Jesús Pilar Amado Álvarez

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-

Calidad Integral del Agua Superficial en la Cuenca Hidrológica del Río Amajac

This paper presents an integral study on the quality of the superficial waters in ponds and streams of the watershed of Amajac river. The problems associated to specific pollutants are identified and alternative solutions are detailed to be used in public policies of water resources. Four ponds, one lake and five streams were considered in the study and runoff volume, water speed, and maximum depth were measured. Also, the main physical, chemical and biological parameters were determined: dissolved oxygen, fecal coli forms, nitrogen, phosphorus, sulphate, carbonate, bicarbonate, chloride, and manganese. According to the results, it is concluded that water of the river in Tulancingo city is strongly contaminated.