Louise Belshaw

Ultrafast Electron Dynamics in Phenylalanine Initiated by Attosecond Pulses

In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub–4.5-femtosecond temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems. Electronic dynamics in a complex polyatomic molecule are tracked faster than the time scale for vibrational motion. A very quick look at phenylalanine Over the past decade, laser technology has pushed back the fastest directly observable time scale from femtoseconds (quadrillionths of a second) to attoseconds (quintillionths of a second). For the most part, attosecond studies so far have probed very simple molecules such as H2 and O2. Calegari et al. now look at a more elaborate molecule—the amino acid phenylalanine. They tracked changes in the electronic structure of the compound after absorption of an ultrafast pulse, before the onset of conventional vibrational motion. Science, this issue p. 336

Observation of Ultrafast Charge Migration in an Amino Acid

We present the first direct measurement of ultrafast charge migration in a biomolecular building block - the amino acid phenylalanine. Using an extreme ultraviolet pulse of 1.5 fs duration to ionize molecules isolated in the gas phase, the location of the resulting hole was probed by a 6 fs visible/near-infrared pulse. By measuring the yield of a doubly charged ion as a function of the delay between the two pulses, the positive hole was observed to migrate to one end of the cation within 30 fs. This process is likely to originate from even faster coherent charge oscillations in the molecule being dephased by bond stretching which eventually localizes the final position of the charge. This demonstration offers a clear template for observing and controlling this phenomenon in the future.

LIAD-fs scheme for studies of ultrafast laser interactions with gas phase biomolecules.

Laser induced acoustic desorption (LIAD) has been used for the first time to study the parent ion production and fragmentation mechanisms of a biological molecule in an intense femtosecond (fs) laser field. The photoacoustic shock wave generated in the analyte substrate (thin Ta foil) has been simulated using the hydrodynamic HYADES code, and the full LIAD process has been experimentally characterised as a function of the desorption UV-laser pulse parameters. Observed neutral plumes of densities >10(9) cm(-3) which are free from solvent or matrix contamination demonstrate the suitability and potential of the source for studying ultrafast dynamics in the gas phase using fs laser pulses. Results obtained with phenylalanine show that through manipulation of fundamental femtosecond laser parameters (such as pulse length, intensity and wavelength), energy deposition within the molecule can be controlled to allow enhancement of parent ion production or generation of characteristic fragmentation patterns. In particular by reducing the pulse length to a timescale equivalent to the fastest vibrational periods in the molecule, we demonstrate how fragmentation of the molecule can be minimised whilst maintaining a high ionisation efficiency.

A comb-sampling method for enhanced mass analysis in linear electrostatic ion traps

In this paper an algorithm for extracting spectral information from signals containing a series of narrow periodic impulses is presented. Such signals can typically be acquired by pickup detectors from the image-charge of ion bunches oscillating in a linear electrostatic ion trap, where frequency analysis provides a scheme for high-resolution mass spectrometry. To provide an improved technique for such frequency analysis, we introduce the CHIMERA algorithm (Comb-sampling for High-resolution IMpulse-train frequency ExtRAaction). This algorithm utilizes a comb function to generate frequency coefficients, rather than using sinusoids via a Fourier transform, since the comb provides a superior match to the data. This new technique is developed theoretically, applied to synthetic data, and then used to perform high resolution mass spectrometry on real data from an ion trap. If the ions are generated at a localized point in time and space, and the data is simultaneously acquired with multiple pickup rings, the method is shown to be a significant improvement on Fourier analysis. The mass spectra generated typically have an order of magnitude higher resolution compared with that obtained from fundamental Fourier frequencies, and are absent of large contributions from harmonic frequency components.

Ultrafast Charge Dynamics in an Amino Acid Induced by Attosecond Pulses

In the past few years, attosecond techniques have been implemented for the investigation of ultrafast dynamics in molecules. The generation of isolated attosecond pulses characterized by a relatively high photon flux has opened up new possibilities in the study of molecular dynamics. In this paper, we report on experimental and theoretical results of ultrafast charge dynamics in a biochemically relevant molecule, namely, the amino acid phenylalanine. The data represent the first experimental demonstration of the generation and observation of a charge migration process in a complex molecule, where electron dynamics precede nuclear motion. The application of attosecond technology to the investigation of electron dynamics in biologically relevant molecules represents a multidisciplinary work, which can open new research frontiers: those in which few-femtosecond and even subfemtosecond electron processes determine the fate of biomolecules. It can also open new perspectives for the development of new technologies, for example, in molecular electronics, where electron processes on an ultrafast temporal scale are essential to trigger and control the electron current on the scale of the molecule.

Femtosecond lasers for mass spectrometry: Proposed application to catalytic hydrogenation of butadiene

Mass spectra from the interaction of intense, femtosecond laser pulses with 1,3-butadiene, 1-butene, and n-butane have been obtained. The proportion of the fragment ions produced as a function of intensity, pulse length, and wavelength was investigated. Potential mass spectrometry applications, for example in the analysis of catalytic reaction products, are discussed.

Fragmentation of Neutral Amino Acids and Small Peptides by Intense, Femtosecond Laser Pulses

AbstractHigh power femtosecond laser pulses have unique properties that could lead to their application as ionization or activation sources in mass spectrometry. By concentrating many photons into pulse lengths approaching the timescales associated with atomic motion, very strong electric field strengths are generated, which can efficiently ionize and fragment molecules without the need for resonant absorption. However, the complex interaction between these pulses and biomolecular species is not well understood. To address this issue, we have studied the interaction of intense, femtosecond pulses with a number of amino acids and small peptides. Unlike previous studies, we have used neutral forms of these molecular targets, which allowed us to investigate dissociation of radical cations without the spectra being complicated by the action of mobile protons. We found fragmentation was dominated by fast, radical-initiated dissociation close to the charge site generated by the initial ionization or from subsequent ultrafast migration of this charge. Fragments with lower yields, which are useful for structural determinations, were also observed and attributed to radical migration caused by hydrogen atom transfer within the molecule. Figureᅟ

Ultrafast electron dynamics in an amino acid measured by attosecond pulses

Electron transfer within a single molecule is the fundamental step of many biological processes and chemical reactions. It plays a crucial role in catalysis, DNA damage by ionizing radiation, photosynthesis, photovoltaics, and for switches based on molecular nano-junctions. The investigation of this process has been the subject of considerable research effort [1-2]. In this work we present the first direct measurement of ultrafast charge migration in a biomolecular building block, the amino acid phenylalanine, using attosecond pulses [3]. Clean plumes of isolated, neutral molecules were produced by laser induced acoustic desorption (LIAD) technique [4]. Phenylalanine molecules were irradiated by a short train of attosecond pulses produced by high-order harmonic generation in Xenon (the train was composed by two attosecond pulses), with a photon energy in the range 16-40 eV, followed at a variable temporal delay by a 6-fs visible/near infrared (500-950 nm, VIS/NIR) probe pulse. The parent and fragment ions produced were then extracted into a linear time of flight device for mass analysis. Figures 1(a)-(b) show the mass spectra obtained individually from the XUV and VIS/NIR pulses.

Ionisation and Fragmentation of Small Biomolecules with Femtosecond LaserPulses

We report on femtosecond laser studies of small biomolecules, produced in the gas phase via laser induced acoustic desorption. In studies of the aromatic amino acid phenylalanine, we have found that the wavelength of the femtosecond pulse can be used to manipulate molecular fragmentation. These preliminary experiments indicate that this technique provides a promising scheme for investigations of ultrafast dynamics in complex molecules. Future studies should enable temporally resolved observations, by employing ultrafast pump-probe techniques.

LIAD-fs: A novel method for studies of ultrafast processes in gas phase neutral biomolecules

A new experimental technique for femtosecond (fs) pulse studies of gas phase biomolecules is reported. Using Laser-Induced Acoustic Desorption (LIAD) to produce a plume of neutral molecules, a time-delayed fs pulse is employed for ionisation/fragmentation, with subsequent products extracted and mass analysed electrostatically. By varying critical laser pulse parameters, this technique can be used to implement control over molecular fragmentation for a range of small biomolecules, with specific studies of amino acids demonstrated.