Ultrashort spectroscopy

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

The first theoretical attempts in the field of time-resolved X-ray diffraction were entirely empirical. More precise theoretical work appeared only in the late 1990s and is due to Wilson et al. [13-16]. However, this theoretical work still remained preliminary. A really satisfactory approach must be statistical. In fact, macroscopic transport coefficients like diffusion constant or chemical rate constant break down at ultrashort time scales. Even the notion of a molecule becomes ambiguous at which interatomic distance can the atoms A and B of a molecule A-B be considered to be free Another element of consideration is that the electric field of the laser pump is strong, and that its interaction with matter is nonlinear. What is needed is thus a statistical theory reminiscent of those from time-resolved optical spectroscopy. A theory of this sort was elaborated by Bratos and co-workers and was published over the last few years [17-19]. 

To carry out a spectroscopy, that is the structural and dynamical determination, of elementary processes in real time at a molecular level necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to resolve in time the molecular motions. Sub-100 fs laser pulses were realised for the first time from a colliding-pulse mode-locked dye laser in the early 1980s at AT T Bell Laboratories by Shank and coworkers by 1987 these researchers had succeeded in producing record-breaking pulses as short as 6fs by optical pulse compression of the output of mode-locked dye laser. In the decade since 1987 there has only been a slight improvement in the minimum possible pulse width, but there have been truly major developments in the ease of generating and characterising ultrashort laser pulses. 

Equation (65) illustrates that in the limit of ultrashort pulses the two-pathway method loses its value as a coherence spectroscopy 8s is fixed at it/2 irrespective of the system parameters. From the physical perspective, when the excitation is much shorter than the system time scales, the channel phase carries no imprint of the system dynamics since the interaction time does not suffice to observe dynamical processes. 

Upon absorption of UV radiation from sunlight the bases can proceed through photochemical reactions that can lead to photodamage in the nucleic acids. Photochemical reactions do occur in the bases, with thymidine dimerization being a primary result, but at low rates. The bases are quite stable to photochemical damage, having efficient ways to dissipate the harmful electronic energy, as indicated by their ultrashort excited state lifetimes. It had been known for years that the excited states were short lived, and that fluorescence quantum yields are very low for all bases [4, 81, 82], Femtosecond laser spectroscopy has, in recent years, enabled a much... 

The identification of excited states in strong field interactions with molecules has lead to some novel forms of molecular spectroscopy, allowing previously inaccessible states to be studied. For the most part, this comes from the ability to do transient spectroscopy in the time domain with ultrashort pulses. But, the strong field interaction also allows for new population mechanisms. 

With development of ultrashort pulsed lasers, coherently generated lattice dynamics was found, first as the periodic modulation in the transient grating signal from perylene in 1985 by De Silvestri and coworkers [1], Shortly later, similar modulation was observed in the reflectivity of Bi and Sb [2] and of GaAs [3], as well as in the transmissivity of YBCO [4] by different groups. Since then, the coherent optical phonon spectroscopy has been a simple and powerful tool to probe femtosecond lattice dynamics in a wide range of solid... 

Summary. This Chapter focuses on the investigation of fast electron transport studies in solids irradiated at relativistic laser intensities. Experimental techniques based upon space-resolved spectroscopy are presented in view of their application to both ultrashort Ka X-ray sources and fast ignition studies. Spectroscopy based upon single-photon detection is unveiled as a complementary diagnostic technique, alternative to well established techniques based upon bent crystals. Application of this technique to the study of X-ray fluorescence emission from fast electron propagation in multilayer targets is reported and explored as an example case. 

D. Heiman, Spectroscopy of Semiconductors at Low Temperatures and High Magnetic Fields A. V. Nurmikko, Transient Spectroscopy by Ultrashort Laser Pulse Techniques A. K. Ramdas and S. Rodriguez, Piezospectroscopy of Semiconductors O. J. Glembocki and B. V. Shanabrook, Photoreflectance Spectroscopy of Microstructures D. G. Seiler, C. L. Littler, and M. H. Wiler, One- and Two-Photon Magneto-Optical Spectroscopy of InSb and Hgj Cd Te... 

Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give us a clue to reveal its significance in chemical reactions. In this study, we employed two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigated the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state. 

My question to Prof. Gerber is the following Could you please explain the different virtues of femtosecond pump-pulse experiments versus ultrashort zero electron kinetic energy (ZEKE) spectroscopy Do they yield complementary information on the molecular dynamics or are there specific domains where one of them should be preferred with respect to the other ... 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

Little is known about the "molecular scale of time". Chemical research in the past has had its accent on highly varied spectroscopic methods, but these have been mainly for the study of spatial and frequency resolution. This spectrum of spectroscopies is incomplete without the inclusion of time. The availability of ultrashort pulses of energy and their application to molecular problems are therefore expected to form an important extension to the field of molecular spectroscopy. During the next decade the creation of new frontiers in chemistry through studies based on such techniques is inevitable. 

Summary. X-ray line emissions from ultrashort high-intensity laser-produced plasma were studied in order to clarify the physics of energy transport associated with the generation of ultrashort X-ray pulses for use in various applications. This article reviews two topics. The first is the application of Ka spectroscopy to the study of energy transport in laser-produced plasma. The second topic is the application of X-ray polarization spectroscopy to measurements of the anisotropy of hot electrons generated with ultrashort high-intensity laser pulses. 

The vibrational spectra of strongly associated liquids display broad bands that are difficult to analyze. The structural and dynamical information contained in these spectral features provided a field of speculation over the last decades. Obviously additional experimental evidence is urgently needed for a reliable interpretation. In this chapter, nonlinear pump-probe spectroscopy with intense ultrashort tunable pulses is considered... 

Heilweil EJ. Ultrashort-pulse multichannel infrared spectroscopy using broadband frequency conversion in LiI03. Optics Lett 1989 14 551-553. 

---