Impulsive excitation

Fragnito H L, Bigot J-Y, Becker P C and Shank C V 1989 Evolution of the vibronic absorption spectrum in a molecule following impulsive excitation with a 6 fs optical pulse Chem. Phys. Lett. 160 101 ... 

Dexheimer S L, Wang Q, Peteanu L A, Pollard WT, Mathies R A and Shank C V 1992 Femtosecond impulsive excitation of nonstationary vibrational states in bacteriorhodopsin Chem. Phys. Lett. 188 61-6... 

The response-factor approach is based on a method in which the response factors represent the transfer functions of the wall due to unit impulse excitations. The real excitation is approximated by a superposition of such impulses (mostly of triangular shape), and the real response is determined by the superposition of the impulse responses (see Figs. 11.33 and 11.34). ... 

Here, h(t) characterize the behaviour of the system, and is called the response function, or the impulse response, because it is identical to the response to a unit impulse excitation. 

For the design of the actively compensated RF pulse, experimental and numerical determination of the response function h(t) of the circuit is necessary. We should also keep in mind that modification to the circuit, such as probe timing, insertion or removal of RF filters, and so on, can alter h(t). In practice, it is convenient to measure the response y t) to a step excitation u(t) instead of that to the impulse excitation. By performing Laplace transformation to... 

Summary. Coherent optical phonons are the lattice atoms vibrating in phase with each other over a macroscopic spatial region. With sub-10 fs laser pulses, one can impulsively excite the coherent phonons of a frequency up to 50THz, and detect them optically as a periodic modulation of electric susceptibility. The generation and relaxation processes depend critically on the coupling of the phonon mode to photoexcited electrons. Real-time observation of coherent phonons can thus offer crucial insight into the dynamic nature of the coupling, especially in extremely nonequilibrium conditions under intense photoexcitation. 

Figure 32. Vibronic periodic orbits of a coupled electronic two-state system with a single vibrational mode (Model IVa). All orbits are displayed as a function of the nuclear position x and the electronic population N, where N = Aidia (left) and N = (right), respectively. As a further illustration, the three shortest orbits have been drawn as curves in between the diabatic potentials Vi and V2 (left) as well as in between the corresponding adiabatic potentials Wi and W2 (right). The shaded Gaussians schematically indicate that orbits A and C are responsible for the short-time dynamics following impulsive excitation of V2 at (xo,po) = (3,0), while orbit B and its symmetric partner determine the short-time dynamics after excitation of Vi at (xo,po) = (3, —2.45).

Figure 6. Association reaction of Cd with benzene. Cd was formed by laser desorption/ionization from a cadmium-contaminated stainless steel surface and allowed to react with benzene at a pressure of about 5 x 10" torr. The spectrum shown is for a 6 s reaction time, after which the ions were excited by impulse excitation and detected by FTICR. The multiplets show the cadmium isotope pattern.

The realization of SPODS via PL, that is, impulsive excitation and discrete temporal phase variations, benefits from high peak intensities inherent to short laser pulses. In view of multistate excitation scenarios, this enables highly efficient population transfer to the target states (see Section 6.3.3). Furthermore, PL can be implemented on very short timescales, which is desirable in order to outperform rapid intramolecular energy redistribution or decoherence processes. On the other hand, since PL is an impulsive scenario, it is sensitive to pulse parameters such as detuning and intensity [44]. A robust realization of SPODS is achieved by the use of adiabatic techniques. The underlying physical mechanism will be discussed next. 

Fig. 2. (a) Coherent Response of the NH mode at 90K after impulsive excitation and the Fourier transform spectra (b), for probe frequencies resonant with the absorption peaks in the linear spectrum at 3295 Cm1 (grey lines) and 3195 cm 1 (black lines). Part of the NH band of crystalline ACN (c) and NMA-D6 (d). The free excitons are marked by dotted lines and the self-trapped states by black bars. Response of the sample upon selective excitation of the free exciton peak (e, f) and the self-trapped states (g, h) for delay times 400 fs (black line) and 4 ps (grey line). The arrows indicate the position of the narrow band pump pulse. 

The implementation of time-resolved CARS for microspectroscopy and its application for vibrational imaging based on RFID was first demonstrated by Volkmer et al. [64] using three incident pulses that are much shorter than the relevant material time scale. Here, a pair of temporally overlapped pump and Stokes femtosecond pulses was used to impulsively polarize the molecular vibrations in the sample. Impulsive excitation with a single ultrashort pulse is also possible provided that the spectral bandwidth of the pulse exceeds the Raman shift of the molecular vibration of interest [152]. The relaxation of the induced third-order nonlinear polarization is then probed by scattering of another pulse at a certain delay time, r. A measurement of the RFID consists of the CARS signal collected at a series of delay times. 

Figure 25a shows the response of the cyclic penta peptide introduced in Section IV.C after semi-impulsive excitation with an intense, ultrashort pump pulse, in the time domain. Pronounced beatings, originating from... 

Some polar lattice vibrations can couple strongly to electromagnetic radiation and move at light-like speeds through the host crystal. In this chapter, new methods are reviewed for impulsive excitation of these modes and for monitoring of their spatial and temporal evolution. 

The accepted room-temperature experimental values, obtained from an impulse excitation technique, are (Lalena et al., 2002) ... 

In 1997, a seminal paper of the time-resolved SHG study on a GaAs surface appeared [23], It was shown that the time-resolved SHG probes not only electronic dynamics but also lattice (phonon) dynamics. The detection scheme is as follows The pump pulse impulsively excites the longitudinal optical (LO) phonon in the GaAs... 

Stability The stability of a system is determined by its response to inputs. A stable system remains stable unless it is excited by an external source, and it should return to its original state once the perturbation is removed and the system cannot supply power to the output irrespective of the input. The system is stable if its response to the impulse excitation approaches 0 at long times or when every bounded input produces a bounded output. Mathematically this means that the function does not have any singularities that caimot be avoided. The impedance Z(s) must satisfy the following conditions Z s) is real when s is real (that is, when 0) and Re[Z(5)] > 0 when v > 0 [ = v -i- ja>, see Section... 

In Eq. (3.1), a = iMcjJl, and the other barred quantities are the usual time-dependent parameters. The formation of the coherent state (3.1) by nonresonant optical impulsive excitation [30] and the dependence of the initial values of the parameters on pulse sequence and molecular orientation are discussed in Appendix A. 

APPENDIX A DEPENDENCE OF OPTICAL IMPULSIVE EXCITATION OF PSEUDOROTATION ON MOLECULAR ORIENTATION... 

In a fluid sample one longitudinal wave is excited. The impulsively excited acoustic wave induces a temporally and spatially periodic variation in the index of refraction of the sample. A third pulse ( 1 pJ, 20 pm diameter, 80 ps duration) selected from the same Q-switched envelope as the excitation pulses is frequency doubled (A,p = 0.532pm) and delayed by a combination of time of flight and mode lock pulse selection to generate the "probe." Observation of the intensity of the Bragg scattering of the probe, by the acoustic or thermo-acoustic grating, as a function of probe delay serves to determine the frequency (v), and hence the adiabatic velocity (c = dv) of the acoustic waves. 

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