Pump-probe scheme

When using a 265 nm pump and a 398 nm probe a 1+3 pump-probe scheme produces ion signal resulting from excitation of the coupled a2, Bi states [4, 6], a 2+1 pump-probe scheme leads to excitation of an absorption band that has been assigned as the F band [5]. Unfortunately, to the best of our knowledge, assignment of the detailed spectroscopic properties of the F band has not been performed. 

The pump-probe scheme that we use is as follows the molecule is initially excited by a pump laser photon (htOj) to the first excited neutral state. The dynamics is then followed with a probe laser by one-photon ionization (hooa). Compared to a free diatomic, solvation by CH3CN brings two new factors come into play. Firstly, the potential energy curves of the diatomic are modified by the presence of the neighboring molecule. Secondly, the fragmentation dynamics of the diatomic is changed as there may be collisions with, and a transfer of energy to, the acetonitrile. 

When extending the pump-probe scheme into the x-ray domain, we deal with two very different pump and probe light sources with respect to their pulse intensities. This poses stringent boundary conditions on the sample design in order to guarantee a feasible experiment [11]. To start with, we estimate the signal-to-noise ratio that one can anticipate with current technology. Hereby, we restrict the calculations to pump-probe experiments in... 

As pointed out hv Scoutland. "The general principles of ultrafast laser experiments are well known. All ultrafast experiments are variants on the pump-probe scheme, in which lime resolution is obtained by spatial delay of a probe pulse relative to the pump, or excitation, pulse (I ps = 3.0 nim)"... 

Although we interpreted our experimental results with the motion of nuclear packets on the light-dressed ground state of the parent ions, a pump-probe scheme could also explain the pulse width dependence. When two successive pulses are interacted with molecules with a proper interval, the second pulse can be adjusted to achieve synchronicity with the motion of the nuclear wave packet of an excited state. This results in energy transitions to... 

In conclusion, the rather straightforward one-color pump-probe scheme of the self-heterodyne method seems particularly appropriate for smaller coupled systems where the vibrational spectra are less congested. The main content of the present approach and the frequency domain method described in Section IV.C are the same, namely the existence and magnitudes of cross peaks and their relationship to couplings between... 

The spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

Wang et al, 2003 Wang et al, 2004). The time resolution of this technique is inversely proportional to the spectral resolution. With 10 cm spectral resolution, a time resolution of -1.5 ps can be obtained. These probe techniques can be combined with an excitation pulse in a pump/probe scheme to measure interface selective dynamics. They have been used to study solvation dynamics at a liquid-liquid interface (Zimdars et al, 1999) and vibrational relaxation on a metal surface (Bonn et al, 2000 Bonn et al, 2001). Although not yet reported, the technique should also be applicable to the study of interfacial electron-transfer dynamics. 

Things are not quite as simple as they seem. In order for the constructive interference, which is at the core of wavepacket interferometry, to occur, not only must (t + At) = (t), but also the phases of apump and aprobe> which depend on the optical phase of the femtosecond laser rather than the molecular phase, must match. A rigorous treatment of the phase coherent pump/probe scheme using optically phase-locked pulse pairs is presented by Scherer, et al., [1990, 1991, 1992] and refined by Albrecht, et al., (1999), who discuss the distinction between and consequences of pulse envelope delays vs. carrier wave phase shifts (see Fig. 9.6). A simplified treatment, valid only for weak optical pulses is presented here. 

In the phase-coherent, one-color pump/probe scheme (see Section 9.1.9) the wavepacket is detected when the center of the wavepacket returns to its to position, (x)to+nT — (x)to, after an integer number of vibrational periods. The pump pulse creates the wavepacket. The probe pulse creates another identical wavepacket, which may add constructively or destructively to all or part of the original pump-produced wavepacket. If the envelope delay and optical phase of the probe pulse (Albrecht, et al, 1999) are both chosen correctly, near perfect constructive or destructive interference occurs and the total spontaneous fluorescence intensity (detected after the pump and probe pulses have traversed the sample) is either quadrupled (relative to that produced by the pump pulse alone) or nulled. As discussed in Section 9.1.9, the probe pulse is delayed, relative to the pump pulse, in discrete steps of At = x/ojl- 10l is selected by the experimentalist from within the range (ljl) 1/At (At is the temporal FWHM of the pulse) to define the optical phase of the probe pulse relative to that of the pump pulse and the average excitation frequency. However, [(E) — Ev ]/K is selected by the molecule in accord with the classical Franck-Condon principle (Tellinghuisen, 1984), also within the (ojl) 1/At range. When the envelope delay is chosen so that the probe pulse arrives simultaneously with the return of the center of the vibrational wavepacket to its position at to, a relative maximum (optical phase at ojl delayed by 2mr) or minimum (optical phase at u>l delayed by (2n + l)7r) in the fluorescence intensity is observed. 

Using the two-color pump/probe scheme, similar to the one depicted in Fig. 9.8, it is possible to measure the time required for the center of the vibrational wavepacket created at to on the B-state to arrive at either the inner i B- or outer fiB+ turning point of the bound, electronically excited B-state. Owing to their different center frequencies, (wi)pump wi)probe, and their non-interchangeable roles in the... 

Rotational recurrences may be detected in polarization selected spontaneous fluorescence (provided the photodetector has a sufficiently fast response) or by a variety of sub-nanosecond pump/probe schemes (Felker and Zewail, 1987 Felker, 1992 Hartland, et al. 1992 Joireman, et al.. 1992 Smith, et al., 2003a,b). 

This somewhat brief description of the PXR system has made evident the general aspects of this unique experimental ultrafast x-ray system. The experimental procedure for time-resolved x-ray diffraction presented is based upon the pump-probe scheme first introduced several years ago in picosecond spectroscopy [17]. The laser in these experiments is used to create x-ray pulses and also functions as an excitation source for the sample. To detect the very weak signals of diffracted x-rays, a unique... 

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