Vibrational Relaxation of D2O Induced by Collision with He: A Rigid Bender Close Coupling Study.

The vibrational relaxation of the first excited bending state of D2O induced by collision with He is studied at the close coupling level and using the Rigid Bender approximation. A new 4D potential energy surface is calculated and reported for this system. It is then used to determine the low-lying bound states of the D2O-He van der Waals complex and to perform scattering calculations. Collision rates are determined for pure rotational transitions as well as for rovibrational transitions within the first excited bending state. The results are compared with those obtained for the collision of D2O with other noble gases such as Ne and Ar. We also analyse the differences observed with respect to the H2O+He collisions and compare our results with experiment.

A Rigid Bender Study of the Bending Relaxation of H2O and D2O by Collisions with Ar.

The bending relaxation of H2O and D2O by collisions with Ar is studied at the Close Coupling level. Two new 4D PES are developed for these two systems. They are tested by performing rigid rotor calculations as well as by computing the D2O-Ar bound states. The results are compared with available theoretical and experimental data. Propensity rules for the dynamics are discussed and compared to those of H2O colliding with Ne or He. The bending relaxation cross sections and rates are then calculated for these two systems. The results are analysed and compared with available experimental data.

An explicitly correlated six-dimensional potential energy surface for the SiCSi + H2 complex.

The first six-dimensional potential energy surface (PES) for the SiCSi + H2 complex is presented in this work. This surface is developed from a large number of ab initio energies computed at the explicitly correlated coupled-cluster level of theory together with the augmented correlation-consistent polarized valence triple zeta basis set (CCSD(T)-F12/aug-cc-pVTZ). These energies are fitted to an analytical function through a procedure that combines spline, least-squares, and kernel-based methods. Two minimums of similar depths were found at the equilibrium geometry of the SiCSi molecule. The dependence of the PES on the bending angle is analyzed. Furthermore, a reduced four-dimensional PES averaged over the H2 orientation is presented. Finally, the six-dimensional PES is used for computing the second virial coefficient of the SiCSi + H2 pair using classical and semi-classical methods.

A close coupling study of the bending relaxation of H2O by collision with He.

We present a close coupling study of the bending relaxation of H2O by collision with He, taking explicitly into account the bending-rotation coupling within the rigid-bender close-coupling method. A 4D potential energy surface is developed based on a large grid of ab initio points calculated at the coupled-cluster single double triple level of theory. The bound states energies of the He-H2O complex are computed and found to be in excellent agreement with previous theoretical calculations. The dynamics results also compare very well with the rigid-rotor results available in the Basecol database and with experimental data for both rotational transitions and bending relaxation. The bending-rotation coupling is also demonstrated to be very efficient in increasing bending relaxation when the rotational excitation of H2O increases.

Study of charge sharing effect and energy resolution of the Timepix hybrid detector based on gallium arsenide compensated with chromium / Estudio del efecto de compartición de cargas y la resolución energética del detector híbrido Timepix de arseniuro de galio compensado con cromo

Abstract Among the latest ionizing radiation detectors, those based on chromium compensated gallium arsenide (GaAs:Cr) are ones of the most competitive for many applications due to their high Z and strong resistance to radiation damage. They have been used in high energy physics research, medical visualization and spatial technologies, geological prospecting, among other advanced fields. The object of this work is a 900 µm GaAs:Cr detector with Timepix readout technology. Some detector characteristics for three experimental conditions were measured and studied by using the X-rays from a synchrotron and an X-ray tube provided with different materials for obtaining the corresponding fluorescence photons. A complex function was used to decompose the differential spectra into the most important contributions involved. As an additional tool for the research, the mathematical modeling of the mobility of charge carriers generated by radiation within the active volume of the detector was used. The results of these charge sharing effect studies showed a noticeable prevalence in the detector of this effect, changing its contribution according to the experiment characteristics. The detector was calibrated for the planned experiments and the energy resolution was determined. From the analysis of all the obtained results and their comparison with those reported in literature, it was confirmed that the detector has a marked charge-sharing effect between neighboring pixels, being its performance more impaired as the energy of incident photons increases.

Resumen Entre los últimos detectores de radiación ionizante, los basados ​​en arseniuro de galio compensado con cromo (GaAs: Cr) son de los más competitivos para muchas aplicaciones debido a su alto Z y fuerte resistencia al daño de la radiación. Se han utilizado en investigación de física de alta energía, visualización médica y tecnologías espaciales, prospección geológica, entre otros campos avanzados. El objeto de este trabajo es un detector de GaAs: Cr de 900 µm con tecnología de lectura Timepix. Algunas características del detector para tres condiciones experimentales se midieron y estudiaron utilizando rayos X de un sincrotrón y un tubo de rayos X provisto de diferentes materiales para obtener los fotones de fluorescencia correspondientes. Se utilizó una función compleja para descomponer los espectros diferenciales en las contribuciones más importantes involucradas. Como herramienta adicional para la investigación, se utilizó el modelado matemático de la movilidad de los portadores de carga generados por la radiación dentro del volumen activo del detector. Los resultados de estos estudios de efecto de carga compartida mostraron una prevalencia notable en el detector de este efecto, cambiando su contribución según las características del experimento. El detector se calibró para los experimentos planificados y se determinó la resolución de energía. A partir del análisis de todos los resultados obtenidos y su comparación con los reportados en la literatura, se confirmó que el detector tiene un marcado efecto de reparto de carga entre píxeles vecinos, y su rendimiento se ve más afectado a medida que aumenta la energía de los fotones incidentes.