Recent experiments have demonstrated the feasibility of orienting rotationally cooled polar molecules in an electric field. The anisotropy of the Stark effect allows molecules in low rotational states to be trapped in "pendular states," confined to librate over a limited angular range about the field direction. We present calculations exhibiting the nature of these pendular states for a linear molecule and characteristic features of infrared and microwave spectra which become observable in strong fields.PACS numbers: 33.10.-n, 33.20.Bx, 33.20.Ea, 33.55.Be Electric-field focusing has long provided an effective means to obtain beams of oriented symmetric top molecules (or equivalent) that have a first-order Stark effect [1,2]. Such molecules in certain rotational states precess rather than tumble and hence maintain a constant projection of the dipole moment on the field direction. State selection by the focusing field thus suffices to pick out molecules with substantial orientation of the figure axis. This has enabled elegant studies of the anisotropic forces governing collisions with reactive atoms [1-4], photons [5,6], or surfaces [7]. However, the technique requires an elaborate apparatus and is not applicable when the Stark interaction is second order, as usual for diatomic, linear, or asymmetric top molecules.A simpler technique applicable to polar molecules with either a first-or second-order Stark interaction has recently been proposed [8,9]. This exploits the extreme rotational cooling attainable in a supersonic expansion to condense a large fraction of a molecular beam into low rotational states. By sending the beam into a strong uniform electron field, these low-/ states can be converted from pinwheeling rotors into pendular librators confined to oscillate over a limited angular range about the field direction. The pendular states are directional hybrids [10,11], comprised of linear combinations of the field-free rotor states |/,A/> with a range of J values but the same fixed value of the M quantum number specifying the projection of the angular momentum on the field direction. For a diatomic or linear molecule with dipole moment ju and rotational constant B in a field of strength , the ratio a)=nG/B governs the extent of hybridization of rotor states and the consequent directional localization of the pendular states. The feasibility of this orientation technique has been demonstrated experimentally for beams of a symmetric top molecule [8,12] (methyl iodide, CH3I) and a diatomic molecule [13,14] (iodine monochloride, IC1). The overall orientation achieved was modest, however, since in those experiments the co values for the highest field strengths used were rather low, only 1.7 (CH3I) and 3.7 (IC1), respectively. Block, Bohac, and Miller [15] have now provided a far more incisive demonstration of pendular orientation, by measuring infrared spectra with subDoppler resolution for the linear trimer of hydrogen cyanide, (HCN>3. By virtue of its extremely large dipole moment and small rotational constant, f...
It is shown that the time-dependent equations (Schrödinger and Dirac) for a quantum system can be always derived from the time-independent equation for the larger object of the system interacting with its environment, in the limit that the dynamical variables of the environment can be treated semiclassically. The time which describes the quantum evolution is then provided parametrically by the classical evolution of the environment variables. The method used is a generalization of that known for a long time in the field of ion-atom collisions, where it appears as a transition from the full quantum mechanical perturbed stationary states to the impact parameter method in which the projectile ion beam is treated classically.
Differential cross sections describing the correlated motion of three electrons in the nuclear Coulomb field during the complete photofragmentation of a four-body system, the lithium atom, are calculated. Two selection rules are derived and their operation illustrated. Two features, not present in the corresponding three-body case are emphasized, namely that the Wannier configuration, in contrast to the three-body photofragmentation of helium, is allowed. Second, the cross section to access certain spin-momentum configurations of the three particles is zero for an uncorrelated but finite for a correlated final state. ͓S1050-2947͑97͒50406-9͔ PACS number͑s͒: 32.80.Fb, 41.60.Ap RAPID COMMUNICATIONS R3982 55 A. W. MALCHEREK, J. M. ROST, AND J. S. BRIGGS
The generation of a streaking spectrogram is based on energy absorption from the streaking laser. Investigating this absorption we show rigorously under which condition the measured time shift is independent of properties of the streaking light. In this case it provides the Wigner-Smith time delay. The latter is infinite for systems with a long-range potential tail, such as Coulomb systems. Here, we suggest to determine the time delay relative to pure hydrogen for meaningful results. Finite delays obtained so far for Coulombic systems without the hydrogen reference are the consequence of a finite streaking frequency and depend on its value as well as on the electron's excess energy. Our analysis also suggests a time-delay measurement technique that avoids the record of a complete streaking scan.
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