Time-delay pulse

The nonresonant background in CARS spectroscopy originates from instantaneous four-mixing processes, while the resonant contribution involves real vibrational states. This provides a basis for possible discrimination against the nonresonant background. To do so, one has to come up with a pair of pulses that excite the vibrational state, and the third, time-delayed pulse will only contribute to the resonant part of the CARS signal. However, to make this scheme work efficiently, one has to overcome certain obstacles. To achieve high spectral resolution, the bandwidth of the third pulse should... 

These decay measurements on the state excited can be repeated by a TRSEP technique (Hineman et al. 1994) to verify the IVR cluster kinetics. This has been done for the aniline(N2)1 P- vibronic excitation. The experiment involves excitation of the P- state, followed by stimulated emission with a time delayed pulse to deplete the P" population. The total emission from the excited Sj cluster and bare molecule as a function of time delay between the excitation and dump... 

In the basic experiment, a first optical pulse (pump) is absorbed by the sample. A second, time-delayed pulse of weaker intensity is used to probe the change in optical response. Commonly, the test pulse probes the change in transmission, given in the small signal limit as... 

Here c.c. refers to the complex conjugate and ej (t) and Uj are the temporal envelope function and unit vector of the polarization of the jth electric field. The frequencies of the five fields are assumed to be identical, i.e., ojj = co0 for j = 1, 2, 3, 4, and 5. This can be experimentally achieved by using time-delayed pulses generated from a common laser oscillator. Although the femtosecond pulses generated in the laboratory have a finite width, for the sake of simplicity the laser pulses are assumed to be impulsive in this section, i.e. ... 

This chapter reviews novel optical responses such as SRRS and SF from low-dimensional crystals of TPCOs. First, we present the general molecular characteristics of TPCOs and the preparation of their low-dimensional crystals. Then, ASE and SRRS phenomena are shown for solution-grown platelets and epitaxially grown needle-like crystals. Finally, our recent studies on time-delayed pulse emission from two-dimensional TPCO crystals are presented. 

Fig. 1.1. Principles of the real-time multiphoton ionization (MPI) (a) and NeNePo (b) spectroscopic technique, (a) Principle of time-resolved MPI spectroscopy. A wave packet is prepared in an excited state of the neutral system by a pump pulse. Since in general the transition probability to the ion state is a function of the wave packet s location on the potential-energy surface, the propagation of the wave packet can be probed by a second, time-delayed pulse, (b) Principle of the time-resolved NeNePo process. Starting in the anion s potential-energy surface, an ultrashort pump pulse detaches an electron cuid prepares a wave packet in the neutrcd. After a certain delay time At a probe pulse photoionizes the neutral. The time-dependent signal of the cation s intensity is detected. For convenience, this method is called NeNePo , Negative-to-Neutrcd-to-Positive...

A useful and common way of describing the reorientation dynamics of molecules in the condensed phase is to use single molecule reorientation correlation functions. These will be described later when we discuss solute molecular reorientational dynamics. Indirect experimental probes of the reorientation dynamics of molecules in neat bulk liquids include techniques such as IR, Raman, and NMR spectroscopy. More direct probes involve a variety of time-resolved methods such as dielectric relaxation, time-resolved absorption and emission spectroscopy, and the optical Kerr effect. The basic idea of time-resolved spectroscopic techniques is that a short polarized laser pulse removes a subset of molecular orientations from the equifibrium orientational distribution. The relaxation of the perturbed distribution is monitored by the absorption of a second time-delayed pulse or by the time-dependent change in the fluorescence depolarization. 

We will explore the effect of three parameters 2 -and < )> that is, the time delay between the pulses, the tuning or detuning of the carrier frequency from resonance with an excited-state vibrational transition and the relative phase of the two pulses. We follow closely the development of [22]. Using equation (Al.6.73). 

Figure Al.6,8 shows the experimental results of Scherer et al of excitation of I2 using pairs of phase locked pulses. By the use of heterodyne detection, those authors were able to measure just the mterference contribution to the total excited-state fluorescence (i.e. the difference in excited-state population from the two units of population which would be prepared if there were no interference). The basic qualitative dependence on time delay and phase is the same as that predicted by the hannonic model significant interference is observed only at multiples of the excited-state vibrational frequency, and the relative phase of the two pulses detennines whether that interference is constructive or destructive.

Figure Al.6.8. Wavepacket interferometry. The interference contribution to the exeited-state fluoreseenee of I2 as a fiinotion of the time delay between a pair of ultrashort pulses. The interferenee eontribution is isolated by heterodyne deteetion. Note that the stnieture in the interferogram oeeurs only at multiples of 300 fs, the exeited-state vibrational period of f. it is only at these times that the wavepaeket promoted by the first pulse is baek in the Franek-Condon region. For a phase shift of 0 between the pulses the returning wavepaeket and the newly promoted wavepaeket are in phase, leading to eonstnietive interferenee (upper traee), while for a phase shift of n the two wavepaekets are out of phase, and interfere destnietively (lower traee). Reprinted from Seherer N F et 0/1991 J. Chem. Phys. 95 1487.

Second-order effects include experiments designed to clock chemical reactions, pioneered by Zewail and coworkers [25]. The experiments are shown schematically in figure Al.6.10. An initial 100-150 fs pulse moves population from the bound ground state to the dissociative first excited state in ICN. A second pulse, time delayed from the first then moves population from the first excited state to the second excited state, which is also dissociative. By noting the frequency of light absorbed from tlie second pulse, Zewail can estimate the distance between the two excited-state surfaces and thus infer the motion of the initially prepared wavepacket on the first excited state (figure Al.6.10 ). 

CN] —> I + CN. Wavepacket moves and spreads in time, with its centre evolving about 5 A in 200 fs. Wavepacket dynamics refers to motion on the intennediate potential energy surface B. Reprinted from Williams S O and lime D G 1988 J. Phys. Chem.. 92 6648. (c) Calculated FTS signal (total fluorescence from state C) as a fiinction of the time delay between the first excitation pulse (A B) and the second excitation pulse (B -> C). Reprinted from Williams S O and Imre D G, as above. 

Figure Al.6.22 (a) Sequence of pulses in the canonical echo experiment, (b) Polarization versus time for the pulse sequence in (a), showing an echo at a time delay equal to the delay between the excitation pulses.

Figure Al.6.25. Modulus squared of tire rephasing, (a), and non-rephasing, R., (b), response fiinetions versus final time ifor a near-eritieally overdamped Brownian oseillator model M(i). The time delay between the seeond and third pulse, T, is varied as follows (a) from top to bottom, J= 0, 20, 40, 60, 80, 100,...

Figure Al.6.28. Magnitude of the excited-state wavefimction for a pulse sequence of two Gaussians with time delay of 610 a.u. = 15 fs. (a) (= 200 a.u., (b) ( = 400 a.u., (c) (= 600 a.u. Note the close correspondence with the results obtained for the classical trajectory (figure Al. 6.27(a) and (b)). Magnitude of the ground-state wavefimction for the same pulse sequence, at (a) (= 0, (b) (= 800 a.u., (c) (= 1000 a.u. Note the close correspondence with the classical trajectory of figure Al.6.27(c)). Although some of the amplitude remains in the bound region, that which does exit does so exclusively from chaimel 1 (reprinted from [52]).

Figure Al.6.30. (a) Two pulse sequence used in the Tannor-Rice pump-dump scheme, (b) The Husuni time-frequency distribution corresponding to the two pump sequence in (a), constmcted by taking the overlap of the pulse sequence with a two-parameter family of Gaussians, characterized by different centres in time and carrier frequency, and plotting the overlap as a fiinction of these two parameters. Note that the Husimi distribution allows one to visualize both the time delay and the frequency offset of pump and dump simultaneously (after [52a]).

Figure Al.6.31. Multiple pathway interference interpretation of pump-dump control. Since each of the pair of pulses contains many frequency components, there are an infinite number of combination frequencies which lead to the same fmal energy state, which generally interfere. The time delay between the pump and...

FigureBl.5.16 Rotational relaxation of Coumarin 314 molecules at the air/water interface. The change in the SFI signal is recorded as a fimction of the time delay between the pump and probe pulses. Anisotropy in the orientational distribution is created by linearly polarized pump radiation in two orthogonal directions in the surface. (After [90].)...

Time delay ber / een laser puJse arid RF-pulse. ms... 

Time delay betzcecn laser poise a.nd RF-pulse, us... 

The intensity autocorrelation measurement is comparable to all of the spectroscopic experunents discussed in the sections that follow because it exploits the use of a variably delayed, gating pulse in the measurement. In the autocorrelation experiment, the gating pulse is just a replica of the time-fixed pulse. In the spectroscopic experiments, the gating pulse is used to mterrogate the populations and coherences established by the time-fixed pulse. 

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