Picosecond Infrared Spectroscopy

Many photochemical and photophysical phenomena occur on a time scale shorter than a nanosecond. In order to follow such fast phenomena by infrared spectroscopy, picosecond to femtosecond time-resolved infrared measurements are required. Since time resolving in this time range cannot be performed by utilizing the fast-response capability of a detector and the time-resolving power of an electronic circuit (gate circuit, etc.), the following optical methods are mainly used (i) a method based on the upconversion (optical gating) process, and (ii) a method which detects pulsed infrared radiation itself. At present, the latter method is commonly used for picosecond to femtosecond time-resolved measurements. 

The complexity of the physical properties of liquid water is largely determined by the presence of a three-dimensional hydrogen bond (HB) network [1]. The HB s undergo continuous transformations that occur on ultrafast timescales. The molecular vibrations are especially sensitive to the presence of the HB network. For example, the spectrum of the OH-stretch vibrational mode is substantially broadened and shifted towards lower frequencies if the OH-group is involved in the HB. Therefore, the microscopic structure and the dynamics of water are expected to manifest themselves in the IR vibrational spectrum, and, therefore, can be studied by methods of ultrafast infrared spectroscopy. It has been shown in a number of ultrafast spectroscopic experiments and computer simulations that dephasing dynamics of the OH-stretch vibrations of water molecules in the liquid phase occurs on sub-picosecond timescales [2-14],... 

Dougherty TP, Heilweil EJ. Dual track picosecond infrared spectroscopy of metal-carbonyl photochemistry. In Lau A, Siebert F, Wemcke W, eds. Proceedings in Physics. Berlin Springer-Verlag, 1994 136-140. 

George MW, Dougherty TP, Heilweil EJ. UV photochemistry of [CpFe(CO)2]2 studied by picosecond time-resolved infrared spectroscopy. J Phys Chem 1996 100 201-206. 

Photodissociation of CO ( flash ) from the reduced enzyme after mixing with O2 ( flow ) in the dark has been the main method of initiating the reaction. Some concern has been voiced as to the possibility that this technique might introduce artifacts due to CO, but results using a rapid O2 mixing system without CO have coirfirmed the applicabihty of the flow/flash technique. Yet, it has been shown by infrared spectroscopy that upon photolysis, the heme 03-bound CO is first transferred in less than a picosecond to the nearby Cub, to which it remains bound for about a microsecond at room temperature before diffusing out of the enzyme, as also verified by EXAFS data. ... 

The use of infrared spectroscopy, either through fingerprint characterisation or by functional group identification, is well established. IR vibrational spectroscopy has thus been applied in spectroelectrochemistry for quite some time. ° The possibility to establish the symmetry of a molecule has made IR-SEC a most valuable tool for mixed-valence chemistry, ° allowing intramolecular electron-transfer rates in the picosecond region to be assessed and electron-transfer isomers to be established. ... 

Ellingson R. J., Asbury J. B., Eerrere S., Ghosh H. N., Sprague J. R., Lian T. and Nozik A. J. (1999), Sub-picosecond injection of electrons from excited [Ru(2,2 -bipy-4,4 -dicarboxy)2(SCN)2] into Ti02 using transient mid-infrared spectroscopy , Z Phys. Chem. (Munchen) 212,11-84. 

One should remember that the atoms and molecules in liquids are not motionless but in a state of flux determined by the intermolecular interactions and temperature. From the study of microwave spectroscopy discussed in chapter 4, it was found that rotational diffusion processes in liquids are characterized by relaxation times the order of a few picoseconds. When a liquid is irradiated with UV or visible radiation which involves frequencies greater than 10 Hz, the atoms and molecules appear to be motionless because the frequency of the electromagnetic radiation is much greater than that associated with molecular motion (10 Hz and lower). The same is true for infrared spectroscopy except in the... 

The 19-electron complexes are short-lived species, but their formation and reactivity has been observed by picosecond timescale infrared spectroscopy. Their dominant mode of reactivity is electron transfer to yield a more stable 18-electron complex. In the absence of a metal-radical trap, the two metal radicals produced by M-M photolysis can recombine and no net reaction will take place. 

In this section attention is focused on the vibrational aspects of the phenomena that can be probed at low temperatures by simple pump and probe" techniques, even without using very fast (femtosecond or picosecond) spectroscopy. The concepts developed for trans-PA can also be used for understanding of the photophysics of other polyconjugated materials. The main interest is to show that, from the viewpoint of infrared spectroscopy, photoexcitation corresponds to some kind of (clean) photodoping that introduces a displacement of charges (charge carrier) within domains of the chain, thus giving rise to photoinduced infrared (PIIR) spectra. It is then expected that DIIR and PIIR spectra may show common features, as was indeed found in photoexcited PA and in many other systems [41]. 

Elsaesser, T. and Kaiser, W. (1986) Visible and infrared spectroscopy of intramolecular proton transfer using picosecond laser pulses. Chem. Phys. Lett., 128, 231-237. 

Okamoto, H. and Tasumi, M. (1996) Picosecond transient infrared spectroscopy of electronically excited 4-dimethylamino-4 -nitrostilbene in the fingerprint region (1640-940cm ). Chem. Phys. Lett., 256, 502-508. 

Noguchi, H., Yoda, E., Ishizawa, N., Kondo, J. N., Wada, A., Kobayashi, H. and Domen, K. (2005) Direct observation of unstable intermediate species in the reaction of trans-2-butene on ferrierite zeolite by picosecond infrared laser spectroscopy. J. Phys. Chem B, 109, 17217-17223. 

Yamakata, A., Uchida, T., Kubota, J. and Osawa, M. (2006) Laser-induced potential jump at the electrochemical interface probed by picosecond time-resolved surface-enhanced infrared absorption spectroscopy./. Phys. Chem. B, 110, 6423-6427. 

Material response in THz frequency region, which corresponds to far- and mid-infrared electromagnetic spectrum, carries important information for the understanding of both electronic and phononic properties of condensed matter. Time-resolved THz spectroscopy has been applied extensively to investigate the sub-picosecond electron-hole dynamics and the coherent lattice dynamics simultaneously. In a typical experimental setup shown in Fig. 3.5, an... 

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