Pump-probe approach

Unfortunately, femtosecond laser pulses are not so readily predisposed to study collisions between atoms and molecules by the pump-probe approach. The reason is that, typically, the time between collisions in the gas phase is on the order of nanoseconds. So, with laser pulses of sub-lOOfs duration, there is only about one chance in ten thousand of an ultrashort laser pulse interacting with the colliding molecules at the instant when the transfer of atoms is taking place in other words, it is not possible to perform an accurate determination of the zero of time. 

In the next section, we describe two pump-probe approaches and review their use in experiments with MbCO. This places the nonlinear optical techniques of transient phase grating introduced in the subsequent section in their proper context. 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

TR methods were originally developed in om laboratories to study excited-state structures and dynamics of transition metal complexes such as Ru + (bpy)s and metaUoproteins. TR measurements rely on a pump-probe approach in which two separate laser pulses are used, one to excite the system and the other to probe the transient Raman spectrum. The time resolution of the experiment is determined by the width of the laser pulses (typically 7 ns for a Q-switched laser or as short as 1 ps for a mode-locked laser). The pulses are variably delayed with respect to one another to achieve time resolution, either by optically dela)dng the probe pulse with respect to the pump pulse or by electronically delaying two independently tunable lasers. Thus, two different approaches are required depending on the time scale of interest. The fastest timescale (from 10 to 10 s) requires optical delay to achieve sufficiently short separation between the pump and probe pulses. In such a scheme, the probe pulse is sent through a fixed path, but the pump pulse is sent through a variable path that can be scanned. Since hght travels about 1 ft per ns, a difference in pathlength of a few feet is sufficient. The second approach typically uses two Q-switched Nd YAG lasers that are electronically delayed with respect to one another, to access... 

We will show that the ILIT method eliminates some of the problems associated with an electrical perturbation and, not surprisingly, creates new, interesting, and challenging problems. Improved electronics developed at the time of the termination of this program, coupled with picosecond or subpicosecond laser pulses, should, in principle, allow the ILIT method to probe interfacial relaxations of the order of 1 ns or less (our published work has used a slower system with a response function of the order of 15 ns). Of course, really dramatic improvement in response time will be achieved only with a pump-probe approach. Nevertheless, even at its present stage of development, ILIT effects significant improvement in time resolution over methods using conventional electrochemical perturbations where the time resolution is limited by solution resistance and interfacial capacitance (see Ref. 41) ... 

The first method extracts / based on optical transient absorption results using a laser only pump-probe approach, where the photoexcitation conditions are set to be as close as possible to those in the XTA measurements. The/ values can be obtained by comparing AOD (change in optical density) and the total OD in the spectral region of the ground state bleach, as long as minimal interference from other spectral features is present in the same region. 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

It is interesting to note that the use of correlation functions in spectroscopy is an old topic, and has been used to derive, for example, infrared (IR) spectra, from classical trajectories [134,135]. Stock and Miller have recently extended this approach, and derived expressions for obtaining electronic and femtosecond pump-probe spectra from classical trajectories [136]. 

A final study that must be mentioned is a study by Haitmann et al. [249] on the ultrafast spechoscopy of the Na3p2 cluster. They derived an expression for the calculation of a pump-probe signal using a Wigner-type density mahix approach, which requires a time-dependent ensemble to be calculated after the initial excitation. This ensemble was obtained using fewest switches surface hopping, with trajectories inibally sampled from the thermalized vibronic Wigner function vertically excited onto the upper surface. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

Many methods of investigation of protein-ligand binding kinetics that are based on linear processes are of a pump-probe type. In this approach an optical pulse, called a pump, starts a photoreaction (such as dissociation of MbCO into Mb and CO), and its progress is probed a time At later. The probe could be, for example, a weak laser pulse, which detects the spectral changes in the heme during the protein-ligand recombination, or an x-ray pulse, which allows determination of the protein structure at a particular instant in time. 

Following another experimental approach, GWgoire et al [9] have tried to understand the influence of an increasing number of solvent molecules on the femtosecond dynamics of diatomic molecules, including the dimers Nal and Csl. Due to its relative simplicity, the isolated Nal molecule has been studied extensively with pump-probe techniques both experimentally [10], and theoretically [11,12], In this report, we investigate theoretically the femtosecond pump-probe ionization of Nal and Csl when aggregated with a molecule of acetonitrile CH3CN. 

A simple approach for the calculation of femtosecond pump-probe spectra for electronically nonadiabatic systems... 

In my talk I surveyed recent advances in the methodology and selected 2D-IR spectra of secondary structures. The results promise to provide structurally based kinetic probes for conformational dynamics, sharp tests of anharmonic potential surfaces and novel information regarding the transient and equilibrium vibrational dynamics of peptides. The heterodyned 2D-IR approach has proven useful in determining structures of peptides in solution and the anharmonic nature of the potential surfaces of peptides and secondary structures [1-10], as have polarized photon echo [2,6,10-12] or pump-probe techniques [4,13-16]. 

A simple approach lor the calculation of Femtosecond pump-probe spectra for electronically nonndiiihuuc systems... 

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 pump-probe spectroscopic time-resolved study of autoionization processes in atoms and molecules uses an ultra-short (100-500 as) XUV pulses for the pump stage in conjunction with an intense (1012-1014 W/cm2), few-cycle IR pulse as probe. Traditional time-independent approaches are inadequate to interpret these kind of experiments. This is so because, on the... 

Y. Tanimura and S. Mukamel. Multistate quantum Fokker-Planck approach to nonadiabatic wave packet dynamics in pump-probe spectroscopy. J. Chem. Phys., 101 3049, 1994. 

Although the presented numerical approach to the coupled master equations has shown that a turnover feature can be seen in vibronic dynamics appearing in the calculated pump-probe stimulated emission spectra as a function of the energy gap between the two relevant vibronic states. It is found that vibronic quantum beats cannot be observed when the energy gap becomes larger in which situation it leads to smaller Franck-Condon overlaps between the energy conserved levels. 

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