Time-resolved vibrational spectroscop

Proceeding of the "International Conference on Time Resolved Vibrational Spectroscopy", Lake Placid, New York, Aug. 16-21, 1982 G. Atkinson, (Department of Chemistry, Syracuse University), Ed.. The references on most of the time resolved vibrational spectroscopic applications between 1976-82 can be obtained from the papers presented in this conference. 

Time-resolved spectroscopic techniques are important and effective tools for mechanistic photochemical studies. The most widely used of these tools, time-resolved ultraviolet-visible (UV-Vis) absorption spectroscopy, has been applied to a variety of problems since its introduction by Norrish and Porter [1] over 50 years ago. Although a great deal of information about the reactivity of organic photochemical intermediates (e.g., excited states, radicals, carbenes, and nitrenes) in solution at ambient temperatures has been amassed with this technique, only limited structural information can be extracted from such investigations because absorption bands are usually quite broad and featureless. Questions of bonding, charge distribution, and solvation (in addition to those of dynamics) are more readily addressed with time-resolved vibrational spectroscopy. 

Compared with other time-resolved spectroscopic methods for measuring electronic transitions (electronic absorption, fluorescence, etc.) in molecules and molecular systems, time-resolved vibrational spectroscopy, which measures vibrational transitions, has the potential for simultaneously providing information on both the molecular structure (and vibrational force field) and the dynamics of a short-lived transient molecular species. Time-resolved vibrational spectroscopy is therefore considered to be one of the most effective means for monitoring changes in molecular structure over a wide time scale, particularly for condensed-phase studies. 

The events taking place in the RCs within the timescale of ps and sub-ps ranges usually involve vibrational relaxation, internal conversion, and photo-induced electron and energy transfers. It is important to note that in order to observe such ultrafast processes, ultrashort pulse laser spectroscopic techniques are often employed. In such cases, from the uncertainty principle AEAt Ti/2, one can see that a number of states can be coherently (or simultaneously) excited. In this case, the observed time-resolved spectra contain the information of the dynamics of both populations and coherences (or phases) of the system. Due to the dynamical contribution of coherences, the quantum beat is often observed in the fs time-resolved experiments. 

One of the most powerful spectroscopic techniques for the detection and characterization of persistent and transient phenoxyls is time-resolved resonance Raman (RR) spectroscopy. Vibrational frequencies and the relative intensities of the resonance-enhanced bands have proven to be sensitive markers for tyrosyl radicals in proteins. For example, Sanders-Loehr and co-workers (31) detected the tyrosyl radical in native ribonucleotide reductase from Escherichia coli by a resonance-enhanced Raman mode at 1498 cm 1 that they assigned to the Ula Wilson mode of the tyrosyl, which is predominantly the u(C=0) stretching mode. 

Bimolecular photoinduced electron transfer between an electron donor and an electron acceptor in a polar solvent may result in the formation of free ions (FI). Weller and coworkers [1] have invoked several types of intermediates for describing this process (Fig.la) exciplex or contact ion pair (CIP), loose ion pair (LIP), also called solvent separated ion pair. The knowledge of the structures of these intermediates is fundamental for understanding the details of bimolecular reactions in solution. However, up to now, no spectroscopic technique has been able to differentiate them. The UV-Vis absorption spectra of the ion pairs and the free ions are very similar [2], Furthermore, previous time resolved resonant Raman investigations [3] have shown that these species exhibit essentially the same high frequency vibrational spectrum. 

The knowledge of the internal-energy distribution is of equal interest for the practical applications indicated in the preceding paragraphs. First spectroscopic obervations of the IR emission from the molecule BC, which is related to the vibrational-state population, were reported by Karl and Polanyi13 on the system Hg + CO. These measurements were subsequently improved and extended.14-16 Recent time-resolved experiments with IR-laser absorption17- 18 and emission techniques19-21 yield more reliable results on the product-state distribution. 

While in the frequency domain all the spectroscopic information regarding vibrational frequencies and relaxation processes is obtained from the positions and widths of the Raman resonances, in the time domain this information is obtained from coherent oscillations and the decay of the time-dependent CARS signal, respectively. In principle, time- and frequency-domain experiments are related to each other by Fourier transform and carry the same information. However, in contrast to the driven motion of molecular vibrations in frequency-multiplexed CARS detection, time-resolved CARS allows recording the Raman free induction decay (RFID) with the decay time T2, i.e., the free evolution of the molecular system is observed. While the non-resonant contribution dephases instantaneously, the resonant contribution of RFID decays within hundreds of femtoseconds in the condensed phase. Time-resolved CARS with femtosecond excitation, therefore, allows the separation of nonresonant and vibrationally resonant signals [151]. 

A variety of spectroscopic methods has been used to determine the nature of the MLCT excited state in the /ac-XRe(CO)3L system. Time-resolved resonance Raman measurements of /ac-XRe(CO)3(bpy) (X = Cl or Br) have provided clear support for the Re -a- n (bpy) assignment of the lowest energy excited state [44], Intense excited-state Raman lines have been observed that are associated with the radical anion of bpy, and the amount of charge transferred from Re to bpy in the lowest energy excited state has been estimated to be 0.84 [45], Fast time-resolved infrared spectroscopy has been used to obtain the vibrational spectrum of the electronically excited states of/ac-ClRe(CO)3(bpy) and the closely related/ac-XRe(CO)3 (4,4 -bpy)2 (X = Cl or Br) complexes. In each... 

By using transient absorption spectroscopic techniques, time-resolved measurements of photo-induced interfacial ET with time-constants shorter than 100 fsec have become possible [51,52,58-60]. When ultrashort laser pulses are used in studying PIET, vibrational coherences (or vibrational wave packet) can often be observed and have indeed been observed in a number of dye-sensitized solar cell systems. This type of quantum beat has also been observed in ultrafast PIET in photosynthetic reaction center [22], It should be noted that when the PIET takes place in the time scale shorter than 100 fsec, vibrational relaxation between the system and the heat bath is slower than PIET this is the so-called vibrationally non-relaxed ET case, and it will be treated in this section. 

Recently, however, experimental studies have cast a doubt on this assumption (see Ref. 1 for a review). For example, spectroscopic studies reveal hierarchical structures in the spectra of vibrationally highly excited molecules [2]. Such structures in the spectra imply the existence of bottlenecks to intramolecular vibrational energy redistribution (IVR). Reactions involving radicals also exhibit bottlenecks to IVR [3]. Moreover, time-resolved measurements of highly excited molecules in the liquid phase show that some reactions take place before the molecules relax to equilibrium [4]. Therefore, the assumption that local equilibrium exists prior to reaction should be questioned. We seek understanding of reaction processes where the assumption does not hold. 

Vibrational spectroscopy has proven to be a powerful method of studying biological molecules. Continued technical improvement (FT spectroscopy, time resolved spectroscopy, etc.) open up new domains of investigation which help solve fundamental problems of structure-function correlation at the molecular level. Many domains are beginning to be explored, and results are expected in the fields of compatible biomaterials, intelligent drug development, and in vivo spectroscopic measurement. 

Several recent reviews have presented broad overviews of ultrafast time-resolved spectroscopy [3-6], We shall concentrate instead on a selected, rather small subset of femtosecond time-resolved experiments carried out (and to a very limited extent, proposed) to date. In particular, we shall review experiments in which phase-coherent electronic or, more often, nuclear motion is induced and monitored with time resolution of less than 100 fs. The main reason for selectivity on this basis is the rather ubiquitous appearance of phase-coherent effects (especially vibrational phase coherence) in femtosecond spectroscopy. As will be discussed, nearly any spectroscopy experiment on molecular or condensed-phase systems is likely to involve phase-coherent vibrational motion if the time scale becomes short enough. Since the coherent spectral bandwidth of a femtosecond pulse often exceeds collective or molecular vibrational frequencies, such a pulse may perturb and be perturbed by a medium in a qualitatively different manner than a longer pulse of comparable peak power. The resulting spectroscopic possibilities are of special interest to these reviewers. 

IR spectroscopy was mainly used to characterize the sorbed species. The zeolite powder was pressed into self supporting wafers and analyzed in situ during all treatments (i.e., activation, sorption, reaction) by means of transmission absorption IR spectroscopy using a BRUKER IPS 88 FTIR spectrometer (resolution 4 cm" ). For the sorption experiments, an IR cell equipped with IR transparent windows which could be evacuated to pressures below 10" mbar was used [11]. The activated zeolite wafer was contacted with a constant partial pressure (0.001 mbar) of the adsorbate at 308 K until adsorption-desorption equilibrium was reached (which was monitored by time resolved IR spectroscopy). For the coadsorption experiments, the catalysts were equilibrated with 0.001 mbar of both adsorbates admitted in sequentional order. The spectra were normalized for the sample thickness by comparing the intensities of the absorption bands of the adsorbate with the integral intensity of the lattice vibration bands of the zeolite between 2090 and 1740 cm". The surface coverage was quantified by calibration with gravimetric measurements (under conditions identical to the IR spectroscopic experiments). 

Temperature-independent anharmonic decay rates of high-frequency modes of myoglobin have been observed in time-resolved spectroscopic studies. Pump-probe vibrational spectra of the amide I band, between 1600 and 1700 cm 1, measured at temperatures from 6 K to 310 K reveal decay rates ranging only from 0.5 to 1 ps-1 [25], similar to the values we calculate. Similarly, pump-probe studies on myoglobin-CO reveal that the decay of the CO stretch, about 1950 cm is also essentially independent of temperature over the same temperature range [28]. 

Lukins, P.B., Rehman, S., Stevens, G.B. and George, D. (2005) Time-resolved spectroscopic fluorescence imaging, transient absorption and vibrational... 

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