Quantum-assisted diamagnetic deflection of molecules

We measure the diamagnetic deflection of anthracene and adamantane in a long-baseline matter-wave interferometer.

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

Multi-mass velocity-map imaging (VMI) is becoming established as a promising method for probing the dynamics of a variety of gas-phase chemical processes. We provide an overview of velocity-map imaging and multi-mass velocity-map imaging techniques, highlighting examples in which these approaches have been used to provide mechanistic insights into a range of photoinduced and electron-induced chemical processes. Multi-mass detection capabilities have also led to the development of two new tools for the chemical dynamics toolbox, in the form of Coulomb-explosion imaging and covariance-map imaging. These allow details of molecular structure to be followed in real time over the course of a chemical reaction, offering the tantalising prospect of recording real-time 'molecular movies' of chemical dynamics. As these new methods become established within the reaction dynamics community, they promise new mechanistic insights into chemistry relevant to fields ranging from atmospheric chemistry and astrochemistry through to synthetic organic photochemistry and biology.

The effect of CO rotation from shaped pulse polarization on reactions that form C2

The effect of CO rotational energy on bimolecular reactions to form electronically excited C₂ is reported here.

Imaging of rotational wave-function in photodissociation of rovibrationally excited HCl molecules

We demonstrate a visualization of quantum mechanical phenomena with the velocity map imaging (VMI) technique, combining vibrationally mediated photodissociation (VMP) of a simple diatomic HCl with the VMI of its H-photofragments. Free HCl molecules were excited by a pump infrared (IR) laser pulse to particular rotational J levels of the v = 2 vibrational state, and subsequently a probe ultraviolet laser photodissociated the molecule at a fixed wavelength of 243.07 nm where also the H-fragments were ionized. The molecule was aligned by the IR excitation with respect to the IR laser polarization, and this alignment was reflected in the angular distribution of the H-photofragments. In particular, the highest degree of molecular alignment was achieved for the J=1←0 transition, which exclusively led to the population of a single rotational state with M = 0. The obtained images were analyzed for further details of the VMP dynamics, and different J states were studied as well. Additionally, we investigated the dynamic evolution of the excited states by changing the pump-probe laser pulse delay; the corresponding images reflected dephasing due to a coupling between the molecular angular momentum and nuclear spin. Our measurements confirmed previous observation using the time-of-flight technique by Sofikitis et al. [J. Chem. Phys. 127, 144307 (2007)]. We observed a partial recovery of the originally excited state after 60 ns in agreement with the previous observation.

Rotational Dynamics of Quantum State-Selected Symmetric-Top Molecules in Nonresonant Femtosecond Laser Fields

Rotational dynamics of quantum state selected and unselected CH₃I molecules in intense femtosecond laser fields has been studied. The orientation and alignment evolutions are derived from a pump-probe measurement and in good agreement with the numerical results from the time-dependent Schrödinger equation (TDSE) calculation. The different rotational transitions through nonresonant Raman process have been assigned from the Fourier analysis of the orientation and alignment revivals. These revivals are derived from a pump-probe measurement and in good agreement with the numerical results from the TDSE calculation. For the molecules in rotational state |1, ±1, ∓1⟩, the transitions can be assigned to ΔJ = ±1, ±2, while for thermally populated molecules, the transitions are ΔJ = ±2. Our results illustrate that the orientation and alignment revivals of the rotational quantum-state-selected molecules give a deep insight into the rotational excitation pathways for the transition of different rotational states of molecules in ultrafast laser fields.

Imaging rotations and vibrations in polyatomic molecules with X-ray scattering

An approach for calculating elastic X-ray scattering from polyatomic molecules in specific electronic, vibrational, and rotational states is presented, and is used to consider the characterization of specific states in polyatomic molecules using elastic X-ray scattering.

On the state selection of linear triatomic molecules by electrostatic hexapole fields

Electrostatic hexapole state-selector is a versatile tool in experimental stereodynamics. The requirement of appropriate models to correctly predict the behavior of molecules in the hexapole motivated us to realize a treatment that predicts the Stark effect of linear triatomic molecules with rotational doublet states. Various perturbative approximations are conventionally adopted to obtain analytic Stark energy derivatives of a truncated Hamiltonian matrix, without utilizing numerical diagonalization of the full Hamiltonian matrix. By including both the low and high field effects, which were alternatively ignored in the analytical formulae of such approximate approaches, herein we demonstrate that the performance of hexapole state selector to linear triatomic molecules can be appropriately predicted via Van Vleck transformation. This method can provide analytic Stark energy derivatives that are acceptably in consistent with the ones obtained via numerical diagonalization of the full Hamiltonian matrix. Particularly, this work is suitable for v2 = 1 level of linear triatomic molecules, due to the following reasons: (1) the Stark energy derivative and the molecular orientation as a function of the electric field are expressed in analytical formulae, hence it is suitable for implementation without involving numerical diagonalization of the full Hamiltonian matrix; (2) a better prediction of the focusing curves with respect to conventional analytical treatments is provided, allowing a reliable determination of the selected state compositions and molecular orientation.

Dynamics of the A-band ultraviolet photodissociation of methyl iodide and ethyl iodide via velocity-map imaging with ‘universal’ detection

Universal ionization combined with velocity-map imaging allows a comprehensive investigation into the photodissociation dynamics of methyl iodide and ethyl iodide at a range of UV wavelengths within their A-bands.

Formation of coherent rotational wavepackets in small molecule-helium clusters using impulsive alignment

We show that rotational line spectra of molecular clusters with near zero permanent dipole moments can be observed using impulsive alignment. Aligned rotational wavepackets were generated by non-resonant interaction with intense femtosecond laser pump pulses and then probed using Coulomb explosion by a second, time-delayed femtosecond laser pulse. By means of a Fourier transform a rich spectrum of rotational eigenstates was derived. For the smallest cluster, C(2)H(2)-He, we were able to establish essentially all rotational eigenstates up to the dissociation threshold on the basis of theoretical level predictions. The C(2)H(2)-He complex is found to exhibit distinct features of large amplitude motion and very early onset of free internal rotor energy level structure.

Role of rotational wave packets in strong field experiments

Complex revival features of rotational wave packets are obtained from the interplay of a molecular rotational distribution and a measured physical observable. The analysis of the measured temporal behavior can be used to retrieve either one or both quantities. We show here the first observation of high order fractional revival (up to 1/12 in CO2) using time-of-flight measurements of ion yields leading to the information required for full reconstruction of the rotational wave packet. We further show via an analysis of higher order fractional revivals in high harmonic generation that new information on the participating ionic channels can be clearly identified, showing the general implication of our results.

Characterising and optimising impulsive molecular alignment in mixed gas samples

Laser induced impulsive molecular alignment has been fully characterized in linear molecules by matching numerical simulations and experimental data of the corresponding rotational wavepacket in the frequency domain. A rigorous procedure for an accurate matching between simulation and experimental data is presented for the first time, making this a versatile technique for experiments where the molecular axis distribution is not directly accessible. Seeding small molecules in Ar as a carrier gas has then been employed to assist cooling and we systematically retrieve the molecule's rotational temperature and alignment distribution for different mixing ratios. For a total backing pressure of 2 bar it was found that seeding 10% N(2) in Ar results in the best cooling. Compared to pure N(2) the rotational temperature was reduced from 24 ± 2 K down to 9 ± 2 K. This leads to an improvement of the peak alignment distribution from <cos(2)θ> = 0.60 to <cos(2)θ> = 0.71. For the same mixing ratio CO(2) was cooled from 34 ± 3 K to 9 ± 1 K improving the alignment distribution from 0.48 to 0.64. In O(2) a cooling from 58 ± 2 K to 37 ± 4 K was observed, corresponding to an alignment distribution improvement from 0.49 to 0.58. The results demonstrate the wide applicability of the characterisation procedure and of seeded supersonic beams to optimise impulsive alignment of small molecules.

Field-free molecular alignment control by phase-shaped femtosecond laser pulse

In this paper, we theoretically show that the field-free molecular alignment can be controlled by shaping the femtosecond laser pulse with a periodic phase step modulation, involving the maximum degree and temporal structure of the molecular alignment. We show that the molecular alignment can be completely suppressed or reconstructed as that by the transform-limited laser pulse, the temporal structure of the alignment transient can be controlled with a desired shape, and the molecular alignment and antialignment for any temporal structure can be switched. Furthermore, we also show that both the degree and direction of the molecular alignment at a fix time delay can be continuously modulated.