phase systems. The techniques employed and the quality of the data obtained have improved significantly over the years that, in turn, presently makes it possible to address a wide variety of problems using both steady-state and time-resolved measurements. The development of spectroscopic methods to monitor other transitions in oxygen, specifically those that involve the singlet sigma state, 02(b1X;), and the incorporation of high-level computational methods provides access to an even broader range of fundamental issues. The expertise presently available to monitor radiative transitions in oxygen, coupled with the current understanding of the effect of solvent on these transitions as achieved through state-of-the-art theoretical modeling makes it possible to consider the next step forward: the incorporation of spatial resolution and the construction of the singlet oxygen microscope.

Langmuir-Blodgett (LB) monomolecular layers of alkylhydroxamic acids and alkylphosphonic acids on copper and iron substrates have been studied by X-ray photoelectron spectroscopy (XPS) and sum-frequency vibrational spectroscopy. According to the XPS results, the structures of the hydroxamic acid and phosphonic acid Langmuir-Blodgett films are very similar: the thickness of the layer of the hydrocarbon tails is typically 1.9-2.1 nm, while the layer of headgroups is about 0.3-0.35 nm thick. The tilt angle of the carbon chains is estimated to be 20-30 degrees with respect to the sample surface normal, and the molecules are connected to the substrate via their headgroups. Analysis of the P 2p and N 1s lines indicates the presence of deprotonated headgroups. The substrate Cu 2p line includes a component which can be assigned to Cu(2+) ions in a thin Cu(OH)(2) layer. The deposition of LB layers led to significant decrease of the hydroxide-related signal, which indicates that binding of the headgroups to the surface is accompanied by the elimination of water molecules. The sum-frequency spectra also clearly indicate that well-ordered monolayers can be formed by the Langmuir-Blodgett technique. Since the non-resonant background from the metal substrates renders the analysis of the spectra more difficult, model system samples on glass were prepared. It was found that the alkyl chains of the adsorbed acids predominantly adopt the all-trans conformation and form an ordered structure. Upper limits for the mean tilt angle of the terminal methyl groups are approximately 10-20 degrees.

The equilibrium adsorption layers of symmetric chain alkyltrimethylammonium alkyl sulfates (Cn+.Cn- for n = 8, 12) were investigated at the air/water interface by sum-frequency vibrational spectroscopy in the function of the bulk surfactant concentration. To ensure the surface purity of the solutions investigated, an improved version of the foam fractionation method was used for the purification of the constituent ionic surfactants and the surface purity of the solutions was also checked. In the monolayer of the C12+.C12- surfactant, a two-dimensional first-order gas/liquid phase transition was observed. At surfactant bulk concentrations just exceeding the concentration corresponding to the phase transition, the monolayer is conformationally disordered, liquidlike, but with increasing bulk surfactant concentration the conformational order of the monolayer increases. The SFG spectra of the C8+.C8- monolayer did not indicate the occurrence of phase transition at room temperature.

In a nanosecond time-resolved infrared spectroscopic study of dissolved oxygen, O 2 (a 1 ∆ g ) absorption, i.e., a 1 ∆ g f b 1 Σ g + , and O 2 (b 1 Σ g + ) emission, i.e., b 1 Σ g + f a 1 ∆ g , were monitored at ∼5200 cm -1 in a number of solvents. The maxima of the respective spectra depend significantly on the solvent, indicating that the O 2 (a 1 ∆ g ) and O 2 (b 1 Σ g + ) energy levels likewise depend significantly on the solvent. The corresponding Stokes shifts, however, are small. The latter, recorded as the difference between the absorption and emission maxima, do not exceed the uncertainty limits that derive from the step-scan Fourier transform spectroscopic measurements (∼ (3 cm -1 ). Nevertheless, the data clearly indicate that the difference between the equilibrium and nonequilibrium solvation energies for the O 2 (a 1 ∆ g ) and O 2 (b 1 Σ g + ) states is not large. Within the error limits, it is not possible to ascertain if the Stokes shifts are solvent dependent. Ab initio computational methods were used to model the data, and the results indicate that both long-and short-range interactions between oxygen and the perturbing solvent must be considered to adequately describe spectroscopic transitions in dissolved oxygen. The computational results indicate that a 1:1 complex between oxygen and the perturbing molecule embedded in a dielectric continuum appears to provide a sufficiently accurate model that can be used to probe subtle solvent-oxygen interactions.

In a time-resolved infrared spectroscopic study, the a1Δg → b1Σg + absorption spectrum of molecular oxygen at ∼5200 cm-1 was recorded in 19 solvents using a step-scan Fourier transform infrared spectrometer. Solvent-dependent changes in the full width at half-maximum of this absorption band covered a range of ∼30 cm-1 and solvent-dependent changes in the position of the band maximum covered a range of ∼55 cm-1. When considered along with solvent-dependent O2(a1Δg) → O2(X3Σg -) emission data, the current results identify features that must be incorporated in computational models of the interaction between oxygen and the surrounding solvent. In particular, data presented herein clearly demonstrate the importance of considering the influence of equilibrium and nonequilibrium solvation when interpreting the effect of solvent on transitions between the X3Σg -, a1Δg, and b1Σg + states of oxygen. The data indicate that the bandwidths of the O2(a1Δg) → O2(b1Σg +) and O2(a1Δg) → O2(X3Σg -) transitions principally reflect the effects of equilibrium solvation, whereas the associated solvent-dependent spectral shifts reflect the effects of both equilibrium and nonequilibrium solvation. These general conclusions make it possible to resolve some long-standing problems associated with early attempts to interpret the effect of solvent on electronic transitions in oxygen

Electronic absorption and resonance Raman spectra of the radical cation of bithiophene are reported. The bithiophene radical cation was produced by γ-radiolysis in a glassy matrix at 77 K, and the Raman spectrum excited in resonance with the two absorption bands at 425 and 590 nm. The electronic states relevant to the observed electronic transitions were identified and characterized by CASSCF calculations. The optical absorption and resonance Raman spectra were calculated by wave packet propagation methods using the ab initio calculated molecular parameters. The calculated spectra agree well with the experimental ones. The importance of carrying out full wave packet propagation calculations is underlined by the fact that in one case the simple Savin formula gave a completely wrong prediction of the resonance Raman spectrum.

The radical cation and the two lowest excited singlet Rydberg states of DABCO (1,4-diazabicyclo[2.2.2]octane) are studied. Experimentally, the radical cation of DABCO is generated by either laser flash photolysis in solution at room temperature or by γ-irradiation in a Freon glass at 77 K, and its electronic absorption and resonance Raman spectra in these two media are reported. The present resonance Raman spectra differ substantially from previous reports given in the literature, and it is concluded that a number of bands attributed previously to the DABCO radical cation are due to other species. Theoretically, the absorption and resonance Raman spectra are interpreted on the basis of density functional theory (DFT; B3LYP/6-31G(d)) calculations and wave packet propagation methods. The same DFT calculations are used to interpret excitation and multiphoton ionization spectra of the two lowest singlet Rydberg states, making use of the close similarity between a Rydberg state and its ionic core. From the combined results it is concluded that DFT calculations with a relatively modest basis set provide a valuable framework to predict potential energy surfaces of radical cations and Rydberg states in terms of minima and Hessians.

The surface properties of three water-soluble and amphiphilic PEO−PPO−PEO triblock copolymers of different composition (Pluronic 6100, 6400, and 6800) are investigated by tensiometry and sum-frequency vibrational spectroscopy. We compared the concentration dependence of the structure of surface layers prepared by three different methods:  (i) adsorption from aqueous solution, (ii) dropwise spreading from an organic solution onto a surface of constant area, and (iii) compression of the spread film in a Langmuir trough. The surface density and conformation of the polymers were deduced from the vibrational band intensities of the methyl groups of the central hydrophobic PPO block and from the surface tension isotherms. A transition range related to a conformational change was observed by tensiometry for the adsorbed and the compressed Langmuir films of Pluronics with short PEO blocks, whereas the Pluronic with longest PEO blocks displays a gradual change of surface pressure without the sign of a phase transition. This difference reveals the effect of the relative lengths of the hydrophilic and hydrophobic blocks on the structural changes in the surface layers of block copolymers. A clear indication of the influence of the hydrophilic blocks on the structure of the whole molecule at the interface was also observed in the sum-frequency experiments. Above a given concentration the Pluronic with longest PEO chains exhibited lower sum-frequency intensities and methyl symmetric/asymmetric amplitude ratios than the other Pluronic compounds for all of the layers formed by the various methods. The differences in the surface properties of the dropwise spread layers and of the compressed Langmuir layers exposed the importance of the kinetic aspects of polymer monolayer formation. In the case of the spread films the possible large degree of nonequilibrium chain entanglements might hinder the accomplishment of preferred orientation and conformation of the polymer chains.