Decoherence cross-section in NO + Ar collisions: experimental results and a simple model.

Quantum decoherence can be viewed as the mechanism responsible for the quantum-to-classical transition as the initially prepared quantum state interacts with its environment in an irreversible manner. One of the most common mechanisms responsible for the macroscopically observed decoherence involves collisions of an atom or molecule, initially prepared in a coherent superposition of states, with gas particles. In this work, a coherent superposition of quantum internal states of NO molecules is prepared by the interaction between the molecule with both a static and a radiofrequency electric field. Subsequently, NO + Ar collision decoherence experiments are investigated by measuring the loss of coherence as a function of the number of collisions. Data analysis using a model based on the interaction potential of the collisional partners allowed to unravel the molecular mechanism responsible for the loss of coherence in the prepared NO quantum superposition of internal states. The relevance of the present work relies on several aspects. On the one hand, the use of radio-waves introduces a new way for the production of coherent beams. On the other hand, the employed methodology could be useful in investigating the Stereodynamics of chemical reactions with coherent reagents.

Discharge source coupled to a deceleration unit for anion beam generation: application to H2- photodetachment.

A cathode discharge source coupled to a deceleration unit for anion beam generation is described. The discharge source, made of stainless steel or duralumin electrodes and Macor insulators, is attached to the exit nozzle valve plate at one end, and to an Einzel lens to the other end. Subsequently, a cylindrical retardation unit is attached to the Einzel lens to decelerate the ions in order to optimize the laser beam interaction time required for spectroscopic investigations. The compact device is able to produce beam intensities of the order of 2 × 10(12) anions/cm(2) s and 20 µrad of angular divergence with kinetic energies ranging from 30 to 120 eV. Using distinct gas mixtures for the supersonic expansion together with a linear time-of-flight spectrometer, anions of great relevance in molecular astrophysics like, for example, H2(-), C3H(-), C2(-), C2H(-), HCN2(-), CO2(-), CO2H(-), C4(-), C4H(-), C5H4(-), C5H6(-), C7N(-), and C10N(-) were produced. Finally, in order to demonstrate the capability of the experimental technique the photodetachment cross-section of the metastable H2(-), predominantly in the (v = 0, J = 26) state, was measured following laser excitation at λexc = 565 nm obtaining a value of σph = 0.04 Ų [corrected]. To the best of our knowledge, it is the first time that this anion cross-section has been measured.

Vibrational excitation of adsorbed molecules by photoelectrons of very low energy: acrylonitrile on Cu(100).

In this study, we report on a powerful method of primary photoelectron scattering by adsorbed species. Specifically, threshold-energy (E(kin,max) < 0.5 eV) two-photon photoelectrons (2PPE) are used to probe acrylonitrile (ACN) molecules chemisorbed onto a Cu(100) substrate, held at room temperature. This has proven to constitute a perfect tool to reveal the ACN vibrational modes in the chemisorbed state. From the dynamics of the directional (perpendicular to the copper surface) electron energy loss we conclude that only a few fundamental vibrational motions of adsorbed ACN are excited, namely the C=C, C≡N and C-H stretch modes. From the excitation probability spectra threshold energies, E(th), of these modes was extracted: E(th)(C=C) = 182(15) meV, E(th)(C≡N) = 248(16) meV--which are shifted noticeably from the equivalent gas phase values; and E(th)(C-H) ∼360-380 meV--which varies only marginally from the gas phase value. The interpretation of the excitation spectra suggests that the di-σ adsorption configuration of the terminal C- and N-atoms dominates, which agrees well with the orientation and bindings predicted in Density Functional Theory (DFT) calculations. Consistent with this is the observation that the contribution to the 2PPE excitation spectra from the C-H stretch motion is by far the largest, which are not directly affected by chemisorption bonding.

Nitric oxide beam intensity oscillations induced by the combined action of a static and a radio frequency electric field.

This paper details an experimental and theoretical investigation in which a simplified version of the molecular beam electric resonance technique is employed that requires the use of a C-field only. In the experiment the forward intensity of a NO beam is measured as a function of the frequency of the oscillating electric field over the 900-1460 kHz range. Specifically, the interaction of the NO beam with a radio frequency (rf) field of 1.12 kV/m amplitude and -610 kV/m (2) of gradient at the horizontal plane during 72 micros produces a series of oscillations in the transmitted beam intensity. The theoretical analysis shows how the interaction between a beam of NO molecules and both a static and oscillating rf field produces interferences in the forward beam intensity and how the observed interferences are due to superposition of molecular internal states. Furthermore, the interference model reproduces satisfactorily the observed beam intensity oscillations. The present technique could be useful for the development of new schemes to achieve coherent control of molecular processes using radiowaves.

Interferences in the transverse profile of a toluene beam induced by a resonant RF electric field.

In this work, the interaction of a supersonic beam of toluene diluted in He with a resonant oscillating RF field is investigated both experimental and theoretically. It is shown how the resonant field induces a peak structure in the transverse beam profile which can be explained by the onset of molecular interferences. Specifically, the interaction of a toluene beam of 0.12 eV of translational energy with a resonant RF field of 1.12 kV/m amplitude, and -610 kV/m(2) of gradient at the horizontal plane, during 160 micros produces a series of maxima in the transverse beam profile. The observed structure was satisfactorily reproduced by a quantum interference model based on the interaction of two coherent superpositions induced by the resonant RF field. It appears the present experimental technique could be useful to investigate the spectroscopy and dynamical behavior of coherent beams of polar molecules.

Interaction of polar molecules with resonant radio frequency electric fields: imaging of the NO molecular beam splitting.

The interaction between a NO supersonic beam and a resonant radio frequency (RF) field is investigated using laser ionization coupled to imaging techniques. It is shown how the resonant interaction leads to a beam splitting of +/-0.2 degrees toward both positive and negative direction perpendicular to the beam propagation axis. This phenomenon is rationalized using a model based on molecular interferences produced by the action of the resonant RF electric field.

Isotopic and internal CX3 (X = D,H) rotational motion effects in the Ba...FCX3 + h nu --> BaF + CX3 intracluster reactions.

Photodepletion and action spectra of the laser-induced Ba...FCD3 fragmentation have been measured over the 16 075-16 380 cm(-1) range. The observed band and peak structures allowed us to estimate the vibrational and rotational structures of the excited complex at the transition state configuration. The relative reaction probability P(R)(E) for the intracluster Ba...FCD3 + h nu --> BaF + CD3 reaction has been determined over the cited energy range. P(R)(E) shows a peak structure with an energy spacing of 8.9 cm(-1) which was attributed to an internal rotation of the CD3 group in the intermediate state. A comparison with previous Ba...FCH3 photofragmentation spectra reveals the dynamical role of the internal CX3 (X = H,D) motion which is manifested by the presence of rotational resonances in the laser-induced intracluster reaction.

trans-Resveratrol and grape disease resistance. A dynamical study by high-resolution laser-based techniques.

Two modern laser-based techniques were synchronously applied to study the dynamics of the trans-resveratrol activity in Botrytis cinerea-infected grapes. Direct analysis of trans-resveratrol in both infected and noninfected grapes (Vitis vinifera, Aledo variety) was performed by using an analytical technique incorporating laser desorption coupled with laser resonant ionization and time-of-flight mass spectrometry. On the other hand, one of the most sensitive on-line methods for trace gas detection, laser photoacoustic spectroscopy, was used to investigate the involvement of the plant hormone ethylene (C(2)H(4)) in the B. cinerea grapes interaction and its temporal relationship with the trans-resveratrol content upon infection. The trans-resveratrol content and the ethylene released by noninfected grapes showed an opposite behavior. In this case, a high trans-resveratrol content corresponds to a low ethylene emission. For the B. cinerea-infected grapes, ethylene emission rises up after 48 h when the analogous content of trans-resveratrol started to decrease irreversibly. Moreover, the activity of trans-resveratrol as natural pesticide has been investigated by exogenous application on grapes. A short submerge (5 s) of the grapes in 1.6 x 10(-4) M solution of trans-resveratrol delays the increase of C(2)H(4) emission with about 48 h and produces a decrease of the C(2)H(4) concentration and its emission rate. The treatment has positive effects on fruit conservation during storage; it doubled the normal shelf-life of grapes at room temperature, maintaining their post-harvest quality within 10 d.

Improving postharvest resistance in fruits by external application of trans-resveratrol.

As it is well-known, one of the main problems of modern agriculture is the postharvest fruit losses due to pathogen's attack and natural senescence during storage. Well established solutions to improve this situation, such as, for example, storage under controlled conditions and the use of synthetic pesticides, are not free of problems due to human health risks and environmental effects caused by chemical pesticides. A new strategy to solve these problems consists of developing methods to improve the natural plant resistance by using, upon their identification, the plant's own defense molecules, in other words, applying methods based on the plant's own natural processes of pest suppression to control spoilage. This requires the identification of components of the natural defense response in plants, which, in turn, demands highly sensitive, fast, and versatile analytical methods especially for trace, nonvolatile, compounds. In this work a laser-based technique has been applied for screening the postharvest elicitation of resveratrol by Botrytis cinerea in grapes. Besides antifungal character, resveratrol is known to present important antioxidant properties, which could also have positive effects on fruit conservation during storage. Consequently, several experiments were carried out in which exogenous application of resveratrol to several fruits maintained their postharvest quality. The quality of both resveratrol-treated and untreated fruits has been studied by the assessment of the biochemical composition and sensory analysis. Indeed, the present work demonstrates that the external application of resveratrol does not alter the sensorial and biochemical properties of the fruit.