Pulsed excitation techniques

Information about the influence of 2D UPD phases on thermodynamics and kinetics of subsequent 3D Me nucleation and growth can be obtained by UPD-OPD transition experiments. In general, the experiment has two stages. In the initial stage i, a 2D Me UPD phase is formed and eventually equilibrated at a selected underpotential AE. The final stage f of the system is characterized by an external potentiostatic pulse to t]f into the OPD range. There are two possibilities for pulse excitation techniques potentiostatic or galvanostatic conditions. 

Galvanostatic pulse excitation technique requires a fast E - i conversion device to switch from potentiostatic to galvanostatic conditions. The analysis of E(t) transients is rather complex since the nucleation and growth kinetics of the 3D Me bulk phase are changed continuously by the varying supersaturation. 

In both of the pulse excitation techniques we describe below, the excitation pulse is also used to provide a tri ering signal that the recording electronics uses to synchronize events, hi the case of flash lamp excitation, this is commonly accomplished using an anteima pick-up . For laser pulse excitation. 

The difference between the various pulse voltammetric techniques is the excitation waveform and the current sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is applied for each drop of mercury when the DME is used. (Both techniques can also be used at solid electrodes.) By controlling the drop time (with a mechanical knocker), the pulse is synchronized with the maximum growth of the mercury drop. At this point, near the end of the drop lifetime, the faradaic current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

This is a dynamic electrochemical technique, which can be used to study electron transfer reactions with solid electrodes. A voltammo-gram is the electrical current response that is due to applied excitation potential. Chapter 18b describes the origin of the current in steady-state voltammetry, chronoamperometry, cyclic voltammetry, and square wave voltammetry and other pulse voltammetric techniques. 

Advances in pulse radiolysis studies in the gas phase have been summarized in several review papers. In a comprehensive review by Sauer [4], a review presented by Firestone and Dorfman [5] in 1971 was referred to as the first review on gas-phase pulse radiolysis. Experimental techniques and results obtained were summarized by one of the present authors [6], with emphasis on an important contribution of pulse radiolysis to gas-phase reaction dynamics studies. Examples were chosen by Sauer [7] from the literature prior to 1981 to show the types of species that were investigated in the gas phase using pulse radiolysis technique. Armstrong [8] reviewed experimental data obtained from gas-phase pulse radiolysis together with those from ordinary steady-state radiolysis. Advances in gas-phase pulse radiolysis studies since 1981 were also briefly reviewed by Jonah et al. [9], with emphasis on an important contribution of this technique to free radical reaction studies. One of the present authors reviewed comprehensively the gas-phase collision dynamics studies of low-energy electrons, ions, excited atoms and molecules, and free radicals by means of pulse radiolysis method [1-3]. An important contribution of pulse radiolysis to electron attachment, recombination, and Penning collision studies was also reviewed in Refs. 10-15. 

We have investigated the reactions of the COs " radicals with double-stranded DNA by laser flash photolysis techniques [15]. In these time-re-solved experiments, the COs radicals were generated by one-electron oxidation of HCOs by sulfate radical anions, SO4 the latter were derived from the photodissociation of persulfate anions, S20s initiated by 308-nm XeCl excimer laser pulse excitation. In air-equilibrated buffer solution containing the self-complementary oligonucleotide duplex d(AACGCGAATTCGCGTT), 208 , and an excess of HCO3., the decay of the CO3 radical anion absorption band at 600 nm is associated with the concomitant formation of the characteristic narrow absorption band of the G(-H) radicals near 310 nm. 

Spectroscopic evidence for the transient formation of the trans-stilbene radical cation could be obtained when colloidal TiOj suspended in an acetonitrile solution containing trans-stilbene (a species which should also be exothermically oxidized by a TiO valence band hole) was excited with a laser pulse The observed transient was identical in spectroscopic features and in lifetime with an authentic sample of the stilbene cation radical generated in the same medium via pulse radiolytic techniques. That the surface influences the subsequent chemistry of this species can be seen in the distribution of products observed under steady state illumination, Eq. (4) 2 . ... 

Spontaneously Detected Photon Echoes in Excited Molecular Ensembles A Probe Pulse Laser Technique for the Detection of Optical Coherence of Inhomogeneously Broadened Electronic Transitions, A. H. Zewail, T. E. Orlowski, K. E. Jones, and D. E. Godar, Chem. Phys. Lett. 4S, 256 (1977). 

The resemblance of the photocurrent to the optical adsorption spectrum has suggested the involvement of molecular excited states in the creation of charge carriers. While this resemblance is by no means universally observed, the concept of carrier creation via exciton interactions at or very near the illuminated electrode has become increasingly favored. Many of the data leading to these conclusions have been obtained by the use of pulsed light techniques (6, 7,3). These methods are virtually independent of electrode effects and the subsequent analysis of the transient current has led to considerable advances in the theory of charge transfer in molecular crystals. 

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... 

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