Communication: Gas-phase structural isomer identification by Coulomb explosion of aligned molecules

Abstract

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...

Multi-mass velocity-map imaging studies of photoinduced and electron-induced chemistry

“…The electron-stripping step is usually achieved either through multiphoton/strong-field ionization with an intense (few mJ) femtosecond infra-red laser, or by X-ray ionization of a core electron followed by an Auger cascade. Both approaches have been used in multi-mass VMI investigations into Coulomb explosions 48,49,50,67,69,70 , with a recent study 48 on difluoroiodobenzene showing that the two approaches lead to similar Coulomb explosion processes despite the very different initiation mechanisms.…”

“…Performing a statistical covariance analysis on the data allows these correlations to be revealed. Transforming the 'raw' velocity-map images into covariance images allows the velocity distribution of one product relative to one or more other fragments to be determined and visualised 67,69,70 . This can reveal important details of both the structure of the molecule under study, and the dynamics of the Coulomb explosion process itself.…”

“…A considerable amount of experimental optimisation is often required in order to make sure that this is the case. As an example of Coulomb explosion and covariance-map imaging, we will briefly summarise a recent study in which the two techniques were used to distinguish between structural isomers of difluoroiodobenzene and dihdroxybromobenzene in the gas phase 67 . Prior to Coulomb explosion, the electric field of an intense linearlypolarised nanosecond laser pulse was used to align the most polarisable molecular axis (containing the C-I bond) parallel to the plane of the image.…”

Approaching black-box calculations of pump-probe fragmentation dynamics of polyatomic molecules

“…7). From the covariance analysis of ionic channels [65,66] we know that a prominent decay channel for the dication is a Coulomb explosion C 14 H 2+ 10 → CnH + x + C 14−n H + 10−x with = 2, 3, 4. Therefore it is interesting to compare the pump-probe yields of these channels in the experiment with our theoretical predictions.…”

Femtosecond laser induced Coulomb explosion imaging of aligned OCS oligomers inside helium nanodroplets

“…For example, a covariance signal at angles θ 1 = θ 2 + 180 • would indicate two ion fragments recoiling in opposite directions to each other. This technique allows information that is otherwise hidden in the raw data to be revealed, and greatly enhances the utility of ion imaging as a structural probe [27,28].…”