Physics
Scientific paper
Jul 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010njph...12g3019b&link_type=abstract
New Journal of Physics, Volume 12, Issue 7, pp. 073019 (2010).
Physics
1
Scientific paper
A quasi-classical model (QCM) of nuclear wavepacket generation, modification and imaging by three intense ultrafast near-infrared laser pulses has been developed. Intensities in excess of 1013 W cm-2 are studied, the laser radiation is non-resonant and pulse durations are in the few-cycle regime, hence significantly removed from the conditions typical of coherent control and femtochemistry. The 1sσ ground state of the D2 precursor is projected onto the available electronic states in D2+ (1sσg ground and 2pσu dissociative) and D++D+ (Coulomb explosion) by tunnel ionization by an ultrashort 'pump' pulse, and relative populations are found numerically. A generalized non-adiabatic treatment allows the dependence of the initial vibrational population distribution on laser intensity to be calculated. The wavepacket is approximated as a classical ensemble of particles moving on the 1sσg potential energy surface (PES), and hence follow trajectories of different amplitudes and frequencies depending on the initial vibrational state. The 'control' pulse introduces a time-dependent polarization of the molecular orbital, causing the PES to be modified according to the dynamic Stark effect and the transition dipole. The trajectories adjust in amplitude, frequency and phase-offset as work is done on or by the resulting force; comparing the perturbed and unperturbed trajectories allows the final vibrational state populations and phases to be determined. The action of the 'probe' pulse is represented by a discrete internuclear boundary, such that elements of the ensemble at a larger internuclear separation are assumed to be photodissociated. The vibrational populations predicted by the QCM are compared to recent quantum simulations (Niederhausen and Thumm 2008 Phys. Rev. A 77 013404), and a remarkable agreement has been found. The applicability of this model to femtosecond and attosecond time-scale experiments is discussed and the relation to established femtochemistry and coherent control techniques are explored.
Bryan W. A.
Calvert C. R.
Greenwood Jason B.
King Robert B.
Nemeth R. A. G. J.
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