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Ahmed Zewail the King of Femtoland ARTICLE-IN-A-BOX Ahmed Zewail The King of Femtoland 1. Introduction Ahmed H Zewail, was awarded the 1999 Chemistry Nobel “for his studies of the transition states of chemical reactions using femtosecond spectroscopy” [1]. Through a series of elegant experiments, he showed that chemical bonds form and break on the ‘femtosecond’ timescale. He thus provided the description of a chemical reaction, in real time. Zewail was born in Egypt on 26 February 1946 [2, 3]. After finish- ing his Masters, he became a demonstrator and started teaching at the University of Alexandria. In 1967, at the age of 21, he landed in USA to start his PhD under the supervision of a renowned professor – Robin M Hochstrasser at the University of Pennsylvania. During his PhD days, he worked on optical and magnetic-resonance spec- troscopy of molecules in triplet states and on excitons. About his PhD days, Zewail said that they used to have all-night duels with lasers and the prize was a most illuminating discussion at breakfast next day, with his supervisor Prof. Hochstrasser. Following his PhD, Zewail moved to Berkeley to work as a postdoc with Prof. C B Harris. Dur- ing this time, he developed interest in the coherence in electronically Prof. Ahmed Zewail excited state dimers [4]. In 1976, Zewail joined Caltech (California Institute of Technology) as a faculty member and remained there for the rest of his life. At Caltech, he combined molecular beam and ultrafast lasers to perform a series of extraordinarily difficult and illuminating experiments and in the 1980s, started publishing many pioneering papers [5]. His (along with co-workers) first paper on femtosecond dynamics of transition states of ICN appeared in 1987 [6]. It was received by the Journal of Chemical Physics on 3 June 1987 and was accepted as a communication on 15 June 1987. The referee of this paper observed, “It (the manuscript) has the smell that the authors are onto some very exciting new stuff.... This manuscript meets all requirements for a communication. It may turn out to be a classic. Publish with all dispatch [1].” In the same year, Zewail published a manuscript on picosecond clocking of chemical reactions in collaboration with R B Bernstein [7]. In the early 80s, R B Bernstein, a giant in the field of molecular beams, shifted to the University of California at Los Angeles. RESONANCE | June 2018 633 ARTICLE-IN-A-BOX Figure 1. Potential energy curve for a typical chemical reaction. 2. Time Scale at Which Chemical Bonds Break As we all know, the typical covalent bond length is of the order of one Angstrom (= 0.1 nm). Zewail addressed a simple question – how much time does an atom take to travel 0.1 nm? To answer this, one needs to know the velocity of an atom. As a first approximation, Zewail assumed that the velocity of an atom was equal to the average speed obtained from the kinetic theory of gases: 8RT c¯ = . (1) πM Substituting M = 12 for carbon, one gets the average speed to be of the order of 1 km/s (0.7 km/s for carbon), i.e., 1012 nm/satT = 300 K. Thus, a free atom typically takes 100 femtoseconds (1 fs = 10−15 s) to travel 0.1 nm, i.e., to break a bond. Obviously, a bonded atom will move slightly slower and would take a little longer than 100 fs to travel the same distance. But, in any case, it is clear that the femtosecond time scale is needed to follow the breaking of a chemical bond. Thus began the subject ‘femtochemistry’. 3. Transition State and Energy Surfaces In a chemical reaction, old bonds are broken and new bonds are formed. The progress of the reaction may be monitored by a plot of the energy (E) as a function of the reaction coordinate. The reaction coordinate is defined in terms of the combined bond distances and angles involved 634 RESONANCE | June 2018 ARTICLE-IN-A-BOX Figure 2. Schematic illustration of a pump-probe set-up. in the reaction. The plot of the energy versus the reaction coordinate is known as the potential energy curve or the potential energy surface (when more than one reaction coordinates are involved). The energy of the system initially rises from that of the reactant to reach a state called the ‘transition state’ (Figure 1) or an ‘activated complex’. While the transition states are associated with the maxima of the potential energy surface, there may be one or more minima in between the reactant and the product. These minima correspond to intermediates. An English translation of the classic paper by Eyring and Polanyi in 1931 on the subject of transition state was reproduced in the January 2018 issue of Resonance. Before the pioneering experiment was carried out by Zewail, chemists believed that only long- lived reaction intermediates could be captured by various spectroscopic techniques. But Zewail demonstrated that it was possible to record the spectrum and even the X-ray structure of an extremely short-lived transition state. 4. Pump-Probe Experiment Ultrafast experiments are based on a simple set-up described in Figure 2 [1]. The basic idea is that a femtosecond laser beam is split into two beams by a beam splitter (BS). One of the beams, called the ‘pump’, is directly reflected by a mirror (M1) into the sample and it produces a transient chemical species (transition state). The second part of the beam is called the ‘probe’ beam. For this, the mirror (M2) reflects the beam into a delay stage, formed by two mirrors RESONANCE | June 2018 635 ARTICLE-IN-A-BOX M3 and M4. This delay stage varies the distance of the probe beam from the sample and sends it after a delay of Δt, after the arrival of the pump beam at the sample. The delay is given by, Δt =ΔL/c, where c is the velocity of light and ΔL is the extra distance travelled by the probe beam. Note that light (c = 3 × 1010 cm/s), moves 30 cm in one nanosecond (10−9 s), 0.3 cm (3 mm) in one picosecond (10−12 s), and only 0.003 mm (3 micron) in one femtosecond (10−15 s). The unit ‘P’ in Figure 2 is a processor unit, which converts the pump laser into a suitable femtosecond optical beam or a femtosecond X-ray or electron beam. In some cases, ‘P’ is a non-linear crystal which produces a new laser with wavelength half (second harmonic) or one- third (third harmonic) of the pump beam. If ‘P’ is an optical parametric amplifier, one can get other wavelengths too. If the probe beam is optical beam, one can get absorption or emission spectra of the transients. To get an electron beam, the pump is allowed to strike a photocathode to produce photoelec- trons. Similarly, to get an X-ray probe, the photoelectrons are allowed to strike an X-ray target. Using femtosecond electron or X-ray beam as probe, one can directly obtain the structure of the transient species through electron or X-ray diffraction. 5. Ionic-Covalent Resonance Any covalent bond has a bit of ionic character. Thus, sodium iodide is a resonance hybrid of two structures, Na–I ↔ Na+ I− In the language of quantum mechanics, the electronic wave function of NaI is a linear combi- nation of two wave functions – ionic and covalent states. ψNaI = c1Φcov + c2Φionic. (2) 2 2 The relative contribution of the covalent state is c1 and that of the ionic state is c2. To verify this, Zewail excited sodium iodide vapor and studied the following reaction, NaI → [Na···I] → Na + I Transition State From the conventional flame taste, we know that sodium chloride gives a bright golden yellow colored flame. This golden yellow color originates from the Na atom (with a 3s electron) and 636 RESONANCE | June 2018 ARTICLE-IN-A-BOX Figure 3. Ionic-covalent resonance (adapted from [1] and [6]). not from the Na+ ion (with no unpaired 3s electron). The Na atom may be produced if the covalent state of NaI dissociates into Na and I atom and not from the ionic state. Zewail and co-workers showed that after photodissociation of NaI, the signal of the Na atom fluctuates between a high value (covalent state) and a low value (ionic state, Figure 3) [7]. This clearly demonstrates that each NaI molecule remains for sometime in the covalent state and sometime in the ionic state. In the case of NaI, the potential energy curves, PECs (i.e., plot of potential energy against the internuclear distance, RNa−I) of the covalent and ionic states intersect at RNa−I = 7 Å [8]. In reality, they avoid each other because they belong to the same symmetry (see the Classics reproduced in this issue [8]). In this experiment, the pump laser (wavelength, λ1) first excites NaI from the ionic state (with energy minimum at 2.7 Å) to the covalent state. In the covalent state, the Na–I bond expands and as it reaches around 7 Å, the wave packet (a linear combination of the ionic and covalent states) shuttles between the covalent and ionic states. In this case, the probe femtosecond laser ∗ (wavelength, λ2) excites the covalent state leading to dissociation into Na atom (and I atom). This results in an oscillatory signal where the probe signal corresponds to the system in the covalent state and the trough to the ionic state of NaI (see Figure 3, [8]). Such an oscillatory signal (in this case, due to the sodium atom) is known as ‘coherent wave-packet dynamics’.
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