Pump-probe experiment performed

An approximate method, described in detail in Ref. (15), was applied to simulate a complementary pump-probe experiment performed with picosecond laser pulses. In this method the interaction with the probe laser pulse is approximated. A complete three-dimensional ab initio simulation, as carried out for the femtosecond experiment, is hardly possible for the picosecond experiment with the computers available today. The free laser pulse parameters were taken from the picosecond experiment duration AffWhm = 1.5 ps, intensity I = 300 MW/cm2, and central wavenumber v = O.QTiEh/h = 16021 cm 1. The dynamics induced by such a laser pulse are illustrated by... 

We have studied the temporal dynamics of CPG in m-LPPP by performing field-assisted pump-probe experiments on LED structures, as described in Section 8.3.2. The narrow line-width PA assigned to polarons (see Section 8.5.2) is a fingerprint of charge generation in m-LPPP. Monitoring the dynamics of these PA band enables us, for the first time, to directly observe the CPG dynamics in a conjugated polymer with sub-picosecond time resolution [40],... 

The results for the A state show that a different mechanism is operative. A series of femtosecond pump-probe experiments were performed at wavelengths corresponding to the Rydberg states A (v = 0,1,2) of ammonia molecules.64-66,68,69 The wavelengths used to access these vibrational levels were 214 nm, 211 nm, and 208 nm for the pump laser and 321 nm, 316.5 nm, and 312 nm for the probe laser, respectively. 

As discussed in Sect. 1.2.3, it is usually not possible to distinguish ESA from 2PA with Z-scan experiments if they are performed with only one excitation pulsewidth. However, since ESA is not an instantaneous process as is 2PA, the pump-probe technique can be successfully used to verify the origin of the nonlinearity for the spectral regions close to the main absorption band. Figure 7 illustrates how the influence of the ESA can be distinguished from the 2PA with pump-probe experiments. The curve labeled (1) shows an instantaneous 2PA response without ESA and the long-lived components of the transmittance change seen in (2) and (3) are due to ESA. 

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. 

It is more difficult to perform ultrafast spectroscopy on neat H20 (than it is on H0D/D20 or HOD/H20) since the neat fluid is so absorptive in the OH stretch region. One innovative and very informative technique, developed by Dlott, involves IR pumping and Raman probing. This technique has a number of advantages over traditional IR pump-probe experiments The scattered light is Stokes-shifted, which is less attenuated by the sample, and one can simultaneously monitor the populations of all Raman-active vibrations of the system at the same time. These experimental have been brought to bear on the spectral diffusion problem in neat water [18, 19, 75 77],... 

Photon echo and IR pump-probe experiments can also be performed on neat water, but one needs a very small sample. Fabrication of nano-fluidic Si3Ni4 sample cells have opened up this new and exciting field, and data from these experiments, performed by the Elsaesser and Miller groups, have recently been reported [73, 74]. At room temperature, spectral evolution occurs within 50 fs, and polarization anisotropy decays within 75 fs. At temperatures just about the freezing point, spectral evolution slows down dramatically [74],... 

A closed loop optimization experiment was presented in order to investigate the ability to control the ionization of NaK clusters in a molecular beam. The analysis of the optimized pulse shows that the maximum intensity of the ion signal was obtained with a pulse structure containing intensity maxima separated by a multiple of half vibrational periods. The proposed simple model in order to explain the pulse structure is based on previously performed pump-probe experiments [6]. 

We performed two-color femtosecond mid-infrared pump-probe experiments on dilute (0.1 M) solutions of HDO dissolved in D2O that contain a high concentration (0.5 - 6 M) of KF, NaCl, NaBr, or Nal. The pulses are tunable between 2.7 and 4 pm, have an energy of 10 pJ, and a duration of approximately 200 fs. The pump and probe pulses are independently tunable and have a bandwidth of 80 and 60 cm-1, respectively. 

We performed femtosecond infrared pump-probe experiments, using pulses with a bandwidth of 200 cm"1 (FWHM). A small fraction of the infrared pulses was split off to obtain a broadband probe pulse, which was spectrally dispersed after interaction with the... 

AT DNA double strand oligomers with sodium counterions and a length of 20 base pairs were obtained from Biotherm, and were dissolved in water and dried on a CaF2 window at 293 K in an atmosphere of 52% relative humidity (saturated solution of NaHSC. IDO at 20° Celsius [60]). This results in DNA samples with approximately 4 to 6 water molecules per base pair [37] (sample thickness 6.5 pm). It has been reported that under these conditions AT DNA oligomers adopt the B -form [35], Femtosecond time-resolved IR pump-probe experiments were performed with two independently tunable femtosecond pulses generated by parametric conversion processes pumped by a regenerative Ti sapphire laser system (800 nm, repetition rate 1 kHz, pulse duration 100 fs) [61]. The central frequency of the pump pulse was varied from 1630 to 1760 cm-1 and the probe was centred around 1650 cm-1 or 3200... 

In this chapter, we describe the density- and temperature-dependent behavior of the vibrational lifetime (TO of the asymmetric CO stretching mode of W(CO)6( 2000 cm-1) in supercritical ethane, fluoroform, and carbon dioxide (C02). The studies are performed from low density (well below the critical density) to high density (well above the critical density) at two temperatures one close to the critical temperature and one significantly above the critical temperature (68-70). In addition, experimental results on the temperature dependence of Ti at fixed density are presented. Ti is measured using infrared (IR) pump-probe experiments. The vibrational absorption line positions as a function of density are also reported in the three solvents (68,70) at the two temperatures. 

In CV, substrates are generally activated during the first scan by electron transfer and the reverse scan is then used to detect the activated substrates. The formation of other electrochemically active intermediates or products can also be revealed by these scans or possibly by the performance of multiscans. The CV technique is, like DPSC, similar to the pump/probe experiment known from photochemistry, but in CV, unique knowledge about the thermodynamic properties of the intermediates... 

Dimer structures containing two O-H - -O (OH/OH dimer) or two O-D- -O (OD/OD dimer) hydrogen bonds were dissolved in CCI4 at concentrations between 0.2 and 0.8 M. Two-color pump-probe experiments with independently tunable pump and probe pulses were performed with a 100 fs time resolution. Approximately 1% of the dimers present in the sample volume were excited by the 1 pj pump pulse. After interaction with the sample, the probe pulses were spectrally dispersed to measure transient vibrational spectra with a spectral resolution of 6 cm 1. 

An interferometric method was first used by Porter and Topp [1,92] to perform a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.1.2 is replaced by an absorbing sample, and then the transmission of the variably delayed pulse of light is measured as a function of the delay i. This approach is known today as a pump probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a function of time. 

With a similar setup as used by Ippen et al. for pump-probe experiments, except for an intensity stabilizer in both beams, we performed experiments on the electronic origin at 6027 A and vibronic transitions at 5933 and 5767 A. The results of these experiments are shown in Fig. 22. Except for minor details, the transient on the purely electronic transition is in agreement with our expectation that the singlet excited state is long lived (19.5 ns) on a picosecond time scale. The transient on the 261 cm vibration confirms what was already known from the optical absorption spectrum, namely, that it is very short lived. From the near Lorentzian lineshape at low temperature we calculate a 3.3 ps relaxation time in... 

One final example of ultrafast kinetics performed at radiolysis facilities is the study of excited states of radical ions. An accelerator pulse can be used to generate radical species, which can then be excited by a pump laser beam and probed with femtosecond resolution by another laser pulse with variable optical delay. This application does not depend on precise correlation of the electron and laser pulses and can be done at almost all radiolysis facilities. The availability of femtosecond lasers in photocathode facilities places all the necessary components to hand. Effective pump-probe measurements will require significant concentrations of radical ions. This can be accomplished by frequency-quadrupling a 5-9 nanosecond Nd YAG pulse to irradiate the photocathode, thereby creating a macropulse containing several tens of nanocoulombs which will produce a high concentration of radicals for the pump-probe experiment. 

Experimental Setup. An obvious extension of the one-color pump-probe experiments is the application of two-color experiments in which two independently tunable dye lasers share the same pump laser. One can use the same high repetition rate and obtain spectral evolutions on excitation at selected wavelengths. The measurements are performed in essentially the same way as one-color experiments.A disadvantage is the broadened instrument function (cross-correlation function) caused by time jitter between the two pulses, since they are not obtained from the same dye laser. This leads to a full-width half-maximum (fwhm) value of the instrument function of approximately 5-10 psec. 

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