Coherent control of the vibrational state population in a nonpolar molecule (original) (raw)
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Physical Review A, 2011
We theoretically study the control of two-photon excitation to bound and dissociative states in a molecule induced by trains of laser pulses, which are equivalent to certain sets of spectral phase modulated pulses. To this end, we solve the time-dependent Schrödinger equation for the interaction of molecular model systems with an external intense laser field. Our numerical results for the temporal evolution of the population in the excited states show that, in the case of an excited dissociative state, control schemes, previously validated for the atomic case, fail due to the coupling of electronic and nuclear motion. In contrast, for excitation to bound states the two-photon excitation probability is controlled via the time delay and the carrier-envelope phase difference between two consecutive pulses in the train.
Shaping coherent excitation of atoms and molecules by a train of ultrashort laser pulses
Physical Review A, 2010
We propose a mechanism to produce a superposition of atomic and molecular states by a train of ultrashort laser pulses combined with weak control fields. By adjusting the repetition rate of the pump pulses and the intensity of the coupling laser, one can suppress a transition, while simultaneously enhancing the desired transitions. As an example various superpositions of states of the K2 molecule are shown.
Visualizing and controlling vibrational wave packets of single molecules
Nature, 2010
The active steering of the pathways taken by chemical reactions and the optimization of energy conversion processes 1-3 provide striking examples of the coherent control of quantum interference through the use of shaped laser pulses. Experimentally, coherence is usually established by synchronizing a subset of molecules in an ensemble 4-7 with ultra-short laser pulses 8 . But in complex systems where even chemically identical molecules exist with different conformations and in diverse environments, the synchronized subset will have an intrinsic inhomogeneity that limits the degree of coherent control that can be achieved. A natural-and, indeed, the ultimate-solution to overcoming intrinsic inhomogeneities is the investigation of the behaviour of one molecule at a time. The single-molecule approach 9,10 has provided useful insights into phenomena as diverse as biomolecular interactions 11-13 , cellular processes 14 and the dynamics of supercooled liquids 15 and conjugated polymers 16 . Coherent state preparation of single molecules has so far been restricted to cryogenic conditions 17 , whereas at room temperature only incoherent vibrational relaxation pathways have been probed 18 . Here we report the observation and manipulation of vibrational wave-packet interference in individual molecules at ambient conditions. We show that adapting the time and phase distribution of the optical excitation field to the dynamics of each molecule results in a high degree of control, and expect that the approach can be extended to achieve single-molecule coherent control in other complex inhomogeneous systems.
Coherent control with shaped femtosecond laser pulses applied to ultracold molecules
Physical Review A, 2006
We report on coherent control of excitation processes of translationally ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond laser pulses. Evolution strategies are applied in a feedback loop in order to optimize the photoexcitation of the Rb2 molecules, which subsequently undergo ionization or fragmentation. A superior performance of the resulting pulses compared to unshaped pulses of the same pulse energy is obtained by distributing the energy among specific spectral components. The demonstration of coherent control to ultracold ensembles opens a path to actively influence fundamental photo-induced processes in molecular quantum gases.
The Journal of Chemical Physics, 2001
We investigate two-photon, selective excitation of diatomic molecules with intense, ultrafast laser pulses. The method involves transfer of a vibrational population between two electronic states by shaping of light-induced potentials ͑LIPs͒. Creation and control of the LIPs is accomplished by choosing pairs of transform-limited pulses with proper frequency detunings and time delays. Depending on the sequence of pulses ͑intuitive or counter-intuitive͒ and on the sign of the detuning ͑below or above the first transition͒ four schemes are possible for population transfer by LIP shaping. We develop a simple analytic model to predict the optimal laser pulses, and to model the adiabatic dynamics in the different schemes. Based on a harmonic, three-state model of the sodium dimer we demonstrate numerically that all four schemes can lead to efficient, selective population transfer. A careful analysis of the underlying physical mechanisms reveals the varying roles played by the adiabatic and diabatic crossings of the LIPs. The detailed mechanisms influence the robustness and experimental applicability of the schemes.
Fine-Tuning Molecular Energy Levels by Nonresonant Laser Pulses †
The Journal of Physical Chemistry A, 2010
We evaluate the shifts imparted to vibrational and rotational levels of a linear molecule by a nonresonant laser field at intensities of up to 10 12 W/cm 2 . Both types of shift are found to be either positive or negative, depending on the initial rotational state acted upon by the field. An adiabatic field-molecule interaction imparts a rotational energy shift which is negative and exceeds the concomitant positive vibrational shift by a few orders of magnitude. The rovibrational states are thus pushed downward in such a field. A nonresonant pulsed laser field that interacts nonadiabatically with the molecule is found to impart rotational and vibrational shifts of the same order of magnitude. The nonadiabatic energy transfer occurs most readily at a pulse duration which amounts to about a tenth of the molecule's rotational period, and vanishes when the sudden regime is attained for shorter pulses. We applied our treatment to the much studied 87 Rb2 molecule in the last bound vibrational levels of its lowest singlet and triplet electronic states. Our calculations indicate that 15 ns and 1.5 ns laser pulses of an intensity in excess of 5 × 10 9 W/cm 2 are capable of dissociating the molecule due to the vibrational shift. Lesser shifts can be used to fine tune the rovibrational levels and thereby to affect collisional resonances by the nonresonant light. The energy shifts due to laser intensities of 10 9 W/cm 2 may be discernible spectroscopically, with a 10 MHz resolution.
Journal of Physics B: Atomic, Molecular and Optical Physics, 2014
High-order harmonic generation from the molecular ion H 2 + exposed to intense laser fields is investigated by the time-dependent quantum wave packet method. Molecular and atomic plateaus of harmonic spectra are effectively distinguished at large internuclear distances, where the harmonic efficiency of the molecular plateau is several orders of magnitude higher than that of the latter. We report on a physical model of the origin of the molecular supercontinua and reveal that the creation of this plateau directly results from the interference of the intramolecular electronic wave packet localized in two potential wells following the laser field. This is our first effort in utilizing the efficient molecular plateau to generate intense isolated attosecond pulses by controlling the dynamics of the nucleus and electrons with a mid-infrared laser. Further, we show that the harmonic plateau is enhanced at the macroscopic level by solving the Maxwell wave equation coupled with the Schrödinger equation.
Ultrafast control of vibrational states of polar molecules with subcycle unipolar pulses
Physical Review A
We investigate theoretically the nonresonant excitation of vibrational levels in polar molecules by unipolar radiation pulses of duration much shorter than the characteristic period of the molecule's vibration. We consider several profiles of the potential of the interaction of atoms in a diatomic molecule and derive analytically the probabilities of the molecule's transition to excited vibrational states when driven by subcycle unipolar pulses. It is shown that the excitation efficiency is governed by the electric pulse area so that unipolar half-cycle pulses turn out to be the most efficient ones. We introduce the characteristic scale of the electric pulse area, which serves as a measure of the pulse action on the vibrational states of the molecule. The results are generalized to the interaction of excited vibrational and rotational states and it is shown that the behavior of the vibrational levels' populations versus the electric pulse area as well as the introduced characteristic scale stays valid.