Ultrafast infrared spectroscopy

2 Experimental Studies of Ion Effects on the Solvent Orientation Dynamics 5.2.2.1 Ultrafast Infrared Spectroscopy 

Pulsed two-frequency (ultrafast, femtosecond) polarization-resolved mid-infrared spectroscopy was used in a series of papers by Bakker and coworkers to study the effect of ions on the structural dynamics of their aqueous solutions [44,120]. The solvent consisted of mixtures of Hp and D O (generally O.IM HDO in D O) and the first, the pump, pulse excited the 0-H or 0-D stretch vibration to the first excited state that then relaxed at a measurable rate. The second, the probe, pulse was red-shifted with respect to the first and probed the decay of this excited state. Generally, fairly concentrated electrolyte solutions were required for the application of this technique, in the range 0.5 to lOM. The rotational anisotropy is as follows  

In pure water, the exponential relaxation rate is described by r =2.5 0.2ps for the 0-D stretch vibration in H O, somewhat shorter than the 3.0 0.2ps for the O-H relaxation in D O. This is because the relaxation depends on the collective motion of neighboring water molecules and the somewhat larger viscosity of the heavier isotopic form of water [120]. 

As the ion concentration increases, it is not possible to disentangle the effects of cations and of anions on the water reorientation dynamics because of the formation of solvent-shared ion pairs. Nevertheless, van der Postand Bakker [121] showed that sodium ions at concentrations up to 6 m slow down the reorientation of water molecules in aqueous NaCl and Nal, compared with CsCl and KI at the same concentration. Small effects are shown as the concentration of Lil increases up to 2 m, but gradual slowing down of is seen in aqueous Cs SO and Mg(C10 )2 and much more so in aqueous Na SO and MgSO, but the effects diminish as the temperature increases from 22 to 70°C as found by Tielrooij et al. [122]. In 6m NaOH solutions, the reorientation time of the OH hydration complex is r =12 2ps, that is, much slowed down relative to bulk water, because it is a large hydrogen-bonded structure that reorients as a whole according to Liu et al. [123]. 

The mutual effects of cations and anions on the water reorientation rates have also been studied. A comparison between solutions of 4 m aqueous LiCl, Csl, and CsF shows that there is a considerably larger amount of slow water in the CsF solutions, but hardly any effects in the former two salt solutions. The combination of a strongly hydrated ion (F ) with a weakly hydrated one (Cs ) is responsible for this effect according to Tiehooij et al. [124]. In aqueous alkali metal formate solutions, the time constant for the slow water was estimated as T j 20ps, and its fraction increased in the order Cs NH Li Na (note the out-of-order position of Na ). This, 

---

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],... 

K.B. Mpller, R. Rey, J.T. Hynes, On Hydrogen Bond Dynamics in Water and Ultrafast Infrared Spectroscopy A Theoretical Study, to appear in J. Phys. Chem. A (2004). 

Bimodal intermolecular proton transfer in water photoacid-base pairs studied with ultrafast infrared spectroscopy... 

As a final example, we discuss the use of broadband ultrafast infrared spectroscopy as a tool for monitoring electron transfer rates in dye-sensitized... 

Dougherty TP, Grubbs WT, Heilweil EJ. Photochemistry of Rh(CO)2 (acetyl-acetonate) and related metal dicarbonyls studied by ultrafast infrared spectroscopy. J Phys Chem 1994 98 9396-9399. 

Arrivo SM, Dougherty TP, Grubbs WT, Heilweil EJ. Ultrafast infrared spectroscopy of vibrational CO-stretch up-pumping and relaxation dynamics in metal hexacarbonyls. Chem Phys Lett 1995 235 247-254. 

Owrutsky JC, Li M, Culver JP, Sarisky MJ, Yodh AG, Hochstrasser RM. Vibrational dynamics of condensed phase molecules studied by ultrafast infrared spectroscopy. In Lau A, Siebert F, Werncke W, eds. Time Resolved Vibrational Spectroscopy IV. Berlin Springer-Verlag, 1993 63-67. 

Ultrafast Infrared Spectroscopy of Vibrational States Prepared by Photoinduced... 

Rini, M., Mohammed, O. F., Magnes, B.-Z., Pines, E., Nibbering, E. T. J, Bimodal intermolecular proton transfer in water photoacid-base pairs studied with ultrafast infrared spectroscopy, in Ultrafast Molecular Events in Chemistry and Biology , J. T. Hynes, M. Martin (Eds.), Elsevier, Amsterdam, 2004,... 

A novel setup was developed to study laser-driven reactions in solid matrices (e.g., polymers) using time-resolved IR spectroscopy. The first experiments have provided one of the first examples of how ultrafast infrared spectroscopy may be used to examine laser-driven reactions in polymeric matrices. The photo chemically as well as the thermally initiated reaction of a model compound has been studied in a PMMA matrix. It is remarkable that both initial reactions happen on a time scale faster than our experimental limit of 20 ps. While the initial reaction products are probably the same, the... 

Hill J R ef a/1996 Ultrafast infrared spectroscopy in biomolecules active site dynamics of heme proteins Biospectroscopy 2 277-99... 

Ultrafast Infrared Spectroscopy of Vibrational States Prepared by Photoinduced Electron Transfer in (CN)5FeCNRu(NH )/-... 

As a generalization, ultrafast infrared spectroscopy in dilute aqueous salt solutions is rather insensitive to the nature of the cation (unless large and hydrophobic) but does respond to the anion by its hydrogen bonding to the 0-D probe. At larger concentrations, solvent-shared ion pairs show a cooperative cation-anion effect on the reorientation rate of the water molecules. 

Lippert, T, Koskelo, A., and Stoutland, P.O., Direct Observation of a Photoinduced Wolff Rearrangement in Pmma Using Ultrafast Infrared Spectroscopy, /. Am. Chem. Soc., 118,1551,1996. Camacho, M.B., Clark, A.E., Liebrecht, T.A., and DeLuca, J.R, A Phenyliodonium Ylide as a Precursor for Dicarboethoxycarbene Demonstration of a Strategy for Carbene Generation, /. Am. Chem. Soc., 122, 5210, 2000. 

---