Attosecond electronic and molecular dynamics

9/03/2006 - H. Fielding

Contents

• Introduction
• Generating a phase-locked pair of femtosecond pulses
• UV-VUV time-resolved photoelectron spectroscopy
• Monitoring organic photochemical reactions
• References

Fig. 1:Photograph of the stabilised Michelson interferometer

Introduction

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).

References

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).