Acetic acid adsorption on the rutile TiO 2 (110) and (011)-2 Â 1 surfaces is compared by temperature-programmed desorption, ultraviolet photoemission spectroscopy, and scanning tunneling microscopy. In contrast to the wellestablished bidentate adsorption of carboxylic acids on the (110) surface, we find a monodentate adsorption on the (011)-2 Â 1 surface as the most likely adsorption geometry. The adsorption dynamics is also very different for the two surfaces. While acetic acid adsorbs homogeneously on the (110) surface, acetic acid adsorbs as quasi-one-dimensional clusters on the (011)-2 Â 1 surface. Initially, acetate adsorbs only at surface defects, but subsequent acetic acid can adsorb along these nucleated acetate clusters and spread to the defect-free terraces. Therefore, the preadsorbed acetic acid facilitates further acetic acid adsorption in a self-catalyzed adsorption mechanism. The acetic acid clusters preferentially grow along the [011] crystallographic surface direction forming up to tens of nanometers long clusters with a usual width of three acetates. In TPD studies acetate reacts in a unimolecular dehydration reaction yielding ketene as the main desorption product at ∼585 K for both surfaces.

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The dephasing of PbS quantum dots has been carefully measured using three pulse four-wave mixing and two-dimensional nonlinear optical spectroscopy. The temperature dependence of the homogeneous linewidth obtained from the two-dimensional spectra indicates significant scattering by acoustic phonons, whereas the excitation density dependence shows negligible excitation induced broadening in agreement with previous results. The rapid dephasing is attributed to elastic scattering by acoustic phonons. However, two dephasing components emerge, the short component that dominates the decay and a weaker longer decay, likely due to 'zero-phonon' dephasing. Quantum beats originating from two separate states can be observed, possibly revealing a ∼23.6 meV splitting of the excitonic ground state. Finally, the emergence of biexcitonic effects enhanced by the high quantum confinement is discussed.

A multidimensional optical nonlinear spectrometer (MONSTR) is a robust, ultrastable platform consisting of nested and folded Michelson interferometers that can be actively phase stabilized. The MONSTR provides output pulses for nonlinear excitation of materials and phase-stabilized reference pulses for heterodyne detection of the induced signal. This platform generates a square of identical laser pulses that can be adjusted to have arbitrary time delays between them while maintaining phase stability. This arrangement is ideal for performing coherent optical experiments, such as multidimensional Fourier-transform spectroscopy. The present work reports on overcoming some important limitations on the original design of the MONSTR apparatus. One important advantage of the MONSTR is the fact that it is a closed platform, which provides the high stability. Once the optical alignment is performed, it is desirable to maintain the alignment over long periods of time. The previous design of the MONSTR was limited to a narrow spectral range defined by the optical coating of the beam splitters. In order to achieve tunability over a broad spectral range the internal optics needed to be changed. By using broadband coated and wedged beam splitters and compensator plates, combined with modifications of the beam paths, continuous tunability can be achieved from 520 nm to 1100 nm without changing any optics or performing alignment of the internal components of the MONSTR. Furthermore, in order to achieve continuous tunability in the spectral region between 520 nm and 720 nm, crucially important for studies on numerous biological molecules, a single longitudinal mode laser at 488.5 nm was identified and used as a metrology laser. The shorter wavelength of the metrology laser as compared to the usual HeNe laser has also increased the phase stability of the system. Finally, in order to perform experiments in the reflection geometry, a simple method to achieve active phase stabilization between the signal and the reference beams has been developed.

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