Transient Pulse Measurements

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

It is the essence of transient measurements that a certain perturbation is applied to the electrode and then the response is recorded as a function of time. Usually, the transition times are short (fractions of a second), and the transient measurements are performed very rapidly with automated data acquisition. 

The transient techniques can be grouped according to various criteria (Fig. 12.6), as follows  

According to the shape of the perturbation. A step (static perturbation curve 

According to the number of perturbations. Single perturbations (curves a and 

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Fig. 2.16. G-mode frequency of SWNTs as a function of pump-probe time delay obtained from transient transmission measurement using a sub-10 fe pulse at 2.1 eV. From [55]...

For capacity measurements, several techniques are applicable. Impedance spectroscopy, lock-in technique or pulse measurements can be used, and the advantages and disadvantages of the various techniques are the same as for room temperature measurements. An important factor is the temperature dependent time constant of the system which shifts e.g. the capacitive branch in an impedance-frequency diagram with decreasing temperature to lower frequencies. Comparable changes with temperature are also observed in the potential transients due to galvanostatic pulses. 

For the investigation of charge tranfer processes, one has the whole arsenal of techniques commonly used at one s disposal. As long as transport limitations do not play a role, cyclic voltammetry or potentiodynamic sweeps can be used. Otherwise, impedance techniques or pulse measurements can be employed. For a mass transport limitation of the reacting species from the electrolyte, the diffusion is usually not uniform and does not follow the common assumptions made in the analysis of current or potential transients. Experimental results referring to charge distribution and charge transfer reactions at the electrode-electrolyte interface will be discussed later. 

Aramaki and Atkinson were also active in work on the spiro-oxazines [65]. They noted that for NOSH in many polar and nonpolar solvents the picosecond time-resolved resonance Raman spectra simply built up over 50 psec with no shape evolution. The same finding was concluded from transient absorption measurements over the same time scale. The spectra/absorbances were then constant for 1.5 nsec. These authors suggest that only two isomers can be expected to contribute to the merocyanine spectra because those trans about the y-methene bridge bond attached to the naphthalene ring are sterically crowded due to short interproton distances. There was no evidence for the X transient in their study however, the 50-psec convoluted pulse profile may be expected to mask this sortlifetime species even if it were present. 

Fig. 3. Long range and one-bond carbon-13 satellite spectrum of a 5% w/w solution of ethanediol in D2O at 94°C. 16 transients were measured on a Varian Associates Unity 500 spectrometer using the sequence of fig. 1, with 2.5 s presaturation, a t value of 100 ms, spin lock pulses of 450 ps, no homospoil pulse, and no homodecoupling during acquisition.

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

We performed transient absorption measurements on BP(OH>2 with a spectrometer based on two noncollinearly phase matched optical parametric amplifiers (NOPAs) pumped by an homebuilt regenerative Ti Sapphire laser system or a CPA 2001 (Clark-MXR) [1,7]. The tunable UV pump pulses are generated by frequency doubling the output of one of the NOPAs. The other NOPA provides the visible probe pulses. The cross correlation between pump and probe pulses has a typical width (FWHM) of 40 fs. The sample is a cyclohexane solution of BP(OH)2 pumped through a flow cell with a 120 pm thick channel. 

Fig. 1 left FTIR spectrum (a) and 2D-IR spectrum (b) of the ground state for perpendicular polarization of pump and probe pulse. A broadband IR probe pulse measures the spectral change as a function of delay and frequency of a narrowband IR pump pulse (c). right time resolved absorption spectrum (d, magic angle polarization) and transient 2D-IR spectrum (e) recorded 20 ps after UV excitation. The T2D-IR spectrum was recorded with magic angle between UV-pump (500 fs, 5 iJ) and IR-pump polarizations and perpendicularly polarised IR-pump and probe beams, pulse sequence (f). 

Fig. 1. Top Reference spectra for femtosecond transient absorption measurements S-S abs. in solution (thin solid lines), oxidized dye (dye+) abs. in solution (thick solid line), fluorescence for solution (dotted line), steady-state absorption ofNKX-2311/ZnO (dotted-dashed line), and absorption of electrons in the conduction band (dashed line). Bottom Transient absorption spectra of NKX-23ll/ZnO in the spectral range between 600 and 1350 nm at the 2 (thick solid line), 10 (dotted line), 100 ps (thin solid line) delay times after excitation at 540 nm by the femtosecond pulse with the intensity of about 10 pJ.

Deff can be measured, either directly by the flux through a catalyst pellet (Wicke-Kallenbach diffusion cell [60]), or by transient pulse method [61]. It is easier, but less accurate, to relate Deff to molecular diffusivity. 

Barker later described some work that involved apparatus like that shown in Figure 28.11. Light was supplied to a continuously renewed mercury pool electrode by a Q-switched, frequency-doubled ruby laser with a pulse width of — 15 ns. The electrode was set initially at any desired potential by a simple polarizing circuit, the response of which was slow enough that the electrode s reaction to the flash could be monitored as a coulostatic transient, AE (measured with respect to the initial potential) versus time. The difference in charge with respect to the initial condition is straightforwardly related to AE,... 

Additionally, in order to examine the charge-recombination dynamics we turned to complementary nanosecond transient absorption measurements. Once more, the spectral fingerprints of the radical ion pair state emerged immediately after the laser pulse and their decays yielded charge-recombination lifetimes in the order of 4.0 ps (Fig. 9.38). 

Early ultrafast transient infrared measurements were typically performed using identical frequency picosecond infrared pulses for both pumping and probing a vibrational mode of condensed-phase molecules... 

Although such a 2-MHz ESR apparatus was very sophisticated, its time resolution was not enough for measurement of CIDEP. In 1973, Fessenden [5] found that the direct ESR measurement without field modulation improved the time resolution, observing CIDEP signals in solution with pulse radiolysis. This method was applied for laser-photolysis measurements in solids [6] and in solution [7]. A spin-echo ESR technique was also found to be useful for CIDEP [8]. Since then, CIDEP experiments with cw-ESR and pulsed-ESR spectrometers without field modulation have become much more popular than before. Through such transient ESR measurements, CIDEP due to not only the radical pair mechanism but also several other mechanisms have been observed in many chemical reactions including biologically important ones such as photosynthesis reactions. In this chapter, we will show several mechanisms for CIDEP with several typical examples. 

Since 1962, when it was first characterized by pulse radiolysis transient absorption measurements in water, the solvated electron has been widely studied in numerous solvents. The solvated electron, denoted by e, is a thermodynamically stable radical, but like most free radicals, it has a short lifetime due to its high chemical reactivity. The solvated electron is a unique chemical moiety whose properties may be compared in many solvents and are not dependent on the method creating the solvated electron. The solvated electron is an important reactive species as it is the simplest electron donor, its reactions correspond to electron transfer reactions and its reactivity may be used to probe electron transfer properties of acceptors. During the last 40 years, due to its optical absorption properties, the... 

Another commonly used waveform is a square wave where the electrode is held at a nonoxidizing potential and transiently pulsed to an oxidizing potential. An example of this form is high-speed chronoamperometry, which uses oxidizing pulses that typically last for around 100 ms and are repeated each second. This method provides information (current) both on the oxidation of compounds at the surface of the electrode and on the reduction of the oxidized material. This additional information that is obtained during each measurement aids in the identification of the analyte. 

The short pulse duration combined with the high photon density of ps-and fs-lasers have provided the means to study the properties of the excited states by emission and transient absorption measurements. Fluorescence of the lowest and higher excited states of azobenzene can be detected, but most work is being directed toward the dynamics of isomerization. Because questions about the isomerization mechanism are prominent in this field, this work will be discussed in Section 1.6 The Isomerization Mechanism. 

Transient absorption measurements have recently been recorded from the organometallic species chromium hexacarbonyl in ethanol solution [94], Absorption of a 65-fs, 310-nm excitation pulse was followed by measurement of excited-state absorption of a 65-fs, 480-nm probe pulse. The data shown in Figure 14 indicate a rapid nonexponential decay at short times followed by a gradual exponential rise. The slower feature was observed previously [95] and is known to correspond to the solvent complexation of Cr(CO)5 to yield Cr(CO)j(MeOH). The initial feature, which is observed at other probe wavelengths as well, is believed to correspond to the initial ligand loss reaction. Note that this case is different from ICN in that the initially excited wavepacket is not on the side of the Sj potential but rather (as is clear from the molecular symmetry) on a local potential maximum. The wavepacket must then spread that is, dissociation along either direction is equally likely. The rapid nonexponential decay was analyzed in terms of classical kinematics along a dissociative potential. 

First, Harrison et al. [3.36] studied Me UPD in the systems Ag(lll)/Pb, Cf, Ag(polycrystalline)/Tl, Cf, and Ag (polycrystalline)/Pb, acetate by cyclic voltammetry and potentiostatic pulse measurements. The authors claimed that a non-monotonous current transient represents a necessary criterion for 2D nucleation and growth involved in the 2D Meads overlayer formation. However, the experimental results presented did not give evidence for a first order phase transition. 

Bewick and Thomas [3.110-3.114, 3.270] measured electrochemically and by optical means different Me UPD systems Ag(A 0/Pb, H, ClOd", acetate and citrate, CnQikt)/ h H C104, acetate, and AgQikt)m SOd with Qikt) = (111), (100), and (110). Potentiostatic pulse measurements showed non-monotonous current transients for Ag(lll) substrates which are attributed to a first order phase transition. As an example, a current transient in the system Pig hkt)/Vf, H, SOd is shown in Fig. 3.46. In the case of Ag(lOO) and Ag(llO) substrates, higher order phase transitions were supposed. Clear evidence of a participation of 2D nucleation and growth steps in the 2D Meads phase formation process was found in the system Cu(lll)/Pb H", ClOd", acetate [3.270]. Non-monotonous current transients and a discontinuity in the q(lsE,fi) isotherm were observed (Fig. 3.13). 

FIGURE 7.1. Experimental set-up for transient EL measurements under steep voltage pulses. 

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