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

Abstract

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. [1][2][3] 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.[4] 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 [5] 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 CH 3 I, 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 CH 3 I at 266 nm. Photodissociation of CH 3 I 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 Ion signals corresponding to the parent ion CH 3 I + and the fragments CH 3 + 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 b...

Surface pump-probe femtosecond-laser mass spectrometry: Time-, mass-, and velocity-resolved detection of surface reaction dynamics

“…8,9 In first experiments, methyl iodide adsorbed at submonolayer coverage on an ultrathin magnesia film was employed as a photochemical model system. [14][15][16][17][18] The photodissociation of methyl iodide adsorbed at submonolayer and multilayer coverages on a magnesia single crystal has been characterized experimentally in detail as well, but without time resolution. The lowest energy photochemical dissociation channel of methyl iodide is induced by UV light excitation to the A-band ͑see, e.g., Refs.…”

Surface‐Aligned Femtochemistry: Real‐Time Dynamics of Photoinduced I2 Formation from CD3I on MgO(100)

“…[11,16] As the exact influence of the surface on the methyl iodide potentials is not known, and because the methyl iodide molecules do only weakly bind to the magnesia surface, [17] the shown gas-phase potentials are assumed to be almost unperturbed by the surface to a first approximation. One 266 nm photon (pump) excites the CD 3 I molecule almost exclusively to the In contrast to the gas phase, where the dissociation proceeds directly leading to a decay of the transition state within less than 40 fs, [9,18] here, the emerging methyl radical heads toward the MgO surface (illustrated by the dashed potential in Figure 3, cf. also Figure 1 a) resulting in an inelastic collision that prevents the direct dissociation.…”

Femtosecond Photodissociation Dynamics by Velocity Map Imaging. The Methyl Iodide Case

“…Considering this mechanism, which is very much in the spirit of the methodology presented by Zhong and Zewail in Ref. [43], the kinetic energy finally present in the methyl ion fragment should contain two contributions. The first contribution arises from the available energy of the neutral A-band dissociation at a given intermediate C-I internuclear distance (E ‡ av ), which will be smaller than the asymptotic available energy (E final av ), since the wave packet cannot have reached a very long internuclear distance at the time of the second absorption (see Fig.…”

“…It is interesting to see how, for times immediately after time zero, intense contributions appear in the methyl ion images, that are not entirely dissimilar from those at the asymptotic limit. Analysis of the energy-angle-and-time resolved channels allowed us to propose that the measurements reflect early-time dynamics of the molecular system in the Aband, as will be discussed below [32,43].…”