Transient grating intensity

Figure 15 shows examples of decays of transient grating signal (the intensity of the diffraction pattern) observed for an n-Ti02 (100) electrode by excitation at 360 nm and probing at 670 nm [33]. The decays are related with the rate of electron-hole recombination near the n-Ti02 surface. 

Figure 3 Folded BOXCARS geometry applied in several transient nonlinear optical spectroscopies. In pump-probe spectroscopy, one of the three beams is blocked and the intensity of one of the incoming beams is monitored as a function of the time delay between the remaining two beams (e.g., beam 3 is blocked and beam 2 is monitored as a function of its delay with respect to beam 1, phase-matching condition would be k2 = ki — ki -I- k2>. Beams 4 and 5 are photon echo signals generated from beams 1 and 2. Beams 6 and 7 can be stimulated photon echo or transient grating signals generated from beams 1,2, and 3. In transient grating two of the beams are time coincident. In coherent anti-Stokes Raman spectroscopy, beams 1 and 3 are time coincident and carry the same frequency the difference between this frequency and that of beam 2 (so-called Stokes beam) matches a vibrational frequency of the system and beam 6 will correspond to the anti-Stokes emission.

In a heterodyne-detected transient-grating (HD-TG) experiment [5,8-11], two infrared laser pulses, typically obtained dividing a single pulsed laser beam, interfere within the sample producing an impulsive spatially periodic variation of the material optical properties. The spatial modulation is characterized by a wave vector which is given by the difference of the two pump wave vectors. The relaxation toward equilibrium of the induced modulation is probed by measuring the Bragg scattered intensity of a second continuous wave laser beam. A sketch of the experimental set-up and details on the laser systems can be found in ref 5 and ref. 10, respectively. 

Figure 1. Laser-induced ultrasonic wave excitation and detection using ISLS. Two 100 picosecond infrared pulses converge spatially and temporally within a transparent sample medium. The time-dependant intensity or strength of the optical transient diffraction grating is monitored by a third frequency doubled pulse which, in the case for relatively low scattering strength, is systematically delayed in time. The time response of stronger gratings can be monitored using a CW laser.

Figure 22. Two different methods of two setup excitation TG methods [143]. Temperature distributions of a sample for the (left) TSETG-I and (right) TSETG-II method are shown, (left) (1) After the prepulse, the temperature rises uniformly by the nonradiative transition from the excited states. (2) The transient absorption by the grating pulses creates the spatially modulated temperature distribution and it produces the TG signal. (3) The stimulated emission decreases the concentration of the excited state and temperature, (right) (1) After the grating pulse, sinusoidally modulated temperature is created. (2) The transient absorption by the boosting laser with a spatially uniform intensity enhances the temperature modulation. (3) The stimulated emission suppresses the modulation.

More substantial information on the vibrational energy transfer in an adsorbate can be obtained from transient IR spectra associated with a vibrationally excited adsorbate. The spectral resolution (< 1 cm ) is achieved by introduction of a grating infrared spectrometer in the detection optics following the pump-probe interaction at the sample (Fig. 4.12). Transient absorption is observed by measuring the relative intensity of the probe beam reflected from the sample, Ip/k, and normalizing it to the same intensity ratio in the absence of the pump, Ip/Is)o- The signal is then defined as... 

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