 FIG. 1: Main
spectral features of
the x-ray absorption spectra of the NO molecule in the strong IR
field. The pump infrared laser prepare a virtual vibrational
state which is probe by a x-ray field. The spectral shape can be
modified for short pulses of the probe signal, which selects the wave
packet in a certain region of the PES. The two picks in the spectra
correspond to the
turning points of the WP in the electronic ground state, where the
velocity of the WP vanishes and changes sign.
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For long x-ray pulse the final spectrum (d in Fig. 1) is phase and time delay time independents. In this case, the spectrum presents two picks that correspond to the turning points of the WP. For short x-ray pulses the wave packet is ``snapshoted'' in a certain region during its dynamics in the PES. Therefore here the spectrum is time delay and phase dependent (spectra b and c in Fig. 1). And, the control of the time delay, for short pulses, permits increase or decrease distinctly the two different picks. But, this is only viable for x-ray time durations t<>x<2p/w and it is valid for pump excitations bellow the dissociative limit. This prediction breaks down for excitations above de dissociation, since the WP packet in this case do not propagate vackwards and forwards in the PES.
Another feature concerning with the short and long pulses is the spectral resolution. In one hand for short x-ray pulses spectra b and c the vibrational resolution vanishes due the uncertainty relation between time and energy (Fig. 1 spectra b and c). In another hand for long x-ray pulses the vibrational profile start to appear (Fig. 1 spectrum d).