Rebeca de Nalda

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.

Velocity Map Imaging and Theoretical Study of the Coulomb Explosion of CH3I under Intense Femtosecond IR Pulses

The Coulomb explosion of CH(3)I in an intense (10-100 TW cm(-2)), ultrashort (50 fs) and nonresonant (804 nm) laser field has been studied experimentally and justified theoretically. Ion images have been recorded using the velocity map imaging (VMI) technique for different singly and multiply charged ion fragments, CH(3)(p+) (p = 1) and I(q+) (q ≤ 3), arising from different Coulomb explosion channels. The fragment kinetic energy distributions obtained from the measured images for these ion fragments show significantly lower energies than those expected considering only Coulomb repulsion forces. The experimental results have been rationalized in terms of one-dimensional wave packet calculations on ab initio potential energy curves of the different multiply charged species. The calculations reveal the existence of a potential energy barrier due to a bound minimum in the potential energy curve of the CH(3)I(2+) species and a strong stabilization with respect to the pure Coulombic repulsion for the higher charged CH(3)I(n+) (n = 3, 4) species.

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-

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.

Dissociative ionization of halogenated ethylenes in intense femtosecond laser pulses

Abstract The interaction of chloro-ethylene (H 2 CCHCl), bromo-ethylene (H 2 CCHBr), and 2-chloro-ethenylsilane (H 3 SiCHCHCl) with 50 fs linearly polarized laser pulses of 800 and 400 nm at intensities up to 2×10 14 and 7×10 12 W cm −2 , respectively, has been studied using a time-of-flight mass spectrometer. At 400 nm extensive fragmentation is observed, while at 800 nm the presence of singly and doubly charged parent ions and of multiply charged atomic ions in the mass spectra attests to a reduction of fragmentation in favour of multiple ionization of the parent followed by Coulomb explosion. The silane compound dissociates more easily, its multicharged atomic fragments appear at lower intensities and are ejected with higher kinetic energies than the chlorine and bromine derivatives. The results reveal that laser–molecule coupling is influenced by the molecular properties and that energy channeling increases with molecular size.

Photodissociation of ketene with a narrow-band tunable laser around 212.5 nm

Abstract The photodissociation of ketene with a narrow-band tunable laser has been studied at 218.0, 212.5 and 207.0 nm. A CH photofragment is formed in the C 2 Σ + (ν′ = 0), B 2 Σ − (ν′ = 0, 1) and A 2 Δ (ν′ = 0, 1) states through a process involving the absorption of two laser photons. The visible emission observed above 580 nm is attributed to the CH 2 (b 1 B 1 ) photofragment formed in a single-photon process. The experimental results strongly suggest that electronically excited CH could be formed in a sequential mechanism involving the absorption of a second photon by the CH 2 (b 1 B 1 ) intermediate. Nevertheless, the simultaneous existence of several mechanisms must be considered to explain the formation of the CH fragment in the various observed electronic states.

Induced HSiCl emission in the UV photodissociation of 2-chloroethenylsilane

Abstract The laser induced fluorescence of the HSiCl radical, detected in the 525–600 nm range, has been observed in the photolysis of 2-chloroethenylsilane at 193 and 212 nm. The results indicate that this radical is produced with considerable rotational and vibrational excitation in the electronic ground state. Quenching with Argon of the excited electronic state A 1 A″ is very slow, on the order of 8×10−13 cm3 molecule−1 s−1. The implications of these observations for the primary photofragmentation paths of the 2-chloroethenylsilane molecule are discussed.

IR and UV laser-induced photolysis of 2-chloroethenylsilane

Abstract The laser-induced decomposition of 2-chloroethenylsilane was studied in the IR with a TEA CO 2 laser and in the UV with a narrow-band, frequency-doubled dye laser at 212.5 nm. Silylene was observed in the IR multiphoton dissociation (MPD) via laser-induced fluorescence (LIF). The nascent silylene fragments are vibrationally excited in the bending mode. Multiphoton UV photolysis yields a fluorescence emission spectrum originating from the SiH (A 2 Δ → X 2 Π ) Δν = 0 system, with several atomic Si transitions and molecular bands corresponding to C 2 (d 3 Π g → a 3 Π g ) Δν = 2, 1, 0, −1 and −2 transitions. The simulation of the spectra originating from diatomic fragments indicates that these possess a high content of internal energy. A high population of Si triplet states is observed.

Multiphoton Dissociation of Phenylsilane Upon Excitation at 212.5 NM

The photodissociation processes that follow the photolysis of phenylsilane with a narrow band laser at 212.5 nm were studied by observing photofragment fluorescence spectra in the 200 to 900 nm range. Emission from several excited states of Si atoms was detected together with emissions from the molecular fragments SiH(A2∆) and C2(d3∏g). Si and SiH emissions show a quadratic dependence with laser energy whereas dependence for C2 emission is cubic, indicating the participation of two and three photon processes in the formation of the respective fragments. The emission spectra of the molecular fragments provides information about their internal energy content and allows discussion of the possible channels responsible for the appearance of those fragments.

Strong field control of predissociation dynamics

Strong field control scenarios are investigated in the CH3I predissociation dynamics at the origin of the second absorption B-band, in which state-selective electronic predissociation occurs through the crossing with a valence dissociative state. Dynamic Stark control (DSC) and pump-dump strategies are shown capable of altering both the predissociation lifetime and the product branching ratio.