All Publications

  • Necklace-structured high-harmonic generation for low-divergence, soft x-ray harmonic combs with tunable line spacing. Science advances Rego, L., Brooks, N. J., Nguyen, Q. L., Roman, J. S., Binnie, I., Plaja, L., Kapteyn, H. C., Murnane, M. M., Hernandez-Garcia, C. 2022; 8 (5): eabj7380


    The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wave function and, in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse necklace-shaped phased array of linearly polarized HHG emitters, where orbital angular momentum conservation allows us to tune the line spacing and divergence properties of extreme ultraviolet and soft x-ray high-harmonic combs. The on-axis HHG emission has extremely low divergence, well below that obtained when using Gaussian driving beams, which further decreases with harmonic order. This work provides a new degree of freedom for the design of harmonic combs-particularly in the soft x-ray regime, where very limited options are available. Such harmonic beams can enable more sensitive probes of the fastest correlated charge and spin dynamics in molecules, nanoparticles, and materials.

    View details for DOI 10.1126/sciadv.abj7380

    View details for PubMedID 35119926

  • Bright, single helicity, high harmonics driven by mid-infrared bicircular laser fields OPTICS EXPRESS Dorney, K. M., Fan, T., Nguyen, Q. D., Ellis, J. L., Hickstein, D. D., Brooks, N., Zusin, D., Gentry, C., Hernandez-Garcia, C., Kapteyn, H. C., Murnane, M. M. 2021; 29 (23): 38119-38128


    High-harmonic generation (HHG) is a unique tabletop light source with femtosecond-to-attosecond pulse duration and tailorable polarization and beam shape. Here, we use counter-rotating femtosecond laser pulses of 0.8 µm and 2.0 μm to extend the photon energy range of circularly polarized high-harmonics and also generate single-helicity HHG spectra. By driving HHG in helium, we produce circularly polarized soft x-ray harmonics beyond 170 eV-the highest photon energy of circularly polarized HHG achieved to date. In an Ar medium, dense spectra at photon energies well beyond the Cooper minimum are generated, with regions composed of a single helicity-consistent with the generation of a train of circularly polarized attosecond pulses. Finally, we show theoretically that circularly polarized HHG photon energies can extend beyond the carbon K edge, extending the range of molecular and materials systems that can be accessed using dynamic HHG chiral spectro-microscopies.

    View details for DOI 10.1364/OE.440813

    View details for Web of Science ID 000716468800091

    View details for PubMedID 34808871

  • Diol isomer revealed as a source of methyl ketene from propionic acid unimolecular decomposition INTERNATIONAL JOURNAL OF CHEMICAL KINETICS Rogers, C. O., Lockwood, K. S., Nguyen, Q. D., Labbe, N. J. 2021

    View details for DOI 10.1002/kin.21532

    View details for Web of Science ID 000697229600001

  • Detection of the keto-enol tautomerization in acetaldehyde, acetone, cyclohexanone, and methyl vinyl ketone with a novel VUV light source PROCEEDINGS OF THE COMBUSTION INSTITUTE Couch, D. E., Nguyen, Q. D., Liu, A., Hickstein, D. D., Kapteyn, H. C., Murnane, M. M., Labbe, N. J. 2021; 38 (1): 1737-1744
  • Highly-excited state properties of cumulenone chlorides in the vacuum-ultraviolet PHYSICAL CHEMISTRY CHEMICAL PHYSICS Nguyen, Q. D., Peters, W. K., Fortenberry, R. C. 2020; 22 (21): 11838-11849


    Recent observations of chloromethane in interstellar environments suggest that other organohalogens, which are known to be critically important in Earth's atmosphere, may also be of significance beyond our own terrestrial veil. This raises the question of how such molecules behave under extreme conditions such as when exposed to vacuum ultraviolet (VUV) radiation. VUV photons promote molecules to highly excited states that fragment in non-statistical patterns controlled by the initial femtosecond dynamics. A detailed understanding of VUV-driven photochemistry in complex organic molecules that consist of more than one functional group is a particularly challenging task. This quantum chemical analysis reports the electronic states and ionization potentials up to the VUV range (6-11 eV) of the chlorine-substituted cumulenone series molecules. The valence and Rydberg properties of lone-pair terminated, π-conjugated systems are explored for their potential resonance with lone pairs from elsewhere in the system. The carbon chain elongation within the family ClHCnO, where n = 1-4, influences the electronic excitations, associated wavefunctions, and ionization potentials of the molecules. The predicted geometries and ionization potentials are in good agreement with the available experimental photoelectron spectra for formyl chloride and chloroketene, n = 1-2. Furthermore, comparison between the regular cumulenone species and the corresponding chlorinated derivatives exhibit similar behaviors especially for n = 3, where the allene backbone in propadienone chloride is severely bent. Most notably for the excited states is that the Rydberg character becomes more dominant as the energy increases, with some retaining valence characters.

    View details for DOI 10.1039/d0cp01835j

    View details for Web of Science ID 000540513500004

    View details for PubMedID 32426777

  • Generation of extreme-ultraviolet beams with time-varying orbital angular momentum SCIENCE Rego, L., Dorney, K. M., Brooks, N. J., Nguyen, Q. L., Liao, C., San Roman, J., Couch, D. E., Liu, A., Pisanty, E., Lewenstein, M., Plaja, L., Kapteyn, H. C., Murnane, M. M., Hernandez-Garcia, C. 2019; 364 (6447): 1253-+


    Light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics, and microparticle manipulation. We introduce a property of light beams, manifested as a temporal OAM variation along a pulse: the self-torque of light. Although self-torque is found in diverse physical systems (i.e., electrodynamics and general relativity), it was not realized that light could possess such a property. We demonstrate that extreme-ultraviolet self-torqued beams arise in high-harmonic generation driven by time-delayed pulses with different OAM. We monitor the self-torque of extreme-ultraviolet beams through their azimuthal frequency chirp. This class of dynamic-OAM beams provides the ability for controlling magnetic, topological, and quantum excitations and for manipulating molecules and nanostructures on their natural time and length scales.

    View details for DOI 10.1126/science.aaw9486

    View details for Web of Science ID 000473271300039

    View details for PubMedID 31249031

  • Controlling the polarization and vortex charge of attosecond high-harmonic beams via simultaneous spin-orbit momentum conservation NATURE PHOTONICS Dorney, K. M., Rego, L., Brooks, N. J., San Roman, J., Liao, C., Ellis, J. L., Zusin, D., Gentry, C., Nguyen, Q. L., Shaw, J. M., Picon, A., Plaja, L., Kapteyn, H. C., Murnane, M. M., Hernandez-Garcia, C. 2019; 13 (2): 123-+
  • Strong field ionization with orbital angular momentum light beams measured with three-dimensional velocity imaging technique Quynh Nguyen, Dorney, K., Peters, W., Couch, D., Wooldridge, L., Li, W., Kapteyn, H., Murnane, M. AMER CHEMICAL SOC. 2018
  • High harmonics with spatially varying ellipticity OPTICA Ellis, J. L., Dorney, K. M., Hickstein, D. D., Brooks, N. J., Gentry, C., Hernandez-Garcia, C., Zusin, D., Shaw, J. M., Nguyen, Q. L., Mancuso, C. A., Jansen, G., Witte, S., Kapteyn, H. C., Murnane, M. M. 2018; 5 (4): 479-485
  • Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Peters, W. K., Couch, D. E., Mignolet, B., Shi, X., Nguyen, Q. L., Fortenberry, R. C., Schlegel, H., Remacle, F., Kapteyn, H. C., Murnane, M. M., Li, W. 2017; 114 (52): E11072-E11081


    Highly excited electronic states are challenging to explore experimentally and theoretically-due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron-ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.

    View details for DOI 10.1073/pnas.1712566114

    View details for Web of Science ID 000418722400003

    View details for PubMedID 29109279

    View details for PubMedCentralID PMC5748188

  • A new electron-ion coincidence 3D momentum-imaging method and its application in probing strong field dynamics of 2-phenylethyl-N, N-dimethylamine JOURNAL OF CHEMICAL PHYSICS Fan, L., Lee, S., Tu, Y., Mignolet, B., Couch, D., Dorney, K., Quynh Nguyen, Wooldridge, L., Murnane, M., Remacle, F., Schlegel, H., Li, W. 2017; 147 (1): 013920


    We report the development of a new three-dimensional (3D) momentum-imaging setup based on conventional velocity map imaging to achieve the coincidence measurement of photoelectrons and photo-ions. This setup uses only one imaging detector (microchannel plates (MCP)/phosphor screen) but the voltages on electrodes are pulsed to push both electrons and ions toward the same detector. The ion-electron coincidence is achieved using two cameras to capture images of ions and electrons separately. The time-of-flight of ions and electrons are read out from MCP using a digitizer. We demonstrate this new system by studying the dissociative single and double ionization of PENNA (2-phenylethyl-N,N-dimethylamine). We further show that the camera-based 3D imaging system can operate at 10 kHz repetition rate.

    View details for DOI 10.1063/1.4981526

    View details for Web of Science ID 000405089400021

    View details for PubMedID 28688410

  • Photophysical properties of pyrrolocytosine, a cytosine fluorescent base analogue PHYSICAL CHEMISTRY CHEMICAL PHYSICS Nguyen, Q. L., Spata, V. A., Matsika, S. 2016; 18 (30): 20189-20198


    The photophysical behavior of pyrrolocytosine (PC), a fluorescent base analogue of cytosine, has been investigated using theoretical approaches. The similarities between the PC and cytosine structures allow PC to maintain the pseudo-Watson-Crick base-pairing arrangement with guanine. Cytosine, similar to the other natural nucleobases, is practically non-fluorescent, because of ultrafast radiationless decay occurring through conical intersections. PC displays a much higher fluorescence quantum yield than cytosine, making it an effective fluorescent marker to study the structure, function, and dynamics of DNA/RNA complexes. Similar to 2-aminopurine, a constitutional isomer of adenine that base-pairs with thymine, PC's fluorescence is quenched when it is incorporated into a dinucleotide or a trinucleotide. In this work we examine the photophysical properties of isolated PC, microhydrated PC, as well as, complexes where PC is either base-stacked or hydrogen-bonded with guanine. Our results indicate that hydration affects the radiationless decay pathways in PC by destabilizing conical intersections. The calculations of dimers and trimers show that the radiative decay is affected by π stacking, while the presence of charge transfer states between PC and guanine may contribute to radiationless decay.

    View details for DOI 10.1039/c6cp01559j

    View details for Web of Science ID 000381428600020

    View details for PubMedID 27251599

    View details for PubMedCentralID PMC5030112

  • Fluorescent properties of DNA/RNA base analog pyrrolocytosine Nguyen, Q., Matsika, S. AMER CHEMICAL SOC. 2014
  • Exciplexes and conical intersections lead to fluorescence quenching in pi-stacked dimers of 2-aminopurine with natural purine nucleobases PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES Liang, J., Nguyen, Q. L., Matsika, S. 2013; 12 (8): 1387-1400


    Fluorescent analogues of the natural DNA bases are useful in the study of nucleic acids' structure and dynamics. 2-Aminopurine (2AP) is a widely used analogue with environmentally sensitive fluorescence behavior. The quantum yield of 2AP has been found to be significantly decreased when engaged in π-stacking interactions with the native bases. We present a theoretical study on fluorescence quenching mechanisms in dimers of 2AP π-stacked with adenine or guanine as in natural DNA. Relaxation pathways on the potential energy surfaces of the first excited states have been computed and reveal the importance of exciplexes and conical intersections in the fluorescence quenching process.

    View details for DOI 10.1039/c3pp25449f

    View details for Web of Science ID 000322448600013

    View details for PubMedID 23625036

    View details for PubMedCentralID PMC5006741