Posts Tagged ‘control’
The motion of the two electrons in the helium atom can be imaged and controlled with attosecond-timed laser flashesSource: MAX-PLANCK-GESELLSCHAFT, MÜNCHEN Original publication:
STSM by Aurelie Chenel, Université Paris-Sud, Orsay (France), with Octavio Roncero Villa, Consejo Superior de Investigaciones Científicas (CSIC), Madrid (ES)
On February 24th, 2014 (26 days)
From FRANCE to SPAIN
Control of the infrared photodissociation of LiHF
We numerically study the competitive dissociation of the Van der Waals complex LiHF into the LiF + H or the Li + HF products. The potential energy surface of the LiHF system has been calculated by Roncero and al. (JCP 107,23 (1997)). Our goal aims at controlling the dissociation of LiHF toward the formation of the Li + HF products, that are not the products predominantly formed. The dynamics will be studied in hyperspherical coordinates. For the control we will use the Local Control Theory (LCT) strategy with a time-dependent approach based on Møller-operators.
The program we want to use for the control is written in hyperspherical coordinates. Furthermore, to be feasible in terms of calculation costs, we achieve it in a reduced two-dimensional model, so that in fact we use polar coordinates. To get such polar coordinates, it is usual to start with the Jacobi coordinates (rjac , Rjac ), as shown in the ﬁgure, where the angle γ is set at a value of 73° .
We ﬁrst started with these coordinates. However, studying the energy potential curves we got, we then saw that they were not appropriate to describe our problem in two dimensions : we cannot have a good description of both rearrangement channels, leading to the LiF + H or the Li + HF products. To describe well both dissociation products, we are going to use bond coordinates, where r = rHF and R = rLiF . However, with these coordinates, we have to make some assumptions to be able to use our program for the control: in particular, we will neglect the cross-terms of the kinetic energy. These assumptions may be too strong, so that we will still have to think if these coordinates are suitable for the control.
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)
From SPAIN to FINLAND
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.
The first meeting of the Working Group 3 will take place in Birmingham Apr. 14th – 16th 2014.
The Working group focuses on the control of chemical reactivity using laser light. There will be 5 sessions covering control strategies, strong field control, measuring the evolving system, control in the condensed phase and applications. Contributions from young scientists are encouraged: as a talk or as a poster.
Registration will be open Feb. 15th.
For further details see www.stchem.bham.ac.uk/~worthgrp/xlic_wg3_2014
STSM by Cristina Sanz-Sanz, Department of Physical Chemistry, Universidad Autónoma de Madrid with Graham Worth, School of Chemistry, University of Birmingham
On September 23rd, 2013 (10 days)
From SPAIN to UNITED KINGDOM
Fitting of field dependent potential energy curves, spin-orbit and elements of the dipole moment matrix of the first 36 states of IBr
The control of the photodissociation of IBr through a curve crossing was studied in the group of Prof. Stolow using the dynamic Stark effect. Until now, theoretical simulations have used a reduced model including just 3 electronic states. However, the system consists on 36 electronic states dissociating to the ground states of atoms. We have computed and fitted the 36 potential energy curves, spin-orbit and elements of the dipole matrix for several electric field strengths and orientations. Those curves will be used in the dynamical calculations to reproduce the experiment.
The electronic structure calculation programs do not maintain the phase of the wavefunction and it translates into jumps in the spin-orbit and transition dipole moment curves. In order to use these curves in dynamical calculation programs the curves have to be fitted. Because of the sudden changes in the curves normal fitting methods do not work. We have used an optimization method to smooth out the spin-orbit and transition dipoles.
During the STSM visit we finished with the fittings of the potential energy curves, spin-orbit couplings and the elements of the dipole moment matrix for several electric field strengths and orientations. The dynamical calculations will be done using MCTDH package and we created the input files and fittings required for the wavepacket calculations. A test calculation was done for a free field example using the 36 spin-orbit states. In addition, we wrote the outline of the first publication of a series of works that will include the global fittings of potential energy curves, spin-orbit couplins and dipole moment matrix.