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The photodissociation dynamics leading to the C–N bond cleavage in methylamine (CH3NH2) are investigated upon photoexcitation in the blue edge of the first absorption A-band, in the 198–204 nm range. Velocity map images of the generated methyl (CH3) fragment detected in specific vibrational modes, i.e., ν = 0, ν1 = 1, and ν2 = 1, through resonance enhanced multiphoton ionization, are presented along with the corresponding translational energy distributions and the angular analysis. The experimental results are complemented by high-level ab initio calculations of potential energy curves as a function of the C–N bond distance. While a similar single Boltzmann-type contribution is observed in all the translational energy distributions measured, the speed-dependent anisotropy parameter obtained through the angular analysis reveals the presence of two different mechanisms. Prompt dissociation through the conical intersection between the A ̃ 1 A ′ first excited state and the ground state located in the exit channel is, indeed, revealed as a minor channel. In contrast, slow dissociation on the ground state, presumably from frustrated N–H bond cleavage trajectories, constitutes the major reaction pathway leading to the methyl formation. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of methylamine (CH3NH2) upon excitation in the blue edge of the first absorption A-band, in the 198–203 nm range, are investigated by means of nanosecond pump–probe laser pulses and velocity map imaging combined with H(2S)-atom detection through resonance enhanced multiphoton ionization. The images and corresponding translational energy distributions for the H-atoms produced show three different contributions associated with three reaction pathways. The experimental results are complemented by high-level ab initio calculations. The potential energy curves computed as a function of the N–H and C–H bond distances allow us to draw a picture of the different mechanisms. Major dissociation occurs through N–H bond cleavage and it is triggered by an initial geometrical change, i.e., from a pyramidal configuration of the C–NH2 with respect to the N atom to a planar geometry. The molecule is then driven into a conical intersection (CI) seam where three outcomes can take place: first, threshold dissociation into the second dissociation limit, associated with the formation of CH3NH ( A ̃) , is observed; second, direct dissociation after passage through the CI leading to the formation of ground state products; and third, internal conversion into the ground state well in advance to dissociation. While the two last pathways were previously reported at a variety of wavelengths in the 203–240 nm range, the former had not been observed before to the best of our knowledge. The role of the CI and the presence of an exit barrier in the excited state, which modify the dynamics leading the two last mechanisms, are discussed considering the different excitation energies used. [ABSTRACT FROM AUTHOR]

The valence-shell dissociative photoionization of acetaldehyde has been investigated by means of the photoion photoelectron coincidence technique in conjunction with tuneable synchrotron radiation. The experimental results consist of threshold photoelectron spectra for the parent ion and for each fragment ion in the 10.2-19.5 eV photon energy range, along with (ion, e) kinetic energy coincidence diagrams obtained from measurements at fixed photon energies. The results are complemented by high-level ab initio calculations of potential energy curves as a function of the C-H bond distance. The nudged elastic band (NEB) method has been employed to connect the parent ion Franck-Condon region to the formation of the HCO+, CH3+ and CH4+ ion fragments. Appearance energies have been determined for six fragment ions with an improved accuracy, including two fragmentation channels, which to the best of our knowledge have not been reported previously, i.e. the formation of CH2CO+, lying at 13.10 x 0.05 eV, and the formation of CH2+ at 15.1 x 0.1 eV. Based on both experimental and theoretical results, the dissociation dynamics following ionization of acetaldehyde into the different fragmentation channels are discussed. [ABSTRACT FROM AUTHOR]

In this work, laser‐induced periodic surface structures (LIPSS) are generated on the surface of thin films of poly(trimethylene terephthalate) (PTT) fabricated by spin coating on silicon. Previous studies have shown the appearance of LIPSS in this type of materials with femtosecond laser pulses in the near‐infrared region of the spectrum, despite the extremely low absorption of the polymer. The present work consists of an exploration of the effect of adding complexity to the femtosecond laser fields employed for irradiation. Changes in the repetition rate of the pulse train, the pulse duration, and the pulse temporal chirp are explored, and conditions for optimum LIPSS formation are identified. The effects of these modifications on the polymer surface topography, assessed by atomic force microscopy, are described and discussed. [ABSTRACT FROM AUTHOR]

The reaction of the nitrogen atom (N) in its first electronically excited state (2D) with cyanoacetylene (HC3N) has been investigated under single-collision conditions by using the crossed molecular beam method with mass spectrometric detection at a collision energy of 31 kJ mol−1. With the support of electronic structure calculations, we found that this reaction proceeds via the barrierless addition of the N(2D) atom to the carbon–carbon triple bond of HC3N, followed by the formation of a cyclic intermediate adduct HC(N)CCN, which dissociates to C(N)CCN + H products or isomerises to a more stable intermediate HNCCCN by H-migration and ring-opening processes. The long-lived HNCCCN complex produces the linear 3Σg– ground state dicyanocarbene (NCCCN) radical plus atomic hydrogen through a barrierless unimolecular dissociation accompanied by a negligible competitive channel forming the NCCCN radical (1A1) with a bent C2v structure plus H. The main product of this neutral-neutral reaction is the 3NCCCN radical that could be a potential precursor to form other nitriles (C2N2, C3N) or more complex organic species in planetary atmospheres, such as that of Titan and Pluto, in cometary comas, and in UV irradiated interstellar environments. [ABSTRACT FROM AUTHOR]

The identification of the primary products and the determination of their branching ratios as a function of translational energy (temperature) for multi-channel elementary (bimolecular) reactions of importance in combustion flames and plasma-assisted combustion still represent a challenge for traditional kinetics experiments. On the other hand, this kind of information is central for the detailed modeling of combustion/plasma systems. In this short review, the significant contribution provided in this area by the crossed molecular beam (CMB) scattering method with "universal" mass-spectrometric detection and time-of-flight analysis is illustrated. In particular, we describe the basics of the CMB technique empowered with "soft" electron-impact ionization as recently implemented in our laboratory, and report on its application to the study of the multi-channel elementary reactions of ground state atomic oxygen, O(3P), with unsaturated hydrocarbons containing two carbon atoms (acetylene and ethylene), three carbon atoms (propyne, propene, and allene), and also four carbon atoms (1-butene, 1,2-butadiene, and 1,3-butadiene), which are of paramount interest in combustion flames and plasma-assisted combustion of hydrocarbons. These studies are usually complemented in a synergistic manner by high-level electronic structure calculations of the underlying potential energy surfaces and related statistical (and dynamical when feasible) calculations of product branching ratios. The complementarity to kinetics studies and the implications of the dynamics results for the modeling of combustion/plasma chemistry will be commented on. [ABSTRACT FROM AUTHOR]