Atomic and Molecular Dynamics
Electrons form the “glue” for matter: they screen the positive charges of the nuclei and form chemical bonds that give each molecule its unique property. In a molecule, electrons and nuclei form a correlated quantum system. We are working on experiments that contribute to our understanding of the coupled dynamics among electrons and nuclei. For that, we are utilizing the process of high harmonic generation (HHG), which gives us the chance to resolve electronic structure with Angstrom scale spatial resolution and attosecond temporal resolution.
High Harmonic Generation from Multiple Orbitals in N2
K. McFarland, J. P. Farrell, P. H. Bucksbaum and M. Gühr
Up to now, the modeling and interpretation of HHG has always assumed that only the highest occupied orbital can be ionized and thus contribute to HHG. However, at least two orbitals need to be observable in order to deduce electron dynamics, because the quantum representation of motion is a coherent superposition of several orbitals. We found multi-orbital effects in HHG for nitrogen molecules and attributed the spectral signatures in the high harmonic spectrum to the highest occupied molecular orbital (HOMO) and the next lower bound orbital, the HOMO-1. We used the control over the molecular alignment with respect to the strong laser field in order to prepare the different orbital contributions. The methodology of multi-orbital HHG is currently developed to follow more complicated electronic dynamics in molecules with HHG spectroscopy.
High harmonic phase and amplitude in molecular nitrogen
B. K. McFarland, J. P. Farrell, P. H. Bucksbaum and M. Gühr
and J. P. Farrell, B. K. McFarland, M. Gühr and P. H. Bucksbaum
In the process of HHG a portion of the highest occupied orbital of an atom or molecule tunnels into the continuum and is accelerated in a strong oscillating optical field. This continuum part of the wave function is treated as a free electron wave packet which interferes coherently with the bound part of the highest occupied orbital when it returns to the molecule, thus creating a time varying dipole that radiates the harmonic light. Depending on the orbital symmetry, characteristic maxima or minima appear in the harmonic spectrum due to interference effects, whereas the minimum is accompanied by a phase jump. We measured this phase jump for molecular nitrogen by interfering the N2 harmonics with harmonic light created from atomic argon.
Calibration of a High Harmonic Spectrometer by Laser Induced Plasma Emission
J. P. Farrell, B. K. McFarland, P. H. Bucksbaum and M. Gühr
The calibration of VUV spectrometer for HHG spectroscopy is usually performed with metal filter edges cutting away part of the harmonic spectrum or directly by counting harmonics assuming that they are integer multiples of the fundamental quantum energy. A fundamental problem in using harmonics for calibration purposes lies in the extreme sensitivity of the harmonic wavelength to the fundamental pulse parameters and to phase matching. The fundamental laser chirp, duration and the HHG phase matching influence the harmonic wavelength; shifts in wavelength of half an odd harmonic are not unusual. We implemented a simple calibration method using plasma emission lines in an HHG spectroscopy set-up. We show that lines emitted by a laser-generated plasma of rare gases and molecular nitrogen can be conveniently used to calibrate harmonic spectrometers. The plasma generation and HHG use the same laser. The intensity conditions for plasma generation on one hand and HHG emission on the other hand are different and in a HHG set-up with optimised phase matching, efficient plasma emission is hindered.
The figure shows the vacuum ultraviolet signal as a function of gas jet position along the laser propagation direction for Xe as a HHG medium. The z=0 corresponds to the laser focus position. Before and after the focus, a spectrally regular harmonic pattern appears, corresponding to harmonics. At the focus, the harmonics are suppressed and plasma line emission dominates. The documented plasma lines are used to calibrate the spectrometer wavelength.
Strongly Dispersive Transient Bragg Grating for High Harmonics
J. P. Farrell, L. S. Spector, M. B. Gaarde, B. K. McFarland, P. H. Bucksbaum and M. Gühr Optics Letters, 35, 2028 (2010) link to article
We are currently working on implementing this high harmonic spectroscopy (HHS) technique to study transient processes on photoexcited electronic states that play a crucial role in photochemistry. However, such processes are weak-field phenomena, which limits the overall excited state population and the HHS sensitivity to the excitation.
Transient gratings are generally applied in the IR to UV range to overcome the reduced sensitivity problem. Two excitation pulses with wave vectors k1 and k2, intersecting under an angle, create an excitation grating in the sample while a third (probe) pulse kp diffracted from the grating. The diffracted signal has high sensitivity to excited state dynamics. We implemented the grating scheme for HHG where the harmonics kHH are dispersed in angle within a diffraction order to enable the spectroscopic analysis without the use of an additional spectrometer. This is achieved by enlarging the angle between k1 and k2 to 180 deg., resulting in a shorter grating period d. This disperses the harmonics to distinguishable angles without an additional grating element. We construct a Bragg grating that is highly selective in its wavelength acceptance resulting in dispersed and distinguishable harmonics.