The DAFNE-Light Laboratory in Frascati: a new brilliant infrared Synchrotron Radiation source for microspectroscopy experiments

E. Burattini,^a,b F. Monti,^a G. Cinque^a and C. Marcelli^b

^aDipartimento Scientifico e Tecnologico, Università degli Studi di Verona, Strada Le Grazie,1 , I-37134 Verona, Italy
^bINFN, Laboratori Nazionali di Frascati, Via E. Fermi, Frascati, Roma, Italy.

DAFNE is the name of the new double storage ring collider for electrons and positrons recently in operation at the INFN National Laboratories in Frascati, Italy (Figure 1). Special magnetic structures allow DAFNE to reach one of the highest beam currents (1–5 A) inside the storage ring, with a high stability of the beam orbit. The DAFNE-Light laboratory has been designed to utilise the intense and stable Synchrotron Radiation (SR) emitted at DAFNE. Two beamlines, collecting photons from a wiggler and from a bending magnet of the electron storage ring, will be in operation at the end of this year. The first one will operate in the soft X-ray range (1–7 keV). The other one, called SINBAD¹ (Synchrotron INfrared Beamline At DAFNE), will work in the infrared (IR) range (1–1000 mm).

Figure 1. The experimental hall of DAFNE, the new double storage ring for electrons at positrons in operation at Frascati.

When electrons (or positrons) at relativistic velocities are forced to turn in the magnetic structures (like in a bending magnet) of a storage ring, they emit electromagnetic radiation (called Synchrotron Radiation) in a narrow cone in the direction of motion.² The vertical emission angle (i.e. perpendicular to the orbital plane) can be as low as a few tenths of mrad and increases with the cube root of wavelength. This behaviour is important to understand the gain in brilliance of IR-SR with respect to a conventional broadband laboratory source. It also determines the required vertical acceptance angle at the extraction port of the ring. At SINBAD, this angle is about 50 mrad.

The SR spectral distribution is continuous from IR to X-rays. In the case of DAFNE, the high beam current gives a very high photon flux, which is independent of the beam energy, depending only on the beam current, in the visible-IR range.

Moreover, SR is an absolute source: the photon flux can be easily calculated once the horizontal collection angle is fixed.

At SINBAD, the combined effect of the spectral flux and angular behaviour of SR with respect to a black-body source (which emits isotropically following Planck’s Law), taking also into account the optical source dimensions, gives rise to a high gain in brilliance (flux divided by the transversal dimensions of the optical source) as a function of wavelength (Figure 2). The minimum value of the brilliance ratio (ABR), as estimated from theoretical calculations, is 25–30 and reaches two orders of magnitude in the NIR towards up to three orders of magnitude in the far-infrared.

Figure 2. Brilliance ratio of the DAFNE SR source to a black-body source (for T=2000 K and for an emitting surface S=0.45 cm2).

If we consider the low noise at DAFNE, thanks to the high stability of the electron beam orbit, it is clear that SINBAD will give improved S/N ratio with respect to a conventional laboratory source in all the IR range for brightness-limited experiments (small sample areas and microspectroscopy).

SR is also a highly polarised source. The radiation is linearly polarised with horizontal polarisation in the plane of the orbit, while above and below the plane a certain fraction of the emitted intensity is vertically polarised, as a function of the vertical angle and depending on the wavelength. At SINBAD, a couple of remotely controlled slits will allow radiation to be collected above and below the orbital plane with the desired flux and degree of circular polarisation.

At the end of the beamline, two experimental stations will be in operation working with an interferometer and a monochromator. One of them will be installed in collaboration with the group of physicists of the University of Verona to carry out spectroscopy experiments in the NIR. The estimated dimensions of the focussed photon beam as obtained by ray-tracing calculations are about 2 ´ 2 mm² (Figure 3).

Figure 3. Intensity distribution for l=10 mm of the last focus of SINBAD, placed at the entrance pupil of the interferometer, as obtained by ray-tracing simulations.

Although the first beam diagnosis, by means of an IR monochromator, have still to be performed, all the theoretical calculations indicate that SINBAD, the first Italian IR beamline, will be one of the most brilliant and stable IR-SR sources in the world and a powerful linearly and circularly polarised source. The scientific programme of the beamline includes IR spectroscopy and microspectroscopy, with special attention to biological and medical applications.^3–5 Moreover, the pulsed time structure of the photon beam—due to the bunch structure of the electron beam—could also allow for pump-probe experiments in the IR during the planned SR dedicated beamtime of the DAFNE ring.

References

1. A. Marcelli, E. Burattini, A. Nucara, P. Calvani, G. Cinque, C. Mencuccini, S. Lupi, F. Monti and M. Sanchez del Rio, in Infrared Synchrotron Radiation, Nuovo Cimento D, Ed. Composizioni Bologna (1998).

2. H. Winick and S. Doniach, Synchrotron Radiation Research. Plenum Press, New York (1980).

3. G.L. Carr, P. Dumas, C.J. Hirschmugl and G. P. Williams, in Infrared Synchrotron Radiation, Nuovo Cimento D, Ed. Composizioni Bologna (1998).

4. M. Jackson et al., Biochimica et Biophysica Acta 1270, 1 (1995).

5. D.C. Malins et al., Cancer 75, 503 (1995).

Nobel Prize for femtosecond spectroscopy

The 1999 Nobel Prize in Chemistry has been awarded to Professor Ahmed H. Zewail, of the California Institute of Technology, Pasadena, USA, for his studies of transition states of chemical reactions by femtosecond spectroscopy. Ahmed H. Zewail was born in 1946 in Egypt where he grew up and studied at the University of Alexandria. After continuing his studies in the USA, he obtained his PhD in 1974 at the University of Pennsylvania. After two years at the University of California at Berkeley he was employed at Caltech where he has the Linus Pauling Chair of Chemical Physics.

Professor Zewal’s achievement of being able to investigate chemical reactions on the time-scale that they occur is the latest development in a series by previous Nobel Prize winners leading to the understanding of chemical reactions and their observations on decreasing time-scales.

Ronald Norrish and George Porter, Nobel Laureates in Chemistry 1967, achieved time resolution in the millisecond to microsecond range by using a flash lamp. They shared the prize with Manfred Eigen, who reached a similar time resolution by exposing his chemical solution to a pressure or electrical shock or a heat shock. Some million times better resolution was later achieved in studies of collisions between molecules in vacuum, work for which the Americans Dudley Herschbach, Yuan Lee and John Polanyi were awarded the Nobel Prize in Chemistry in 1986.

Ahmed Zewail’s work towards the study of reactions on the fs time-scale began when he realised, from an experiment in the 1970s on anthracene molecules at low temperature, that molecules could be brought to vibrate in step. This “coherent preparation” of a sample system is a key point in all his experiments.

Zewail mixes the reactants in the form of molecular beams in a vacuum chamber. An ultrafast laser, which produces pulses lasting a few tens fs each, is then used to inject two pulses: first, a powerful pump pulse which excites the molecule to a higher energy state and starts the reaction. The pump pulse is followed by a second (weaker) pulse, the probe pulse, at a wavelength chosen to detect, for example, the original molecule or an altered form of it. This pulse is timed to arrive at precise intervals after the reaction has been started, thus enabling the course of the reaction to be studied and transitions states detected. The time interval between pulses can be varied simply using mirrors to increase or decrease the distance travelled by the probe pulse.

In his first series of experiments Zewail studied the unimolecular disintegration of iodine cyanide into iodine atom and cyano radical: ICN ® I + CN. In 1987 his research team managed to observe a transition state corresponding to that of the I–C bond in the action of breaking—the whole reaction being over in 200 fs.

In another important experiment (published in 1989) Zewail and his group studied the dissociation of sodium iodide (NaI): Na⁺I^– ® Na + I. The pump pulse excites the ion pair Na⁺I^– which has an equilibrium distance of 2.8 Å between the nuclei (Figure 1) to an excited form [Na–I]^* which at this short bond distance assumes a covalent bonding character. However, its properties change when the molecule vibrates: when the Na and I atoms are at their outer turning points, 10–15 Å apart the electron structure is ionic, [Na⁺ … I^–]^*, as one electron has moved from Na to I. When the atoms move back together the bonding becomes covalent again: [Na–I]^*, and so forth.

Figure 1(a). Potential energy curves showing the energies of ground state (bottom curve with deep minimum) and excited state (top curve) for NaI as function of the distance between the nuclei.

Figure 1(b). Experimental observations of coherent vibrations (so-called wave-packet motion) in femtosecond-excited NaI, on one hand manifested in terms of amount of activated complex [Na–I]* at covalent (short) distance, on the other as bursts of free Na atoms.

A critical point during the vibration is when the distance is 6.9 Å. As seen in Figure 1(a), this is where the excited state (upper curve) and ground state (lower curve) are very close to each other. At this point there is a great probability that the excited [Na–I]^* will either fall back to its ground state [Na–I] or decay into sodium and iodine atoms.

What Zewail and his coworkers found was that they could follow the activated complex as it moved back and forth between covalent and ionic structures and, moreover, that bursts of free sodium atoms were produced in phase with these oscillations. The explanation was that the pump pulse had created a sample of excited molecules all starting at the point 2.8 Å and thereafter vibrating in synchrony. Then they will all pass the magic point 6.9 Å at the same time, explaining why they reacted almost at the same time and thus the oscillatory production of sodium atoms in the experiment.

This is the crucial “coherent preparation” of the system, which means that although the measurement by the later probe pulse will include a huge number of molecules, their behaviour will not be random but coherent. In turn this enables observation of the movements of the nuclei during the vibration and thus a characterisation of the transition state with high spatial resolution (about 0.1 Å in NaI).

Femtosecond spectroscopy has fundamentally changed our view of chemical reactions. From a phenomenon described in terms of rather vague metaphors such as “activation” and “transition state” it is now possible to see the actual movements of individual atoms.

Analytical chemistry award

The Heinrich-Emanuel-Merck Award for Analytical Chemistry has, since 1988, been sponsored by Merck KgaA of Darmstadt, Germany. It is worth DM25,000 and is open to chemists up to the age of 45, working in particular on developing new methods of chemical analysis and their applications in the area of human interest. Their work should be directed towards the improvement of our conditions of life, providing solutions to analytical problems in the areas of life sciences, material sciences or environment.

The prize will be awarded for the sixth time during Analytica 2000 in Munich, Germany, which is being held between 10 and 14 April. Papers published during the last three years by applicants will be judged by a jury of internationally recognised scientists. The Chairman of the jury is Professor Manfred Grasserbauer, to whom applications and requests for further information should be made. The deadline for applications is 31 December 1999. Professor Dr M. Grasserbauer, TU Wien, Institut für Analytische Chemie, Getreidemarkt 9, A- 1060 Wien, Austria, sek151@email.tuwien.ac.at. The award also has its own web site at www.hem2000.de.

Bran+Luebbe bought

Bran+Luebbe has been bought by the US company, United Dominion Industries, which is based in Charlotte, North Carolina, and has 12,500 employees and an annual turnover of $2 billion. Bran+Luebbe was previously owned by Anglo-American investment bankers, following a leveraged buy-out in 1993 from the Swedish company Tetra-Laval.

Bran+Luebbe, who manufacture NIR instruments, wet chemistry analysers and metering pumps, will become part of the Flow Technology Segment of United Dominion. www.bran-luebbe.de, www.uniteddominion.com.