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H2MOL

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Multiphoton Processes

Laser-driven dissociative ionization of H2

The redistribution of energy between electrons and nuclei in the heating of molecular chains and clusters by high-frequency light can be best studied using ultrashort (femtosecond) intense laser pulses which can resolve processes on molecular vibration timescales. The hydrogen molecule H2 and its isotopes (HD, D2) have become the focus of experiments [1, 2] using ultrashort (femtosecond) intense laser pulses and represents a system which has the potential to be simulated by theory to a high level of precision. Further development of the Consortium fixed-nuclei code, H2MOL [3], to include nuclear vibration will provide the first opportunity world-wide to handle the fundamental system of laser-driven vibrating H2 and its isotopes properly.

QUB has made a world-leading contribution using grid and DVR methods [4] to study the 5+1 dimensional PDE controlling electronic dynamics in the diatoms H2 and D2 for ultrashort intense X-ray radiation [4] with nuclei fixed. Extending the highly-scalable code, H2MOL, to optical wavelengths (>248 nm) requires memory and processing power exceeding the resources available on green and thus demands a growing HPCx service. It is known, from simulations performed on green on the molecular ions H2+, that redistribution of energy from electronic to nuclear motion strongly modifies both the electron spectra and high-order harmonic generation yields. Simulation of femtosecond pulses in molecules involves coupling the nuclear dynamics to H2MOL. Ultrashort laser pulses are characterized by irregular pulse shapes and bandwidths which necessitate a time-dependent theory. Classical nuclear motion is feasible using the present resources of green, but it is known from our simulations at CSAR that intense laser pulses create fragmented and distorted nuclear wavepackets and quantal dynamics are required. For ultrashort (femtosecond) high-frequency (X-ray) pulses the rotational dynamics are frozen but vibrational relaxation occurs over these timescales and is isotope dependent. The dimensionality of the problem with nuclear dynamics requires wavepacket storage of the order 0.1-0.5 Terabytes. The new code (DYMOL) including quantal nuclear vibration can be propagated by successive operations of the Hamiltonian [5] requiring large-scale processing power that can be harnessed efficiently through parallel processors. A quantal treatment of dissociative multiple ionization at optical wavelengths, with intensities exceeding 1015 W/cm2 over picosecond timescales would stretch the power of even a growing HPCx service. Our aim is to produce the first reliable estimates of the energy and angular distributions of the fragments (electrons, nuclei and photons) resulting from dissociative ionization, and to characterize the process in terms of laser intensity, laser wavelength, pulse shape and bandwidth in the ultraviolet and X-ray region.


References

1
Posthumus J H et al, J Phys B, 33 L563 (2000); Williams I et al, J Phys B, 23 2743 (2000); Sandig et al, Phys Rev Lett, 85 4876 (2000); Walsh T D G et al, J Phys B, 30 2167 (1997); Gibson G N et al, Phys Rev Lett, 79 2022 (1997)

2
Dörner et al, Phys Rev Lett, 81 5776 (1998)

3
Dundas D, Meharg K, McCann J F and Taylor K T to be published

4
Dundas D et al, J Phys B 33 3261 (2000); Dundas D et al preprint (2001)

5
Smyth E S, Parker J S and Taylor K T Comput Phys Commun 114 1 (1998)

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