Time domain vibrational spectroscopy

The electronic absorption spectra of complex molecules at elevated temperatures in condensed matter are generally very broad and virtually featureless. In contrast, vibrational spectra of complex molecules, even in room-temperature liquids, can display sharp, well-defined peaks, many of which can be assigned to specific vibrational modes. The inverse of the line width sets a time scale for the dynamics associated with a transition. The relatively narrow line widths associated with many vibrational transitions make it possible to use pulse durations with correspondingly narrow bandwidths to extract information. For a vibration with sufficiently large anharmonicity or a sufficiently narrow absorption line, the system behaves as a two-level transition coupled to its environment. In this respect, time domain vibrational spectroscopy of internal molecular modes is more akin to NMR than to electronic spectroscopy. The potential has already been demonstrated, as described in some of the chapters in this book, to perform pulse sequences that are, in many respects, analogous to those used in NMR. Commercial equipment is available that can produce the necessary infrared (IR) pulses for such experiments, and the equipment is rapidly becoming less expensive, more compact, and more reliable. It is possible, even likely, that coherent IR pulse-sequence vibrational spectrometers will... 

In recent years there has been significant interest in the extension of nonlinear optical spectroscopy to higher orders involving multiple time and/or frequency variables. The development of these multidimensional techniques is motivated by the desire to probe the microscopic details of a system that are obscured by the ensemble averaging inherent in linear spectroscopy. Much of the recent work to extend time domain vibrational spectroscopy to higher dimensionality has involved the use of nonresonant Raman-based techniques. The use of Raman techniques has followed directly from the rapid advancements in ultrafast laser technology for the visible and near-IR portions of the spectrum. Time domain nonresonant Raman spectroscopy provides access to an extremely... 

Low-frequency vibrations of molecular submonolayers detected by time-domain raman spectroscopy. /. Mol. Struct., 735-736, 169-177. 

Both population and coherence experiments provide information on the dynamics and interactions of condensed matter systems. In addition, time domain vibrational experiments can extract spectroscopic information that is hidden in a conventional measurement of the infrared or Raman spectra. This book will provide the reader with a picture of the state of the art and a perspective on future developments in the field of ultrafast infrared and Raman spectroscopy. 

Following the initial proposal, fifth-order time domain Raman spectroscopy received considerable attention both theoretically (11-20) and experimentally (21-32). For the case of intermolecular motions, the majority of the experimental efforts have involved probing the intermolecular modes of liquid CS2, a standard system in nonresonant Raman spectroscopy due to its very large polarizability and the wealth of available experimental results. Experimental efforts to probe intramolecular vibrations are fewer in number, with the only published example probing modes of liquid CH3C1 and CCU (24). It was quickly realized that, owing to the direct transfer of the first vibrational coherence to the second, the experiment offered substantially more information than had initially been... 

Ultrafast vibrational spectroscopy offers a variety of techniques for unraveling the microsopic dynamics of hydrogen bonds occurring in the femto- to picosecond time domain. In particular, different vibrational couplings can be separated in nonlinear experiments by measuring vibrational dynamics in real-time. Both coherent vibrational polarizations and processes of population and energy relaxation have been studied for a number of hydrogen bonded systems in liquids [1],... 

Beyond imaging, CARS microscopy offers the possibility for spatially resolved vibrational spectroscopy [16], providing a wealth of chemical and physical structure information of molecular specimens inside a sub-femtoliter probe volume. As such, multiplex CARS microspectroscopy allows the chemical identification of molecules on the basis of their characteristic Raman spectra and the extraction of their physical properties, e.g., their thermodynamic state. In the time domain, time-resolved CARS microscopy allows recording of ultrafast Raman free induction decays (RFIDs). CARS correlation spectroscopy can probe three-dimensional diffusion dynamics with chemical selectivity. We next discuss the basic principles and exemplifying applications of the different CARS microspectroscopies. 

Raman and infrared vibrations are mutually exclusive and consequently use of both techniques is required in order to obtain a set of vibrational bands for a molecule. The advent of powerful computer-controlled instrumentation has greatly enhanced the sensitivity of these vibrational spectroscopies by the use of Fourier transform (FT) techniques, whereby spectra are recorded at all frequencies simultaneously in the time domain and then Fourier transformed to give conventional plots of absorbance versus frequency. The wide range of applications of FT Raman spectroscopy is discussed by Almond et al. (1990). Specific examples of its use in metal speciation are the observation of the Co-C stretch at 500 cm-1 in methylcobalamin and the shift to lower frequency of the corrin vibrations when cyanide is replaced by the heavier adenosyl in going from cyanocobalamin to adenosylcobalamin (Nie et al., 1990). 

E>vib and Our also show up in the theory of spontaneous Raman spectroscopy describing fluctuations of the molecular system. The functions enter the CARS interaction involving vibrational excitation with subsequent dissipation as a consequence of the dissipation-fluctuation theorem and further approximations (21). Equations (2)-(5) refer to a simplified picture a collective, delocalized character of the vibrational mode is not included in the theoretical treatment. It is also assumed that vibrational and reori-entational relaxation are statistically independent. On the other hand, any specific assumption as to the time evolution of vib (or or), e.g., if exponential or nonexponential, is made unnecessary by the present approach. Homogeneous or inhomogeneous dephasing are included as special cases. It is the primary goal of time-domain CARS to determine the autocorrelation functions directly from experimental data. 

We shall conclude this chapter with a few speculative remarks on possible future developments of nonlinear IR spectroscopy on peptides and proteins. Up to now, we have demonstrated a detailed relationship between the known structure of a few model peptides and the excitonic system of coupled amide I vibrations and have proven the correctness of the excitonic coupling model (at least in principle). We have demonstrated two realizations of 2D-IR spectroscopy a frequency domain (incoherent) technique (Section IV.C) and a form of semi-impulsive method (Section IV.E), which from the experimental viewpoint is extremely simple. Other 2D methods, proposed recently by Mukamel and coworkers (47), would not pose any additional experimental difficulty. In the case of NMR, time domain Fourier transform (FT) methods have proven to be more sensitive by far as a result of the multiplex advantage, which compensates for the small population differences of spin transitions at room temperature. It was recently demonstrated that FT methods are just as advantageous in the infrared regime, although one has to measure electric fields rather than intensities, which cannot be done directly by an electric field detector but requires heterodyned echoes or spectral interferometry (146). Future work will have to explore which experimental technique is most powerful and reliable. 

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. 

Time resolved coherent anti-Stokes Raman spectroscopy of condensed matter has been recently extended to the femtosecond domain allowing direct and detailed studies of the fast relaxation processes of molecular vibrations in liquids. The vibrational phase relaxation (dephasing) is a fundamental physical process of molecular dynamics and has attracted considerable attention. Both experimental and theoretical studies have been performed to understand microscopic processes of vibrational dephasing. Developments in ultrafast coherent spectroscopy enables one now to obtain direct time-domain information on molecular vibrational dynamics. Femtosecond time-resolved coherent anti-Stokes Raman scattering measuring systems have been constructed (see Sec. 3.6.2.2.3) with an overall time resolution of less than 100 fs (10 s). Pioneering work has been per-... 

Since the development of ultrashort lasers, nudear wavepacket dynamics of various matters have attracted continuing attention [1,2]. The research targets extend from gas phase molecules [3, 4] to molecules in solution [5, 6], and solids [7]. In general, an excitation of matter by an ultrashort pulse with sufficient bandwidth leads to the creation of coherence between vibrational (or vibronic) eigenstates [1]. The induced nuclear wavepacket then starts to evolve on a certain potential energy surface and the dynamics is probed by a suitable pump-probe spectroscopy. The direct time-domain observation of the nudear motion provides us with valuable information on photochemical reaction dynamics, vibrational excitation/relaxation mechanisms, electron-vibration (phonon) coupling, and so on. 

In picosecond time-resolved Raman spectroscopy, the sample is pumped and probed by energetically well-defined optical pulses, producing a full vibrational spectrum over a 1000 2000 cm 1 window.207 One would expect vibrational spectroscopy to be restricted to the picosecond time domain and above by the Heisenberg uncertainty principle (Equation 2.1), because a 1 ps transform-limited pulse has an energy width of... 

An electronic or vibrational excited state has a finite global lifetime and its de-excitation, when it is not metastable, is very fast compared to the standard measurement time conditions. Dedicated lifetime measurements are a part of spectroscopy known as time domain spectroscopy. One of the methods is based on the existence of pulsed lasers that can deliver radiation beams of very short duration and adjustable repetition rates. The frequency of the radiation pulse of these lasers, tuned to the frequency of a discrete transition, as in a free-electron laser (FEL), can be used to determine the lifetime of the excited state of the transition in a pump-probe experiment. In this method, a pump energy pulse produces a transient transmission dip of the sample at the transition frequency due to saturation. The evolution of this dip with time is probed by a low-intensity pulse at the same frequency, as a function of the delay between the pump and probe pulses.1 When the decay is exponential, the slope of the decay of the transmission dip as a function of the delay, plotted in a log-linear scale, provides a value of the lifetime of the excited state. 

Rotational reorientation of frans-stilbene in alkane solution at room temperature occurs in the 10 to 30-ps time domain [347]. Rare-gas complexes with trons-stilbene were studied by purely rotational coherence spectroscopy [51,364]. Moreover, the decay kinetics of excited trans-stil-bene-cyclodextrin complexes were examined [366], It is worth mentioning that great progress has also been made in high-resolution spectroscopy [52, 369-372], Resonance coherent Raman spectroscopy showed a large enhancement of the electronic hyperpolarizability of t with respect to ground state trons-stilbene [374]. Vibrational motions were observed with ps transient Raman spectroscopy [375]. 

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