Robin M. Hochstrasser

Intense terahertz pulses by four-wave rectification in air

We describe a new four-wave rectification method for the generation of intense, ultrafast terahertz (THz) pulses from gases. The fundamental and second-harmonic output of an amplified Ti:sapphire laser is focused to a peak intensity of ~5x10(14)W/cm (2) . Under these conditions, peak THz fields estimated at 2 kV/cm have been observed; the measured power spectrum peaks near 2 THz. Phase-dependent measurements show that this is a coherent process and is sensitive to the relative phases of the fundamental and second-harmonic pulses. Comparable THz signals have been observed from nitrogen and argon as well as from air.

Wide-field subdiffraction imaging by accumulated binding of diffusing probes

A method is introduced for subdiffraction imaging that accumulates points by collisional flux. It is based on targeting the surface of objects by fluorescent probes diffusing in the solution. Because the flux of probes at the object is essentially constant over long time periods, the examination of an almost unlimited number of individual probe molecules becomes possible. Each probe that hits the object and that becomes immobilized is located with high precision by replacing its point-spread function by a point at its centroid. Images of lipid bilayers, contours of these bilayers, and large unilamellar vesicles are shown. A spatial resolution of ≈25 nm is readily achieved. The ability of the method to effect rapid nanoscale imaging and spatial resolution below Rayleigh criterion and without the necessity for labeling with fluorescent probes is proven.

Two-photon absorption by rotating diatomic molecules

Explicit forms for the two-photon absorption cross section in linearly and circularly polarized light for rotating diatomic molecules have been developed. The ratio (σλλ/σ cc ) of the cross sections for individual rotational transitions is precisely two-thirds for all branches of ΔΩ = 0, ±1, ±2 transitions except for the unique case—the Q branch of a ΔΩ=0 transition. In the latter case the ratio becomes J-dependent and may range from infinity to 4/1 (for high J) for different relative contributions to the absorption cross section of intermediate states having either the same or different electronic symmetry. These contributions are thus experimentally accessible in this favourable situation.

Perturbations between Electronic States in Aromatic and Heteroaromatic Molecules

The low‐temperature electronic spectra of several aromatic azines in regions of nπ* and ππ* overlap are presented and compared. It is shown that transitions to the higher‐energy state are generally quite diffuse compared with those to the lower. Comparisons are made with aromatic hydrocarbons where ππ* states overlap and aromatic carbonyl compounds where nπ* and ππ* states overlap. The factors which determine the severity of broadening of higher states are discussed. It is concluded that when two states of different orbital type are nearby, there is a more severe breakdown of the Born–Oppenheimer approximation. This breakdown is less severe if the two states are far apart or if the molecule has a similar geometry in both states. The presentation includes accounts of the low‐temperature singlet–triplet and/or singlet–singlet crystal spectra or mixed‐crystal spectra of pyrazine, 2.6‐dimethylpyrazine, tetramethyl pyrazine, pyrimidine, pyridazine, quinoxaline, phenazine, and diazaphenanthrene. The spectra of ...

Solvent influence on photoisomerization dynamicsa)

The rate constants of the nonradiative decay process of the excited singlet state of trans‐stilbene and trans‐1,1′‐biindanylidene (‘‘stiff ’’ stilbene) were measured in liquid n‐alkanes at room temperature. These data were compared with the two models for isomerization in condensed phases proposed by Kramers and by Grote and Hynes. For trans‐stilbene, the agreement with Kramers’ model was unsatisfactory, whereas better agreement was obtained with Grote and Hynes’ model. The data for stiff stilbene agreed well with Kramers’ model in the Smoluchowski limit. These results were explained in terms of the shape of the potential energy barrier which separates the trans from the twisted configuration of the excited singlet state of these molecules. It is suggested that the deviation between theory and experiment is a result of a qualitative difference in the microscopic solvent–solute collisional interaction when the solvent molecules are longer than stilbene. The best fit of theory to experiment for trans‐stilbe...

Two-dimensional IR spectroscopy can be designed to eliminate the diagonal peaks and expose only the crosspeaks needed for structure determination

The power of two-dimensional (2D) IR spectroscopy as a structural method with unprecedented time resolution is greatly improved by the introduction of IR polarization conditions that completely eliminate diagonal peaks from the spectra and leave only the crosspeaks needed for structure determination. This approach represents a key step forward in the applications of 2D IR to proteins, peptides, and other complex molecules where crosspeaks are often obscured by diagonal peaks. The technique is verified on the model compound 1,3-cyclohexanedione and subsequently used to clarify the distribution of structures that the acetylproline-NH2 dipeptide adopts in chloroform. In both cases, crosspeaks are revealed that were not observed before, which, in the case of the dipeptide, has led to additional information about the structure of the amino group end of the peptide.

Two-dimensional spectroscopy at infrared and optical frequencies

This Perspective on multidimensional spectroscopy in the optical and infrared spectral regions focuses on the principles and the scientific and technical challenges facing these new fields. The methods hold great promise for advances in the visualization of time-dependent structural changes in complex systems ranging from liquids to biological assemblies, new materials, and fundamental physical processes. The papers in this special feature on multidimensional spectroscopy in chemistry, physics, and biology are typical of many recent advances.

Applications of 2D IR spectroscopy to peptides, proteins, and hydrogen-bond dynamics

Following a survey of 2D IR principles, this article describes recent experiments on the hydrogen-bond dynamics of small ions, amide-I modes, nitrile probes, peptides, reverse transcriptase inhibitors, and amyloid fibrils.

Femtosecond laser studies of the cis‐stilbene photoisomerization reactions

Femtosecond laser studies have been performed on the photoisomerization reactions of cis‐stilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the ∼150 fs time scale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans‐stilbene product using transient fluorescence and transient absorption, confi...

Two-dimensional infrared spectroscopy: a promising new method for the time resolution of structures

Recently, new methods for determining time-evolving structures using infrared analogs of NMR spectroscopy have been introduced that have outstanding potential in structural biology. Already, within the past two years, structures of dipeptides, tripeptides and pentapeptides have been determined on much faster timescales than the conformational dynamics. Also, two-dimensional infrared correlation spectra of some proteins and isotopically edited alanine-rich helices have been examined.