TY - JOUR A1 - Christiansen, Lars A1 - Nielsen, Jens H. A1 - Christensen, Lauge A1 - Shepperson, Benjamin A1 - Pentlehner, Dominik A1 - Stapelfeldt, Henrik T1 - Laser-induced Coulomb explosion of 1,4–diiodobenzene molecules: Studies of isolated molecules and molecules in helium nanodroplets JF - Physical Review A N2 - Coulomb explosion of 1,4–diiodobenzene molecules, isolated or embedded in helium nanodroplets, is induced by irradiation with an intense femtosecond laser pulse. The recoiling ion fragments are probed by time-of-flight measurements and two-dimensional velocity map imaging. Correlation analysis of the emission directions of I+ ions recoiling from each end of the molecules reveals significant deviation from axial recoil, i.e., where the I+ ions leave strictly along the I-I symmetry axis. For isolated molecules, the relative angular distribution of the I+ ions is centered at 180∘, corresponding to perfect axial recoil, but with a full width at half maximum of 30∘. For molecules inside He droplets, the width of the distribution increases to 45∘. These results provide a direct measure of the accuracy of Coulomb explosion as a probe of the spatial orientation of molecules, which is particularly relevant in connection with laser-induced molecular alignment and orientation. In addition, our studies show how it is possible to identify fragmentation pathways of the Coulomb explosion for the isolated 1,4–diiodobenzene molecules. Finally, for the 1,4–diiodobenzene molecules in He droplets, it is shown that the angular correlation between fragments from the Coulomb explosion is preserved after they have interacted with the He environment. KW - Isolated Molecules KW - Helium nanodroplets KW - Coulomb explosion Y1 - 2016 IS - 93 ER - TY - JOUR A1 - Christiansen, Lars A1 - Nielsen, Jens H. A1 - Pentlehner, Dominik A1 - Underwood, Jonathan G. A1 - Stapelfeldt, Henrik T1 - Alignment enhancement of molecules embedded in helium nanodroplets by multiple laser pulses JF - Physical Review A N2 - We show experimentally that field-free one-dimensional (1D) alignment of 1,4-diiodobenzene molecules embedded in helium nanodroplets, induced by a single, linearly polarized 200-fs laser pulse, can be significantly enhanced by using two or four optimally synchronized laser pulses. The strongest degree of 1D alignment is obtained with four pulses and gives ⟨cos2θ⟩>0.60. Besides the immediate implications for molecular frame studies, our results pave the way for more general manipulation of rotational motion of molecules in He droplets. KW - Isolated Molecules KW - Helium nanodroplets Y1 - 2015 IS - 92 ER - TY - JOUR A1 - Hansen, Jonas A1 - Omiste, Juan J. A1 - Nielsen, Jens H. A1 - Pentlehner, Dominik A1 - Küpper, Jochen A1 - González-Férez, Rosario A1 - Stapelfeldt, Henrik T1 - Mixed-field orientation of molecules without rotational symmetry JF - The Journal of Chemical Physics N2 - The mixed-field orientation of an asymmetric-rotor molecule with its permanent dipole moment nonparallel to the principal axes of polarizability is investigated experimentally and theoretically. We find that for the typical case of a strong, nonresonant laser field and a weak static electric field complete 3D orientation is induced if the laser field is elliptically polarized and if its major and minor polarization axes are not parallel to the static field. For a linearly polarized laser field solely the dipole moment component along the most polarizable axis of the molecule is relevant resulting in 1D orientation even when the laser polarization and the static field are nonparallel. Simulations show that the dipole moment component perpendicular to the most-polarizable axis becomes relevant in a strong dc electric field combined with the laser field. This offers an alternative approach to 3D orientation by combining a linearly polarized laser field and a strong dc electric field arranged at an angle equal to the angle between the most polarizable axis of the molecule and its permanent dipole moment. KW - Asymmetric-rotor molecule KW - Mixed-field orientation Y1 - 2013 IS - 139 ER - TY - JOUR A1 - Pentlehner, Dominik A1 - Nielsen, Jens H. A1 - Christiansen, Lars A1 - Slenczka, Alkwin A1 - Stapelfeldt, Henrik T1 - Laser Induced Adiabatic Alignment of Molecules Dissolved in Helium Nanodroplets JF - Physical Review A N2 - A moderately intense, nonresonant, linearly polarized nanosecond laser pulse is used to induce one-dimensional (1D) adiabatic alignment of 1,4 diiodobenzene (C6H4I2), iodobenzene (C6H5I), and methyliodide (CH3I) molecules dissolved in helium nanodroplets. The alignment sharpens as the laser intensity is increased, similar to the behavior in gas phase. For diiodobenzene the highest degree of alignment, ⟨cos2θ2D⟩=0.90, is essentially identical to the value obtained for isolated molecules in a supersonic beam, whereas the highest degrees of alignment for iodobenzene and for methyliodide in helium droplets fall below the values obtained for isolated molecules. We believe this is due to the deviation from axial recoil in the Coulomb explosion process that probes the alignment of the molecules in the He droplets. KW - Helium nanodroplets KW - Linearly polarized nanosecond laser pulse Y1 - 2013 IS - 87 ER - TY - JOUR A1 - Pentlehner, Dominik A1 - Nielsen, Jens H. A1 - Slenczka, Alkwin A1 - Mølmer, Klaus A1 - Stapelfeldt, Henrik T1 - Impulsive Laser Induced Alignment of Molecules Dissolved in Helium Nanodroplets JF - Physical Review Letters N2 - We show that a 450 fs nonresonant, moderately intense, linearly polarized laser pulse can induce field-free molecular axis alignment of methyliodide (CH3I) molecules dissolved in a helium nanodroplet. Time-resolved measurements reveal rotational dynamics much slower than that of isolated molecules and absence of the sharp transient alignment recurrences characteristic of gas phase molecules. Our results presage a range of new opportunities for exploring both molecular dynamics in a dissipative environment and the properties of He nanodroplets. KW - Helium nanodroplets KW - mpulsive Laser Induced Alignment Y1 - 2013 IS - 110 ER -