Ultrafast pump-probe techniques

The power of these techniques in elucidating the detailed mechanisms of ion-molecule reactions is well demonstrated by this example. Undoubtedly, we can expect to see increasing use of the ultrafast pump-probe technique in the field of ion-molecule reaction dynamics. 

Other classes of reactions that have received much attention are proton transfer and electron transfer reactions. These processes can often be initiated by light. This characteristic has made it possible to monitor their time evolution by ultrafast pump-probe techniques. An example is the acid-base reaction of 1-naphthol with ammonia. On the ground electronic surface... 

Recently, Eisenthal and coworkers have developed time-resolved surface second harmonic techniques to probe dynamics of polar solvation and isomerization reactions occurring at liquid liquid, liquid air, and liquid solid interfaces [22]. As these experiments afford subpicosecond time resolution, they are analogous to ultrafast pump probe measurements. Specifically, they excite a dye molecule residing at the interface and follow its dynamics via the resonance enhance second harmonic signal. 

Absorption and Ensuing Ion-Molecule Reactions via Ultrafast Laser Pump-Probe Techniques.196... 

Optical pump-probe techniques have been particularly useful in the study of magnetic metals. The recent discovery of the ultrafast demagnetisation... 

Several laboratories now make use of the ultrafast continuum pump-probe technique in the study of ultrafast processes in biological molecules or molecular complexes.Notable molecules and complexes under study are the photosynthetic reaction centers of purple bacteria, the reaction centers of photosystems I and II of green plants. 

The time resolution of the point-by-point and step-scan FTIR approaches is limited by the rise time of the fast IR detector used in the experiment (ca. 10 ns). However, many photochemical and photophysical events take place on the subnanosecond timescale, which require a faster technique. Ultrafast IR spectroscopy is a variant of the pump-probe technique, where time resolution is achieved by spatially delaying the probe pulse with respect to the pump pulse (Figure 5). 

The ILIT program at Brookhaven National Laboratory has been terminated. We have demonstrated that our best instrumentation can now attain nanosecond and possibly subnanosecond time resolution (Sec. IV.E Fig. 6). The holy grail for those who study ultrafast interfacial kinetics remains the development of a pump-probe technique whose time resolution would be limited only by the operative physical chemical processes (see Sec. V.E). We did not achieve that goal, but ILIT could be a component in... 

To analyze the ultrafast dynamics of small molecules and clusters, the method of multiphoton ionization (MPI) spectroscopy combined with the pump probe technique was chosen. Adiabatic expansion or sputtering was used to produce the molecules and clusters of interest. Quadrupole mass spectrometers enabled a mass-selective detection of the ionized species. 

Optical detection methods (streak-camera and pump-probe techniques) exist that are even faster then those discussed here. Ultrafast spectroscopy is discussed in Sect. 9.5. 

FIGURE 2 An illumination of typical pump-probe technique used In ultrafast spectroscopies. The delay time between the pump and probe pulses is controlled by varying the difference of path length of two laser pulses. The measured signal can be the linear or nonlinear reflection/transmission, Raman scattering, luminescence, or any other signal from the probe pulse. 

Recent intensive investigations using pump-probe techniques and femtosecond anisotropy measurements have revealed that ultrafast energy transfer ( 80 fsec) from one of the two accessory bacteriochlorophyUs to the special pair and internal conversion ( 120 fsec) within the special pair occur prior to electron transfer from the special pair to one of the two bacteriopheophytins. 

One effective means to investigate ultrafast processes has been the pump-probe technique, shown schematically in Fig. 6.14 A pump laser pulse excites the... 

Fig. 6.14. Schematic of the pump-probe technique used for examining ultrafast processes in solids.

This chapter focuses on the ultrafast ESIPT found in molecules containing an H-chelate ring (figure 4.1). We will introduce the most popular experimental techniques and discuss what kind of information can be extracted from the spectral signatures associated with the ESIPT and subsequent processes. In the remainder of the introduction, we introduce the investigated molecular systems. The subsequent experimental section describes different pump-probe techniques. Then the transient spectroscopic signatures and their interpretation and evaluation... 

It is more difficult to perform ultrafast spectroscopy on neat H20 (than it is on H0D/D20 or HOD/H20) since the neat fluid is so absorptive in the OH stretch region. One innovative and very informative technique, developed by Dlott, involves IR pumping and Raman probing. This technique has a number of advantages over traditional IR pump-probe experiments The scattered light is Stokes-shifted, which is less attenuated by the sample, and one can simultaneously monitor the populations of all Raman-active vibrations of the system at the same time. These experimental have been brought to bear on the spectral diffusion problem in neat water [18, 19, 75 77],... 

Ultrafast molecular elimination of iodine from IF2C-CF2I has been studied using the velocity map ion imaging technique in combination with femtosecond pump-probe laser excitation.51 By varying the femtosecond delay between pump and probe pulse, it has been found that elimination of molecular iodine is a concerted process, although the two carbon-iodine bonds are not broken synchronously. 

The initial charge separation in PS I and PS II can be followed by what are known as ultrafast optical spectroscopy techniques. Several variations on this method exist, but they can be grouped into pump-probe absorbance difference and transient fluorescence methods (25, 26). In the first instance, the sample is irradiated with a pump pulse to initiate the electron transfer and the absorbance is measured using a probe pulse at a... 

Abstract A challenging task in surface science is to unravel the dynamics of molecules on surfaces associated with, for example, surface molecular motion and (bimolecular) reactions. As these processes typically take place on femtosecond time scales, ultrafast lasers must be used in these studies. We demonstrate two complementary approaches to study these ultrafast molecular dynamics at metal surfaces. In the first, the molecules are studied after desorbing from the surface initiated by a laser pulse using the so called time-of-flight technique. In the second approach, molecules are studied in real time during their diffusion over the surface by using surface-specific pump-probe spectroscopy. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

---