Insights into the structure of cyclohexane from femtosecond degenerate four-wave mixing spectroscopy and ab initio calculations

In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B(0) with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order M?ller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A(1g) modes, a justification of which is provided by an analysis of these modes.     

Ultrafast Electron Transfer from Photoexcited CdSe Quantum Dots to Methylviologen

The ultrafast charge separation at the quantum dot (QD)/molecular acceptor interface was investigated in terms of acceptor concentration and the size of the QD. Time‐resolved experiments revealed that the electron transfer (ET) from the photoexcited QD to the molecular acceptor methylviologen (MV²⁺) occurs on the fs time scale for large acceptor concentrations and that the ET rate is strongly reduced for low concentrations. The increase in the acceptor concentration is accompanied with a growth in the overlap of donor and acceptor wavefunctions, resulting in a faster reaction until the MV²⁺ concentration reaches a saturation limit of 0.3–0.4 MV²⁺ nm^?2. Moreover, we found significant QD size dependence of the ET reaction, which is explained by a change of the free energy (ΔG).     

Observation of pH-dependent back-electron-transfer dynamics in alizarin/TiO2 adsorbates: importance of trap states

The dependence of the interfacial electron transfer in alizarin-sensitized TiO2 nanoparticles on the sample pH has been examined via transient absorbance spectroscopy in the visible spectral region (443-763 nm). Excitation of the alizarin/TiO2 system with visible pump pulses (lambdaexc = 500 nm) leads to a very fast electron injection (tauinj < 100 fs) over a wide pH range. Back electron transfer shows complicated multiphasic kinetics and strongly depends on the acidity of the solution. The strong dependence of back-electron-transfer dynamics on the ambient pH value is explained by a Nernstian-type change in the semiconductor band energy. Indeed, a variation of pH values over 7 units leads to a approximately 0.42 eV change of the conduction band edge position (i.e., the nominal free energy of the electron in the electrode). Assuming a pH-independent redox potential of the dye, this change was sufficient to push the system to a condition where direct photoinitiated electron injection to intraband gap surface states could be investigated. The existence of an electron-transfer pathway via surface trap states is supported by the similarity of the observed back-electron-transfer kinetics of alizarin/TiO2 at pH 9 and alizarin/ZrO2 reported in earlier work (J. Phys. Chem. B 2000, 104, 8995), where the conduction band edge is approximately 1 eV above the excited state of the dye. The influence of surface trap states on interfacial electron transfer has been studied, and a detailed analysis of their population, depopulation, and relaxation kinetics is performed. Therefore, alizarin adsorbed on the surface of TiO2 nanoparticles is an ideally suited system, where pH-dependent investigations allow a detailed study of the electron dynamics in trap states of TiO2 nanoparticles.     

Rotational recurrences in thermal ensembles of nonrigid molecules

An approach has been developed to calculate the periods of rotational recurrences (RRs) in thermal ensembles of nonrigid molecules. Starting from the standard nonrigid rotor Hamiltonian with centrifugal distortion (CD) terms, the explicit analytical formulas were derived for CD-induced corrections to the periods of J, H, and K transients for symmetric top molecules. The corrections are shown to be proportional to the CD constants and the temperature. In addition, they depend considerably on the rotational constants of the molecule. Generally, the CDs are demonstrated to induce a shift in the periods of the RRs, which depends on the order (n=1,2,…) of RRs. Only the first few RRs exhibit n-independent shifts, and this is the situation in which our theory is exact. For higher order RRs, our approach starts to exaggerate slightly the values of the CD-induced corrections, giving their upper boundary.     

Coherent longitudinal-optical ground-state phonon in CdSe quantum dots triggered by ultrafast charge migration

We observe the CdSe longitudinal-optical ground-state phonon in the electron transfer system composed of CdSe quantum dots and methylviologen directly by femtosecond absorption spectroscopy. A significant phase shift indicates that the coherent oscillations are triggered by an ultrafast charge migration, which is the consequence of an electron transfer from the photoexcited quantum dot to the molecular acceptor methylviologen. In contrast, the observed coherent phonons in isolated quantum dots stem from the frequency modulation of the quantum dot excited-state spectrum. From the probe wavelength dependence of the longitudinal-optical phonons in the electronic ground state and excited state it is possible to determine a biexciton binding energy of 35?meV.     

Ultrafast Photoinduced Processes in Alizarin‐Sensitized Metal Oxide Mesoporous Films

Close to the edge : Photoexcitation of alizarin coupled to the surface of mesoporous TiO₂ films leads to ultrafast electron transfer to the TiO₂ conduction band (see picture). Complex kinetics after photoexcitation depend on the excitation energy, and indicate a position of the alizarin excited state close to the TiO₂ conduction band edge, where the density of acceptor states is reduced.

     

High aspect ratio microstructuring of transparent dielectrics using femtosecond laser pulses: method for optimization of the machining throughput

High average power, high repetition rate femtosecond lasers with μJ pulse energies are increasingly used for material processing applications. The unique advantage of material processing with sub-picosecond lasers is efficient, fast and localized energy deposition, which leads to high ablation efficiency and accuracy in nearly all kinds of solid materials. This work focuses on the machining of high aspect ratio structures in transparent dielectrics, in particular chemically strengthened Xensation? glass from Schott using multi-pass ablative material removal. For machining of high aspect ratio structures, among others needed for cutting applications, a novel method to determine the best relation between kerf width and number of overscans is presented. The importance of this relation for optimization of the machining throughput will be demonstrated.