Aligning symmetric and asymmetric top molecules via single photon excitation

Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H3O+-Ar and H+(H2O)2-Ar

Infrared photodissociation of protonated water clusters with an Ar atom, namely H₃O⁺-Ar and H⁺(H₂O)₂-Ar, was investigated by an imaging technique for mass-selected ions, to reveal the intra- and intermolecular vibrational dynamics. The presented system has the advantage of achieving fragment ion images with the cluster size- and mode-selective photoexcitation of each OH stretching vibration. Translational energy distributions of photofragments were obtained from the images upon the excitation of the bound (ν_b) and free (ν_f) OH stretching vibrations. The energy fractions in the translational motion were compared between ν_b^I and ν_f^I in H₃O⁺-Ar or between ν_b^II and ν_f^II in H⁺(H₂O)₂-Ar, where the labels "I" and "II" represent H₃O⁺-Ar and H⁺(H₂O)₂-Ar, respectively. In H₃O⁺-Ar, the ν_f^I excitation exhibited a smaller translational energy than ν_b^I. This result can be explained by the higher vibrational energy of ν_f^I, which enabled it to produce bending (ν₄) excited H₃O⁺ fragments that should be favored according to the energy-gap model. In contrast to H₃O⁺-Ar, the ν_b^II excitation of an Ar-tagged H₂O subunit and the ν_f^II excitation of an untagged H₂O subunit resulted in very similar translational energy distributions in H⁺(H₂O)₂-Ar. The similar energy fractions independent of the excited H₂O subunits suggested that the ν_b^II and ν_f^II excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H₂O-H⁺-H₂O moiety. However, the translational energy distributions for H⁺(H₂O)₂-Ar did not agree with a statistical dissociation model, which implied another aspect of the process, that is, Ar dissociation via incomplete energy randomization in the whole H⁺(H₂O)₂-Ar cluster.

Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion

Vibrational predissociation processes of the H₂O⁺Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H₂O⁺ unit were studied by photofragment ion imaging. The anisotropy parameters, β, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, β > 0 for the P-branch excitations, while β < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H₂O⁺ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H₂O⁺ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H₂O⁺ fragment ions was discussed based on the steric configuration of the H₂O⁺ and Ar units at the moment of dissociation.

Chirality in molecular collision dynamics

Chirality is a phenomenon that permeates the natural world, with implications for atomic and molecular physics, for fundamental forces and for the mechanisms at the origin of the early evolution of life and biomolecular homochirality. The manifestations of chirality in chemistry and biochemistry are numerous, the striking ones being chiral recognition and asymmetric synthesis with important applications in molecular sciences and in industrial and pharmaceutical chemistry. Chiral discrimination phenomena, due to the existence of two enantiomeric forms, very well known in the case of interaction with light, but still nearly disregarded in molecular collision studies. Here we review some ideas and recent advances about the role of chirality in molecular collisions, designing and illustrating molecular beam experiments for the demonstration of chiral effects and suggesting a scenario for a stereo-directional origin of chiral selection.

Hexapole-Oriented Asymmetric-Top Molecules and Their Stereodirectional Photodissociation Dynamics

Molecular orientation is a fundamental requisite in the study of stereodirected dynamics of collisional and photoinitiated processes. In this past decade, variable hexapolar electric filters have been developed and employed for the rotational-state selection and the alignment of molecules of increasing complexity, for which the main difficulties are their mass, their low symmetry, and the very dense rotational manifold. In this work, for the first time, a complex molecule such as 2-bromobutane, an asymmetric top containing a heavy atom (the bromine), was successfully oriented by a weak homogeneous field placed downstream from the hexapolar filter. Efficiency of the orientation was characterized experimentally, by combining time-of-flight measurements and a slice-ion-imaging detection technique. The application is described to the photodissociation dynamics of the oriented 2-bromobutane, which was carried out at a laser wavelength of 234 nm, corresponding to the breaking of the C-Br bond. The Br photofragment is produced in both the ground Br ((2)P3/2) and the excited Br ((2)P1/2) electronic states, and both channels are studied by the slice imaging technique, revealing new features in the velocity and angular distributions with respect to previous investigations on nonoriented molecules.

Defect-Tolerant Aligned Dipoles within Two-Dimensional Plastic Lattices

Carboranethiol molecules self-assemble into upright molecular monolayers on Au{111} with aligned dipoles in two dimensions. The positions and offsets of each molecule's geometric apex and local dipole moment are measured and correlated with sub-Ångström precision. Juxtaposing simultaneously acquired images, we observe monodirectional offsets between the molecular apexes and dipole extrema. We determine dipole orientations using efficient new image analysis techniques and find aligned dipoles to be highly defect tolerant, crossing molecular domain boundaries and substrate step edges. The alignment observed, consistent with Monte Carlo simulations, forms through favorable intermolecular dipole-dipole interactions.

A new perspective: imaging the stereochemistry of molecular collisions

The concept of the steric effect plays a central role in chemistry. This Perspective describes how the polarization of reactant molecules in space can be used to probe directly the steric effect, and highlights some of the new measurements that are made possible by coupling reactant orientation and alignment with ion imaging techniques.

Selective OD bond dissociation of HOD: photodissociation of vibrationally excited HOD in the 5nu(OD) state

Exclusively selective OD bond dissociation of HOD has been demonstrated by the ultraviolet photodissociation at 243.1 nm through the fourth overtone state of the OD stretching mode (5nu(OD)). Branching ratio between the OH and OD bond dissociation channels has been determined by detecting H and D atoms, utilizing a (2+1) resonance-enhanced multiphoton ionization (REMPI) process. The OD bond dissociation has been solely observed with the branching ratio phi(D+OH)/phi(H+OD)>12, which has been determined by the detection limit for the H atom. Time-dependent wave-packet calculations reveal two important features for the highly selective OD bond dissociation: (1) strong local-mode character of the 5nu(OD) state and (2) limitation of the total excitation energy lower than the saddle point between the OH and OD dissociation channels in the A state. Additionally, the recoil velocity and angular distribution of the nascent D atom are roughly evaluated by analyzing the Doppler-resolved REMPI spectrum. Based on these results, the dynamics of the selective OD dissociation has been discussed in detail.

Steric effects in state-to-state scattering of OH (Π3∕22,J=3∕2,f) by HCl

In this paper we address stereo-dynamical issues in the inelastic encounters between OH (chi2pi) radicals and HCl (chi1sigma+). The experiments were performed in a crossed molecular-beam machine at the nominal collision energy of 920 cm(-1). Prior to the collisions, the OH molecules were selected using a hexapole in a well-defined rotational state v=0, omega=32, J=32, M(J)=32, f, and subsequently oriented in a homogeneous electrical field. We have measured rotationally resolved relative cross sections for collisions in which OH is oriented with either the O side or the H side towards HCl, from which we have calculated the corresponding steric asymmetry factors S. The results are presented in comparison with data previously obtained by our group for the inelastic scattering of OH by CO (E(coll)=985 cm(-1)) and N2 (E(coll)=985 cm(-1)) studied under similar experimental conditions. The dissimilarity in the behavior of the OH+HCl system revealed by this comparison is explained on the basis of the difference in the anisotropy of the interaction potential governing the collisions. The interpretation of the data takes into account the specific features of both nonreactive and reactive parts of the potential-energy surface. The results indicate that the scattering dynamics at this collision energy may be influenced by the HO-HCl van der Waals well and by reorientation effects determined by the long-range electrostatic forces and, furthermore, may involve reactive collisions.

Mapping rotational coherences onto time-resolved photoelectron imaging observables

We explore the information content of time-resolved photoelectron imaging, a potentially powerful pump-probe technique whose popularity has been rapidly growing in recent years. To that end, we identify a mapping of the alignment properties of time-evolving wave packets onto the moments of the photoelectron images and investigate its origin and consequences theoretically and numerically.

Two-photon state selection and angular momentum polarization probed by velocity map imaging: Application to H atom photofragment angular distributions from the photodissociation of two-photon state selected HCl and HBr

A formalism for calculating the angular momentum polarization of an atom or a molecule following two-photon excitation of a J-selected state is presented. This formalism is used to interpret the H atom photofragment angular distributions from single-photon dissociation of two-photon rovibronically state selected HCl and HBr prepared via a Q-branch transition. By comparison of the angular distributions measured using the velocity map imaging technique with the theoretical model it is shown that single-photon dissociation of two-photon prepared states can be used for pathway identification, allowing for the identification of the virtual state symmetry in the two-photon absorption and/or the symmetry of the dissociative state. It is also shown that under conditions of excitation with circularly polarized light, or for excitation via non-Q-branch transitions with linearly polarized light the angular momentum polarization is independent of the dynamics of the two-photon transition and analytically computable.

Optical-optical double resonance photoionization spectroscopy of nf Rydberg states of nitric oxide

The spectra of vibrationally excited nf Rydberg states of nitric oxide were recorded by monitoring the photoion current produced using two-photon double resonance excitation via the NO A (2)Sigma(+) state followed by photoexcitation of the Rydberg state that undergoes autoionization. The optical transition intensities from NO A state to nf Rydberg states were calculated, and the results agree closely with experiment. These results combined with circular dichroism measurements allow us to assign rotational quantum numbers to the nf Rydberg states even in a spectrum of relatively low resolution. We report the positions of these nf (upsilon,N,N(c)) Rydberg levels converging to the NO X (1)Sigma(+) upsilon(+) = 1 and 2 ionization limits where N is the total angular momentum excluding electron and nuclear spin and N(c) represents the rotational quantum number of the ion core. Our two-color optical-optical double resonance measurements cover the range of N from 15 to 28, N(c) from 14 to 29, and the principal quantum number n from 9 to 21. The electrostatic interaction between the Rydberg electron and the ion core is used to account for the rotational fine structure and a corresponding model is used to fit the energy levels to obtain the quadrupole moment and polarizability of the NO(+) core. Comparison with a multichannel quantum defect theory fit to the same data confirms that the model we use for the electrostatic interaction between the nf Rydberg electron and the ion core of NO well describes the rotational fine structure.