Recording molecular movies on ultrafast timescales has been a longstanding goal for unravelling detailed information about molecular dynamics. Here we present the direct experimental recording of very-high-resolution and -fidelity molecular movies over more than one-and-a-half periods of the laser-induced rotational dynamics of carbonylsulfide (OCS) molecules. Utilising the combination of single quantum-state selection and an optimised two-pulse sequence to create a tailored rotational wavepacket, an unprecedented degree of field-free alignment, 〈cos 2 θ 2D 〉 = 0.96 (〈cos 2 θ 〉 = 0.94) is achieved, exceeding the theoretical limit for single-pulse alignment. The very rich experimentally observed quantum dynamics is fully recovered by the angular probability distribution obtained from solutions of the time-dependent Schrödinger equation with parameters refined against the experiment. The populations and phases of rotational states in the retrieved time-dependent three-dimensional wavepacket rationalises the observed very high degree of alignment.

The gas-phase structures of four difluoroiodobenzene and two dihydroxybromobenzene isomers were identified by correlating the emission angles of atomic fragment ions created following femtosecond laser-induced Coulomb explosion. The structural determinations were facilitated by confining the most polarizable axis of each molecule to the detection plane prior to the Coulomb explosion event using one-dimensional laser-induced adiabatic alignment. For a molecular target consisting of two difluoroiodobenzene isomers, each constituent structure could additionally be singled out and distinguished. a) These authors contributed equally to this work. b) ElectronicLaser-induced molecular Coulomb explosion is the process whereby an intense femtosecond laser pulse detaches several electrons from a molecule, breaking it into cationic fragments. If the axial recoil approximation is satisfied, the created fragment ions recoil along the original bond axes of their parent molecule, allowing laser-induced Coulomb explosions to be used for two purposes: the first concerns how molecules are oriented in space at the instant the laser pulse arrives, and is measured by determining the emission directions of the fragment ions with respect to one or more fixed axes. This method has been used as a probe for a large number of laser-induced alignment and orientation experiments. 1-4 The second concerns molecular structures, which can be identified by correlating the measured fragment momenta. These latter experiments have revealed static molecular geometries and, with femtosecond pump-probe schemes, structural and reaction dynamics. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Coulomb explosion research on diatomic and triatomic molecules has demonstrated that covariance and coincidence analyses are efficient and powerful approaches for identifying the correlations between the emission directions of different fragment ions. 20-25 This is even more evident in studies of larger and more complex molecules, towards which interest has turned over the past decade. 10,16,26,27 Covariance analysis of Coulomb explosion fragments has also been applied to molecules that were pre-aligned by adiabatic laser pulses. [26][27][28] In these experiments, this alignment placed the molecules in well-defined spatial orientations with respect to the imaging detector, and thereby increased the structural information obtained from the recorded fragment momenta. The purpose of this work is to further develop Coulomb explosion imaging of pre-aligned targets as a method to determine the structures of polyatomic molecules. In particular, we demonstrate that four difluoroiodobenzene (DFIB) isomers can be unequivocally distinguished by analyzing the angular correlations of specific fragment ions. Furthermore, we show that for a mixture of two DFIB isomers the constituent structures can be singled out and identified. Finally, to demonstrate that our method is also capable of identifying isomers when only one substituent is a halogen atom, experiments were carr...

Measurements on the strong-field ionization of carbonyl sulfide molecules by short, intense, 2 µm wavelength laser pulses are presented from experiments where angle-resolved photoelectron distributions were recorded with a high-energy velocity map imaging spectrometer, designed to reach a maximum kinetic energy of 500 eV. The laser-field-free elastic-scattering cross section of carbonyl sulfide was extracted from the measurements and is found in good agreement with previous experiments, performed using conventional electron diffraction. By comparing our measurements to the results of calculations, based on the quantitative rescattering theory (QRS), the bond lengths and molecular geometry were extracted from the experimental differential cross sections to a precision better than ±5 pm and in agreement with the known values.

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Strong-field ionization of carbonyl sulphide (OCS) molecules, induced by a linearly polarized mid-infrared (mid-IR) probe laser pulse is investigated experimentally and theoretically. We focus on the dependence of the single-ionization yield on the alignment of the molecular axis with respect to the probe pulse polarization axis. In the experiment, the OCS molecules are 1-dimensionally adiabatically aligned and ionized by a 12-femtosecond pulse centered at 1850 nm. The alignment-dependent ionization yields are compared to the theory based on the two-step model for strong-field ionization. Overall the measurements are consistent with the theoretical results.