Laser-driven H-Bond Breaking

An approach to achieving H-bond fragmentation in a selective manner is shown in Fig. 4.8(b). It consists of using a few-cycle IR pulse followed by a UV laser 

A laser field (cf Eq. (4.3)) consisting of one or several half cycle pulses takes the form 

11 shows the anion and neutral V PES of FHF. Moreover, we replace the series of three half-cycle pulses by a smooth, switch-on-switch-off sin -shaped laser field of the type given in Eq. (4.3) and shown in Fig. 4.12(b). The frequency is chosen, as in the ID model, to equal the H-bond oscillations, which in the 2D case correspond to longer vibrational periods of ca. 1500 cm , therefore f iR =1516 cm i (0.188 eV). In Fig. 4.12(b) it can be seen how encompasses the maximum amplitude of each half cyde. In passing we note that with increasing displacement of the wave packet its dispersion increases as well. Thus the optimum time delay between the IR and the UV pulse calls for a compromise between maximum displacement and minimum dispersion. As shown in Fig. 4.11(b), at t = 41.5 fs the wave packet remains relatively compact in the anion surface Va and the mean value of has reached 0.207 A. When the IR pulse achieves maximum displacement, an ultrashort (r = 5 fs) resonant ( uv = 43548 cm = 

5 eV) sin -shaped UV pulse is applied to photodetadi an electron, preparing the system on the PES of the neutral spedes. The maximum intensity needed for this selective preparation of the wave packet is ca. I = 3.3 TW cm. For a laser pulse of duration r = 5 fs, mildly focused to a diameter of s = 1 mm, this corresponds to a pulse energy of approximately 500 pj, which can be provided by state of the art laser systems. This few-cyde UV pulse excites the displaced wave packet to a downhill domain of Vjj where it evolves predominantly along one dissociation 

In these wave packet simulations, the molecular axis of the FHF system is assumed to be aligned along the space-fixed axis Z electric field vector. This assumption involves a maximum interaction of the IR and UV laser pulses with the system. Recalling that the time-dependent interaction potential is given by the scalar product of the electric field vector and the dipole vector, i.e. (t) /j, cos 9, it is clear that for field polarizations perpendicular to the molecular axis [9 = 90°) the interaction of the IR laser pulse with the anion vanishes, and for any molecular orientation different from 0= 0° or 180° the interaction is less efficient. Consider now an ensemble of randomly oriented FHF molecules, as in Fig. 4.13(c). Since the UV pulse is tuned to match the energy gap between anion and neutral 

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