10-15 ≈ FEMTOChemistry: New Frontier Exponent after NANOChemistry

 

Priya R. Modiya, Palakben K. Parikh, Deepa R. Parmar, Dhrubo Jyoti Sen and Vidhi R. Patel

Department of Pharmaceutical Chemistry, Shri Sarvajanik Pharmacy College, Gujarat Technological University,

Arvind Baug, Mehsana-384001, Gujarat, India

*Corresponding Author E-mail: dhrubosen69@yahoo.com

 

ABSTRACT:

The study of chemical reactions on a very short time scale, often using pulsed lasers is femtochemistry. Etymology: From femtosecond + chemistry. Femto-(symbol f) is a prefix in the metric system denoting a factor of 10−15 or 0.000000000000001. Adopted by the 11th Conférence Générale des Poids et Mesures, it was added in 1964 to the SI. It is derived from the Danish word femten, meaning "fifteen". Example of use: a proton has a diameter of about 1.6 to 1.7 femtometres. The femtometre shares the unit symbol (fm) with the older non-SI unit fermi, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.

 

KEYWORDS: Femto, Nano, Pump pulse, Probe plus

 


 

INTRODUCTION:

Femtochemistry is the branch of chemistry that studies elementary (often very fast) chemical reactions as they occur; the experimental methods are often based on the use of femtosecond laser pulses chemical science, chemistry -the science of matter; the branch of the natural sciences dealing with the composition of substances and their properties and reactions. It is the science that studies

 

chemical reactions on extremely short timescales, approximately 10–15 seconds (one femtosecond, hence the name). In 1999, Ahmed H. Zewail received the Nobel Prize in Chemistry for his pioneering work in this field. Zewail’s technique uses flashes of laser light that last for a few femtoseconds SI prefixes.

 

 


Prefix

Symbol

1000m

10n

Decimal

Short scale

Long scale

Since

yotta

Y

10008

1024

1000000000000000000000000

Septillion

Quadrillion

1991

zetta

Z

10007

1021

1000000000000000000000

Sextillion

Trilliard

1991

exa

E

10006

1018

1000000000000000000

Quintillion

Trillion

1975

peta

P

10005

1015

1000000000000000

Quadrillion

Billiard

1975

tera

T

10004

1012

1000000000000

Trillion

Billion

1960

giga

G

10003

109

1000000000

Billion

Milliard

1960

mega

M

10002

106

1000000

Million

1960

kilo

k

10001

103

1000

Thousand

1795

hecto

h

10002⁄3

102

100

Hundred

1795

deca

da

10001⁄3

101

10

Ten

1795

10000

100

1

One

deci

d

1000−1⁄3

10−1

0.1

Tenth

1795

centi

c

1000−2⁄3

10−2

0.01

Hundredth

1795

milli

m

1000−1

10−3

0.001

Thousandth

1795

micro

µ

1000−2

10−6

0.000001

Millionth

1960

nano

n

1000−3

10−9

0.000000001

Billionth

Milliardth

1960

pico

p

1000−4

10−12

0.000000000001

Trillionth

Billionth

1960

femto

f

1000−5

10−15

0.000000000000001

Quadrillionth

Billiardth

1964

atto

a

1000−6

10−18

0.000000000000000001

Quintillionth

Trillionth

1964

zepto

z

1000−7

10−21

0.000000000000000000001

Sextillionth

Trilliardth

1991

yocto

y

1000−8

10−24

0.000000000000000000000001

Septillionth

Quadrillionth

1991

1.  The metric system was introduced in 1795 with six prefixes. The other dates relate to recognition by a resolution of the CGPM.

2.  The 1948 recognition of the micron by the CGPM was abrogated in 1967.


Professor Ahmed H. Zewail is an Egyptian-American scientist, and the winner of the 1999 Nobel Prize in Chemistry for his work on femtochemistry. He is the Linus Pauling Chair Professor Chemistry and Professor of Physics at the California Institute of Technology. California Institute of Technology, Pasadena, USA for showing that it is possible with rapid laser technique to see how atoms in a molecule move during a chemical reaction. The Academy's citation: For his studies of the transition states of chemical reactions using femtosecond spectroscopy.

 

Nobel laureate in Chemistry is being rewarded for his pioneering investigation of fundamental chemical reactions, using ultra-short laser flashes, on the time scale on which the reactions actually occur. Professor Zewail's contributions have brought about a revolution in chemistry and adjacent sciences, since this type of investigation allows us to understand and predict important reactions. Femtochemistry is the area of physical chemistry that addresses the short time period in which chemical reactions take place and investigates why some reactions occur but not others. Zewail’s picture-taking technique made possible these investigations. One of the first major discoveries of femtochemistry was to reveal details about the intermediate products that form during chemical reactions, which cannot be deduced from observing the starting and end products. Many publications have discussed the possibility of controlling chemical reactions by this method, but this remains controversial1.

 

 

The simplest approach and still one of the most common techniques is known as pump-probe spectroscopy. In this method, two or more optical pulses with variable time delay between them are used to investigate the processes happening during a chemical reaction. The first pulse (pump) initiates the reaction, by breaking a bond or exciting one of the reactants. The second pulse (probe) is then used to interrogate the progress of the reaction a certain period of time after initiation. As the reaction progresses, the response of the reacting system to the probe pulse will change. By continually scanning the time delay between pump and probe pulses and observing the response, workers can follow the progress of the reaction in real time.

 

 

The topic of femtochemistry is surveyed from both theoretical and experimental points of view. A time-dependent wave packet description of the photodissociation of the O—C—S molecule reveals vibrational motion in the transition-state region and suggests targets for direct experimental observation. Theoretical approaches for treating femtosecond chemical phenomena in condensed phases are featured along with prospects for laser-controlled chemical reactions by using tailored ultrashort chirped pulses. An experimental study of the photoisomerization of retinal in the protein bacteriorhodopsin is discussed with an aim to gain insight into the potential energy surfaces on which this remarkably efficient and selective reactions proceeds. Finally, a prospective view of new frontiers in femtochemistry is given2.

 

Figure4-8-1

 

Development of femtochemistry rewarded

What would a football match on TV be without "slow motion" revealing afterwards the movements of the players and the ball when a goal is scored? Chemical reactions are a similar case. The chemists' eagerness to be able to follow chemical reactions in the greatest detail has prompted increasingly advanced technology. Ahmed H. Zewail, has studied atoms and molecules in "slow motion" during a reaction and seen what actually happens when chemical bonds break and new ones are created. Zewail's technique uses what may be described as the world's fastest camera. This uses laser flashes of such short duration that we are down to the time scale on which the reactions actually happen-femtoseconds (fs). One femtosecond is 10-15 seconds, that is, 0.000000000000001 seconds, which is to a second as a second is to 32 million years. This area of physical chemistry has been named femtochemistry.

Femtochemistry enables us to understand why certain chemical reactions take place but not others. We can also explain why the speed and yield of reactions depend on temperature. Scientists the world over are studying processes with femtosecond spectroscopy in gases, in fluids and in solids, on surfaces and in polymers. Applications range from how catalysts function and how molecular electronic components must be designed, to the most delicate mechanisms in life processes and how the medicines of the future should be produced.

 

How fast are chemical reactions?

Chemical reactions can, as we all know, take place at very varying velocities - compare a rusting nail and exploding dynamite! Common to most reactions is that their velocity increases as temperature rises, i.e. when molecular motion becomes more violent. For this reason researchers long believed that a molecule first needs to be activated, 'kicked' over a barrier, if it is to react. When two molecules collide, nothing normally happens, they just bounce apart. But when the temperature is high enough the collision is so violent that they react with one another and new molecules form. Once a molecule has been given a sufficiently strong 'temperature kick' it reacts incredibly fast, whereupon chemical bonds break and new ones form. This also applies to the reactions that appear to be slow (e.g. the rusting nail). The difference is only that the 'temperature kicks' occur more seldom in a slow reaction than in a fast one.

 

The barrier is determined by the forces that hold atoms together in the molecule (the chemical bonds) roughly like the gravitational barrier that a moon rocket from Earth must surmount before it is captured by the Moon's force field. But until very recently little was known about the molecule's path up over the barrier and what the molecule really looks like when it is exactly at the top, its 'transition state'.

 

Hundred years of research:

Svante Arrhenius (Nobel laureate in Chemistry 1903), inspired by van't Hoff (the first Nobel laureate in Chemistry, 1901) presented just over a hundred years ago a simple formula for reaction speed as a function of temperature. But this referred to many molecules at once (macroscopic systems) and relatively long times. It was not until the 1930s that H. Eyring and M. Polanyi formulated a theory based on reactions in microscopic systems of individual molecules. The theoretical assumption was that the transition state was crossed very rapidly, on the time scale that applies to molecular vibrations. That it would ever be possible to perform experiments over such short times was something no-one dreamed of.

 

But this is exactly what Zewail set out to do. At the end of the 1980s he performed a series of experiments that were to lead to the birth of the research area called femtochemistry. This involves using a high-speed camera to image molecules in the actual course of chemical reactions and trying to capture pictures of them just in the transition state. The camera was based on new laser technology with light flashes of some tens of femtoseconds. The time it takes for the atoms in a molecule to perform one vibration is typically 10-100 fs. That chemical reactions should take place on the same time scale as when the atoms oscillate in the molecules may be compared to two trapeze artists "reacting" with each other on the same time scale as that on which their trapezes swing back and forth.

What did the chemists see as the time resolution was successively improved? The first success was the discovery of substances formed along the way from the original one to the final product, substances termed intermediates. To begin with these were relatively stable molecules or molecule fragments. Each improvement of the time resolution led to new links in a reaction chain, in the form of increasingly short-lived intermediates, being fitted into the puzzle of understanding how the reaction mechanism worked.

 

The contribution for which Zewail is to receive the Nobel Prize means that we have reached the end of the road: no chemical reactions take place faster than this. With femtosecond spectroscopy we can for the first time observe in 'slow motion' what happens as the reaction barrier is crossed and hence also understand the mechanistic background to Arrhenius' formula for temperature dependence and to the formulae for which van't Hoff was awarded his Nobel Prize.

 

Femtochemistry in practice:

In femtosecond spectroscopy the original substances are mixed as beams of molecules in a vacuum chamber. An ultrafast laser then injects two pulses: first a powerful pump pulse that strikes the molecule and excites it to a higher energy state, and then a weaker probe pulse at a wavelength chosen to detect the original molecule or an altered form of this. The pump pulse is the starting signal for the reaction while the probe pulse examines what is happening. By varying the time interval between the two pulses it is possible to see how quickly the original molecule is transformed. The new shapes the molecule takes when it is excited-perhaps going through one or more transition states-have spectra that may serve as fingerprints. The time interval between the pulses can be varied simply by causing the probe pulse to make a detour via mirrors. Not a long detour: the light covers the distance of 0.03 mm in 100 fs! To better understand what happens, the fingerprint and the time elapsing are then compared with theoretical simulations based on results of quantum chemical calculations (Nobel Prize in Chemistry 1998) of spectra and energies for the molecules in their various states3.

 

The first experiments

In his first experiments Zewail studied the disintegration of iodocyanide: ICN     I + CN. His team was able to observe a transition state exactly when the I-C bond was about to break: the whole reaction takes place in 200 femtoseconds. In another important experiment Zewail studied the dissociation of sodium iodide (NaI): NaI     Na + I. The pump pulse excites the ion pair Na+ and I- which has an equilibrium distance of 2.8 Å between nuclei (Fig. 1) to an activated form [NaI]* which then assumes covalent bonding. However, its properties change when the molecules vibrate; when the nuclei are at their outer turning points, 10-15 Å apart, the electron structure is ionic, while at short distances it is covalent. At a certain point on the vibration cycle, just when the nuclei are 6.9 Å apart, there is a great probability that the molecule will fall back to its ground state or decay into sodium and iodine atoms.

Zewail also studied the reaction between hydrogen and carbon dioxide: H + CO2      CO + OH a reaction that takes place in the atmosphere and in combustion. He showed that the reaction crosses a relatively long state of HOCO (1 000 fs).

 

A question that has occupied many chemists is why certain chemical bonds are more reactive than others and what happens if there are two equivalent bonds in one molecule: will they break simultaneously or one at a time? To answer this kind of question Zewail and his co-workers studied the disassociation of tetrafluordiiodethane (C2I2F4) into tetrafluorethylene (C2F4) and two iodine atoms (I):

 

Figure8-2

 

Potential energy curves showing ground state and excited state for NaI. The upper curve shows the molecule vibrations in excited NaI. When the distance between the sodium nucleus and the iodine nucleus is short the covalent bond dominates, while the ion bond dominates at a greater distance. The vibrations may be compared to those of a marble rolling back and forth in a dish. As the 6.9 Å point is passed there is a chance that the marble will roll down to the lower curve. There it may end up in the pit to the left (return to ground state) or fly out to the right (decay into sodium and iodine atoms respectively).

 

They discovered that the two C-I bonds, despite their equivalence in the original molecule, break one at a time. Research is extra interesting when the results are unexpected. Zewail studied what may be thought the simple reaction between benzene, a ring of six carbon atoms (C6H6) and iodine (I2), a molecule consisting of two iodine atoms. When the two molecules become sufficiently close together they form a complex. The laser flash causes an electron to be shot from the benzene molecule into the iodine molecule. This then becomes negatively charged while the benzene molecule becomes positively charged. The negative and positive charges cause the benzene and the nearest iodine atom to be rapidly drawn to one another. The bond between the two iodine atoms is stretched when one of them is sucked in towards the benzene, whereupon the other atom breaks free and flies away. All this happens within 750 fs. Zewail found, however, that this is not the only way individual iodine atoms can be formed: sometimes the electron falls back onto benzene. But it is already too late for the iodine atoms: like a stretched rubber band breaking, the bond between the two atoms breaks and they fly apart4.

 

Figure10-3

Research explosion:

A much studied model reaction in organic chemistry is the ring opening of cyclobutane to yield ethylene or the reverse, the combining of two ethylene molecules to form cyclobutane. The reaction may thus go directly via one transition state with a simple activation barrier. Alternatively, it may proceed through a two-stage mechanism (right) so that first one bond breaks and tetramethylene is formed as an intermediate. After crossing another activation barrier the tetramethylene in turn is converted to the final product. Zewail and his co-workers showed with femtosecond spectroscopy that the intermediate product was in fact formed, and had a lifetime of 700 fs.

 

Another type of reaction studied with femtosecond technology is the light-induced conversion of a molecule from one structure to another, photoisomerisation. The conversion of the stilbene molecule, which includes two benzene rings, between the cis- and trans- forms was observed by Zewail and his co-workers5.

Figure16-4

They concluded that during the process the two benzene rings turn synchronously in relation to one another. Similar behaviour has also recently been observed for the retinal molecule, which is the colour substance in rodopsin, the pigment in the rods of the eye. The primary photochemical step, when we perceive light, is a cis-trans conversion around a double bond in retinal. With femtosecond spectroscopy other researchers have found that the process takes 200 fs and that a certain amount of vibration remains in the product of the reaction. The speed of the reaction suggests that energy from the absorbed photon is not first redistributed but is localised directly to the relevant double bond. This would explain the high efficiency (70%) and hence the eye's good night vision. Another biologically important example where femtochemistry has explained efficient energy conversion is in chlorophyll molecules, which capture light in photosynthesis6.

 

Femtosecond studies following Zewail's work are being performed intensively the world over, using not only molecular beams but also processes on surfaces (e.g. to understand and improve catalysts), in liquids and solvents (to understand mechanisms of the dissolving of and reactions between substances in solution) and in polymers (e.g. to develop new material for use in electronics). Another important research field is studies of biological systems. Knowledge of the mechanisms of chemical reactions is also important for our ability to control the reactions. A desired chemical reaction is often accompanied by a series of unwanted, competing reactions that lead to a mixture of products and hence the need for separation and cleansing. If the reaction can be controlled by initiating reactivity in selected bonds, this could be avoided. Femtochemistry has fundamentally changed our view of chemical reactions. From a phenomenon described in relatively vague metaphors such as 'activation' and 'transition state', we can now see the movements of individual atoms as we imagine them. They are no longer invisible. Here lies the reason why the femtochemistry research initiated by this year's Nobel Laureate has led to explosive development. With the world's fastest camera available, only the imagination sets bounds for new problems to tackle7.

 

The essence of the chemical industry and indeed of life is the making and breaking of molecular bonds. The elementary steps in bond making and breaking occur on the time scale of molecular vibrations and rotations, the fastest period of which is ≈10 femtoseconds (10−14 s). Chemical reactions are, therefore, ultrafast processes, and the study of these elementary chemical steps has been termed “femtochemistry”. A primary aim of this field is to develop an understanding of chemical reaction pathways at a molecular level. Given this information, one can better conceive of new methods to control the outcome of a chemical reaction. Because chemical reaction pathways for all but the simplest of reactions are complex, this field poses challenges both theoretically and experimentally. Nevertheless, much progress is being made, and systems as complex as biomolecules can now be investigated in great detail. In the following, we will present results from theoretical and experimental efforts to probe deeply into the nature of chemical reactions in complex systems8.

 

Theory:

Ultrafast dynamics of molecules have long been studied theoretically by integrating a relevant equation of motion. The time-dependent wave packet approach has proven to be particularly promising for following femtosecond chemical reactions in real time. Briefly, a molecular system can be characterized by electronic potential energy surfaces on which wave packets propagate. For example, an electronic excited-state potential of the O—C—S molecule is depicted as a function of interatomic separation between C and O (Rco) and C and S (Rcs). The time evolution of the wave packet has been investigated by integrating the Schoedinger equation as the wave packet propagates on this potential energy surface. The initial wave packet (t = 0) was prepared on the excited electronic state surface by photoexcitation from the ground electronic state9. It is apparent from wave packet evolution that one portion of the wave packet propagates along the dissociation coordinate fairly rapidly, whereas another portion remains localized near the Franck–Condon region. The localized portion moves along the in-phase-stretching coordinate with comparable vibrational amplitudes for both CO and CS stretching motions. This oscillatory wave packet motion can be regarded as motion along an unstable periodic orbit on the excited state potential energy surface, which would give rise to prominent spectral peaks in the experimental UV absorption spectrum of O—C—S. A wave packet calculation on the ab initio potential energy surface reproduces the qualitative features of the autocorrelation function that has been experimentally extracted from photofragment excitation spectroscopy10. A recurrence period of 48 fs was predicted theoretically, which is comparable to the experimental value of 42 fs. This agreement between experimental and theoretical vibrational periods in the transition-state region highlights the power of recent theoretical approaches11.

 

Time evolution of a dissociating wave packet on an electronically excited state of O—C—S. The initial wave packet at (a) t = 0 (red) and its snapshots at (b) t = 12 fs (green) and (c) t = 24 fs (blue).

 

Experimental efforts in the field of femtochemistry have exploited the pump-probe technique, where a “pump” laser pulse initiates a chemical reaction and a “probe” laser pulse records a “snapshot” of the chemical reaction at a time controlled by the temporal delay between the pump and probe pulses. By recording snapshots as a function of the temporal delay, one can follow the time evolution of a chemical reaction with time resolution limited only by the duration of the laser pulses. Beyond monitoring the outcome of a normal photoreaction, the phase and frequency of a femtosecond “pump” pulse can be tailored, as prescribed by theory, to drive a molecular state to a target location on its potential energy surface and then steer it toward a channel that favors a particular photochemical outcome. For example, the excitation pulse might be a femtosecond linear chirped laser pulse, which can interact with the wave packet through a so-called intra-pulse pump–dump process. A negatively chirped pulse (frequency components shift in time from blue to red) might be tailored to maintain resonance with the wave packet as it evolves along the excited state surface. In contrast, a positively chirped pulse might quickly go off resonance with the wave packet, and the photoexcitation would be nonselective. This intra-pulse pump–dump effect, first proposed theoretically, has been demonstrated experimentally quite recently12. It was also shown that ultrashort chirped laser pulses can be used for laser-controlling photodissociation processes of NaI and CO.

 

When molecules are studied in the gas phase, interactions between neighboring molecules can be neglected. However, a vast range of chemical phenomena occurs preferentially or even exclusively in a liquid environment. When a reaction occurs in a condensed phase, the theoretical problem becomes significantly more complex. This difficulty is because motion of the surrounding molecules leads to fluctuations in the structures and energy levels of neighboring molecules, thereby promoting or hindering thermally activated processes in these systems. Understanding molecular motions and how they couple to the reaction coordinate is, therefore, crucial for a comprehensive description of the underlying microscopic processes. This problem is particularly challenging because molecules exhibit strong mutual interactions, and these interactions evolve on the femtosecond time scale because of random thermal motion of the molecules. In essence, understanding the dynamics of a molecular system in the condensed phase boils down to a problem of nonequilibrium statistical physics. Combined with an impressive increase in computational capacity, recent developments in theoretical methodology such as molecular dynamics, path integral approaches and kinetic equation approaches for dissipative systems have enlarged dramatically the scope of what is now theoretically tractable13.

 

Experiment:

Whereas ultrafast spectroscopic methods used to be confined to a handful of research labs, recent advances in femtosecond laser technology have led to the introduction of commercial lasers that generate optical pulses shorter than 100 fs. The widespread availability of such short pulses has, in turn, led to the study of a diverse range of molecular systems as they transform from “reactants” to “products.” Systems of ever-increasing complexity have now been investigated, including small-molecule reactions in the condensed phase and photo-induced reactions in biological systems. Here, we focus our attention on the photoisomerization of retinal in the protein bacteriorhodopsin with an aim to gain further insight into the potential energy surfaces on which this photoisomerization reaction proceeds14.

 

Bacteriorhodopsin is a trans-membrane protein found in a simple archaebacterium, Halobacterium halobium. This protein has a photosynthetic function and converts light energy to chemical energy by pumping protons across a cell membrane. The chromophore responsible for light absorption is a retinal that is covalently bound within the interior of a seven-helix protein in the form of a protonated Schiff base15. On absorbing a photon, the retinal isomerizes from its all-trans form to its 13-cis form with a quantum efficiency of about 64%16. This primary process triggers a cascade of events, which ultimately leads to the translocation of a proton across the cell membrane and the return of the retinal to its all-trans form17. The pathway of the primary photoisomerization is the focus of this experimental work. Time-resolved spectra recorded over a broad spectral range suggest a reaction pathway that involves three electronic states whose surfaces cross in two regions along the photoisomerization pathway18. Theoretical support for this notion has also been presented19. The three-state model provides a compelling explanation for the origins of selectivity and efficiency demonstrated by this biological reaction. Not all aspects of this reaction are fully understood, however. The measured dynamics for the isomerization reaction evolve biexponentially with 200-fs and 700-fs components in a 2:1 ratio. Although the origin of this nonexponential process is yet to be explained, ongoing efforts to model the kinetics in finer detail appear promising20.

 

New Frontiers:

To extend beyond the view afforded by ultrafast spectroscopy, ultrafast structural techniques are being developed. For example, time-resolved x-ray crystal structures of photolyzed carbon monoxymyoglobin (MbCO + hn* Mb + CO) have been acquired at the European Synchrotron and Radiation Facility in Grenoble, France, with 7.5-ns time resolution. For these experiments, the “probe” is an x-ray pulse, not a laser pulse, and the sample x-ray diffraction is recorded instead of its absorbance spectrum21. Further experiments by Wulff and collaborators are being carried out with about 150-ps time resolution. In addition to x-ray approaches, ultrafast electron diffraction is also being developed. Because of the current pace of new technological developments, the opportunities to directly monitor the motion of nuclei throughout the course of chemical reactions are becoming ever more numerous22. Furthermore, by monitoring nuclear motion on a time scale that is accessible to theory, comparisons can be made that test directly the validity of various theoretical approaches23. Such detailed comparisons can then serve to guide the design of laser-controlled chemical reactions. Femtochemistry, therefore, affords the opportunity to relate the formal study of nonequilibrium statistical physics to the real world of molecular science24.

 

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Received on 31.03.2010        Modified on 20.04.2010

Accepted on 16.06.2010        © AJRC All right reserved

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 837-843