Double probe pulse detection

Por IR-Raman experiments, a mid-IR pump pulse from an OPA and a visible Raman probe pulse are used. The Raman probe is generated either by frequency doubling a solid-state laser which pumps the OPA [16], or by a two-colour OPA [39]. Transient anti-Stokes emission is detected with a monocliromator and photomultiplier [39], or a spectrograph and optical multichannel analyser [40]. 

Real-Time Detection of Laser-Pulse-Induced Molecular Dynamics. To study silver aggregates the amplified laser pulses are, for example, efficiently 40%) frequency-doubled in a BBO crystal. The second harmonic is split into pump and probe pulses, with the probe pulse delayed with respect to the pump pulse by a computer-controlled translation stage. The pump and probe laser beams are imaged into the trap collinearly with the ion trajectories and overlap throughout the whole length of the trap (see Fig. 2.26). The electrons of the stored cluster anions are detached by the pump pulse and, subsequently, after a certain delay time At, the newly created neutrals are ionized by the probe pulse. This is achieved by nonresonant TPI rather than by REMPI [281, 282]. Figure2.34 presents typical mass spectra for silver anions and cations. 

A method less common for lifetime measurements is the so-called pump-probe or double-pulse approach. Like time- and frequency-domain detection, the technique originates in non-spatially-resolved fluorescence spectroscopy [ 19]. In this technique, two very short excitation pulses follow each other. The first pulse excites fluorochromes inside the detection volume to full or partial saturation. The second pulse, or probe pulse, arrives at a variable (ns) time delay. If the time delay between the pulses is short compared to the fluorescence lifetime, most of the fluorochromes will still be in the excited state when the second pulse arrives so that the second pulse cannot excite additional fluorochromes and thus does not lead to additional fluorescence. If the time delay is long, most fluorochromes will have relaxed back to their ground state, so that the second pulse leads to... 

Although the idea of generating 2D correlation spectra was introduced several decades ago in the field of NMR [1008], extension to other areas of spectroscopy has been slow. This is essentially on account of the time-scale. Characteristic times associated with typical molecular vibrations probed by IR are of the order of picoseconds, which is many orders of magnitude shorter than the relaxation times in NMR. Consequently, the standard approach used successfully in 2D NMR, i.e. multiple-pulse excitations of a system, followed by detection and subsequent double Fourier transformation of a series of free-induction decay signals [1009], is not readily applicable to conventional IR experiments. A very different experimental approach is therefore required. The approach for generation of 2D IR spectra defined by two independent wavenumbers is based on the detection of various relaxation processes, which are much slower than vibrational relaxations but are closely associated with molecular-scale phenomena. These slower relaxation processes can be studied with a conventional... 

This behavior is exploited in SEP experiments [51] where the lowering of the population of level 2 for double-resonance conditions is probed by laser-induced fluorescence (LIF) or ion detection (ion dip experiments) by ionizing the molecules in level 2 with a third laser pulse. It is obvious from the rate equations that no dip depth larger than 50% of the maximum off-resonant signal can be obtained as long as no fast decays of the final levels must be considered. (However, for fast-decaying final levels deeper dips can be expected and the dip depth has been used for an estimate of the decay rate [53].)... 

Access to subpicosecond electron pulses has already been achieved at Osaka University by a new double-decker accelerator concept. In order to reduce the time jitter for the detection of the optical absorption signals in pulse radiolysis studies, the light pulse used for the pump-probe system is Cerenkov emission which is produced in the same cell by a synchronized second electron beam and is concomitant with the electron path. The distance between the axes of the two beams is 1.6 mm. The pulse durations of these electron pulses, which are both produced by delayed beams issued from the same laser, are 430 + 25 fs and 510 20 fs, respectively, and the charge per pulse is 0.65 nC. An electron bunch of 100 fs and 0.17 nC has already been generated. 

Heteronuclear NMR experiments, which can be performed with the standard equipment of practically all modern spectrometers, require in general three separate radiofrequency (RF) channels for both spectrometer and probe head. The first two channels deliver the H (for decoupling) and "X frequencies to the sample, and the third channel is commonly tuned to D and operates the field frequency lock. In most standard probe heads, these three frequencies are delivered via two concentric coils. The inner coil with the higher Q factor is generally used for detection, the outer one only for the application of pulses and decouphng. TWo general designs are in use in normal or forward probe heads, which are optimized for direct detection of X nuclei, the inner coil is a tuneable X coil and the outer coil is normally double tuned to and the lock frequency, while in inverse probe heads which are optimized for indirect detection of "X resonances via H, this order is reversed. 

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