583 research outputs found
Recent advances in experimental techniques to probe fast excited-state dynamics in biological molecules in the gas phase : dynamics in nucleotides, amino acids and beyond
In many chemical reactions, an activation barrier must be overcome before a chemical transformation can occur. As such, understanding the behaviour of molecules in energetically excited states is critical to understanding the chemical changes that these molecules undergo. Among the most prominent reactions for mankind to understand are chemical changes that occur in our own biological molecules. A notable example is the focus towards understanding the interaction of DNA with ultraviolet radiation and the subsequent chemical changes. However, the interaction of radiation with large biological structures is highly complex, and thus the photochemistry of these systems as a whole is poorly understood. Studying the gas-phase spectroscopy and ultrafast dynamics of the building blocks of these more complex biomolecules offers the tantalizing prospect of providing a scientifically intuitive bottom-up approach, beginning with the study of the subunits of large polymeric biomolecules and monitoring the evolution in photochemistry as the complexity of the molecules is increased. While highly attractive, one of the main challenges of this approach is in transferring large, and in many cases, thermally labile molecules into vacuum. This review discusses the recent advances in cutting-edge experimental methodologies, emerging as excellent candidates for progressing this bottom-up approach
A sub-Doppler resolution double resonance molecular beam infrared spectrometer operating at chemically relevant energies (~2 eV)
A molecular beam spectrometer capable of achieving sub-Doppler resolution at 2 eV (~18 000 cm^–1) of vibrational excitation is described and its performance demonstrated using the CH stretch chromophore of HCN. Two high finesse resonant power-buildup cavities are used to excite the molecules using a sequential double resonance technique. A v = 0-->2 transition is first saturated using a 1.5 µm color center laser, whereupon a fraction of the molecules is further excited to the v = 6 level using an amplitude modulated Ti:Al2O3 laser. The energy absorbed by the molecules is detected downstream of both excitation points by a cryogenically cooled bolometer using phase sensitive detection. A resolution of approximately 15 MHz (i.e., three parts in 10^8) is demonstrated by recording a rotational line in the v = 6 manifold of HCN. Scan speeds of up to several cm^–1/h were obtained, with signal-to-noise ratios in excess of 100. The high signal-to-noise ratio and a dynamic range of 6×10^4 means that future experiments to study statistical intramolecular vibrational energy redistribution in small molecules and unimolecular isomerizations can be attempted. We would also like to point out that, with improved metrology in laser wavelengths, this instrument can also be used to provide improved secondary frequency standards based upon the rovibrational spectra of molecules
Enhanced selectivity towards O2 and H2 dissociation on ultrathin Cu films on Ru(0001)
The following article appeared in Journal of Chemical Physics 137.7 (2012): 074706 and may be found at http://scitation.aip.org/content/aip/journal/jcp/137/7/10.1063/1.4746942The reactivity of Cu monolayer (ML) and bilayer films grown on Ru(0001) towards O2 and H2 has been investigated. O2 initial sticking coefficients were determined using the King and Wells method in the incident energy range 40-450 meV, and compared to the corresponding values measured on clean Ru(0001) and Cu(111) surfaces. A relative large O2 sticking coefficient (∼0.5-0.8) was measured for 1 ML Cu and even 2 ML Cu/Ru(0001). At low incident energies, this is one order of magnitude larger than the value observed on Cu(111). In contrast, the corresponding reactivity to H2 was near zero on both Cu monolayer and bilayer films, for incident energies up to 175 meV. Water adsorption on 2 ML Cu/Ru(0001) was found to behave quite differently than on the Ru(0001) and Cu(111) surfaces. Our study shows that Cu/Ru(0001) is a highly selective system, which presents a quite different chemical reactivity towards different species in the same range of collision energiesThe authors gratefully acknowledge the financial support by the Ministerio de Educación y Ciencia through projects CONSOLIDER-INGENIO 2010 on Molecular Nanoscience and FIS2007-61114 and Comunidad de Madrid through the program NANOMAGNET S-0505/MAT/0194. P.P. acknowledges support through the Marie Curie AMAROUT EU action and the Spanish MICINN “Juan de la Cierva” contrac
Velocity-selected molecular pulses produced by an electric guide
Electrostatic velocity filtering is a technique for the production of
continuous guided beams of slow polar molecules from a thermal gas. We extended
this technique to produce pulses of slow molecules with a narrow velocity
distribution around a tunable velocity. The pulses are generated by
sequentially switching the voltages on adjacent segments of an electric
quadrupole guide synchronously with the molecules propagating at the desired
velocity. This technique is demonstrated for deuterated ammonia (ND),
delivering pulses with a velocity in the range of and a
relative velocity spread of at FWHM. At velocities around
, the pulses contain up to molecules each. The data are
well reproduced by Monte-Carlo simulations, which provide useful insight into
the mechanisms of velocity selection.Comment: 8 pages, 6 figure
Using Cold Atoms to Measure Neutrino Mass
We propose a beta decay experiment based on a sample of ultracold atomic
tritium. These initial conditions enable detection of the helium ion in
coincidence with the beta. We construct a two-dimensional fit incorporating
both the shape of the beta-spectrum and the direct reconstruction of the
neutrino mass peak. We present simulation results of the feasible limits on the
neutrino mass achievable in this new type of tritium beta-decay experiment.Comment: 10 pages, 5 figure
Stark deceleration of lithium hydride molecules
We describe the production of cold, slow-moving LiH molecules. The molecules
are produced in the ground state using laser ablation and supersonic expansion,
and 68% of the population is transferred to the rotationally excited state
using narrowband radiation at the rotational frequency of 444GHz. The molecules
are then decelerated from 420m/s to 53m/s using a 100 stage Stark decelerator.
We demonstrate and compare two different deceleration modes, one where every
stage is used for deceleration, and another where every third stage decelerates
and the intervening stages are used to focus the molecules more effectively. We
compare our experimental data to the results of simulations and find good
agreement. These simulations include the velocity dependence of the detection
efficiency and the probability of transitions between the weak-field seeking
and strong-field seeking quantum states. Together, the experimental and
simulated data provide information about the spatial extent of the source of
molecules. We consider the prospects for future trapping and sympathetic
cooling experiments.Comment: 14 pages, 6 figures; minor revisions following referee suggestion
Digestibilidad y energía metabolizable en Amaranthus greggii S. Wats.
Amaranthus greggii S. Wats. es una especie de posible valor alimenticio, perenne, susceptible a heladas, cultivable bajo cubierta, a la que se le han estudiado algunos caracteres organolépticos, composición química y posibles aplicaciones hortícolas
Adiabatic orientation of rotating dipole molecules in an external field
The induced polarization of a beam of polar clusters or molecules passing
through an electric or magnetic field region differs from the textbook
Langevin-Debye susceptibility. This distinction, which is important for the
interpretation of deflection and focusing experiments, arises because instead
of acquiring thermal equilibrium in the field region, the beam ensemble
typically enters the field adiabatically, i.e., with a previously fixed
distribution of rotational states. We discuss the orientation of rigid
symmetric-top systems with a body-fixed electric or magnetic dipole moment. The
analytical expression for their "adiabatic-entry" orientation is elucidated and
compared with exact numerical results for a range of parameters. The
differences between the polarization of thermodynamic and "adiabatic-entry"
ensembles, of prolate and oblate tops, and of symmetric-top and linear rotators
are illustrated and identified.Comment: 18 pages, 4 figure
High-energy-resolution molecular beams for cold collision studies
Stark deceleration allows for precise control over the velocity of a pulsed
molecular beam and, by the nature of its limited phase-space acceptance,
reduces the energy width of the decelerated packet. We describe an alternate
method of operating a Stark decelerator that further reduces the energy spread
over the standard method of operation. In this alternate mode of operation, we
aggressively decelerate the molecular packet using a high phase angle. This
technique brings the molecular packet to the desired velocity before it reaches
the end of the decelerator; the remaining stages are then used to
longitudinally and transversely guide the packet to the detection/interaction
region. The result of the initial aggressive slowing is a reduction in the
phase-space acceptance of the decelerator and thus a narrowing of the velocity
spread of the molecular packet. In addition to the narrower energy spread, this
method also results in a velocity spread that is nearly independent of the
final velocity. Using the alternate deceleration technique, the energy
resolution of molecular collision measurements can be improved considerably.Comment: 12 pages, 9 figure
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