Jonas L. Hansen

Photoelectron angular distributions from strong-field ionization of oriented molecules

An experimental study shows how a polar molecule can be oriented in three dimensions by using a combination of laser and electrostatic fields. The approach should help to obtain molecular-frame information about strong-field ionization processes in molecules for which the orientation cannot be determined after ionization.

Quantum-state selection, alignment, and orientation of large molecules using static electric and laser fields

Supersonic beams of polar molecules are deflected using inhomogeneous electric fields. The quantum-state selectivity of the deflection is used to spatially separate molecules according to their quantum state. A detailed analysis of the deflection and the obtained quantum-state selection is presented. The rotational temperatures of the molecular beams are determined from the spatial beam profiles and are all approximately 1 K. Unprecedented degrees of laser-induced alignment (=0.972) and orientation of iodobenzene molecules are demonstrated when the state-selected samples are used. Such state-selected and oriented molecules provide unique possibilities for many novel experiments in chemistry and physics.

Laser-induced 3D alignment and orientation of quantum state-selected molecules

A strong inhomogeneous static electric field is used to spatially disperse a rotationally cold supersonic beam of 2,6-difluoroiodobenzene molecules according to their rotational quantum state. The molecules in the lowest-lying rotational states are selected and used as targets for 3-dimensional alignment and orientation. The alignment is induced in the adiabatic regime with an elliptically polarized, intense laser pulse and the orientation is induced by the combined action of the laser pulse and a weak static electric field. We show that the degree of 3-dimensional alignment and orientation is strongly enhanced when rotational state-selected molecules, rather than molecules in the original molecular beam, are used as targets.

Pure Samples of Individual Conformers: The Separation of Stereoisomers of Complex Molecules Using Electric Fields

Many complex molecules have multiple structural isomers; that is, multiple local minima on their potential energy surface. About twenty-five years ago, it was observed that multiple conformers of tryptophan are present even at the low temperatures of a few Kelvin in a supersonic jet. These conformers have been studied extensively since then with sophisticated spectroscopic techniques. Individual conformers can be identified from their distinct electronic or microwave spectra. Information on the conformational structures can be obtained using microwave or multipleresonance infrared spectroscopy, for example. In similar experiments it was even possible to obtain information on the barriers separating the conformers. The preparation of spatially separated conformers would provide unique possibilities for advanced further investigations. The chemical properties of the individual species and their differences could be directly studied in reactive scattering experiments. Such pure samples would also enable a new class of experiments, such as electron and X-ray diffraction or tomographic imaging experiments of complex molecules in the gas phase. Molecular-frame photoelectron angular distributions, ultrafast time-resolved photoelectron spectroscopy, and ultrafast dynamics studies would also benefit from the availability of these pure samples. For charged species, the separation of molecules with different shapes has been demonstrated by utilizing ion mobility in drift tubes. For neutral molecules, the abundance of the conformers in molecular beams can be partly influenced by selective over-the-barrier excitation in the early stage of the expansion or by changing the carrier gas. Herein, we demonstrate that electrostatic deflection, a classic molecular beam manipulation method that dates back to the 1920s, allows the spatial separation of the conformers of a neutral molecule when it is applied to intense beams of rotationally cold molecules produced by a state-ofthe-art pulsed supersonic expansion source. The idea of exploiting electrostatic deflection to separate quantum states was already conceived by Stern in 1926 for light diatomic molecules, and these ideas were recently extended to proposals for the separation of conformers of large molecules. Polar molecules experience a force in an inhomogeneous electric field. This force is due to the spatial variation in the potential energy of the molecules, and is given by F ! = meff·r ! E. The effective dipole moment meff of a molecule in a given quantum state is the negative gradient of the potential energy with respect to the electric field strength E. This force has been used to decelerate small molecules in a supersonic jet to a standstill and subsequently trap them. Similarly, large neutral molecules have been deflected, focused, and decelerated. Passing polar molecules through a strong inhomogenous electric field will spatially disperse them according to their effective dipole moment. In particular, the conformers of a specific biomolecule all have the same mass m, but differ by the relative orientations of their functional groups. Typically, these functional groups have large local dipole moments associated with them, and the vectorial sum of these local dipole moments largely determines the overall dipole moment of the molecule. Herein, we show that the resulting different overall dipole moments of the conformers can be exploited to select individual conformers using an electrostatic deflector. The cis and trans conformers of 3-aminophenol (Figure 1) are used herein as prototypical structural isomers of complex molecules. From the precisely known rotational constants and dipole moments, the energies of the rotational states of cis3-aminophenol and trans-3-aminophenol are calculated as a function of electric field strength. Figure 1 shows the resulting Stark curves for the lowest rotational states of both species. From Figure 1, it is obvious that the effective dipole moments meff of the states of cis-3-aminophenol are considerably larger than for trans-3-aminophenol, and therefore, a strong spatial separation of the conformers is expected. The results on the separation of the cis and trans conformers of 3-aminophenol are complementary to our previous experiments on the separation of the same species using an ac (alternating current) focusing device, and we will briefly discuss the merits of the individual techniques in the Summary. The experimental setup is shown in Figure 2, and a detailed description is given elsewhere. 3-aminophenol (Sigma–Aldrich, 98%), seeded in 90 bar of helium, is released from a pulsed valve into high vacuum. The molecular beam is [*] F. Filsinger, Dr. J. K pper, Prof. Dr. G. Meijer Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+49)30-8413-5892 http://www.fhi-berlin.mpg.de/mp/jochen E-mail: jochen@fhi-berlin.mpg.de

Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules

The yield of strong-field ionization, by a linearly polarized probe pulse, is studied experimentally and theoretically as a function of the relative orientation between the laser field and the molecule. Experimentally, carbonyl sulphide (OCS), benzonitrile and naphthalene molecules are aligned in one or three dimensions before being singly ionized by a 30 fs laser pulse centred at 800 nm. Theoretically, we address the behaviour of these three molecules. We consider the degree of alignment and orientation and model the angular dependence of the total ionization yield by molecular tunnelling theory accounting for the Stark shift of the energy level of the ionizing orbital. For naphthalene and benzonitrile, the orientational dependence of the ionization yield agrees well with the calculated results, in particular, we observe that ionization is maximized when the probe laser is polarized along the most polarizable axis. For OCS the observation of the maximum ionization yield when the probe is perpendicular to the internuclear axis contrasts the theoretical results.

Ionization of one- and three-dimensionally-oriented asymmetric-top molecules by intense circularly polarized femtosecond laser pulses

We present a combined experimental and theoretical study on strong-field ionization of a three-dimensionally-oriented asymmetric top molecule, benzonitrile (C{sub 7}H{sub 5}N), by circularly polarized, nonresonant femtosecond laser pulses. Prior to the interaction with the strong field, the molecules are quantum-state selected using a deflector and three-dimensionally (3D) aligned and oriented adiabatically using an elliptically polarized laser pulse in combination with a static electric field. A characteristic splitting in the molecular frame photoelectron momentum distribution reveals the position of the nodal planes of the molecular orbitals from which ionization occurs. The experimental results are supported by a theoretical tunneling model that includes and quantifies the splitting in the momentum distribution. The focus of the present article is to understand strong-field ionization from 3D-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals. In the preceding article [Dimitrovski et al., Phys. Rev. A 83, 023405 (2011)] the focus is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons.

Mixed-field orientation of molecules without rotational symmetry

The mixed-field orientation of an asymmetric-rotor molecule with its permanent dipole moment nonparallel to the principal axes of polarizability is investigated experimentally and theoretically. We find that for the typical case of a strong, nonresonant laser field and a weak static electric field complete 3D orientation is induced if the laser field is elliptically polarized and if its major and minor polarization axes are not parallel to the static field. For a linearly polarized laser field solely the dipole moment component along the most polarizable axis of the molecule is relevant resulting in 1D orientation even when the laser polarization and the static field are nonparallel. Simulations show that the dipole moment component perpendicular to the most-polarizable axis becomes relevant in a strong dc electric field combined with the laser field. This offers an alternative approach to 3D orientation by combining a linearly polarized laser field and a strong dc electric field arranged at an angle equal to the angle between the most polarizable axis of the molecule and its permanent dipole moment.

Conformer separation of 3-aminophenol using an electrostatic deflector

In chemistry the word conformer describes one of a set of stereoisomers, all having the same molecular formula and identical constitution (connectivities between the atoms) but differing by the spatially orientations of their functional groups in the molecular frame. Often these structural differences between individual conformers are subtle. For 3-aminophenol (C6H7NO), hereafter 3AP, only the position of one single hydrogen atom distinguishes the two stable conformers cis- and trans-3AP, see fig. 2 a). In the case of 3AP this modest change gives rise to a significant rise in the permanent dipole moment (trans-3AP 0.77D vs. cis-3AP 2.3D).

Enhanced laser-induced alignment and orientation of molecules using quantum-state-selection

We demonstrate the use of strong inhomogeneous electric field to spatially disperse rotational states of a molecule[1, 2]. In the experiment a supersonic beam of iodobenzene molecules is formed and subsequently dispersed in an electric field. The molecules are then aligned using a focused ns YAG laser or oriented using a combination of the laser and a static electric field. The molecules are ionized with an intense fs laser pulse and the ionic fragments are detected using a velocity map image spectrometer. The recoil direction provides information about the spacial orientation of the molecules. By moving the laser focus it is possible to probe different rotational states. The state selection uses the fact that different rotational states experience different Stark shifts. The rotational ground state of iodobenzene will experience the largest Stark shift and will therefore be most strongly deflected in the electric field. The deflected profile of the molecular pulse has been measured and agrees well with trajectory simulations with a temperature of 1 K. By selecting molecules in the lowest rotational states a higher degree of alignment can be obtained even at modest laser fields. The combination of a laser field and a weak static electric field lifts the degeneracy of states pointing up and down through the interaction with the molecular dipole moment. Fig 1 shows examples of the enhanced alignment and orientation. The first row shows alignment, while the second and third shows the orientation for various angles β, between the laser and static electric field. In fig 2 the orientation is shown as a function of β. It is obvious that the quantum-state selection gives a large advantage with more than 70 % of the molecules being oriented the same way. In conclusion the use of an electrostatic deflector enables us to select the lowest lying rotational quantum-states. This allows creation of unprecedented alignment and orientation by laser and static fields