Laser-irradiated temperature-jump

With M = He, experimeuts were carried out between 255 K aud 273 K with a few millibar NO2 at total pressures between 300 mbar aud 200 bar. Temperature jumps on the order of 1 K were effected by pulsed irradiation (< 1 pS) with a CO2 laser at 9.2- 9.6pm aud with SiF or perfluorocyclobutaue as primary IR absorbers (< 1 mbar). Under these conditions, the dissociation of N2O4 occurs within the irradiated volume on a time scale of a few hundred microseconds. NO2 aud N2O4 were monitored simultaneously by recording the time-dependent UV absorption signal at 420 run aud 253 run, respectively. The recombination rate constant can be obtained from the effective first-order relaxation time, A derivation analogous to (equation (B2.5.9). equation (B2.5.10). equation (B2.5.11) and equation (B2.5.12)) yield... 

The laser heating technique can be applied to perform temperature jumps by irradiating short laser pulses at the sample container. Ernst et al. (54) used such a temperature jump protocol to perform stop-and-go experiments. After the start of the laser pulse, the temperature inside the sample volume is raised to the reaction temperature, the conversion of the adsorbed reactants proceeds, and the H MAS NMR measurement is performed. After the laser pulse is stopped, the temperature inside the sample volume decreases to ambient temperature, and the C MAS NMR measurement is made. Subsequently, the next laser pulse is started and, in this way, the reaction is recorded as a function of the reaction time. By use of the free-induction decay and the reaction time as time domains and respectively, a two-dimensional Fourier transformation leads to a two-dimensional spectrum, which contains the NMR spectrum in the Ej-dimension and the reaction rate information in the Ts-dimension (54,55). 

The LDH/NADH pyruvate ternary complex concentration is quite low, and it was found that the concentration of LDH/NADH + pyruvate equals approximately that of the LDH/NAD+ lactate. A temperature increase tips the equilibrium from right to left. Figure 15.10 shows the time-resolved fluorescence emission of NADH at 450 nm in response to a T-jump from 10 to 23 °C. There are two instrument response times one near 30 ns, which is the pulse width of the laser irradiation heating the sample, and the second is diffusion of heat out from the laser interaction volume that occurs around 15 ms (the latter response is not shown). Fitting the data (solid line) with a function of multiexponentials yielded four rates, as indicated on Fig. 15.10, in addition to these instrument response functions. 

Picosecond laser pulses in the UV range do not result in better ablation behavior than nanosecond laser pulses. This is different for doped polymers. Experiments with doped PMMA (an IR-absorber, i.e., IR-165 for ablation with near-IR laser and diazomeldrum s acid (DMA) for ablation with UV lasers) with nanosecond and picosecond laser irradiation in the UV (266 nm) and near-IR (1064 nm) range have shown that, in the IR, neat features could be produced with picosecond laser irradiation, while nanosecond irradiation only results in rough surface features [105]. This corresponds well with the different behavior of the two absorbers. With IR-165 the polymer is matrix is heated by a fast vibrational relaxation and multiphonon up-pumping [106]. This leads to a higher temperature jump for the picosecond irradiation, which causes ablation, while for nanosecond pulses only lower temperatures are reached. 

Figure 8.7.5 Schematic diagram of apparatus employed for the temperature-jump method. The laser pulse is passed through a neutral density filter (ND) and irradiates the thin film electrode at the bottom of the cell. The dark rectangles are an auxiliary electrode and a QRE for measurement of the potential change. The potentiostat (Pot.), which adjusts the electrode potential before irradiation, is disconnected immediately before the laser pulse. The change in potential is measured with a fast amplifier (Amp.). [Reprinted from J. F. Smalley, L. Geng, S. W. Feldberg, L. C. Rogers, and J. Leddy, J. Electroanal. Chem., 356, 181 (1993), with permission from Elsevier Science.]...

The RDE can also be modulated thermally by irradiating the back of the disk electrode with a laser (Figure 9.6.4) (31). This method is the RDE analog of the temperature jump... 

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