J. G. Izquierdo

A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH3I

The real time photodissociation dynamics of CH(3)I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH(3) fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH(3)(nu=0) and I((2)P(32)) and I(*)((2)P(12)) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I(*) channels when the CH(3) fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.

Femtosecond multichannel photodissociation dynamics of CH3I from the A band by velocity map imaging.

The reaction times of several well-defined channels of the C-I bond rupture of methyl iodide from the A band, which involves nonadiabatic dynamics yielding ground state I(2P3/2) and spin-orbit excited I*(2P1/2) and ground and vibrationally excited CH3 fragments, have been measured by a combination of a femtosecond laser pump-probe scheme and velocity map imaging techniques using resonant detection of ground state CH3 fragments. The reaction times found for the different channels studied are directly related with the nonadiabatic nature of this multidimensional photodissociation reaction.

Femtosecond Transition‐State Imaging of the A‐Band CH3I Photodissociation

Since the early days of femtosecond transition state spectroscopy, both the clocking of the reaction (on-resonance experiments) and the detection of transient species along the reaction coordinate (off-resonance experiments) have been at the heart of femtochemistry. In the pioneering experiments carried out by Zewail and co-workers, the real time photodissociation of ICN was studied by tuning the probe laser on-resonance to the first electronic excited state of the CN fragment which then emits fluorescence. The resonant probe laser opens up an optical coupling region on the potential energy surface (determined by its bandwidth), which allows the clocking of the reaction from the initial Franck-Condon wave packet to the free fragments in the asymptotic region. However, the beauty of femtochemistry arises from the detection of the transient species between the initial and asymptotic wave packets by tuning the probe laser off-resonance to the free fragment. The introduction of femtosecond time-resolved kinetic energy time-of-flight (KETOF) by Zhong and Zewail showed the possibility of observing the dynamics of transition states and final products at the same time, using one wavelength for the probe and only resolving the kinetic energy. By following the time evolution of the kinetic energy of the fragment ion, the dissociation dynamics from the initial transition state to the final products could be studied and several examples, in particular the A-band photodissociation of CH3I, were presented. Using this method, the evolution of the kinetic-energy-resolved cations is followed by accessing ionic surfaces to study the dissociation dynamics of neutral molecules. Only one probe laser is used to detect the transition states and final products. Herein, we combine off-resonance multiphoton ionization for the probing step using a femtosecond laser pulse at 800 nm and velocity map imaging for ion detection to follow the time evolution of transition states and final products in the A-band photodissociation dynamics of CH3I at 266 nm. Photodissociation of CH3I in the near UV proceeds via excitation in the A-band, a broad featureless absorption continuum (220–350 nm) involving three optically allowed transitions from the ground state: two weak perpendicular transitions to the Q1 and Q1 states that correlate to the ground state I ACHTUNGTRENNUNG( P3/2) and a strong parallel transition to the Q0 state that correlates to the spin–orbit excited state I* ACHTUNGTRENNUNG(P1/2). The concerted theoretical and experimental efforts show that most of the absorption can be attributed to the Q0 state, and that the IACHTUNGTRENNUNG( P3/2) fragment observed in the experiments is the result of a non-adiabatic transition at the conical intersection between the Q0 and Q1 states. Ion signals corresponding to the parent ion CH3I + and the fragments CH3 + and I are measured from the oscilloscope trace as a function of the time delay between the 266 nm (pump) and the 800 nm (probe) pulses. We observe that all three ions are produced separately by each of the laser pulses, but we work under intensity conditions where such signals are minimized. When the delay time between the pump and the probe pulses is small, a strongly enhanced ion signal, lasting approximately 300 fs, is measured for the parent and all the fragments. When the probe pulse is fired later, the fragment ions show a weak, enhanced signal lasting longer than 100 ps, whereas the parent ion signal does not show any measurable enhancement. This behaviour can be seen in Figure 1 for the CH3I + and the CH3 + ions, where a delay of 50 fs is measured between the parent and the methyl transients. In the experi-

Femtosecond spectral pulse shaping with holographic gratings recorded in photopolymerizable glasses.

The majority of the applications of ultrashort laser pulses require a control of its spectral bandwidth. In this paper we show the capability of volume phase holographic gratings recorded in photopolymerizable glasses for spectral pulse reshaping of ultrashort laser pulses originated in an Amplified Ti: Sapphire laser system and its second harmonic. Gratings with high laser induce damage threshold (LIDT) allowing wide spectral bandwidth operability satisfy these demands. We have performed LIDT testing in the photopolymerizable glass showing that the sample remains unaltered after more than 10 million pulses with 0,75 TW/cm2 at 1 KHz repetition rate. Furthermore, it has been developed a theoretical model, as an extension of the Kogelniks theory, providing key gratings design for bandwidth operability. The main features of the diffracted beams are in agreement with the model, showing that non-linear effects are negligible in this material up to the fluence threshold for laser induced damage. The high versatility of the grating design along with the excellent LIDT indicates that this material is a promising candidate for ultrashort laser pulses manipulations.

Modification of the LM124 Single Event Transients by Load Resistors

The influence of a load resistor on the shape of the single event transients was investigated in the LM124 operational amplifier by means of laser tests. These experiments indicated that, as a general rule, load resistors modify the size of the transients. SPICE simulations helped to understand the reasons of this behavior and showed that the distortion is related to the necessity of providing or absorbing current from the load resistor, which forces the amplifier to modify its operation point. Finally, load effects were successfully used to explain the distortion of single event transients in typical feed-back networks and the results were used to explain experimental data reported elsewhere.

Intracellular pH-Induced Tip-to-Tip Assembly of Gold Nanorods for Enhanced Plasmonic Photothermal Therapy

The search for efficient plasmonic photothermal therapies using nonharmful pulse laser irradiation at the near-infrared (NIR) is fundamental for biomedical cancer research. Therefore, the development of novel assembled plasmonic gold nanostructures with the aim of reducing the applied laser power density to a minimum through hot-spot-mediated cell photothermolysis is an ongoing challenge. We demonstrate that gold nanorods (Au NRs) functionalized at their tips with a pH-sensitive ligand assemble into oligomers within cell lysosomes through hydrogen-bonding attractive interactions. The unique intracellular features of the plasmonic oligomers allow us to significantly reduce the femtosecond laser power density and Au NR dose while still achieving excellent cell killing rates. The formation of gold tip-to-tip oligomers with longitudinal localized surface plasmon resonance bands at the NIR, obtained from low-aspect-ratio Au NRs close in resonance with 800 nm Ti:sapphire 90 fs laser pulses, was found to be the key parameter for realizing the enhanced plasmonic photothermal therapy.

SPICE Simulations of Single Event Transients in Bipolar Analog Integrated Circuits Using Public Information and Free Open Source Tools

This paper proposes a technique to build SPICE micromodels of integrated circuits in bipolar technology appropriate to simulate single event transients. First of all, we will show how to obtain SPICE models of the internal transistors from texts in the scientific and academic literature. Next, several strategies to figure out the internal structure of the integrated circuits and bias point will be shown. Finally, simulation results will be compared to data issue from experiments, either performed by the authors or by other researchers. As the simulations do not require expensive software or hardware, this paper can be a start point for research groups with small budget or for academic purposes at universities.

LASER system for space environment emulation

The Sensors and Electronic Instrumentation Group of the University Complutense of Madrid has developed a system to emulate the cosmic radiations effects on electronic devices by LASER irradiations. Of great interest to the nuclear industry and space electronics, this project was carried out partially supported by the company ALTER SPAIN S.A., specialized in engineering, quality management and testing of high reliability electronic components for space applications.

Ablation dynamics of Co/ZnS targets under double pulse femtosecond laser irradiation.

Femtosecond lasers, used as tools to investigate the ablation dynamics of solids, can help to develop strategies to control the deposition of nanomaterials by pulsed laser ablation. In this work, Co/ZnS targets, potential candidates for the synthesis of diluted magnetic semiconductor materials, are irradiated by sequences of two femtosecond laser pulses delayed in the picosecond time scale. The ionic composition of the ablation plasma and the dependence of the ion signals on the interpulse delay and relative fluence are determined by time-of-flight mass spectrometry. The results show that, when pulses of different fluence are used, highly asymmetric ion yields are obtained, with more intense ion signals detected when the lower fluence pulse is temporally ahead. The comparison between asymmetric and equal fluence double pulse ablation dynamics provides some understanding of the different processes that modify the properties of the layer irradiated by the first pulse and of the mechanisms affecting the coupling of the delayed pulse into the material. The final outcome of the double pulse irradiation is characterized through the analysis of the deposits produced upon ablation.

Peak Detector Effect in Low-Dropout Regulators

The peak detector effect is a phenomenon that makes single event transients much longer once an error amplifier switches from linear to saturation zone due to the presence of external capacitors. This is so-called since it was discovered in a simple voltage reference in which a parasitic lossy peak detector was unwillingly built in the output stage. In this paper, peak detector effect is generalized to explain the appearance of long duration pulses in typical low dropout voltage regulator built with discrete devices. This effect has been related to the way in which the negative feedback loop is closed and to the kind of pass device in the output stage. Thus, if the linear voltage regulator consists in an error amplifier the output of which controls a current source, the peak detector effect will occur if the current source is unidirectional, the output load does not drain enough current and is in parallel with an external capacitor.