Spectroscopy, femtosecond

In Section 9.2.2 we saw that a pulsed Tp sapphire laser can produce pulses less then 10 fs in length. There are also other laser techniques which can be used to produce pulse lengths 

So far in this book we have seen how vibrational energy levels can be investigated spectroscopically through the absorption or emission of energy. Direct measurement of the time taken for vibration to occur, and the translation of these times into energy levels, provides an alternative means of accessing these levels. 

We shall consider just two examples of the use of femtosecond lasers in spectroscopy. One is an investigation of the transition state in the dissociation of Nal and the other concerns the direct, time-based observation of vibrational energy levels in an excited electronic state of I2. 

All heteronuclear diatomic molecules, in their ground electronic state, dissociate into neutral atoms, however strongly polar they may be. The simple explanation for this is that dissociation into a positive and a negative ion is much less likely because of the attractive force between the ions even at a relatively large separation. The highly polar Nal molecule is no exception. The lowest energy dissociation process is 

The region of the avoided crossing for Nal is the region where the molecule is in a transition state, a state intermediate between those in which the molecule is fully bound or dissociafed. If is fhis region of fhe pofenfial energy curves which had remained inaccessible before investigation wifh femtosecond lasers became possible. 

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Seidner L, Stock G and Domcke W 1995 Nonperturbative approach to femtosecond spectroscopy - general theory and application to multidimensional nonadiabatic photoisomerization processes J. Chem. Phys. 103 4002... 

Vos M H, Rappaport F, Lambry J-C, Breton J and Martin J-L 1993 Visualization of the coherent nuclear motion in a membrane protein by femtosecond spectroscopy Nature 363 320-5... 

Breton J, Martin J-L, Fleming G R and Lambry J-C 1988 Low-temperature femtosecond spectroscopy of the initial step of electron transfer in reaction centers from photosynthetic purple bacteria Biochemistry 27 8276... 

Vos M H, Jones M R, Hunter C N, Breton J, Lambry J C and Martin J L 1996 Femtosecond spectroscopy and vibrational coherence of membrane-bound RCs of Rhodobacfe/ sp/raero/des genetically modified at positions M210 and LI 81 The Reaction Center of Photosynthetic Bacteria—Structure and Dynamics ed M E Michel-Beyerle (Berlin Springer) pp 271-80... 

M. C. Nuss, W. Zinth, W. Kaiser, E. Kolling, and D. Oesterhelt. Femtosecond spectroscopy of the first events of the photochemical cycle in bacteriorhodopsin. Chem. Phys. Lett, 117(l) l-7, 1985. 

In order to define how the nuclei move as a reaction progresses from reactants to transition structure to products, one must choose a definition of how a reaction occurs. There are two such definitions in common use. One definition is the minimum energy path (MEP), which defines a reaction coordinate in which the absolute minimum amount of energy is necessary to reach each point on the coordinate. A second definition is a dynamical description of how molecules undergo intramolecular vibrational redistribution until the vibrational motion occurs in a direction that leads to a reaction. The MEP definition is an intuitive description of the reaction steps. The dynamical description more closely describes the true behavior molecules as seen with femtosecond spectroscopy. 

In the chapter on reaction rates, it was pointed out that the perfect description of a reaction would be a statistical average of all possible paths rather than just the minimum energy path. Furthermore, femtosecond spectroscopy experiments show that molecules vibrate in many dilferent directions until an energetically accessible reaction path is found. In order to examine these ideas computationally, the entire potential energy surface (PES) or an approximation to it must be computed. A PES is either a table of data or an analytic function, which gives the energy for any location of the nuclei comprising a chemical system. 

These two examples show how femtosecond spectroscopy allows the observation, in real time, of processes whose real-time behaviour could only be inferred, previously, from experimental observations made on a much longer time scale. 

New to the fourth edition are the topics of laser detection and ranging (LIDAR), cavity ring-down spectroscopy, femtosecond lasers and femtosecond spectroscopy, and the use of laser-induced fluorescence excitation for stmctural investigations of much larger molecules than had been possible previously. This latter technique takes advantage of two experimental quantum leaps the development of very high resolution lasers in the visible and ultraviolet regions and of the supersonic molecular beam. 

Singlet diradicals are usually extremely short-lived intermediates. For example, trimethylene (TM, 2) was observed to have a fast decay time of 120 fs by femtosecond spectroscopy [84, 85]. Since the localized 1,3-cyclopentanediyl diradical (62) was characterized by Buchwalter and Closs in 1975 [81, 82], experimental efforts have been made to prepare and characterize the persistent, localized singlet 1,3-diradicals. Some experimental achievements of the localized diradicals are collected in Fig. 25 and Table 3. It should be mentioned that the literature of experimental studies selected here is not exhaustive and more related references can be found in [83-115] and others. 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

Figure 1.4. Experimental and theoretical femtosecond spectroscopy of IBr dissociation. Experimental ionisation signals as a function of pump-probe time delay for different pump wavelengths given in (a) and (b) show how the time required for decay of the initally excited molecule varies dramatically according to the initial vibrational energy that is deposited in the molecule by the pump laser. The calculated ionisation trace shown in (c) mimics the experimental result shown in (b).

Figure 1.5. Femtosecond spectroscopy of bimolecular collisions. The cartoon shown in (a illustrates how pump and probe pulses initiate and monitor the progress of H + COj->[HO. .. CO]->OH + CO collisions. The huild-up of OH product is recorded via the intensity of fluorescence excited hy the prohe laser as a function of pump-prohe time delay, as presented in (h). Potential energy curves governing the collision between excited Na atoms and Hj are given in (c) these show how the Na + H collision can proceed along two possible exit channels, leading either to formation of NaH + H or to Na + H by collisional energy exchange.

Note, however, that the 1999 Nobel Prize for Chemistry was awarded to A. Zewail for his studies of the transition states of chemical reactions using femtosecond spectroscopy (Academy s citation, October 12,... 

Onidas D, Gustavsson T, Lazzarotto E, Markovitsi D (2007) Fluorescence of the DNA double helices (dAdT)n-(dAdT)n studied by femtosecond spectroscopy. Phys Chem Chem Phys 9 5143-5148... 

From the analyses presented in this section, it can be seen that, at least in principle, the enormous complexity of a solute surrounded by a solvent can be reduced to problems of smaller complexity. The interesting point is that these relatively simple model situations have been most useful to correlate experimental results derived from femtosecond spectroscopy. 

By means of femtochemistry, investigation of elementary reactions on a timescale of femtoseconds (10-15s) is possible. The method employs a combination of pulsed-laser and molecular-beam technologies. Investigation of a unimolecular reaction by femtosecond spectroscopy involves two ultra-fast laser pulses being passed into a beam of reactant molecules. 

F. Laermer, T. Elsaeser, and W. Kaiser, Femtosecond spectroscopy of excited-state proton-transfer in 2-(2 -hydroxyphenyl)benzothiazole, Chem, Phys. Lett. 148, 119(1988). 

Two examples given to illustrate the importance of Picosecond and Femtosecond spectroscopy in... 

The year 2003 is the tenth anniversary of the first Femtochemistry Conference and the fiftieth anniversary of Watson and Crick s celebrated discovery of the DNA double helix [1], Remarkable progress has been made in both fields femtosecond spectroscopy has made decisive contributions to Chemistry and Biology, and advances in the elucidation of static nucleic acid structures have profoundly transformed the biosciences. However, much less is known about the dynamical properties of these complex macromolecules. This is especially true of the dynamics of the excited electronic states, including their evolution toward the photoproducts that are the end result of DNA photodamage [2],... 

In addition to ab initio quantum simulations of the experimental femtosecond and picosecond pump-probe spectra, traditional continuous-wave (CW) spectra could also be simulated using the time-dependent approach to absorption spectroscopy [13]. The results show that femtosecond/picosecond versus CW spectroscopy is complementary in the present case, the radial and angular pseudorotation and the symmetric stretch are observed, and simulated with preferential sensitivity using CW picosecond, and femtosecond spectroscopy, respectively. 

The laser source in our spectrometer is an amplified femtosecond dye laser with a much larger repetition rate than many of the existing amplified laser systems used for femtosecond spectroscopy. The amplification is necessary to improve the signal intensity which actually depends on roughly the third power of the laser intensity. The large repetition rate helps average over pulse-to-pulse fluctuations of the laser. 

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