Fs pump-probe signals

We have investigated the vibrational relaxation of Na3F by time-resolved photoionization at the threshold. Among the two isomers of Na3F, we have studied the excited electronic states of the C2v one. The pump-probe signal clearly shows damped oscillations, the period of which is fitted to 390 8 fs, close to twice the previously measured bending mode of Na2F,[l] while the relaxation time is 1275 50 fs. 

Fig. 2. Fit (solid line) of the pump-probe signal (empty circle symbols) using Eqs. (1-3) with a crosscorrelation signal of 180 fs FWHM (dot line).

Fig. 5. Spectrogram of periodically oscillating components of pump-probe signals of polyacetylene probed at 750 nm shown in Fig. 4 and calculated using a Gaussian window function with a HWHM At = 96 fs. S and D denote the stretching modes of single and double bonds respectively. Short-lived satellite-bands (S , S and D , D associated with S and D modes, respectively) indicate the modulation induced by the breather state.

Figure 18. Transient Na3+ signal for strongly attenuated 80-fs pump-probe laser pulses of 620 nm. The frequencies observed in the Fourier transform are due to vibrational wavepacket motion on the B state potential.

The modification of Eq. (7) allows one to use this formulation for the simulation of the fs pump-probe or pump-dump signals involving the ground and excited electronic states for the pump step and the cationic or the ground state for the probing of dynamics in the excited states. Moreover, the expression for the signals can be extended to treat, in addition to adiabatic dynamics, also nonadiabatic dynamics, which will be addressed in Section II.H.3. 

It has been shown that in the limit of ultrashort laser pulses the stimulated-emission pump-probe signal is proportional to the population probability of the initially excited diabatic state [Tf)) Eq. (59) and Refs. 7, 99 and 141. As has been emphasized in Chapter 9, the electronic population probability P2 t) represents a key quantity in the discussion of internal-conversion processes, as it directly reflects the non-Born-Oppenheimer dynamics (in the absence of vibronic coupling, P2 t) = const ). It is therefore interesting to investigate to what extent this intramolecular quantity can be measured in a realistic pump-probe experiment with finite laser pulses. It is clear from Eq. (33) that the detection of P2(t) is facilitated if a probe pulse is employed that stimulates a major part of the excited-state vibrational levels into the electronic ground state, that is, the probe laser should be tuned to the maximum of the emission band. Figure 4(a) compares the diabatic population probability P2(t) with a cut of the stimulated-emission spectrum for uj2 3.4 eV, i.e. at the center of the red-shifted emission band. Apart from the first 20 fs, where the probe laser is not resonant with the emission [cf Fig. 2(b)], the pump-probe signal is seen to capture the overall time evolution of electronic population probability. Pump-probe experiments thus have the potential to directly monitor electronic populations and thus non-Born-Oppenheimer dynamics in real time. ... 

Figure 4(a) also demonstrates that laser pulses of finite duration tend to smooth out the details of molecular time-dependent observables. To give a representative example of the dependence of the pump-probe signals on the pulse duration. Fig. 4 compares pump-probe signals obtained for pulse durations (a) ti = T2 = 20 fs, (b) t =0, T2 = 20 fs, and (c) ti = T2 = 40 fs. It is interesting to note that impulsive preparation of the molecular system with a (5-function pulse (b) results only in minor changes of the pump-probe signal. This indicates that in the present case the impulsive limit is virtually achieved by resonant 20 fs pulses, as the pulse duration is shorter than the characteristic (e.g. vibrational) time scales of the molecular system. The... 

The approximate QD pump probe signal (Fig. 3.47) still yields a good agreement with the experimental and the exact QD theoretical results. The 320 fs oscillation structure dominates in all cases. Moreover, one of the slower pseudorotational vibrations with a period of about 1 ps can also be detected by our approximate QD method. A detailed analysis by means of the Fourier transform of the corresponding autocorrelation function [378] and of the induced wave packet dynamics reveals that this oscillation is caused by a slow pseudorotational vibration in the coordinate (f. As the Fourier spectrum shows the wave packet prepared in the B state is centered around 621 nm, i.e. between the vibrational states = 5 and 6 of the (f mode. The energy distance between these two eigenstates corresponds to a vibrational period of about 1 ps. It is one of the fastest pseudorotational vibrations which can be observed in the absorption spectrum and is the next slower vibration after the Qs mode. This interpretation of the 1 ps oscillation is confirmed by analyzing the induced wave packet dynamics. Here, an accumulation of the... 

As already discussed, the approximate QD signal of a 120fs pulse exhibits two dominant vibrations which are caused by the Qs mode (310 to 320 fs) on the one hand, and by a slow pseudorot at ional mode (1 ps) on the other hand. Interestingly, the vibration along the Qs mode loses its pronounced character after 3ps (see Fig.3.51), i.e. the wave packet spreads out along this coordinate, as a movie of the three-dimensional wave packet dynamics of the B state shows [388]. This observation has already been interpreted as an effect of the anharmonicity of the PES and as intramolecular vibrational redistribution from dominantly Qs to g and (f vibration [62, 81]. The behavior described is also reflected by the approximate QD pump probe signal where the oscillation of period 310 to 320 fs caused by the Qs mode vanishes after a time of 4 to 5ps (see also Fig. 3.52). 

For the hypothetical case of a Ofs pulse a similar behavior of the approximate QD pump probe signal concerning the Qs and the slow pseu-dorotational

[Pg.120]

By Fourier transformation of the pump/probe signal, the frequency spectra can be obtained and thus the origin of the oscillation proved. For example, in a CdS/HgS-QDQW system the oscillatory modulations for low delay times originate from the coupling of the exciton, delocalized in the CdS core, with CdS LO phonons. The population lifetime of this delocalized exciton is about 400 fs. For longer delay times, the exciton localizes in the HgS well and couples weakly to the HgS LO phonons [79]. 

Fig. 1.1. TOF data for I2 as a function of time delay in a pump-probe experiment showing a variety of vibrational signals in the I+ signal. The horizontal band at 5,145 ns corresponds to metastable t] 1. The pair of bands at 5,020 and 5,270 ns corresponds to the I+ + I+ dissociation channel, while the signal in between these bands corresponds to I+ + I. Pump and probe are both 25 fs, 800 nm pulses...

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