William A Bryan

• Physics PhD (University of London)
• Professor (Associate) at Swansea University

Recent developments in ultrafast electron microscopy have shown that spatial and temporal information can be collected simultaneously on very small and fast scales. In the present work, an instrumental design study with application to nanoscale dynamics, we optimize the conditions for a femtosecond transmission electron microscope (fs-TEM). The fs-...

Femtosecond electron microscopy produces real-space images of matter in a series of ultrafast snapshots. Pulses of electrons self-disperse under space-charge broadening, so without compression, the ideal operation mode is a single electron per pulse. Here, we demonstrate femtosecond single-electron point projection microscopy (fs-ePPM) in a laser-p...

Femtosecond vacuum ultraviolet pulses from a monochromated high harmonic generation source excite vibrational wavepackets in the B1 Sg+ state of D2. The wavepacket motion is measured through strong field ionization into bound and dissociative ion states yielding D2+ and D+ products. The time dependence of the D2+ and D+ ion signals provides a sensi...

A novel application of velocity-map imaging (VMI) is demonstrated, whereby the momentum distribution of photoelectrons ejected from a tungsten nanoscale metal tip (< 50 nm radius) is recorded following illumination with an ultrafast laser pulse. The electrostatic conditions in the VMI instrument are optimized through finite element modelling, takin...

Extreme-ultraviolet high-order-harmonic pulses with 1.6·10⁷ photons/pulse at 32.5eV have been separated from multiple harmonic orders by a time-preserving monochromator using a single grating in the off-plane mount. This grating geometry gives minimum temporal broadening and high efficiency. The pulse duration of the monochromatized harmonic pulses...

Femtosecond vacuum ultraviolet pulses from a monochromated high harmonic generation source excite vibrational wavepackets in the ${B}^{1}{{\rm{\Sigma }}}_{{\rm{g}}}^{+}$ state of D2. The wavepacket motion is measured through strong field ionization into bound and dissociative ion states yielding ${{\rm{D}}}_{2}^{+}$ and D+ products. The time depend...

Accelerator based Ultrafast Electron Diffraction (UED) is a technique for static and dynamic structural studies in material and biological sciences. The recently commissioned VELA accelerator at the Daresbury Laboratory provides multi-MeV beams for science and industry and will provide a test bed for the UK electron diffraction community. We presen...

High 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 frag...

Electron dynamics underlie the charge and energy transfer processes in molecules. These processes, although extremely fast, can be resolved by sub-femtosecond light pulses and the electron motion can, to some extent, be controlled. A novel scheme for electron localization using intense IR lasers has recently been developed using phase-locked multip...

Results will be presented from recent experimental studies of intense femtosecond (fs) laser pulse interactions with small organic molecules. By electrostatically trapping the ionisation products to obtain high resolution mass spectra, we have characterised how fragment spectra change for a range of laser pulse parameters.

The use of strong-field (i.e. intensities in excess of 10^13 Wcm-2) few-cycle ultrafast (durations of 10 femtoseconds or less) laser pulses to create, manipulate and image vibrational wavepackets is investigated. Quasi-classical modelling of the initial superposition through tunnel ionization, wavepacket modification by nonadiabatically altering th...

XUV pulses produced through high harmonic generation can probe electron dynamics in complex solid materials and in gas-phase atoms and molecules. This is demonstrated in gas-phase and condensed matter experiments at the Artemis facility.

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 f...

Extreme ultraviolet (XUV) pulses with a duration of tens of femtoseconds initiate 4s−1 or 4p−1 photoionization of krypton, which populates highly excited satellite states through the electron correlation. The excited ions are then tunnel ionized to Kr2+4s−14p−1 or 4p−2 by a strong-field near-infrared (NIR) pulse of a similar duration. The XUV pulse...

A new HHG XUV beamline at Artemis, user open-access facility at CLF, offers unique capabilities optimised for Tr-ARPES. Current result on ultrafast melting of Mott and charge order in TaS2 will be presented.

The use of strong-field (i.e. intensities in excess of 10(13) Wcm(-2)) few-cycle ultrafast (durations of 10 femtoseconds or less) laser pulses to create, manipulate and image vibrational wavepackets is investigated. Quasi-classical modelling of the initial superposition through tunnel ionization, wavepacket modification by nonadiabatically altering...

Few-cycle high peak power laser pulses produced in gas-filled hollow fibres have a EH11 Bessel mode. Control of the intensity and phase-matching conditions requiring no additional dispersion is introduced, and is often achieved by closing a hard aperture around the beam. We use the Huygens–Fresnel diffraction integral to propagate such a beam throu...

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...

We present an experimental demonstration of nonresonant manipulation of vibrational states in a molecule by an intense ultrashort laser pulse. A vibrational wave packet is generated in D{sub 2}{sup +} through tunnel ionization of D{sub 2} by a few-cycle pump pulse. A similar control pulse is applied as the wave packet begins to dephase so that the...

A quasi-classical model (QCM) of nuclear wavepacket generation, modification and imaging by three intense ultrafast near-infrared laser pulses has been developed. Intensities in excess of 1013 W cm-2 are studied, the laser radiation is non-resonant and pulse durations are in the few-cycle regime, hence significantly removed from the conditions typi...

Recent advances in the study of quantum vibrations and rotations in the fundamental hydrogen molecules are reported. Using the deuterium molecules (D2+ and D2) as exemplars, the application of ultrafast femtosecond pump-probe experiments to study the creation and time-resolved imaging of coherent nuclear wavepackets is discussed. The ability to stu...

Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected. Such electronic interactions are predicted to be weak, the exc...

The Artemis facility for ultrafast XUV science is constructed around a high average power carrier-envelope phasestabilised system, which is used to generate tuneable pulses across a wavelength range spanning the UV to the far infrared, few-cycle pulses at 800nm and short pulses of XUV radiation produced through high harmonic generation. The XUV pul...

A novel scheme for enhancing electron localization in intense-field dissociation is outlined. Through manipulation of a bound vibrational wavepacket in the exemplar deuterium molecular ion, simulations demonstrate that the application of multiple phase-locked, few-cycle IR pulses can provide a powerful scheme for directing the molecular dissociatio...

An electrostatic trapping scheme for use in the study of light-induced dissociation of molecular ions is outlined. We present a detailed description of the electrostatic reflection storage device and specifically demonstrate its use in the preparation of a vibrationally cold ensemble of deuterium hydride (HD+) ions. By interacting an intense femtos...

H+3 is the simplest triatomic molecule and plays an important role in laboratory and astrophysical plasmas. It is very stable both in terms of its electronic and nuclear degrees of freedom but is difficult to study in depth in the laboratory due to its ionic nature. In this communication, experimental results are presented for the strong field diss...

The capability of intense ultrashort laser pulses to initiate, control and image vibrational wavepacket dynamics in the deuterium molecular ion has been simulated with a view to inform and direct future femtosecond pump–control–probe experiments. The intense-field coherent control of the vibrational superposition has been studied as a function of p...

We have explored the use of laser driven high-order harmonic generation to probe the electronic structure and symmetry of conjugated polyatomic molecular systems. We have investigated non-adiabatically aligned samples of linear symmetric top, nonlinear symmetric top and asymmetric top molecules, and we have observed signatures of their highest occu...

Tunnel ionization of room-temperature D2 in an ultrashort (12 femtosecond) near infrared (800 nm) pump laser pulse excites a vibrational wavepacket in the D2+ ions; a rotational wavepacket is also excited in residual D2 molecules. Both wavepacket types are collapsed a variable time later by an ultrashort probe pulse. We isolate the vibrational wave...

Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be...

A coherent superposition of rotational states in D2 has been excited by nonresonant, ultrafast (12 fs), intense (2×1014 W cm−2) 800 nm laser pulses, leading to impulsive dynamic alignment. Field-free evolution of this rotational wave packet has been mapped to high temporal resolution by a time-delayed pulse, initiating rapid double ionization, whic...

The dynamics of dissociation of pre-ionized D+2 molecules using intense (1012-1015 W cm-2), ultrashort (50 fs), infrared (lambda = 790 nm) laser pulses are examined. Use of an intensity selective scan technique has allowed the deuterium energy spectrum to be measured over a broad range of intensity. It is found that the dominant emission shifts to...

A novel technique is proposed to control the dissociation mechanism of small diatomic molecules. This technique, relying upon the creation of a coherent nuclear wavepacket, uses intense (>1014 W cm-2), ultrashort (~10 fs) infrared laser pulses in a pump and probe scheme. In applying this technique to D+2 good agreement has been observed between a q...

DOI:https://doi.org/10.1103/PhysRevLett.98.239903

The creation of a vibrational wavepacket within a molecular system shows great promise as an active method of both tracking and controlling nuclear motion on femtosecond timescales. However the coherent excitation of the vibrational eigenstates of the molecule, and the subsequent imaging of the bond vibration, requires pulse durations on the order...

The effects of electronic structure and symmetry are observed in laser driven high-order harmonic generation for laser aligned conjugated polyatomic molecular systems. The dependence of the harmonic yield on the angle between the molecular axis and the polarization of the driving laser field is seen to contain the fingerprint of the highest occupie...

Few-cycle laser pulses are used to “pump and probe” image the vibrational wavepacket dynamics of a HD molecular ion. The quantum dephasing and revival structure of the wavepacket are mapped experimentally with time-resolved photodissociation imaging. The motion of the molecule is simulated using a quantum-mechanical model predicting the observed st...

The experimental study of molecular dissociation of H2+ by intense laser pulses is complicated by the fact that the ions are initially produced in a wide range of vibrational states, each of which responds differently to the laser field. An electrostatic storage device has been used to radiatively cool HD+ ions enabling the observation of above thr...

Ultrashort intense laser pulses have been used to initiate and image a dissociating D+2nuclear wavepacket. Using a pump probe technique, the temporal evolution of the dissociation process has been observed as well as interferometric effects and signatures of bound wavepacket motion.

The vibrational wavepacket revival of a basic quantum system is demonstrated experimentally. Using few-cycle laser pulse technology, pump and probe imaging of the vibrational motion of D+2 molecules is conducted, and together with a quantum-mechanical simulation of the excited wavepacket motion, the vibrational revival phenomenon has been character...

Short intense laser pulses have recently become a viable and efficient method of impulsively inducing alignment of molecules, ranging from simple diatomics to more exotic structures. Key to the widespread applicability of this technique is that the maxima and minima of alignment occur under field-free conditions at delayed periodic intervals corres...

Ultrashort intense laser pulses have been used to initiate and image a dissociating D-2(+) nuclear wavepacket. Using a pump probe technique, the temporal evolution of the dissociation process has been observed as well as interferometric effects and signatures of bound wavepacket motion.

The vibrational wavepacket revival of a basic quantum system is demonstrated experimentally. Using few-cycle laser pulse technology, pump and probe imaging of the vibrational motion of D-2(+) molecules is conducted, and together with a quantum-mechanical simulation of the excited wavepacket motion, the vibrational revival phenomenon has been charac...

The two- and three-body Coulomb explosion of carbonyl sulfide (OCS) by 790 nm, 50 fs laser pulses focused to approximate to 10(16) W cm(-2) has been investigated by the three-dimensional covariance mapping technique. In a triatomic molecule, a single charge state, in this case the trication, has been observed to dissociate into two distinct energy...

Dissociation of the singly ionized CO2+ ion has been investigated in an intense ultrafast (55 fs) laser field, by employing an intensity selective scan technique and comparing the signals from linearly and circularly polarized pulses. Non-sequential contributions have been observed unambiguously for the first time, highlighting the role of rescatte...

Dissociation of the CO{sub 2}{sup +} ion has been investigated in an intense ultrafast (55 fs) laser field by employing an intensity-selective scan technique and comparing the signals from linearly and circularly polarized pulses. Nonsequential contributions have been observed, highlighting the role of rescattering in the dissociative process.

A detailed investigation has been carried out of N2 molecules in intense 55 and 220fs , linear and circular polarized, 790nm laser pulses. Using an intensity selective scanning technique, ionization, dissociation, and dissociative ionization channels have been studied. Remarkably similar enhancements of signal with linear polarization observed for...

We report an experimental technique for the comparison of ionization processes in ultrafast laser pulses irrespective of pulse ellipticity. Multiple ionization of xenon by 50 fs 790 nm, linearly and circularly polarized laser pulses is observed over the intensity range 10 TW∕cm2 to 10 PW∕cm2 using effective intensity matching (EIM), which is couple...

The probability of multiple ionization of krypton by 50 femtosecond circularly polarized laser pulses, independent of the optical focal geometry, has been obtained for the first time. The excellent agreement over the intensity range 10 TWcm-2 to 10 PWcm-2 with the recent predictions of A. S. Kornev et al [Phys. Rev. A v.68, art.043414 (2003)] provi...

Application of a parallel-projection inversion technique to z-scan spectra of multiply charged xenon and krypton ions, obtained by non-resonant field ionization of neutral targets, has for the first time permitted the direct observation of intensity-dependent ionization probabilities. These ionization efficiency curves have highlighted the presence...

We report a novel experimental technique for the comparison of ionization processes in ultrafast laser pulses irrespective of pulse ellipticity. Multiple ionization of xenon by 50 fs 790 nm, linearly and circularly polarized laser pulses is observed over the intensity range 10 TW/cm^2 to 10 PW/cm^2 using Effective Intensity Matching (EIM), which is...

Non-sequential processes in the multiple ionization of Xe and Xe+ targets subject to intense femtosecond laser pulses have been investigated. A precise ratio has been determined for the direct comparison of ionization using circular and linear polarized fields. Suppression of non-sequential effects where an ionic target is compared to a neutral ato...

The interaction of a 60 fs 790 nm laser pulse with beams of Ar$^+$, C$^+$, H$_2^+$, HD$^+$ and D$_2^+$ are discussed. Intensities up to 10$^{16}$ Wcm$^{-2}$ are employed. An experimental $z$-scanning technique is used to resolve the intensity dependent processes in the confocal volume. ultiphoton ionization and excitation to highly excited states (...

The interaction of an intense laser field with a beam of atomic ions has been investigated experimentally for the first time. The ionization dynamics of Ar+ ions and Ar neutrals in a 60 fs, 790 nm laser pulse have been compared and contrasted at intensities up to 10(16) W cm-2. Our results show that nonsequential ionization from an Ar+ target is st...

The Coulomb explosion of OCS in laser pulses of 55 fs and peak intensity of 2×1015 W cm-2 has been investigated using an ion-momentum imaging technique to determine the shape of the exploding molecule. The results, which are interpreted using a generalized, classical, enhanced ionization model show a range of geometries consistent with earlier meas...

There is considerable current interest in the study of atoms and molecules in very strong electric fields. The advent of short pulse high power lasers has enabled new experimental studies in this area leading to the discovery of new effects. We describe an experiment designed to extend these studies to targets of positive ions produced from convent...

We have observed the variation in ion signal as a function of intensity within a focused laser spot. Using an aperture detector, the ion signals from narrow bands of the laser focus have been observed. By moving the laser focus along the direction of propagation, regions of different intensities are exposed to the detector. This has allowed detaile...

A fast beam of H2+ ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, product yields as a function of focused las...

The Coulomb explosion of carbon dioxide in a 55 fs laser pulse of intensity 1.5-3 × 1016 W cm-2 has been studied using a variety of techniques based on time-of-flight mass spectroscopy. Covariance mapping has been used to identify Coulomb explosion channels and to measure the associated kinetic energy release. By comparing time-of-flight spectra ta...

A fast beam of H [iopmath latex="$_{2}^+$"] 2+ [/iopmath] ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, prod...

The Coulomb explosion of H2O in laser pulses of 50 fs and intensity of 3×1016W/cm2 has been investigated using an ion momentum-imaging technique to determine the shape of the exploding molecule. The molecule is found to straighten substantially during the ionization process with the bend distribution at a maximum between 130° and 180°.

The viability of using beams of molecular ions as a target for strong field fragmentation studies using intense ultra-short laser pulses is demonstrated. In this way the production mechanism for multiply charged ions in strong fields may be elucidated.

Two 50 fs laser pulses with orthogonal laser E-fields are superposed with a time delay of less than the pulse duration, producing a single peaked pulse with a continuously varying ratio between the horizontal and vertical laser E-fields. Since the ejection angle of fragments in the multiphoton dissociative ionization of is dependent upon the laser...

We report on direct measurements of the angular distributions of fragment ions produced in interactions with 60 fs laser pulses at 750 nm. These distributions show dramatically the bent nature of the dissociating molecule, and allow observation of the alignment of the different dissociation channels. It is found that for the lowest channel of multi...

In this work we report the first measurements of the interaction of 60 fs laser pulses at 750 nm with vibrationally excited 0953-4075/31/2/004/img9. Comparison with ground state 0953-4075/31/2/004/img9 is made and enhanced kinetic energies for 0953-4075/31/2/004/img11 and 0953-4075/31/2/004/img12 observed. These observations are consistent with unu...

The first detailed observations of the multielectron-dissociative-ionization of sulphur hexafluoride have been made using 60 fs laser pulses at 750 nm and 0953-4075/30/20/013/img10. The fragmentation pattern displays features similar to those found in experiments with diatomic and triatomic molecules, such as a tendency towards symmetrically dissoc...

Quantum mechanics describes internal molecular dynamics in terms of the amplitude and phase of vibrational wavepackets, which are coherent superpositions of states and contain all information about the associated populations and phases. In the energy or frequency domain, the quantum beating is observable by resonant photonic processes; in the tempo...

Advances in laser technology have allowed the experimentalist to subject matter to electric fields of sufficient strength to multiply ionize atoms and molecules. In the present study, the results of two complementary experiments will be presented, where the manner in which carbonyl sulfide (OCS) is ionized, and the evolution of the molecular geomet...

2 Introduction Presented here is a study of the revival of a rotational wavepacket in deuterium observed in a pump-probe experiment, which demonstrates impulsive alignment of the molecule to the laser field polarization direction. The polarization angle of the ultrafast pump pulse was rotated, causing impulsive alignment of the molecule to this axi...

Synopsis We present experimental evidence of the manipulation of vibrational phase and population by light-induced dynamic potential surface deformation. A vibrational wavepacket is created in D + 2 in a 10 fs NIR pump pulse; the evolution of the wavepacket is non-destructively modified by a control pulse. Depending on the pump-control delay, popul...