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Interaction of intense laser light with solid targets causes a cascade of electronic, thermal and mechanical processes that eventually lead to ablation, i.e. ejection of material in the direction perpendicular to the target surface. Even for a fixed target, the characteristics of the laser pulses employed for ablation ¿mainly spectral region, fluence and pulse duration¿ can cause dramatically different transient states in the first stages of the processes, in terms of electron density, mechanical stress and temperature distribution, which in turn can produce ablation media of very different nature. On the other hand, in spite of the multiplicity of factors affecting the composition and dynamics of laser ablation plasmas, it has been found that for some areas of the parameter space, common characteristics appear. One notable example is the short pulse ablation of solids in the fluence range from 0.1 to 10 J cm-2, which is known to produce the ejection of matter largely in the form of nanoparticles [1], a phenomenon that has been observed for metals, semiconductors and insulators. Another is the characteristic spatial segregation of aggregates of different sizes [2], with lighter species concentrated in the leading part of the expanding medium and heavier species predominantly found in the trailing edge. In this work we experimentally explore common and distint features of laser ablation metal plasmas (Cu, Ag, Al) generated by femtosecond vs. nanosecond laser irradiation. The diagnosis of choice has been the measurement of the nonlinear optical response of the medium [3,4] through third harmonic generation, whose lack of species selectivity is compensated by its universality. With this methodology we have found species that are otherwise elusive, like middle-sized aggregates or cold nanoparticles, and through this universal detection, we have explored the limits of the degree of control attainable in the composition of the expanding media through the laser ablation co

The photochemistry of the ethyl radical following excitation to the 3p Rydberg state is investigated in a joint experimental and theoretical study. Velocity map images for hydrogen atoms detected from photoexcited isotopologues CH3CH2, CH3CD2 and CD3CH2 at 201 nm, are discussed along with high-level ab initio electronic structure calculations of potential energy curves and non-adiabatic coupling matrix elements (NACME). A novel mechanism governed by a conical intersection allowing prompt site-specific hydrogen-atom elimination is presented and discussed. For this mechanism to occur, an initial rovibrational excitation is allocated to the radical permitting to access this reaction pathway and thus to control the ethyl photochemistry. While hydrogen-atom elimination from cold ethyl radicals occurs through internal conversion into lower electronic states followed by slow statistical dissociation, prompt site-specific Ca elimination into CH3CH + H, occurring through a fast non-adiabatic crossing to a valence bound state followed by dissociation through a conical intersection, is accessed by means of an initial ro-vibrational energy content into the radical. The role of a particularly effective vibrational promoting mode in this prompt photochemical reaction pathway is discussed.

The photodissociation dynamics of the ethyl radical following excitation into the 3s and 3p Rydbergstates are revisited in a joint experimental and theoretical study. Two different methods to produce theethyl radical, pyrolysis andin situphotolysis, are employed in order to modify the initial ro-vibrationalenergy distribution characterizing the ethyl radical beam. H-atom velocity map images followingexcitation of the radical at 243 nm and at 201 nm are presented and discussed along withab initiopotential energy curves focussing on the bridgedC2vgeometry. The reported results show that thedynamics following excitation to the 3s Rydberg state is insensitive to the initial internal energy of theparent radical, in contrast to the dynamics on the 3p Rydberg state, which is strongly modified. The roleof the bridgedC2vgeometry on both photodynamics is highlighted and discussed.

A coherent control scheme is suggested to modify the output of photodissociation in a polyatomic system. The performance of the scheme is illustrated by applying it to the ultrafast photodissociation of CH3I in the A-band. The control scheme uses a pump laser weak field that combines two pulses of a few femtoseconds delayed in time. By varying the time delay between the pulses, the shape of the laser field spectral profile is modulated, which causes a change in the initial relative populations excited by the pump laser to the different electronic states involved in the photodissociation. Such a change in the relative populations produces different photodissociation outputs, which is the basis of the control achieved. The degree of control obtained over different photodissociation observables, like the branching ratio between the two dissociation channels of CH3I yielding I(2P3/2) and I*(2P1/2) and the fragment angular distributions associated with each channel, is investigated. These magnitudes are found to oscillate strongly with the time delay, with the branching ratio changing by factors between two and three. Substantial variations of the angular distributions also indicate that the scheme provides a high degree of control. Experimental application of the scheme to general polyatomic photodissociation processes should be straightforward.

One of the most important and distinctive features of plasmonic nanostructures is their ability to confine large electromagnetic fields on nanometric volumes; i.e., the so-called hot spots. The generation, control and characterization of the hot spots are fundamental for several applications, like surface-enhanced spectroscopies. In this work, we characterize the near-field distribution and enhancement of nanostructured gold thin films fabricated by glancing angle deposition magnetron sputtering. These films are composed of columnar nanostructures with high roughness and high density of inter-columnar gaps, where the electromagnetic radiation can be confined, generating hot spots. As expected, the hot spots are localized in the gaps between adjacent nanocolumns and we use scattering-type scanning near-field optical microscopy to image their distribution over the surface of the samples. The experimental results are compared with finite-difference time-domain simulations, finding an excellent agreement between them. The spectral dependence of the field-enhancement is also studied with the simulations, together with surface-enhanced Raman spectroscopy at different excitation wavelengths in the visible-NIR range, proving a broad-band response of the substrates. These findings may result in interesting applications in the field of surface-enhanced optical spectroscopies or sensing.

Coherent radiation in the extreme ultraviolet (XUV) range obtained by high harmonic generation (HHG) brings Femtochemistry studies beyond their traditional realm. This XUV light source can be used as a short pump pulse which ionizes neutral molecules leaving the molecular cation in a combination of electronic states. The cation dynamics thus initiated can be later probed using short conventional IR or visible laser pulses. Such schemes have proven successful for the study of the dynamics of homonuclear diatomic cations. In this work we show the results of an XUV-IR experiment which demonstrates that these schemes can be extended to polyatomic molecules.

The notion that strong laser light can intervene and modify the dynamical processes of matter has been demonstrated and exploited both in gas and condensed phases. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place. The central objective of laser control schemes has been the modification of branching ratios in chemical processes, under the philosophy that conveniently tailored light can steer the dynamics of a chemical mechanism towards desired targets.

Coulomb explosion in molecules occurs when more than one electron is ejected from the system, and its charged components undergo explosion due to the electrostatic repulsion. The complete measurement of the final velocity vectors of the resulting fragments can provide information on the geometry of molecules or clusters, the alignment or orientation of a molecular ensemble in the laboratory frame, and can even provide a tool for the identification of isomers or enantiomers. The potential offered by Coulomb explosion imaging to map nonadiabatic molecular dynamics will be discussed using the example of CH3I A-band dissociation.

Experiments aimed at understanding ultrafast molecular processes are now routine, and the notion that external laser fields can constitute an additional reagent is also well established. The possibility of externally controlling a reaction with radiation increases immensely when its intensity is sufficiently high to distort the potential energy surfaces at which chemists conceptualize reactions take place. In recent experiments, we have studied strong laser field control scenarios of ultrafast molecular photodissociation dynamics. The control has been exerted on different observables of the photochemical reaction, such as quantum yields and lifetimes or even on fragment translational energies. The case study involves photodissociation of the polyatomic prototype methyl iodide (CH3I), whose ultrafast photodissociation dynamics has been studied in our laboratory for some years both in the A-band and B-band, under strong femtosecond or picosecond near-IR laser pulses. The control is achieved by opening new strong-field-induced reaction channels or by creating light-induced conical intersections and modulating the potentials around them by light-induced potentials. In particular, control of the fragment spatial distribution (the stereodynamics) in the predissociation of methyl iodide has been achieved by using strong picosecond laser pulses and the results will be presented at the Conference.

Coulomb explosion imaging is proposed as a method to directly map the presence of conical intersections encountered by a propagating wave packet in a molecular system. The case of choice is the nonadiabatic coupling between two dissociative surfaces in the methyl iodide molecule, probed by Coulomb explosion with short, intense near-infrared pulses causing multiple ionization. On-the-fly multidimensional trajectory calculations with surface hopping using perturbation theory and including spin-orbit coupling are performed to visualize the dynamics through the conical intersection and compare with experimental results. The possibilities and limitations of the technique are examined and discussed.

XUV wavelength-selected pulses obtained with high harmonic generation are studies of cation dynamics with state-by-state resolution. We demonstrate this by pump-probe experiments on CH3I+ cations and identify both resonant and non-resonant dynamics.

We present a tutorial to realize a multidimensional fitting procedure capable of extracting all the relevant information contained in a sequence of charged particle images acquired as a function of time in femtosecond pump–probe experiments. The images are reproduced using a 3D fitting method, which provides the velocity (or center-of-mass kinetic energy) and angular distributions contained in the images and their time evolution. A detailed example of the method is shown through the analysis of the time-resolved predissociation dynamics of CH3I on the B-band origin (Gitzinger et al., J. Chem. Phys.2010, 133, 234313). We show that the multidimensional approach is essential for the analysis of complex images that contain several overlapping contributions where reduced dimensionality analyses cannot provide a reliable description of the features present in the image sequence. This methodology can be generalized to many types of multidimensional data analysis.

Several radical molecules of astrochemical interest have been studied theoretically in order to characterize the electronic states (and the couplings beteeen them) involved in the different photodissociation pathways leading to fragmentation of the radical. Specifically, the radicals studied are CH3, CH3CH2, and the methoxy family CH3X (X = O, S). Highly correlated ab initio techniques have been used in the studies. In the cases of CH3 [1]-[3] and CH3X (X = O, S) [4], the potential-energy surfaces and couplings for the corresponding ground and excited electronic states were characterized using a two-dimensional representation that includes the CH2-H and CH3-X dissociative coordinate and the CH2 and CH3 umbrella angle, respectively. For the CH3CH2 radical, there are three possible dissociative coordinates (namely CH3-CH2, CH3CH-H, and H-CH2CH2), and the different electronic states involved in those dissociation pathways have been characterized along with their corresponding couplings by using a one-dimensional representation in the above three dissociative coordinates [5]. The possible photodissociation mechanisms of the radical molecues studied are discussed in the light of the ab initio data obtained. Recent experimental results on the photodissociation of CH3 [1], [2] have been successfully interpreted with the aid of the present theoretical results.

Chemical reaction dynamics and, particularly, photodissociation in the gas phase are generally studied using pump-probe schemes where a first laser pulse induces the process under study and a second one detects the produced fragments. Providing an efficient detection of ro-vibrationally state-selected photofragments, the resonance enhanced multiphoton ionization (REMPI) technique is, without question, the most popular approach used for the probe step, while non-resonant multiphoton ionization (NRMPI) detection of the products is scarce. The main goal of this work is to test the sensitivity of the NRMPI technique to fragment vibrational distributions arising from molecular photodissociation processes. We revisit the well-known process of methyl iodide photodissociation in the A-band at around 280 nm, using the velocity-map imaging technique in conjunction with NRMPI of the methyl fragment. The detection wavelength, carefully selected to avoid any REMPI transition, was scanned between 325 and 335nm seeking correlations between the different observables-the product vibrational, translational and angular distributions-and the excitation wavelength of the probe laser pulse. The experimental results have been discussed on the base of quantum dynamics calculations of photofragment vibrational populations carried out on available ab initio potential-energy surfaces using a four-dimensional model.

The notion that strong laser light can intervene and modify the dynamical processes of matter has been demonstrated and exploited both in gas and condensed phases. The central objective of laser control schemes has been the modification of branching ratios in chemical processes, under the philosophy that conveniently tailored light can steer the dynamics of a chemical mechanism towards desired targets. Less explored is the role that strong laser control can play on chemical stereodynamics, i.e. the angular distribution of the products of a chemical reaction in space. This work demonstrates for the case of methyl iodide that when a molecular bond breaking process takes place in the presence of an intense infrared laser field, its stereodynamics is profoundly affected, and that the intensity of this laser field can be used as an external knob to control it.

The electronic states and the spin–orbit couplings between them involved in the photodissociation process of the radical molecules CH3X, CH3X → CH3 + X (X = O, S), taking place after the Ã(2A1) ← [X with combining tilde](2E) transition, have been investigated using highly correlated ab initio techniques. A two-dimensional representation of both the potential-energy surfaces (PESs) and the couplings is generated. This description includes the C–X dissociative mode and the CH3 umbrella mode. Spin–orbit effects are found to play a relevant role in the shape of the excited state potential-energy surfaces, particularly in the CH3S case where the spin–orbit couplings are more than twice more intense than in CH3O. The potential surfaces and couplings reported here for the present set of electronic states allow for the first complete description of the above photodissociation process. The different photodissociation mechanisms are analyzed and discussed in light of the results obtained.