Laser induced pressure pulse method

FVgura 21 Setup for laser-induced pressure pulse method for charge measuremesi (367). 

Bias-induced reverse piezoelectric response Broadband dielectric spectroscopy (BDS) Dielectric permittivity spectrum Dielectric resonance spectroscopy Elastic modulus Ferroelectrets Electrical breakdown Acoustic method Characterization Dynamic coefficient Interferometric method Pressure and frequency dependence of piezoelectric coefficient Profilometer Quasistatic piezoelectric coefficient Stress-strain curves Thermal stability of piezoelectricity Ferroelectric hysteresis Impedance spectroscopy Laser-induced pressure pulse Layer-structure model of ferroelectret Low-field dielectric spectroscopy Nonlinear dielectric spectroscopy Piezoelectrically generated pressure step technique (PPS) Pyroelectric current spectrum Pyroelectric microscopy Pyroelectricity Quasistatic method Scale transform method Scanning pyroelectric microscopy (SPEM) Thermal step teehnique Thermal wave technique Thermal-pulse method Weibull distribution... 

Instead of temperature fields, one can use pressure discontinuities in order to probe charge or polarization distributions in dielectrics. According to the pressure excitation method, one can distinguish three basic configurations (a) the LIPP (laser-induced pressure pulse. Fig. 21) (Sessler et al. 1981) or PWP (pressure wave propagation) technique (Alquie et al. 1981), (b) the piezoelectrically generated pressure step technique (PPS, cf. Fig. 22) (Eisenmenger and Haardt 1982), and (c) the pulsed electroacoustic method (PEA, cf Fig. 23) (Takada et al. 1987). 

The most commonly used method is the direct measurement of a decay rate by pulsed excitation and time resolved detection. The most straightforward example of this technique is laser induced fluorescence applied to alkali Rydberg atoms. Alkali atoms are typically contained in a glass cell, which also holds a known pressure of perturber gas. The alkali atoms are excited to the Rydberg state at time t = 0 and the time resolved fluorescence from the Rydberg atoms is detected... 

The time to measure spectra of this quality under high-pressure conditions has been about I min. The absolute time scale of the experiment depends on the method of initiation. In thermally initiated (spontaneous) polymerizations reaction time can be several hours or even days. In contrast, in excimer laser-initiated free radical polymerizations application of a few laser pulses each of about 20 ns duration can induce changes between subsequent spectra as on this figure. 

A good representative example for such a photo-induced chemical reaction on a cluster is that of Ba and CI2 on argon clusters (Ar) , with up to w = 800. The experiment was carried out in the set-up shown schematically in Figure 24.20. The Ar-clusfer beam passes through a B a cell, of the form of a small cylinder that is moderately heated to produce a low-pressure Ba vapour ( 10 mbar), where the clusters pick up the Ba atoms. The experimental conditions are optimized to pick up one single atom of Ba on every Ar cluster. Once the Ba - -Cl2 complex is produced on the Ar cluster, a laser pulse excites the Ba this subsequently reacts with the CI2, leading to product chemiluminescence. This method offers an excellent means to compare both cluster-phase and free gas-phase chemiluminescent reactions. 

Collision-induced transitions between fine-structure components where the relative orientation of the electron spin with respect to the orbital angular momentum is changed have been studied in detail by laser-spectroscopic techniques [13.64]. One of the methods often used is sensitized fluorescence, where one of the fine-structure components is selectively excited and the fluorescence of the other component is observed as a function of pressure [13.65]. Either pulsed excitation and time-resolved detection is used [13.66] or the intensity ratio of the two fine-structure components is measured under cw excitation [13.67]. 

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