Strongly driven quantum pendulum of the carbonyl sulfide molecule

Abstract

We demonstrate and analyze a strongly driven quantum pendulum in the angular motion of state-selected and laser-aligned carbonyl sulfide molecules. Raman couplings during the rising edge of a 50-ps laser pulse create a wave packet of pendular states, which propagates in the confining potential formed by the polarizability interaction between the molecule and the laser field. This wave-packet dynamics manifests itself as pronounced oscillations in the degree of alignment with a laser-intensity-dependent period. Pendular states, directional superpositions of field-free rotational states, are created by the anisotropic interaction between an isolated molecule and a strong electric field [1][2][3][4]. In a classical sense, this corresponds to the free rotation of the molecule changing into a restricted angular motion, where a molecular axis librates about the field direction. In the case of a strong static electric field the pendular states result from the interaction with the permanent dipole moment. This was exploited, for instance, for the simplification of spectroscopic signatures of large molecular clusters [5]. In the case of a nonresonant laser field the pendular states are formed due to the interaction with the molecular polarizability. This interaction constitutes the basis for laser-induced alignment [3,4], the confinement of molecular axes to laboratory-fixed axes defined by the polarization of the alignment field. Notably, in the limit where the laser field is turned on slowly compared to the inherent rotational period(s) of the molecule, the initial field-free rotational states are converted into the corresponding pendular states. This process is called adiabatic alignment [4] and it has found widespread use in molecular sciences [6][7][8][9][10][11]. The pendular states persist for as long as the laser field is turned on and the molecules return to their initial field-free rotational states upon turning off the laser field, provided this occurs slowly compared to the rotational period(s), τ rot .Pendular states were investigated through frequencyresolved spectroscopy [12,13] and by photodissociation or Coulomb explosion imaging [4,14]. The former approach probes the field-induced changes of the rotational energy levels, thus the pendular state energies, while the latter approach probes the way the molecules are confined in space, i. e., the orientational character of the pendular states. So far these studies were all performed in the adiabatic limit where the classical signature of the pendular states, i. e., the librational motion of a molecular axis about the field direction, cannot be observed directly. To observe this motion it would * jochen.kuepper@cfel.de; http://desy.cfel.de/cid/cmi be necessary to create a coherent superposition of pendular states.Here, we demonstrate that such pendular motion can be induced through the use of a laser pulse with a duration τ laser ∼ τ rot in between the common limits of adiabatic (τ laser τ rot ) and impulsive (τ laser τ rot ) alignment. The intermediate regim...

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“…These experiments require pure samples delivered into a micrometer-sized interaction region, which can be achieved using electrostatic control techniques applied to cold molecular beams [19][20][21]. Moreover, these techniques also allow the dispersion of rotational states, such that the coldest molecules from a molecular beam can be selected, yielding, e. g., higher degrees of molecular one-and three-dimensional alignment and orientation [22][23][24][25][26][27].…”

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