Transient Gratings

Fourkas J T and Payer M D 1992 The transient grating a holographic window to dynamic processes Acc. Chem. Res. 25 227-33... 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

Hirose et al. [26] proposed a homodyne scheme to achieve the background-free detection of the fourth-order field. With pump irradiation in a transient grating configuration, the fourth-order field propagates in a direction different from that of the second-order field because of different phase match conditions. The fourth-order field is homodyned to make ffourth(td. 2 D) and spatially filtered from the second-order response hecond td, 2 D). 

Moog RS, Ediger MD, Boxer SG, Fayer MD (1982) Viscosity dependence of the rotational reorientation of rhodamine B in mono- and polyalcohols. Picosecond transient grating experiments. J Phys Chem 86 4694-4700... 

The lifetime of formylcarbene was determined by transient absorption and grating spectroscopies.39 Photolysis of formyldiazomethane produces formylcarbene which can isomerize, kr, to ketene or react with pyridine (Scheme 4). Using the pyridine ylide method, the lifetime of singlet formylcarbene was estimated to be 150-730 ps in CH2CI2. This is in reasonable agreement with the lifetime of 900 ps determined by transient grating spectroscopy. 

With development of ultrashort pulsed lasers, coherently generated lattice dynamics was found, first as the periodic modulation in the transient grating signal from perylene in 1985 by De Silvestri and coworkers [1], Shortly later, similar modulation was observed in the reflectivity of Bi and Sb [2] and of GaAs [3], as well as in the transmissivity of YBCO [4] by different groups. Since then, the coherent optical phonon spectroscopy has been a simple and powerful tool to probe femtosecond lattice dynamics in a wide range of solid... 

A. Miller, Transient Grating Studies of Carrier Diffusion and Mobility in Semiconductors... 

FIGURE 1.8 Phase matching diagram for transient grating experiments. Significance of the symbols k 2 = wave vectors of the excitation beams = wave vector of the probe... 

B. Overview of Transient Grating Results 1. MbCO in Aqueous Solutions... 

Analysis of pump-probe absorption data of Walther et al. (2005 not shown here) indicated two stretched exponentials characterizing the geminate recombination. In agreement with pump-probe results, their transient grating data (reproduced in Figure 1.12) have been fitted by an equation,... 

Determination of Certain Physical Parameters from Pump-Probe and Transient Grating Measurements... 

Rogers, J. A., and Nelson, K. A. 1996. A new photoacoustic/photothermal device for real-time materials evaluation An automated means for performing transient grating experiments. Physica B 56264 219-220. 

Picosecond-resolved thermochemical information can be extracted from the evolution of a transient grating produced by the crossing of two laser pulses and interrogated with a third short pulse of light. Several groups have applied this method to thermodynamic questions about the decay of excited states and the evolution of excited states into reactive intermediates. 

Photon Echo (PE) and Reverse Transient Grating (RTG) measurements of the loss of electronic coherence between the X( 2g+) and B(3FIou+) states of molecular iodine are measured by three pulse four wave mixing (FWM). Two cases are considered, relatively low pressure isolated iodine (1-2 Torr), and iodine at high pressure (>10torr) or in the presence of several hundred Torr of buffer gas. 

Fig. 3. Multiplex transient grating spectrum measured with the complex PMDA/DEA in ACN.

The peak shift data in Fig. 17 show oscillatory character, as is our first two examples (I2 and LH1). This arises from vibrational wavepacket motion. In addition, the very fast drop in peak shift to about 65% of the initial value in -20 fs results from the interference between the wavepackets created in different intramoleculear modes. This conclusion follows directly from obtaining the frequencies and relative coupling strengths of the intramolecular modes from transient grating studies of IR144, carried out in the same solvents (data not shown). Thus, by visual inspection of Fig. 17, an answer to a long-standing question—What fraction of the spectral width arises from intra- and intermolecular motion —is immediately apparent. 

Figure 18. The normalized electronic transition frequency correlation function M(t) 1= S(i)] obtained from the experimental three-pulse photon echo peak shifts and transient grating data for IR144 in ethanol (—) total W(t) ( ) ultrafast Gaussian component in M(t) ( ) oscillatory component that arises from intramolecular vibrational motion.

Figure 19. Comparison of calculated and measured signals using the M(t) shown in Fig. 18. (a) Three-pulse echo peak shift, (b) transient grating, and (c) transient absorption. A pulse duration of 16 fs (20 fs for transient grating) and a detuning of 230 cm-1 are used in the calculated signals. The peak near T = 0 in the transient grating and transient absorption signals, usually referted to as the coherent artifact, arises from the ultrafast decay (sum of intramolecular vibrational contribution and -100 fs ultrafast solvation dynamics) in M(t).

The description of pump-probe signals presented in the preceding section can be immediately generalized to heterodyne-detected transient grating spectroscopy as well as to other four-wave mixing techniques. Heterodyne detection involves mixing the scattered field with an additional heterodyne field 4(r). The signal in the ks direction can then be written in terms of the polarization Ts(t) as... 

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