Pump/probe phase coherent

The pump induced transient polarisation of the medium modifies the polarisation state of a time delayed probe pulse. Phenomenologically, this process can be regarded as a transient pump induced linear or circular birefringence, also called the Specular Optical Kerr Effect (SOKE) and the Specular Inverse Faraday Effect (SIFE) [18], These are cubic non-linear effects and are predicted to exist from symmetry arguments. Both effects consist of coherent and incoherent parts. For the coherent part, the pump drives the coherent electron-hole pair that affects the probe polarisation. The effect depends upon the probe phase relative to that of the electron-hole pair, and hence, that of the pump. For the incoherent part of the SIFE and the SOKE, the relative pump-probe phase is not important, since the probe pulse polarisation is modified by the pump induced sample polarisation that survives after the decoherence of the electron-hole pair. 

The protons of the hydroxy groups were deuterated by dissolving BP(OH)2 in cyclohexane and shaking the solution with deuterated water for several hours. After precipitation pump-probe measurements of BP(OD)2 in cyclohexane were recorded and are compared to BP(OH)2 in Fig. 4. Both samples were excited at 350 nm and probed at 505 nm. The delay of the emission rise of about 50 fs is equal in both cases and the coherent excitation of the vibrations is identical with respect to frequencies, phases and amplitudes. The ESIPT dynamics is obviously not altered by the deuteration and the mass of the proton has no influence on the transfer speed. This excludes that tunneling of the proton determines the speed of the transfer and the measurements provide the first proof for the passive behavior of the proton in the ESIPT. 

Unlike the case of simple diatomic molecules, the reaction coordinate in polyatomic molecules does not simply correspond to the change of a particular chemical bond. Therefore, it is not yet clear for polyatomic molecules how the observed wavepacket motion is related to the reaction coordinate. Study of such a coherent vibration in ultrafast reacting system is expected to give us a clue to reveal its significance in chemical reactions. In this study, we employed two-color pump-probe spectroscopy with ultrashort pulses in the 10-fs regime, and investigated the coherent nuclear motion of solution-phase molecules that undergo photodissociation and intramolecular proton transfer in the excited state. 

A main feature of ultrafast processes under consideration takes place in the time scale shorter than picoseconds. Thus, it is necessary to employ the laser with pulse-duration 10 fsec to study these ultrafast processes. From the uncertainty principle AE At h/2 it can be seen that using this pulse-duration, numerous vibronic states can be coherently pumped (or excited) and thus the probing signal in a pump-probe experiment will contain the information of the dynamics of both population and coherence (or phase). In other words, in order to obtain the information of ultrafast dynamics it is... 

In Eq. (4), one can see that there are two terms that depend on the space-phase factor. Eq. (5) clearly shows this situation coherence is generated between the two molecules. This coherent excitation can be called as inter-molecular coherence. This type of coherent excitation can be seen in femtosecond pump-probe non-linear coherent optical scattering spectroscopy. This chapter shall only focus on ultrafast radiationless transition as well as the dynamics of the intramolecular coherence and its relaxation. 

Measurements of electronic phase coherence and its decay are discussed in the next section. Pump-probe experiments with two incident laser pulses and... 

Figure 3. Experimental geometry for dephasing measurements by three-pulse scattering technique. Pump pulses (1 and 2) produce a grating response that diffracts probe pulse (3) into two orders. In grating experiments described in Section III (nuclear phase coherence), t = 0 and probe pulse is incident at the phase-matching angle for Bragg diffraction into only one order.

Since the development of ultrashort lasers, nudear wavepacket dynamics of various matters have attracted continuing attention [1,2]. The research targets extend from gas phase molecules [3, 4] to molecules in solution [5, 6], and solids [7]. In general, an excitation of matter by an ultrashort pulse with sufficient bandwidth leads to the creation of coherence between vibrational (or vibronic) eigenstates [1]. The induced nuclear wavepacket then starts to evolve on a certain potential energy surface and the dynamics is probed by a suitable pump-probe spectroscopy. The direct time-domain observation of the nudear motion provides us with valuable information on photochemical reaction dynamics, vibrational excitation/relaxation mechanisms, electron-vibration (phonon) coupling, and so on. 

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. 

For 900 nm radiation, the path difference between pump and probe is stepped in 0.9 /j,m increments (or integer multiples), which corresponds to At, = 3 fs. This crucial modification is called phase coherent wavepacket interferometry (Scherer, et al, 1990, 1991, 1992) and it results in a profound simplification of... 

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. 

In femtosecond experiments, as shown in Fig. 4.1, the pump-probe methods are most commonly used to study the dynamic processes in chemical compounds or materials. It should be noted that for probing, one can use the optical excitation, photoionization up-conversion, and stimulated emission [18]. From the uncertainty principle, AEAt w /2, we can see that AE depends on the pumping-pulse duration At. For short At, both population and coherence (or phase) can be created. In other words, in this case, both population and coherence dynamics have to be... 

Figure 3 Folded BOXCARS geometry applied in several transient nonlinear optical spectroscopies. In pump-probe spectroscopy, one of the three beams is blocked and the intensity of one of the incoming beams is monitored as a function of the time delay between the remaining two beams (e.g., beam 3 is blocked and beam 2 is monitored as a function of its delay with respect to beam 1, phase-matching condition would be k2 = ki — ki -I- k2>. Beams 4 and 5 are photon echo signals generated from beams 1 and 2. Beams 6 and 7 can be stimulated photon echo or transient grating signals generated from beams 1,2, and 3. In transient grating two of the beams are time coincident. In coherent anti-Stokes Raman spectroscopy, beams 1 and 3 are time coincident and carry the same frequency the difference between this frequency and that of beam 2 (so-called Stokes beam) matches a vibrational frequency of the system and beam 6 will correspond to the anti-Stokes emission.

In all these time-resolved experiments the principles of pump-probe laser spectroscopy are the key element in the experimental design. A laser pulse is optically split into two components of unequal amplitude. The intense fraction, acting as a pump piilse, is directed towards the target or sample cell to trigger the molecular event under study. The much attenuated probe pulse monitors the absorption, raman scattering, polarization, coherence, or phase shift, which is linked explicitly to the dynamical observable under investigation. Extremely precise time... 

With femtosecond pump-probe experiments "fast motion pictures" of a vibrating molecule may be obtained and the time behaviour of the wave packets of coherently excited and superimposed molecular vibrations can be mapped. This will be illustrated by the following examples dealing with the dynamics of molecular multiphoton ionisation and fragmentation of Na2, and its dependence on the phase of the vibrational wave packet in the in-... 

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