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 spectroscopy 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.

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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

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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

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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 photoelectrons. 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 combination 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

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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’. Every time the wave packet approaches the crossing region, a fraction leaks into the covalent surface with an elongated bond (Na···I) and the other fraction continues in the ionic curve. The period of oscillation from the ionic to the covalent state depends on the shape of the PECs and the total energy.

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6. Wave-packet Dynamics: Coherence

The classic experiment on ionic-covalent resonance demonstrates a brilliant interplay between the concepts of quantum mechanics and ultrafast spectroscopy. Historically, this experiment was preceded by a series of works in which the researchers studied the transition to a state, which is a linear combination of diﬀerent eigenstates. Thus, (2) may be generalized as:

Ψ − c1Φ1 + c2Φ2. (3)

The coeﬃcient ci contains phase factors such as exp(-iEt/). Thus, Ψ given in (3) refers to a non-stationary state. In other words, ΨΨ∗ contains time dependent cross terms of the form sin (ΔEt/ + φ) where ΔE = E2 − E1 and φ is a phase factor. This gives rise to the ‘beats’ (oscillating signals) that are shown in Figure 3. The major achievement of Zewail’s group was the observation of such beats in large-sized molecules because of the existence of close-lying vibrational levels. Such transitions to linear combinations of vibrational levels are known as ‘intramolecular vibrational relaxations’ (IVR) [1]. When he started these experiments, many people were skeptical about detecting such oscillations experimentally. To simplify the situ- ation, Zewail used supersonic jets containing isolated molecules at very low vibrational and rotational temperatures [5–8]. Subsequently, many other groups detected such oscillations in- volving electronic energy levels in large molecules in solution [9, 10].

7. Ultrafast Diﬀraction

Zewail’s initial (ultrafast) studies involved recording the optical spectra or mass spectra of the transients. During the later part, he along with several other groups became interested in eluci- dating the structure of the transients by electron diffraction. In ultrafast electron diffraction, the probe femtosecond beam produces photoelectrons by photoelectric effect. This electron beam is energized by a linear accelerator and the energized beam is used for electron diffraction. For such studies, one uses a femtosecond electron or X-ray beam. The associated experiment and in particular, the data analysis is very complicated. Note that we are dealing with a few transient species with a lifetime of femtoseconds or picoseconds duration and hence, the experiment and analysis are far more difficult than for diffraction in a crystal. This field is still restricted to small-sized transients containing only a few atoms.

As a proof of concept, Zewail’s group ﬁrst reported the structure of CF3 radical from the dissociation of CF3–I [11]. They published nearly 100 papers after that. Zewail summarized the progress of this ﬁeld, which he called 4D electron microscopy, in a book [12].

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Immediately after that several groups generated femtosecond X-ray beams for ultrafast X-ray diﬀraction [13, 14]. In such an experiment, the energized electron beam is allowed to strike an X-ray target to knock out the core electrons and this leads to a femtosecond X-ray beam.

8. Biological Processes

Apart from ultrafast spectroscopy, mass spectrometry, and diﬀraction in supersonic jets, Ze- wail made signiﬁcant contributions to femtosecond studies in solutions of biological molecules (proteins and DNA). In the last phase of his life, he developed deep interest in the dynamics of biological water, i.e., water in the vicinity of proteins and DNA [15, 16]. It is now well estab- lished that the biological water contains a slow component of relaxation, which is 100–1000 times slower than that of bulk water [17].

9. Family and Last Days

Zewail came to USA with his first wife in 1967. They had two daughters. Though he spent nearly 50 years in USA, he never lost touch with his country–Egypt. He was a great fan of Egyptian music and used to play them quite loudly in his office at Caltech. He was associated with the scientific planning in Egypt, and he also tried to set up a world class university in Egypt. At one time, his name was considered, very seriously, as the Presidential candidate of Egypt. In 1989, he went to Saudi Arabia to receive the King Faisal International Prize. There he met Dema, whose father had come to receive the same prize in literature. Ahmed and Dema married in September, 1989. They had two sons. Dema remained his friend and confidante till the end of his life. In 2013 Zewail was diagnosed with bone cancer. He passed away on 02 August 2016, at the age of 70. He was cremated in the family cemetery at the outskirts of Cairo, with full military honor led by the Egyptian President, Abdel Fatah El-Sisi.