Posts Tagged ‘HHG’

STSM by Zdenek Masin, The Open University (UK) with Olga Smirnova, Max Born Institute, Berlin (DE)
On October 19th, 2014 (15 days)

High Harmonic Generation from pyrazine

Irradiation of an atom or a molecule by an intense laser pulse leads to the emission of photons with frequency that is an integer multiple of the frequency of the laser field. This is the high harmonic radiation. The spectrum of the radiation depends sensitively on the sub-cycle dynamics in the ion and can be used as a sensitive probe of the electronic and nuclear dynamics. The aim of our collaboration is to construct a theoretical model for the High Harmonic Generation (HHG) for the biological molecule pyrazine.

Our theoretical description is based on the well-known three-step model of HHG. This STSM has allowed us to complete the set of data required to construct the model: transition dipole matrix elements, ionization amplitudes for the low-lying states of the pyrazine cation (carried out by Serguei Patchkovskii, MBI) and the sub-cycle laser-induced dynamics in the cation.

The transition dipole matrix elements were obtained using the UKRmol suite, the UK implementation of the molecular R-matrix method. We have benchmarked our calculations against the known photoionization cross sections for pyrazine. Our results show the strong role of correlation in the photoionization dynamics involving the deeper lying valence states: the scattering model using ~5000 configurations significantly underestimates the partial photoionization cross sections for these states. In order to obtain accurate cross sections for these states a much larger set of ~135000 configurations was needed.

Combining all the obtained data our preliminary analysis has identified several interesting channels that may play role in the process of HHG: ionization into the cationic ground state with recombination into the second excited single-hole state (B1u) and vice versa. However, the potentially most interesting dynamics involves a low-lying satellite state of B2u symmetry: ionization into the satellite state with recombination into the cationic ground state.

Finally, we have developed routines for the UKRmol suite that allow us to visualize the Dyson orbitals produced by the CDENPROP module, see Figure 1.

Further collaboration will focus on analysis of the role of the different cross channels and on generation of the harmonic yields.


Figure 1: Dyson orbitals for ground state of pyrazine cation, 1 Ag, and its two lowest-lying excited single-hole states: 1 B1g, 1 B1u. For each cationic state the label in the parentheses corresponds to the singly occupied molecular orbital in the most dominant configuration in the CAS-CI expansion of the wavefunction.

STSM by Jorge Alejandro Budagosky Marcilla, Institute for Biocomputation and Physics of Complex Systems (Zaragoza), with Esa Räsänen, Tampere University of Technology (Tampere)
On February 16th, 2014 (7 days)


Optimal control of high harmonic generation

At sufficiently high intensities, matter reacts non-linearly to light, and may re-emit at integer multiples (harmonics) of the frequency of the incoming source. The spectrum of atoms and molecules exposed to very intense laser pulses was found to present unexpectedly high harmonics, and its shape was observed to have a plateau extending over many orders of magnitude – a process known as high harmonic generation (HHG). The light emitted in this manner is coherent and may reach the extreme ultraviolet and soft X-ray frequency regime. These properties can be of paramount importance for many technological and scientific purposes.

We examine computationally the possibility of optimizing the HHG spectrum of Hydrogen atoms by shaping a laser pulse in the THz range. The spectra are computed with a fully quantum mechanical description, by explicitly computing the time-dependent dipole moment of the systems, which are modeled in one dimension. Specifically, by the optimal control theory (OCT), we studied the possibility of arbitrarily adjusting the plateau extension in harmonic spectra.

Preliminary results obtained so far show that it is possible to optimize the HHG spectrum in order to arbitrarily extend the plateau length. The length of the plateau can be controlled not only by using a frequency window (target) or by increasing the pulse intensity, but increasing the length of this. In general, we have observed the presence of characteristic structures in the pulses that can be directly associated with particular processes (ionization, recombination, etc.). The latter is still under discussion.

STSM by Zdenek Masin, The Open University (London), with Olga Smirnova, Max-Born-Institute (Berlin)
On February 2nd, 2014 (14 days)

High Harmonic Generation from biological molecules

High Harmonic molecular spectra provide insights into attosecond electronic dynamics in the target molecule. Accurate values of the dipole matrix elements between bound and continuum molecular states are required to generate the spectra. This STSM has allowed Zdeněk Mašín to learn to use the codes developed at Max Born Institute to generate the dipole matrix elements from the results of the R-matrix calculations and to apply them to calculations of photoionization cross sections.

The calculations were performed using the UKRmol suite of codes implementing the molecular R-matrix method. In order to benchmark the quality of our calculated dipole matrix elements for pyrazine we calculated the photoionization cross sections and photoelectron angular distributions (shown in the Figure). The results, performed using a simple Close-Coupling model including the lowest-lying 40 electronic states, show an encouraging agreement with previous experiments and theory. However, the description of the correlation/polarization interaction was limited in this model and therefore the shape resonances, such as the broad resonance in the 6ag cross section, appear too high in energy. This deficiency will be removed in more sophisticated models which we are developing.

This STMS has allowed us to obtain the first photoelectron angular distributions for pyrazine and to identify directions for further improvement of our calculations so that accurate High Harmonic spectra can be obtained. The next steps in the collaboration will include:

  1. Improving on the target CI description in the scattering calculations by including dynamical correlation.
  2. Calculating the population transfer in the target cation induced by the laser field.
  3. Analyzing the Dyson orbitals generated using the R-matrix codes.
  4. Generating the High Harmonic spectra.