Chemical time scales

A principal application of the SwissFEL will be the study of molecular dynamics and reactivity in solution and in heterogeneous catalysis. (Note that applications in biochemistry are the subject of Chapter IV.) Time scales of a range of chemical phenomena in solution are shown in Figure II.1. At the slow end of the scale is the diffusional rotation of a molecule in a solvent. The recombination of photo-dissociated molecules occurs on the ps to ns scales, depending upon whether the recombining reactants arise from the same (geminate) or different (non-geminate) parent molecules. Several types of deactivation processes may occur in an intact photoexcited molecule: “Internal conversion” (10–100 fs) is an electronic reconfiguration in which the total electronic spin is conserved, while slower “intersystem crossing” (1–10 ns) involves a change of spin. Molecular cooling and vibrational energy transfer from a high to a low vibrational state generally occur in 1–100 ps. The exchange of a water molecule in the solvation shell surrounding an ion requires approximately 250 ps, and fur ther ultrafast processes in ionized water are shown schematically in Figure II.2, including the formation and stabilization of the solvated electron within 1 ps, which has proven to be critically impor tant in both photocatalysis and electrochemistry [1].

Catalysis is the enabling technology in a large number of processes for the production of goods, the provision of clean energy, and for pollution abatement. Heterogeneous catalysis occurs at the inter face between a gas or liquid and a solid catalytic sur face; the sur face of a catalytically active solid provides an energy landscape which enhances reactivity. A large number of processes are active here on the nanometer scale (see Fig. II.3), with characteristic times ranging from sub-fs, for electron transfer, to minutes, for the oscillating patterns studied by Ertl et al. [3] and to even months, for the deactivation of catalytic processes. The car toon in Figure II.3 “schematically depicts, at the molecular level, the richness of the phenomena involved in the transformation of reactants to products at the sur face of a material. A molecule may scatter off the sur face, experiencing no or some finite degree of energy exchange with the sur face. Alternatively, molecule-sur face energy transfer can lead to accommodation and physical adsorption or chemical adsorption. In some cases, physisorption is a precursor to chemisorption, and in some cases, bond dissociation is required for chemisorption. Charge transfer plays a critical role in some adsorption processes. Once on the sur face, the adsorbed intermediates may diffuse laterally with a temperature-dependent rate, sampling surface features including adatoms, vacancies and steps. They may become tightly bound to a defect site. Various adsorbed intermediates may meet, either at defect sites or at regular lattice sites, and form short-lived transition state structures and ultimately product molecules. Finally, products desorb from the sur face with a temperature dependent rate, impar ting some fraction of the energy of the association reaction to the sur face” [2]. Within the Born-Oppenheimer approximation, as a chemical system moves from a configuration of high energy towards a potential minimum, the electrons are assumed to instantaneously follow the motion of the much heavier nuclei. The uncertainty in nuclear position during the reaction is then represented by a wavepacket, and the time development during the reaction can be viewed as the motion of the packet along a trajectory on the potential energy sur face. A schematic impulsive photo-excitation process and the resulting wavepacket motion are shown in Figure II.4, where one sees that for a typical 0.1 nm atomic displacement and 100 fs vibrational timescale, the velocity for wave-packet motion is of order 1000 m/s. It should be noted that the Born-Oppenheimer approximation may break down, par ticularly for light chemical elements. Such non-adiabatic couplings are prevalent at critical points on the potential energy sur face, where two sur faces repel one another at an avoided crossing, or where they meet at a point – a conical intersection. It is at these critical points that chemical reactions occur. Also at metal sur faces, non-adiabatic couplings are frequently observed between adsorbate motion and electronic excitations in the metal substrate.

It is clear that predominant mechanisms of solution and sur face catalytic processes are active on the nm length and fs to ns time scales. This richness of behavior serves as a strong justification for the SwissFEL, as a power ful tool for the study of ultrafast chemistry.

THz initiation of surface reactions

The intensity (power/area) delivered by an electromagnetic wave is I = e0E02c / 2 , implying that a halfcycle 100 μJ pulse of THz radiation, focused to 1 mm2, will produce a peak electric field E0 = 4 × 108 V/m, which is comparable to the field exerted on an adsorbed atom by a scanning tunneling microscope tip used to manipulate atoms [15]. One thus envisages the collective control of atomic positions using the THz pulse (Fig. II.i1). Since the phonon frequencies of the host and the local vibration frequencies of adsorbed atoms and molecules lie in the teraher tz region, THz pulses can resonantly excite these modes. Finally, E0 from the THz pulse is of order 1‰ of the field felt by the electron in a Bohr H-atom, opening the possibility of using dynamic Stark control to influence a chemical reaction (Fig. II.i2).

A micro-liquid jet in vacuum

The investigation with the SwissFEL of chemical dynamics in solution will require a rapidly renewable liquid sample, which is compatible with the beamline vacuum. These requirements can be met with a highvelocity fluid jet from a small nozzle, at the limit for laminar low. Fig. II.i3 shows the example of a 6 μm diameter jet, with a flow velocity of 120 m/s, which, when outfitted with a small-aperture skimmer, can be attached to an X-ray photoelectron spectrometer at 10-9 mbar.