All attosecond time-resolved measurements have so far relied on the use of intense near-infrared laser pulses. In particular, attosecond streaking, laser-induced electron diffraction and high-harmonic generation all make use of non-perturbative light-matter interactions. Remarkably, the effect of the strong laser field on the studied sample has often been neglected in previous studies. Here we use high-harmonic spectroscopy to measure laser-induced modifications of the electronic structure of molecules. We study high-harmonic spectra of spatially oriented CH3F and CH3Br as generic examples of polar polyatomic molecules. We accurately measure intensity ratios of even and odd-harmonic orders, and of the emission from aligned and unaligned molecules. We show that these robust observables reveal a substantial modification of the molecular electronic structure by the external laser field. Our insights offer new challenges and opportunities for a range of emerging strong-field attosecond spectroscopies.

The field of cytometry has grown in scope and importance ever since the early 20th century with leaps in technology introducing the Coulter counter and the flow cytometer. Cytometry methods have brought about a revolution for the medical and biotechnology industry by providing fast and accurate analysis of cell and particle suspensions. Recent developments in the field aim at improving current cytometers and to provide miniaturized low-cost cytometry systems for point-of-care clinical diagnostics or research. In an attempt to address the need for particle positioning which is important for both impedance and optically based cytometers we present a microfluidic system which precisely positions cells and particles, using acoustic forces and subsequently performs measurements using an integrated and simple planar electrode Coulter-type impedance cytometer without the need for sheath flows. Data is presented to show how the acoustic method improves the accuracy of the impedance cytometer when prefocusing is employed to particles and cells (diluted whole blood). Confocal imaging and simulations support the findings and provide the basis for further improvements. The acoustophoretic prefocusing technique opens a path towards small, low cost cytometers while also providing an easy way to improve current systems.

Experiments in the gas phase usually involve averaging observables over a random molecular axis alignment distribution. This deleterious averaging limits insights gained by probes of molecular dynamics, but can be overcome by prealigning molecular axes using laser-alignment methods. However, the transformation from the laboratory frame to the molecular frame of reference requires quantitative knowledge of the axis alignment distribution. The latter is often hard to obtain directly from experimental data, particularly for polyatomic molecules. Here we describe a general maximum-likelihood classification procedure for non-adiabatic numerical alignment simulations with free parameters that employs experimental data from an alignment-dependent probe. This method delivers (i) the most probable molecular frame angular dependence of the probe, and (ii) the most likely laboratory frame axis alignment distribution of the sample, each with a confidence interval. This procedure was recently used for studies of angle- and channel-resolved strong field ionization of 1,3-butadiene in the molecular frame [Mikosch et al., Phys. Rev. Lett. 110, 023004 (2013)], used here as an illustrative example.

We investigate the electromagnetic design of whispering gallery mode (WGM) terahertz (THz) resonators. Terahertz radiation is generated by difference-frequency mixing of two electrically pumped high-order near-infrared laser WGM's at room temperature in the active cavity. Due to the leaky nature of the low-order THz WGM, the generated radiation can be efficiently emitted into free space. The inherent high symmetry of the THz WGM prohibits efficient THz emission in the normal direction from the device. We investigate techniques based on concentric surface plasmon polariton gratings to break this symmetry by modification of the dielectric environment of the resonator, and demonstrate a fabrication-optimized structure based on a concentric grating design which efficiently couples the emitted radiation into a narrow, near-gaussian forward-propagating cone of well-defined linear or circular polarization.

Chalcogenide glasses are receiving a lot of attention due to their unique optical properties. In this paper we study the optical properties of As2S3 and GaLaS glasses in a broad terahertz (THz) frequency range (0.2-18 THz). Complex dispersion behavior with drastic changes of refractive index and absorption coefficient is found for both glasses. We observe the breakdown of the universal power-law dependence of the absorption coefficient due to atomic vibrations observed at low THz frequencies in disordered materials, and see the transition to localized vibrational dynamics for the As2S3 compound at higher frequencies. In addition, As2S3 displays two transparency regions, at 7-8 THz and 12.2 THz, of potential interest for future nonlinear applications in the THz range.

We make use of an inhomogeneous electrostatic dipole field to impart a quantum-state-dependent deflection to a pulsed beam of OCS molecules, and show that those molecules residing in the absolute ground state, X(1)Σ(+), |00(0)0>, J = 0, can be separated out by selecting the most deflected part of the molecular beam. Past the deflector, we irradiate the molecular beam by a linearly polarized pulsed nonresonant laser beam that impulsively aligns the OCS molecules. Their alignment, monitored via velocity-map imaging, is measured as a function of time, and the time dependence of the alignment is used to determine the quantum state composition of the beam. We find significant enhancements of the alignment ( = 0.84) and of state purity (>92%) for a state-selected, deflected beam compared with an undeflected beam.

The study of many fundamental processes in chemistry relies on the understanding of the dynamics of the valence electrons, which make and break chemical bonds. A laser method now provides direct information on the dynamics of the valence electrons—separate from any vibrational motion—during a polyatomic chemical reaction, without the need for strong laser fields that unavoidably influence the motions of these electrons. Chemical reactions are manifestations of the dynamics of molecular valence electrons and their couplings to atomic motions. Emerging methods in attosecond science can probe purely electronic dynamics in atomic and molecular systems1,2,3,4,5,6. By contrast, time-resolved structural-dynamics methods such as electron7,8,9,10 or X-ray diffraction11 and X-ray absorption12 yield complementary information about the atomic motions. Time-resolved methods that are directly sensitive to both valence-electron dynamics and atomic motions include photoelectron spectroscopy13,14,15 and high-harmonic generation16,17: in both cases, this sensitivity derives from the ionization-matrix element18,19. Here we demonstrate a time-resolved molecular-frame photoelectron-angular-distribution (TRMFPAD) method for imaging the purely valence-electron dynamics during a chemical reaction. Specifically, the TRMFPADs measured during the non-adiabatic photodissociation of carbon disulphide demonstrate how the purely electronic rearrangements of the valence electrons can be projected from inherently coupled electronic–vibrational dynamics. Combined with ongoing efforts in molecular frame alignment20 and orientation21,22, TRMFPADs offer the promise of directly imaging valence-electron dynamics during molecular processes without involving the use of strong, highly perturbing laser fields23.

The photoelectron spectrum shows that multiphoton ionization of amyl nitrite, C(5)H(11)ONO, using ultrafast laser pulses deposits up to 3.7 eV of energy into internal degrees of freedom. As a result, the molecules fragment to produce various daughter ions of masses 87, 71, 60, 57, 41, 30, 29, and 27. Absorption of an additional photon with 3 eV of energy by the ions yields transients with picosecond decay times, revealing the time scale of the decomposition dynamics of the initially prepared parent ion. Each mass peak has a distinct time constant, in the range of 1.2 to 7.9 ps, emphasizing the dependence of the fragmentation mechanism on the ion internal energy.

Random orientation of molecules within a sample leads to blurred observations of chemical reactions studied from the laboratory perspective. Methods developed for the dynamic imaging of molecular structures and processes struggle with this, as measurements are optimally made in the molecular frame. We used laser alignment to transiently fix carbon disulfide molecules in space long enough to elucidate, in the molecular reference frame, details of ultrafast electronic-vibrational dynamics during a photochemical reaction. These three-dimensional photoelectron imaging results, combined with ongoing efforts in molecular alignment and orientation, presage a wide range of insights obtainable from time-resolved studies in the molecular frame.

We describe theoretically and experimentally a laser-based method to control the rotations of asymmetric top molecules in three-dimensional space. Our method relies on keeping one axis of a molecule essentially fixed in space along the polarization vector of a nanosecond laser pulse (termed the long pulse) and forcing the molecule to rotate about the aligned axis by an orthogonally polarized, femtosecond laser pulse (termed the short pulse). Experimentally, we use femtosecond timed Coulomb explosion to image the three-dimensional (3D) alignment of the 3,5-difluoroiodobenzene molecule as a function of time after the short pulse. Strong 3D alignment is observed a few picoseconds after the short pulse and is repeated periodically, reflecting directly the revolution of the molecular plane about the aligned axis. Our numerical results, based on nonperturbative solution of the time-dependent Schrodinger equation, are in good agreement with the experimental findings and serve to unravel the underlying physical mechanism of the observations. The experiments and theory explore the influence of the laser parameters on the rotational control, in particular the role played by the intensity of the long and the short laser pulses. To illustrate the generality of our method, we illustrate its applicability to a molecule (3,4-dibromothiophene), with significantly different inertia and polarizability tensors. Finally, our theory shows that the strong 3D alignment obtained by the combined laser pulse method can be converted in to field-free alignment by rapid truncation of the long laser pulse.

We present a summary of recent advances in the understanding of the UV photophysics of the isolated DNA base adenine, emphasizing a discussion of the mechanisms behind the ultrafast relaxation following excitation to the pipi* band. Drawing on our femtosecond time-resolved photoelectron spectroscopy experiments, we discuss differences in the ultrafast relaxation of adenine and 9-methyladenine and consider the relative merits of the various proposed mechanisms.

Using the positively charged aniline ion (C6H5NH2+) as a test molecule, we demonstrate that it is possible to study consecutive photodissociation of complex molecular ions at the single molecule level in an ion trap. When a single C6H5NH2+ ion is exposed to laser light at 397 nm and 294 nm, direct or consecutive photodissociation leads to the production of a range of smaller polyatomic molecular ions such as C5H6+ and C3H3+. The applied method is very versatile and can, e. g., be used in combination with free electron lasers or synchrotron radiation sources.

We combine newly developed experimental and theoretical methods to explore and control the rotations of asymmetric top molecules. In the low-temperature limit, the revival spectra of laser-driven rotational wave packets exhibit a fascinating quantum-mechanical structure, which is qualitatively different for asymmetric tops compared to symmetric or linear tops. With increasing fluence, the structure gradually simplifies, illustrating a controllable transformation of the rotational dynamics. Our theoretical and experimental predictions are in qualitative agreement.

We briefly review recent experimental developments of laser induced alignment of molecules with pulses that are either long (adiabatic) or short (nonadiabatic) compared to the molecular rotational periods. The main focus is on one-dimensional (1D) alignment obtained with linearly polarized laser pulses. Both linear and asymmetric top molecules are used to illustrate the alignment methods. Three-dimensional (3D) alignment, where all three molecular axes of a rigid asymmetric top molecule are orientationally confined, is also discussed.

We introduce a new tool to control the nonadiabatic alignment dynamics in asymmetric top molecules. Alignment is induced using nonresonant, linearly polarized laser pulses with pulse durations that are short compared to the rotational periods of the molecule. Through a series of nonresonant Raman transitions a rotational wave packet is created. It is concluded with an interesting connection between the duration of the alignment pulse and the wave packet dynamics, providing a new tool to transform the non-periodic revival structure of an asymmetric top, into stable periodic rotations about the C-axis of the molecule. The experimentally observed features are also reproduced qualitatively in numerical simulations.

The alignment dynamics of a symmetric top molecule, tert-butyliodide, induced by two picosecond, nonresonant, linearly polarized pulses is studied experimentally and numerically using a combination of time-resolved photofragment imaging and nonperturbative solution of the time-dependent Schrodinger equation. It is shown that the strongest degree of one-dimensional (single axis) field-free alignment obtainable with a single pulse can be enhanced using the two-pulse sequence in a parallel polarization geometry. The conditions for alignment enhancement are: (1) The second pulse must be sent near the time where the degree of alignment, due to the first pulse, peaks. (2) The intensity of the first pulse must be smaller than the intensity of the second pulse, the optimal intensity ratio being determined by the duration of the pulses. Numerically, we also discuss the existence of an optimal pulse duration, the effect of focal averaging on the degree of alignment, and compare two-pulse alignment to adiabatic alignment.