Transient grating spectroscopy

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). ° ... 

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. 

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... 

N. Tamai and H. Masuhara, Intersystem crossing of benzophenone by femtosecond transient grating spectroscopy, Chem. Phys. Lett., 198, 41.3 418 (1992). 

The vibronic spectra of Do — Di — D2 electronic states recoded by da Silva Filho et al. [45] revealed resolved vibrational structures of the Do and D2 electronic states and a broad and structureless band for the Di state. A slow ( 3-20 ps) and fast k, 200 fs) relaxation components are estimated for the Dq D2 transition in a (femto)picosecond transient grating spectroscopy measurements [16]. The fast component is attributed to the Do D2 transition and a nonradiative relaxation time of 212 fs is also estimated from the cavity ringdown (CRD) spectroscopy data [42]. Electronic structure results of Hall et al. [107] suggest that the nonradiative Do D2 relaxation occurs via two consecutive sloped type CIs [66,108]. We developed a global model PESs for the Do — Di— D2 electronic states and devised a vibronic coupling model to study the nuclear dynamics underlying the complex vibronic spectrum and ultrafast excited state decay of N +[20]. 

The technique of transient grating spectroscopy has been reviewed, with particular emphasis on its application to monitoring non-radiative deactivation. A unified theory of time-resolved fluorescence anisotropy and Stokes shift spectroscopy has appeared. A separate review has considered the chemical and photophysical events occurring from upper excited states as accessed by multiphoton absorption techniques. ... 

One of the most important experimental techniques to appear in the last few years has been transient grating spectroscopy and this methodology has numerous important applications. The technique has been used to measure the quantum yield for photodissociation of diphenyl disulfide, and compared to photoacoustic and transient absorption spectoscopic methods. Additional studies have used transient grating spectroscopy to monitor the energetics and dynamics of charge separation of an ion pair into free ions. The approach permits direct measurement of the enthalpy change accompanying formation of the free ions... 

Transient grating spectroscopy, photoacoustic spectroscopy and transient... 

Transient grating spectroscopy is relatively easily handled compared with the transient absorption spectroscopy, and is often used to study carrier dynamics at semiconductor electrodes [32]. Figure 14 schematically shows the principle of transient grating spectroscopy. A femtosecond laser pulse for sample excitation is split into two beams, which are crossed again at the semiconductor surface to produce an optical striped interference pattern. The interference pattern produces a striped pattern of the densities of photo-generated electrons and holes near the semiconductor surface. The latter striped pattern gives rise to a striped pattern of optical refractive index near the semiconductor surface, which is monitored by measuring a diffraction pattern of a second probe laser... 

Fig. 14 A schematic view of a sample region in transient grating spectroscopy to explain its principle.

Fig. 5.33 Measured signal from high-harmonic transient grating spectroscopy of NO2 (left) and computed diabatic state population (right). Reprinted form [221. (Reprinted with permission from P. M. Kraus et al., Phys. Rev. A 85, 043409 (2012)).

Morgan, G.P., S.Z. Chen and W.M. Yen, 1986b, Transient Grating Spectroscopy of LaP50 Nd in Dynamics Gratings and Four Wave Mixing, ed. H.J. Eichler, IEEE J. Quantum Electron. QE-22, 1360. 

TRUV-VIS studies have also provided evidence to support the role of conformation in the rearrangement reaction. For example, a series of simple diazoketones RCONjH, where R = H, Me, i-Pr, and t-Bu, has been examined by laser flash photolysis methods and also by picosecond transient grating spectroscopy. Within this series, as the steric bulk of R increases, the extent of the anti diazoketone conformer present at equilibrium decreases relative to the syn conformer. Consistent with a direct rearrangement pathway leading to ketene via only the syn conformer, the amount of trappable carbene intermediate decreases as R becomes more bulky. 

Goodno, G.D., Dadusc, G., Miller, R.J.D. (1998). Ultrafast heterodyne-detected transient-grating spectroscopy using diffractive optics. J. Opt. Soc. Am. B 15 1791-1794. 

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