Pump-probe conductivity

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

A second way to overcome the high reactivity of carbenes and so permit their direct observation is to conduct an experiment on a very short timescale. In the past five years this approach has been applied to a number of aromatic carbenes. These experiments rely on the rapid photochemical generation of the carbene with a short pulse of light (the pump beam), and the detection of the optical absorption (or emission) of the carbene with a probe beam. These pump-probe experiments can be performed on timescales ranging from picoseconds to milliseconds. They provide an important opportunity absent from the low temperature experiments, namely, the capability of studying chemical reactions of the carbene under normal conditions. Before proceeding to discuss the application of these techniques to aromatic carbenes, a few details illuminating the nature of the data obtained and the limitations of the experiment need to be introduced. 

While the first experiments of time-resolved IR spectroscopy were conducted with pulse durations exceeding 10 ps, the improved performance of laser systems now offers subpicosecond (12) to femtosecond (13-15) pulses in the infrared spectral region. In addition, the pump-probe techniques have been supplemented by applications of higher-order methods, e.g., IR photon echo observations (16). 

Vibrational echo and vibrational pump-probe experiments were conducted on the CO asymmetric stretching mode of Rh(CO)2acac (2010 cm-1) in DBP from 3.4 to 250 K. Figure 2 shows vibrational echo data taken at 3.4 K... 

Fig. 15 Rate data for photoinduced charge separation and subsequent charge recombination in the dyads 18(h)- Charge separation rates, cs, in THF at 20°C were determined both from fluorescence lifetimes92,102 and by pump-probe (time-resolved transient absorption) spectroscopic measurements.105 The mean lifetimes towards charge recombination, rcn were obtained from time-resolved conductivity measurements in 1,4-dioxane.99 101,103,104...

The Ti-sapphire oscillator is extremely useful as a stand-alone source of femtosecond pulses in the near-IR region of the spectrum. Some ultrafast experiments, especially of the pump-probe variety (see below), can be conducted with pulses obtained directly from the oscillator or after pulse selection at a lower repetition rate. Far-IR (terahertz) radiation is usually generated using a semiconductor (usually GaAs) substrate and focused Ti-sapphire oscillator pulses [7]. If somewhat higher-energy pulses are required for an experiment, the Ti-sapphire oscillator can be cavity dumped by an intracavity acousto-optical device known as a Bragg cell. 

The first ODMR experiments, in which magnetic sub-levels of optically excited states were preferentially populated by excitation with circularly polarized light in a magnetic field ( optical pumping ) were conducted almost forty years ago by Geschwind and co-workers [54], Since then it has developed into a powerful tool to probe optical and electronic processes in a wide variety of semiconducting [55-57] and biological materials [58]. 

Equations (20) and (21) predict that the anisotropy should initially decay rapidly from r(0) S 0.4 with lifetime 2F, then decay more slowly from r = 0.4 with dynamics that resemble Forster kinetics in the limit of weak coupling. No anisotropies r(0) > 0.4 have been reported in numerous pump-probe studies of energy transport in bacterial and green plant antennae, conducted at time resolutions varying from around 10 psec to about 300 fsec, For these systems, (2F) therefore appears to be bounded from above by (conservatively) approximately 1 psec. Anisotropies r(0) 0.7 have been observed in magnesium tetraphenylporphyrin, a system to which the homodimer model appears to be directly applicable. 

A group at Stanford studied second-order nonhnear susceptibility of the conduction band and valence band quantum well (QW) stractures extracted from the interference between second harmonic Adds of QWs and GaAs substrate as determined by the azimuthal dependence of the second harmonic power. This is the first demonstration of difference frequency generation of mid-lR in any QW. Groups at Stanford also studied vibrational dynamics in glass-forming liquids. These are the first vibrational photon echo experiments and first comprehensive temperature-dependent pump probe measurements on ar r condensed matter system. 

Figure 10.3 illustrates an application of a pump-probe system in a study of the excited states dynamics of DNA and RNA bases cytosine, guanine, thymine, and uracil [8], The experiment was conducted using a TOF mass analyzer equipped with a photoionization ion source. The samples were heated and desorbed using an oven. The vapor was carried into the ionization region using a supersonic argon jet. The pump and probe lasers... 

Bernini U, Lettieri S, Maddalena P, Vitiello R, Di Francia G (2001) Evaluation of the thermal conductivity of porous silicon layers by an optical pump-probe method. J Phys Condens Matter... 

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