Short adiabatic pulses

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. 

Figure 2. Temperature-time curves for adiabatic type calorimeters (with a low time constant). Curves A and C show curves following the release of a short heat pulse in an ideal adiabatic calorimeter and a semiadiabatic calorimeter, respectively. Curves B and D show the curves from experiments where a constant thermal power was released between t, and t2 for an ideal adiabatic calorimeter and a semiadiabatic calorimeter, respectively. For the ideal adiabatic instrument the slope of the curve during the heating period is proportional to the thermal power, P.

H.P. Breuer, M. Holthaus, Adiabatic control (f molecular excitation and tunneling by short laser pulses, J. Phys. Chem. 97 (1993) 12634. 

The adiabatic potential diagram of Nal (Fig. 6.99) is characterized by an avoided crossing between the repulsive potential of the two interacting neutral atoms Na -I-1 and the Coulomb potential of the ions Na" " + which is mainly responsible for the strong binding of Nal at small intemuclear distances R. If Nal is excited into the repulsive state by a short laser pulse at the wavelength A. i, the excited molecules start to move toward larger values of R with a velocity v R) = [(2/ix)(E - E R)V. ... 

Let us briefly discuss the characteristics of the nonadiabatic dynamics exhibited by this model. Assuming an initial preparation of the S2 state by an ideally short laser pulse. Fig. 1 displays in thick lines the first 500 fs of the quantum-mechanical time evolution of the system. The population probability of the diabatic state shown in panel (b) exhibits an initial decay on a timescale of w 20 fs, followed by quasi-periodic recurrences of the population, which are damped on a timescale of a few hundred femtoseconds. Beyond 500 fs (not shown) the S2 population probability becomes quasi-stationary, fluctuating statistically around its asymptotic value of 0.3. The time-dependent population of the adiabatic S2 state, displayed in panel (a), is seen to decay even faster than the diabatic population — essentially within a single vibrational period — and to attain an asymptotic value of 0.05. The finite asymptotic value of is a consequence of the restricted phase space of the three-mode model. The population Pf is expected to decay to zero for systems with many degrees of freedom. 

In contrast to the traditional adiabatic method of measuring Cp (a short heat pulse applied to an isolated sample), experiments in the frequency domain are possible if we apply a ty-periodic energy power and measure the consequent T oscillations at that frequency. The experimental time must be longer than the sample thermal-diffusion time td = Cpd (k (with k the thermal diffusivity and d the distance) and shorter than the time it takes the sample to decay at the surrounding heat bath temperature. Then td l/a> tj. 

Next, consider the effect of pumping laser. With a short pulse pumping laser, both population excitation and coherence excitation can be created and the non-adiabatic processes like PIET can take place afterwards. With a similar derivation as that shown in this section one obtains the coherence created by the pumping laser with electric field Epu and frequency topu... 

In this sense, the control of electronic transitions of wavepackets using short quadratically chirped laser pulses of moderate intensity is a very promising method, for two reasons. First, only information about the local properties of the potential energy surface and the dipole moment is required to calculate the laser pulse parameters. Second, this method has been demonstrated to be quite stable against variations in pulse parameters and wavepacket broadening. However, controlling of some types of excitation processes, such as bond-selective photodissociation and chemical reaction, requires the control of wavepacket motion on adiabatic potential surfaces before and/or after the localized wavepacket is made to jump between the two adiabatic potential energy surfaces. 

Time evolution of the ground state hole as well as fluorescence spectra initiated by a short pulse laser irradiation in solution has been conventionally explained in terms of the two-dimensional configuration coordinate model by Kinoshita . According to his theory, two adiabatic potential curves corresponding to the ground and excited states are assumed to have the same curvature but have the different potential minimum in the configuration coordinate. 

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