@article{ChristiansenNielsenChristensenetal.2016, author = {Christiansen, Lars and Nielsen, Jens H. and Christensen, Lauge and Shepperson, Benjamin and Pentlehner, Dominik and Stapelfeldt, Henrik}, title = {Laser-induced Coulomb explosion of 1,4-diiodobenzene molecules: Studies of isolated molecules and molecules in helium nanodroplets}, series = {Physical Review A}, journal = {Physical Review A}, number = {93}, year = {2016}, abstract = {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.}, language = {en} } @article{ChristiansenNielsenPentlehneretal.2015, author = {Christiansen, Lars and Nielsen, Jens H. and Pentlehner, Dominik and Underwood, Jonathan G. and Stapelfeldt, Henrik}, title = {Alignment enhancement of molecules embedded in helium nanodroplets by multiple laser pulses}, series = {Physical Review A}, journal = {Physical Review A}, number = {92}, year = {2015}, abstract = {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.}, language = {en} } @article{PentlehnerNielsenChristiansenetal.2013, author = {Pentlehner, Dominik and Nielsen, Jens H. and Christiansen, Lars and Slenczka, Alkwin and Stapelfeldt, Henrik}, title = {Laser Induced Adiabatic Alignment of Molecules Dissolved in Helium Nanodroplets}, series = {Physical Review A}, journal = {Physical Review A}, number = {87}, year = {2013}, abstract = {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.}, language = {en} }