A novel application of vibrational sum frequency generation (VSFG) is developed to study the molecular properties of the surface of submicron particles in suspension. The Rayleigh-Gans-Debye scattering theory is extended to extract the local molecular response from the macroscopic nonlinearly scattered spectral intensity. These results demonstrate the use of VSFG to investigate quantitatively the surface molecular properties of submicron particles, dispersed in solution. It provides information on the order and density of alkane chains and allows us to determine the elements of the local second-order surface susceptibility.

We report on the construction and performance of a novel photoelectron-photoion coincidence machine in our laboratory in Amsterdam to measure the full three-dimensional momentum distribution of correlated electrons and ions in femtosecond time-resolved molecular beam experiments. We implemented sets of open electron and ion lenses to time stretch and velocity map the charged particles. Time switched voltages are operated on the particle lenses to enable optimal electric field strengths for velocity map focusing conditions of electrons and ions separately. The position and time sensitive detectors employ microchannel plates (MCPs) in front of delay line detectors. A special effort was made to obtain the time-of-flight (TOF) of the electrons at high temporal resolution using small pore (5 microm) MCPs and implementing fast timing electronics. We measured the TOF distribution of the electrons under our typical coincidence field strengths with a temporal resolution down to sigma=18 ps. We observed that our electron coincidence detector has a timing resolution better than sigma=16 ps, which is mainly determined by the residual transit time spread of the MCPs. The typical electron energy resolution appears to be nearly laser bandwidth limited with a relative resolution of DeltaE(FWHM)/E=3.5% for electrons with kinetic energy near 2 eV. The mass resolution of the ion detector for ions measured in coincidence with electrons is about Deltam(FWHM)/m=14150. The velocity map focusing of our extended source volume of particles, due to the overlap of the molecular beam with the laser beams, results in a parent ion spot on our detector focused down to sigma=115 microm.

The ablation of ionized silicon is studied using near-infrared femtosecond laser pulses in the fluence regime from 1 to 9 J/cm2. Two major peaks are observed in the mass spectrum corresponding to Si+ and Si2+. In the time-of-flight transients of Si+, a bimodal structure is observed. The fast Si+ peak corresponds to a velocity half of that observed for Si2+. This momentum scaling is clearly indicative of a Coulomb explosion.

The origin of the resonant structures in the spectrum of the predissociative part of the A state in the NO-Ar van der Waals cluster has been investigated. We have employed direct excitation to the predissociative part of the NO-Ar A state followed by rotational state selective ionization of the NO fragment. Velocity map imaging of the NO ion yields the recoil energy of the rotational state-selected fragment. A substantial contribution of rotational hotbands to the resonant structures is observed. Our data indicate that a centrifugal barrier as the origin of these resonances can be ruled out. We hypothesize that after the NO-Ar cluster is excited to the A state sufficient mixing within the rotating cluster takes place as it changes geometry from being T shaped in the NO͑X͒ -Ar state to linear in the NO͑A͒ -Ar state. This mixing allows the low energy and high angular momentum ͑J Ϸ 4.5͒ tumbling motion of the initially populated hotbands in the ground state NO͑X͒ -Ar complex to be converted into NO͑A , n =2͒ spinning rotation in the A state of the complex. The electronically excited spinning complex falls apart adiabatically producing rotationally excited NO͑A , n =2͒ at the energetic threshold. This interpretation indicates that the resonances can be attributed to some type of vibrational Feshbach resonance. The appearance energy for the formation of NO͑A , n =0͒ + Ar is found to be 44294.3Ϯ 1.4 cm −1 .

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The increasing awareness of the adverse effects of exposure to UV radiation in combination with the conclusion that presently employed sunscreen agents are far from optimal has led to the need to develop novel UV filters with improved absorption and stability characteristics. Studies of natural sunscreens can provide fundamental insight into strategies to come to a rational design of such filters. Here, we use high-resolution laser spectroscopic methods to study the spectroscopy and excited-state dynamics of methyl sinapate, a prominent plant sunscreen derivative. We find that our experimental approach based on two-color Resonance Enhanced Two-Photon Ionisation spectroscopy enables us to observe a hitherto not observed decay pathway leading to a long-lived state, albeit with a significantly lower yield than in cinnamates and coumarates. In combination with extended excitation spectra and employing the results of quantum chemical calculations a comprehensive picture is obtained of the internal conversion pathways that are accessible to this compound. Similar high-resolution studies on clusters of methyl sinapate with water show how solvent-solute interactions affect the electronic structure and excited-state decay dynamics of the chromophore.

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Herein, we review the current capabilities and potential of advanced single-particle imaging techniques to study photodynamics in isolated molecules. These reaction microscopes are able to measure the full three-dimensional energy and angular distribution of (correlated) particles such as electrons and molecular fragments ejected after photoexcitation of molecules. In particular, we discuss the performance and capabilities of a novel photoelectron-photoion coincidence imaging spectrometer constructed at LaserLaB Amsterdam. This microscope was developed for the study of nonadiabatic effects in ultrafast time-resolved experiments. It is specifically targeted at optimal control studies of photodynamics to foster and advance our understanding of mechanisms in optimal control with shaped ultrafast laser pulses. We review a few recent experimental results illustrating the wealth of detailed information that can be obtained in such imaging experiments about the interplay between (shaped) laser fields, molecular dynamics, ionization processes and competing multichannel pathways. Furthermore, the recently developed photoelectron-circular-dichroism imaging technique to detect enantiomers and to study chirality effects will be discussed, as a further illustration of the potential of modern reaction microscopes in stereochemistry.