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)
