Pump/probe phase locked

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. 

Phase-Locked Interferometric Pump-Probe Transient Signal from Iodine Vapor... 

For this reason, a perspective alternative may be a photonic crystal based on opal-vanadium dioxide (VO2) composite, with a semiconductor- metal phase transition in VO2. Experiments on pump-probe measurements of the VO2 phase transition, which use pulses from mode-locked lasers, have shown that this transition occurs for a period of several hundreds of femtoseconds [6]. 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

A Varian Unity Inova 600-MHz NMR instrument (Palo Alto, CA) equipped with a H C/ N pulse field gradient triple resonance microliow NMR probe (flow cell 60pL 3mm O.D.) was used. Reversed-phase HPLC of the samples was carried out on a Varian modular HPLC system (a 9012 pump and a 9065 photodiode array UV detector). The Varian HPLC software was also equipped with the capability for programmable stop-flow experiments based on UV peak detection. An LCQ classic MS instrument, mentioned in the previous section, was connected on-line to the HPLC-UV system of the LC-NMR by contact closure. The H resonance of the D2O was used for field-frequency lock, and the spectra were centered on the ACN methyl resonance. Suppression of resonances from HOD and methyl of ACN and its two C satellites was accomplished using a train of four selective WET pulses, each followed by a Bo gradient pulse and a composite 90-degree read pulse [41]. 

When these wavelengths superimpose in phase they are said to be mode-locked and behave as a wave packet or pulse. The pump and probe pulses are focused into a chamber containing gaseous molecules for study. The pump pulse excites a change (e.g., chemical reaction) while absorption of the probe pulse monitors the course of structural change as time passes on the fs scale. Zewail has likened the process to the effects of a strobe light that furnishes stop-action pictures of a fast process. 

If the probe laser-induced fluorescence Im(X2) is monitored through a lock-in amplifier at the frequency /i, one obtains negative OODR signals for all transitions 1) -> m) and positive signals for the transitions 2) m) (Fig. 5.17). From the phase of the lock-in signal it is therefore, in principle, possible to decide which of the two possible types of probe transitions is detected. Since this double-resonance technique selectively detects transitions that start from or terminate at levels labeled by the pump lever, it is often called labeling spectroscopy. 

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