Fotodisociación y ionización de moléculas orgánicas con pulsos láser de distinta duración

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Óptica, leída el 29-09-1999En este trabajo se ha abordado el estudio de procesos de fotodisociación y fotoionización de moléculas orgánicas inducidos por láseres pulsados en un rango de duración entre 15 ns y 20 ps, lo que implica el estudio de procesos en distintos regímenes de intensidad láser. Los estudios han involucrado el uso de distintas técnicas de detección (fluorescencia espontánea de fotofragmentos, fluorescencia inducida por láser, medida de la ionización total y espectrometría de masas por tiempo de vuelo). En particular, se ha realizado un extenso estudio de los procesos de fotodisociación de la molécula de cloroetenilsilano (HCIC=CHSiH3) a diferentes longitudes de onda en el ultravioleta, que ha confirmado las dos vías primarias de disociación que se habían postulado en la literatura y ha indicado la existencia de una tercera vía que da lugar al radical HSiCl. En la región del ultravioleta cercano, se han realizado estudios comparativos de los procesos de fotodisociación y ionización en las moléculas de cetena (CH2CO) y ciclohexano (C6H12) inducidos por pulsos láser de 4 ns y 20 ps de duración. El análisis comparativo, realizado con la ayuda de modelos cinéticos, ha proporcionado información sobre los mecanismos multifotónicos operativos en estas moléculas. Por último, se presentan resultados de generación de armónicos elevados de la radiación proporcionada por un láser ultraintenso de 780 nm y una duración de pulso de 200 fs en moléculas orgánicas (benceno y ciclohexano). Los estudios demuestran la capacidad de estas especies para generar armónicos elevados con alta eficiencia y mayor selectividad que los gases noblesDepto. de ÓpticaFac. de Ciencias FísicasTRUEpu

Femtosecond XUV induced dynamics of the methyl iodide cation

Ultrashort XUV wavelength-selected pulses obtained with high harmonic generation are used to study the dynamics of molecular cations with state-to-state resolution. We demonstrate this by XUV pump - IR probe experiments on CH3I+ cations and identify both resonant and non-resonant dynamics

Fresnel phase retrieval method using an annular lens array on an SLM

Wavefront aberrations play a major role when focusing an ultrashort laser pulse to a high-quality focal spot. Here, we report a novel method to measure and correct wavefront aberrations of a 30-fs pulsed laser beam. The method only requires a programmable liquid-crystal spatial light modulator and a camera. Wavefront retrieval is based on pupil segmentation with an annular lens array, which allows us to determine the local phase that mini- mizes focusing errors due to wavefront aberrations. Our method provides accurate results even when implemented with low dynamic range cameras and polychromatic beams. Finally, the retrieved phase is added to a diffractive lens codified onto the spatial light modulator to experi- mentally demonstrate near-diffraction-limited femtosecond beam focusing without refractive components

Structural dynamics effects on the ultrafast chemical bond cleavage of a photodissociation reaction

The correlation between chemical structure and dynamics has been explored in a series of molecules with increasing structural complexity in order to investigate its influence on bond cleavage reaction times in a photodissociation event. Femtosecond time-resolved velocity map imaging spectroscopy reveals specificity of the ultrafast carbon–iodine (C–I) bond breakage for a series of linear (unbranched) and branched alkyl iodides, due to the interplay between the pure reaction coordinate and the rest of the degrees of freedom associated with the molecular structure details. Full-dimension time-resolved dynamics calculations support the experimental evidence and provide insight into the structure–dynamics relationship to understand structural control on time-resolved reactivity

Programmable quasi-direct space-to-time pulse shaper with active wavefront correction

We experimentally demonstrate an extremely compact and programmable pulse shaper composed of a single phase mask encoded into a spatial light modulator. Its principle of operation is similar to the previously theoretically introduced quasi-direct space-to-time pulse shaper [Opt. Express 16, 16993 (2008)], which is based on diffractive optics. The proposed pulse shaper exhibits not only real-time temporal modulation, but also high-efficiency output pulses thanks to an active correction of the wavefront aberrations.This research was funded by the Spanish Ministerio de Ciencia e Innovación and the Generalitat Valenciana through Consolider Programme (SAUUL CSD2007-00013), and Prometeo Excellence Programme (PROMETEO/2012/021), and projects FIS2010-15746, P11B2010-26, and CTQ2008- 02578. We also thank the European network ITN FAS- TQUAST (PITN-GA-2008-214962). Authors are also very grateful to the SCIC of the Universitat Jaume I for the use of the femtosecond laser

Femtochemistry under scrutiny: Clocking state-resolved channels in the photodissociation of CH3I in the A -band

The following article appeared inThe Journal of Chemical Physics 152.1 (2020): 014304 and may be found at https://doi.org/10.1063/1.5134473Clocking of electronically and vibrationally state-resolved channels of the fast photodissociation of CH3I in the A-band is re-examined in a combined experimental and theoretical study. Experimentally, a femtosecond pump-probe scheme is employed in the modality of resonant probing by resonance enhanced multiphoton ionization (REMPI) of the methyl fragment in different vibrational states and detection through fragment velocity map ion (VMI) imaging as a function of the time delay. We revisit excitation to the center of the A-band at 268 nm and report new results for excitation to the blue of the band center at 243 nm. Theoretically, two approaches have been employed to shed light into the observations: first, a reduced dimensionality 4D nonadiabatic wavepacket calculation using the potential energy surfaces by Xie et al. [J. Phys. Chem. A 104, 1009 (2000)]; and second, a full dimension 9D trajectory surface-hopping calculation on the same potential energy surfaces, including the quantization of vibrational states of the methyl product. In addition, high level ab initio electronic structure calculations have been carried out to describe the CH3 3pz Rydberg state involved in the (2 + 1) REMPI probing process, as a function of the carbon-iodine (C–I) distance. A general qualitative agreement is obtained between experiment and theory, but the effect of methyl vibrational excitation in the umbrella mode on the clocking times is not well reproduced. The theoretical results reveal that no significant effect on the state-resolved appearance times is exerted by the nonadiabatic crossing through the conical intersection present in the first absorption band. The vibrationally state resolved clocking times observed experimentally can be rationalized when the (2 + 1) REMPI probing process is considered. None of the other probing methods applied thus far, i.e., multiphoton ionization photoelectron spectroscopy, soft X-ray innershell photoelectron spectroscopy, VUV single-photon ionization, and XUV core-to-valence transient absorption spectroscopy, have been able to provide quantum state-resolved (vibrational) clocking times. More experiments would be needed to disentangle the fine details in the clocking times and dissociation dynamics arising from the detection of specific quantum-states of the molecular fragmentsM.L.M.-S. acknowledges financial support through a predoctoral contract from Universidad Complutense de Madrid. M.E.C. is grateful to the Spanish MINECO for a contract through Programa de Técnicos de Apoyo a Infraestructuras. This work was financially supported by the Spanish MINECO and MICIU (Grant Nos. CTQ2016-75880-P, FIS2016-77889-R, and PGC2018-096444- B-I00). This research was carried out within the Unidad Asociada Química Física Molecular between the Departamento de Química Física of Universidad Complutense de Madrid and CSIC. The facilities provided by the Center for Ultrasfast Lasers of Universidad Complutense de Madrid are acknowledge

Femtosecond XUV–IR induced photodynamics in the methyl iodide cation

The time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV-IR pump-probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited state potential energy surface. Photoelectrons and photofragment ions - and I+ - are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state, considering the presence of the IR pulse. The and I+ pump-probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the state, mainly through the coupling to the ground state and to the excited state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of fragments after the cation is transferred to the ground states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization. © 2021 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft

Femtosecond XUV–IR induced photodynamics in the methyl iodide cation

The time-resolved photodynamics of the methyl iodide cation (CH3I+) are investigated by means of femtosecond XUV–IR pump–probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state potential energy surface. Photoelectrons and photofragment ions—${\mathrm{C}\mathrm{H}}_{3}^{+}$ and I+—are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH3I+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$, considering the presence of the IR pulse. The ${\mathrm{C}\mathrm{H}}_{3}^{+}$ and I+ pump–probe transients reflect the role of the IR pulse in controlling the photodynamics of CH3I+ in the $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state, mainly through the coupling to the ground state $\tilde {X}\enspace {}^{2}\mathrm{E}_{3/2,1/2}$ and to the excited $\tilde {B}\enspace {}^{2}\mathrm{E}$ state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of ${\mathrm{C}\mathrm{H}}_{3}^{+}$ fragments after the cation is transferred to the ground $\tilde {X}\enspace {}^{2}\mathrm{E}_{3/2,1/2}$ states and an enhancement of I+ fragments by absorption of IR photons yielding dissociative photoionization