and molecular dynamics
Fig. 1:Photograph of the stabilised
The interest of our group at UCL is in exploiting the attosecond
light source technology to investigate extremely rapid electron dynamics
in Rydberg systems and to develop a new detection technique for investigating
organic photochemical reactions on the timescale of a few tens of
femtoseconds.The electronic motion in atoms and molecules occurs on
a very fast timescale: the Bohr orbit of the ground state electron
in the hydrogen atom corresponds to 150 attoseconds. If the electron
is in a Rydberg state, the motion is slower, on the order of femtoseconds
to picoseconds, and can be followed by a (sub-) femtosecond laser
pulse. The aim of our group within the attosecond project is to follow
electron dynamics from electrons that have been excited with very
energetic photons. These photons result from high-order harmonic generation
with the few-cycle driving femtosecond laser at Imperial College.
Generating a phase-locked pair of femtosecond pulses
We have designed and built a stabilised Michelson interferometer
(Figure 1) for generating phase-locked pairs of few-cycle femtosecond
laser pulses. The stabilisation is achieved by monitoring interference
fringes of a HeNe laser and applying feedback to a mirror in one of
the interferometer arms. This interferometer will be employed to generate
Ramsey interference fringes in doubly excited Rydberg states of noble
gas atoms. High-order harmonic generation from the phase-locked pair
of femtosecond pulses results in a pair of sub-100 nm pulses.
Fig. 2:Schematic of UV-VUV time-resolved
photoelectron spectroscopy to detect Rydberg wave packets. The photoelectron
spectrum will provide a fingerprint of the position of the electron
on its orbit. When the electron is close to the core, the attosecond
VUV photon ionises a neutral atom (green) but when the electron is
at the outer turning point, the attosecond VUV photon ionises a singly
positively charged ion (red).
UV-VUV time-resolved photoelectron spectroscopy
We are developing a new detection technique for probing the dynamics
of low n Rydberg states based on UV-VUV time-resolved photoelectron
spectroscopy (Figure 2). An electron is excited to a Rydberg state
by a UV photon, and a VUV photon is applied with a delay. Depending
on the location of the Rydberg electron at the time of ionisation,
this can result in ionisation of the Rydberg electron and/or another
electron and a singly or doubly charged ion. The process can be retrieved
from the kinetic energy of the photoelectrons.
Monitoring organic photochemical reactions
The dynamics of highly excited states of small organic molecules
in the 200 - 400 nm range (e.g. benzene S2) frequently occur on the
timescale of a few tens of femtoseconds. We plan to investigate such
ultrafast dynamics using a few femtosecond UV pump pulse and synchronised
VUV probe pulse. These investigations complement investigations of
the control of photochemical dynamics in organic molecules being carried
out at UCL in collaboration with Robb/Bearpark (IC Chemistry) and
Worth (Birmingham Chemistry).
Time-resolved inner-electron ionisation in krypton using XUV attosecond
pulses, E. Heesel, C. Glendinning, H.H. Fielding, S. Gundry, C.
Haworth , J. Robinson, J.P. Marangos, R.A. Smith, J.W.G. Tisch, J.
Steele-Davies, M.J. Stankiewicz, and L.J. Frasinski,XTRA Summer
School, Porquerolles, France (May 2005).
Monitoring highly excited electron dynamics in the noble gases
with high-order harmonic pulses, E. Heesel, C. Glendinning, H.H.
Fielding, S. Gundry, C. Haworth , J. Robinson, J.P. Marangos, R.A.
Smith, J.W.G. Tisch, J. Steele-Davies, M.J. Stankiewicz, and L.J.
Frasinski,ICOLS 2005, Aviemore, U.K. (June 2005).