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Attosecond spectroscopic studies of atomic and molecular dynamics

  • Author(s): Beck, Annelise
  • Advisor(s): Leone, Stephen R
  • Neumark, Daniel M
  • et al.
Abstract

Isolated attosecond pulses with photon energies in the extreme ultraviolet have provided a new capability to study few-femtosecond and sub-femtosecond dynamics in atomic and molecular systems. A novel regime of attosecond transient absorption measurements, in which the near-infrared pulse follows the attosecond pulse, is reviewed. The observed timescales of the decay of an absorption feature and oscillations due to quantum beating are derived and experimental considerations for these measurements are discussed. Attosecond transient absorption spectroscopy is then applied to observe quantum beating between the 2s22p5(2P3/2)3d and 2s22p5(2P1/2)3d electronically excited states of neon. The quantum beating is observed more prominently in one of the absorption features, and theoretical models of the effect of the near-infrared pulse are proposed to explain this asymmetry and characterize the observed beating. In the model that most closely agrees with the measurement, the beating is detected via Rabi cycling induced by the near-infrared pulse through an intermediate state (likely one of the 3p states), and this mechanism is shown to be strongly dependent on the spectrum of the near-infrared pulse.

The isolated attosecond pulses used in these experiments are characterized using the photoelectron streaking method and the duration of these pulses is measured by iteratively reconstructing the streaking spectrograms. The effect on the streaking spectrogram of various pulse characteristics, such as duration, chirp, and the presence of satellite pulses, is described. Streaking spectrograms using low photon energy attosecond pulses are typically asymmetric. The reasons for this asymmetry are discussed and the challenges of reconstructing these low energy pulses are addressed. Finally, attosecond pulses are used to investigate superexcited states, or doubly excited states above the ionization potential, of the nitrogen molecule (N2). These states can decay via two competing channels, autoionization and predissociation. Time-of-flight mass spectrometry is applied to measure lifetimes of these states. Preliminary results are presented and improvements to the experimental design are proposed.

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