Optical transient technique

An optical detector with appropriate electronics and readout. Photomultiplier tubes supply good sensitivity for wavelengths in the visible range, and Ge, Si, or other photodiodes can be used in the near infrared range. Multichannel detectors like CCD or photodiode arrays can reduce measurement times, and a streak camera or nonlinear optical techniques can be used to record ps or sub-ps transients. 

In the next section, we describe two pump-probe approaches and review their use in experiments with MbCO. This places the nonlinear optical techniques of transient phase grating introduced in the subsequent section in their proper context. 

The most convenient means of making time-resolved SH measurements on metallic surfaces is to use a cw laser as a continuous monitor of the surface during a transient event. Unfortunately, in the absence of optical enhancements, the signal levels are so low for most electrochemical systems that this route is unattractive. A more viable alternative is to use a cw mode-locked laser which offers the necessary high peak powers and the high repetition rate. The experimental time resolution is typically 12 nsec, which is the time between pulses. A Q-switched Nd YAG provides 30 to 100 msec resolution unless the repetition rate is externally controlled. The electrochemical experiments done to date have involved the application of a fast potential step with the surface response to this perturbation followed by SHG [54, 55,116, 117]. Since the optical technique is instantaneous in nature, one has the potential to obtain a clearer picture than that obtained by the current transient. The experiments have also been applied to multistep processes which are difficult to understand by simple current analysis [54, 117]. 

Figure 5 presents the frequency ranges that, by optimistic estimates, can be covered by the optical techniques discussed above. We have intentionally left out of the discussion several other optical spectroscopic methods, such as coherent anti-Stokes resonant scattering (CARS), or induced transient grating (TG) spectroscopy, mainly because they are rarely used in the present context and/or are much less straightforward to interpret. 

Transient turbidity is an optical technique for measuring the size of magnetic particles [63,64], It does this by aligning particles in an electric field, removing the field, and following their return to random orientation induced by Brownian motion. Their relaxation is measured by turbidity and this can be related to particle size distribution if assumptions are made... 

Optical techniques like photoluminescence (10) and infrared photothermal spectroscopy (11.) work well for the characterization of shallow level impurities, while electrical techniques work well for deep level impurities. There are a number of methods that have been used for electrical characterization. I will only discuss deep level transient spectroscopy (DLTS), however, because it has become the most popular and gives a fairly complete characterization. 

The relationship between the structure of a polymer chain and it dynamics has long been a focus for work in polymer science. It is on the local level that the dynamics of a polymer chain are most directly linked to the monomer structure. The techniques of time-resolved optical spectroscopy provide a uniquely detailed picture of local segmental motions. This is accomplished through the direct observation of the time dependence of the orientation autocorrelation function of a bond in the polymer chain. Optical techniques include fluorescence anisotropy decay experiments (J ) and transient absorption measurements(7 ). A common feature of these methods is the use of polymer chains with chromophore labels attached. The transition dipole of the attached chromophore defines the vector whose reorientation is observed in the experiment. A common labeling scheme is to bond the chromophore into the polymer chain such that the transition dipole is rigidly affixed either para 1 lei (1-7) or perpendicular(8,9) to the chain backbone. 

A review of rheo-optical techniques by Sherman et al. (1996) notes that there has been an increase in the use of rheo-optic set-ups both for FT-IR dichroism and for dynamic IR dichroism spectroscopies for polymer melts and polymer blends. Skytt et al. (1996) highlight the use of simultaneous measurement of the transient or steady-state rheological properties and IR dichroism to characterize orientation in polymer melts. However, there is little reference to dual spectroscopic-rheological techniques for reactive polymer systems in the literature. 

Recently Burrell and Bhattacharyya (4) pulse radiolyzed a solution of allyl bromide in cyclohexane and observed a transient ultraviolet absorption band at 310 m/x which they attributed to the allyl free radical. Their optical technique did not permit them to study the absorption bands below 290 m/x. Another recent investigation was that of Callear and Lee (5) who flash-photolyzed gas mixtures of 1-butene with argon and isobutylene with argon. The most intense band observed by them in the case of 1-butene was at 225 m/x which they attributed to the allyl free radical in essential agreement with the work of Hamill et al. (II) mentioned above. For the isobutylene photolysis, the most intense observed band was at 238 m/x and this was assigned to the /J-methallyl radical, see Table I. 

A variety of optical techniques have been used to measure gas temperatures in combustion applications, particularly in flames. There are potentially some important advantages of optical techniques compared to contact techniques such as suction pyrometers (see Figure 5.7). Optical measurement techniques do not disturb the flow, where thermocouples may have a significant impact on the fluid dynamics. Optical techniques can potentially measure higher temperatures as there are not the materials issues compared to thermocouples. For some optical techniques, temperature profiles can be measured at one point in time without the need to make multiple individual measurements over some length of time. Optical techniques often have a much faster response time compared to contact methods. This is particularly important in turbulent and transient flows. 

What we have attempted to do here is to present rheological tests for identifying the development and relaxation of orientation and structure in liquid crystalline polymers. Because these fluids are typically quite turbid, it is difficult to use rheo-optical techniques. The interpretation of the rheological tests must then come partly from studies on quenched solid specimens. In summary, it is believed that a detailed set of rheological tests based on the transient response of LCP can be used to evaluate various liquid crystalline polymers and identify processing conditions which will lead to the optimum physical properties. 

In most experiments, ultraviolet or infrared absorption, resonance fluorescence, or laser-induced fluorescence (LIF) is used to follow how transient concentrations change after the photolysis pulse. These optical techniques vary considerably in their sensitivity and hence to the extent to which they isolate the primary reaction. LIF is extremely sensitive, enabling one to follow decays of concentrations from an initial value of 10 ° cm , but its use is restricted to species with a bound-bound electronic transition within the range of tunable dye lasers. LIF has been used to follow the kinetics of reactions of, inter alia, the radicals OH [12-14], CN [15] and CH3O [16,17]. It is more difficult to apply to radical atoms vihich usually have allowed electronic transitions only in the vacuum ultraviolet. Some LIF measurements utilising two-photon excitation of atoms have been reported [18]. 

A slightly modified method can be applied if the investigated molecules are adsorbed on a nontransparent substrate. In this case the vibrational relaxation of molecules excited by an IR pulse can be monitored by the recovery of the surface absorption to its equilibrium value. The transient change in absorption is determined from the relative intensities of the reflected p and s components of the probe beam, Ip/k- This technique was used, for example, to measure the transient response of vibrationally excited CO molecules adsorbed on a Pt(lll) surface (Beckerle et al. 1990). Alternatively, one can probe the vibrational relaxation of excited admolecules by the use of nonlinear optical techniques which will be considered in Chapter 6. 

In Section 10.8.1 it was noted that molecular orientation results in flow birefringence in a polarizable polymer, and if the melt is transparent, optical techniques can be used to determine the three components of the stress tensor in uniform, shear flows [91-93]. To determine the transient normal stress differences, the phase-modulated polarization technique was developed by Frattini and Fuller [ 126]. Kalogrianitis and van Egmond [ 127] used this technique to determine the shear stress and both the normal stress differences as functions of time in start-up of steady simple shear. Optical techniques are particularly attractive for measurements of normal stress differences, since such methods do not require the use of a mechanical transducer, whose compliance plagues measurements of normal stress differences by mechanical rheometry. 

The technique of transient or quasi steady slate photoinduced absorption (PIA allows the population of excited states and also probing the optical transitions... 

A number of variations of the transient hot-wire method have been devised, and an optical method to detect the temperature rise has been used. A modified transient hot-wire technique using a mercury-incapillary probe was introduced by Nagashima et al., in which a thin mercury thread was used as a heater-thermometer and the capillary wall as an insulator. Using this method, they measured the thermal conductivity in mixture systems such as (Na, K)N03, (Li, Na)N03, and HTS(KN03-NaN03-NaN02, 44-7-49 mol.%). ... 

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