Long-lasting molecular alignment: Fact or fiction?

Ortigoso, Juan; Rodrı́guez, Mirta; Santos, J.; Karpati, A.; Szalay, Viktor

doi:10.1063/1.3312533

Cited by 4 publications

(4 citation statements)

References 52 publications

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“…Enhancement of molecular alignment by a train of short laser pulses was shown by [118,119]. The possibility to maintain molecular alignment for arbitrarily long time by using an appropriate periodic sequence of laser pulses was proposed in [120,121]. The alignment mechanism was described in terms of the coherent control of rotational wave packets [122].…”

Section: Alignment By Static Fields and Linearly Polarized Nanosecond...mentioning

confidence: 99%

Quantum control of molecular rotation

2019

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The angular momentum of molecules, or, equivalently, their rotation in three-dimensional space, is ideally suited for quantum control. Molecular angular momentum is naturally quantized, time evolution is governed by a well-known Hamiltonian with only a few accurately known parameters, and transitions between rotational levels can be driven by external fields from various parts of the electromagnetic spectrum. Control over the rotational motion can be exerted in one-, two-and many-body scenarios, thereby allowing to probe Anderson localization, target stereoselectivity of bimolecular reactions, or encode quantum information, to name just a few examples. The corresponding approaches to quantum control are pursued within separate, and typically disjoint, subfields of physics, including ultrafast science, cold collisions, ultracold gases, quantum information science, and condensed matter physics. It is the purpose of this review to present the various control phenomena, which all rely on the same underlying physics, within a unified framework. To this end, we recall the Hamiltonian for free rotations, assuming the rigid rotor approximation to be valid, and summarize the different ways for a rotor to interact with external electromagnetic fields. These interactions can be exploited for control -from achieving alignment, orientation, or laser cooling in a one-body framework, steering bimolecular collisions, or realizing a quantum computer or quantum simulator in the many-body setting. IntroductionMolecules, unlike atoms, are extended objects that possess a number of different types of internal motion. In particular, the geometric arrangement of their constituent atoms endows molecules with the basic capability to rotate in three-dimensional space. Rotations can couple to vibrations of the atomic nuclei as well as to the orbital and spin angular momentum of the electrons. The resulting complexity of the energy level structure [1,2,3,4] may be daunting. It offers, on the other hand, a variety of knobs for control and thus is at the core of numerous applications, from the classic example of the ammonia maser [5] all the way to recent measurements of the electron's electric dipole moment in a cryogenic molecular beam of thorium monoxide [6].A key advantage of internal degrees of freedom such as rotation is that they occupy the low-energy part of the energy spectrum. Quantization of the rotational motion has been an early hallmark of quantum mechanics due to its connection to selection rules that govern all light-matter interactions [7]. Nowadays, rotational states and rotational molecular dynamics feature prominently in all active areas of AMO physics research as well as in neighbouring fields such as physical chemistry and quantum information science. Control over the rotational motion is crucial in one-body, two-body and many-body scenarios. For example, rotational state-selective excitation could pave the way towards separating left-and right handed enantiomers of chiral molecules [8,9]. Still within the one-body scenar...

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Section: Alignment By Static Fields and Linearly Polarized Nanosecond...mentioning

confidence: 99%

Quantum control of molecular rotation

2019

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“…Technically, two different cases may exist for Floquet Hamiltonians [31]: (i) The spectrum of F, Eq. (A1), is singular continuous at least for some ω values.…”

Section: Appendix A: Rotational Cyclic Statesmentioning

confidence: 99%

Preparation and control of aligned cyclic rotational states

2013

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Molecules in selected rotational states can remain well aligned for extended periods of time in the presence of an appropriate periodic train of nonresonant laser pulses. Here, we show that these states can be prepared by slowly switching the electromagnetic field amplitude during a sequence of laser pulses. For low-temperature ensembles, a high degree of alignment can be achieved by designing pulse trains that take into account the distribution of avoided crossings between quasienergy curves. Additionally, we present calculations that illustrate several effects causing misalignment. A discussion of subtleties concerning systems with unbounded rotational spectra is also included.

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“…This operator may have no normalizable eigenvectors if its spectrum is continuous. However, heuristic evidence has been given that the Floquet Hamiltonian for a rotating molecule interacting with external fields has a pure point spectrum [36]. Eigenfunctions, χ, of F in this space, can be expanded in terms of a Fourier time basis and spatial basis functions φ k (x) The time-evolved wave function, for an initial Ψ(t 0 ), can be expanded as…”

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confidence: 99%

Mechanism of molecular orientation by single-cycle pulses

Ortigoso

2012

The Journal of Chemical Physics

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Significant molecular orientation can be achieved by time-symmetric single-cycle pulses of zero area, in the THz region. We show that in spite of the existence of a combined time-space symmetry operation, not only large peak instantaneous orientations, but also nonzero time-average orientations, over a rotational period, can be obtained. We show that this unexpected phenomenon is due to interferences among eigenstates of the time-evolution operator, as was described previously for transport phenomena in quantum ratchets. This mechanism also works for appropriate sequences of identical pulses, spanning a rotational period. This fact can be used to obtain a net average molecular orientation regardless of the magnitude of the rotational constant.

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Long-lasting molecular alignment: Fact or fiction?

Cited by 4 publications

References 52 publications

Quantum control of molecular rotation

Quantum control of molecular rotation

Preparation and control of aligned cyclic rotational states

Mechanism of molecular orientation by single-cycle pulses

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