Collisional depolarization of NO(A) by He and Ar studied by quantum beat spectroscopy

Zeeman and hyperfine quantum beat spectroscopies have been used to measure the total elastic plus inelastic angular momentum depolarization rate constants at 300 K for NO (A 2 σ+) in the presence of He and Ar. In the case of Zeeman quantum beats it is shown how the applied magnetic field can be used to allow measurement of depolarization rates for both angular momentum orientation and alignment. For the systems studied here, collisional loss of alignment is more efficient than loss of orientation. In the case of NO (A) with He, and to a lesser extent NO (A) with Ar, collisional depolarization is found to be a relatively minor process compared to rotational energy transfer, reflecting the very weak long-range forces in these systems. Detailed comparisons are made with quantum mechanical and quasiclassical trajectory calculations performed on recently developed potential energy surfaces. For both systems, the agreement between the calculated depolarization cross sections and the present measurements is found to be very good, suggesting that it is reasonable to consider the NO (A) bond as frozen during these angular momentum transferring collisions. A combination of kinematic effects and differences in the potential energy surfaces are shown to be responsible for the differences observed in depolarization cross section with He and Ar as a collider. © 2009 American Institute of Physics

High-order Harmonic Spectroscopy of the Cooper Minimum in Argon: Experimental and Theoretical Study

We study the Cooper minimum in high harmonic generation from argon atoms using long wavelength laser pulses. We find that the minimum in high harmonic spectra is systematically shifted with respect to total photoionization cross section measurements. We use a semi-classical theoretical approach based on Classical Trajectory Monte Carlo and Quantum Electron Scattering methods (CTMC-QUEST) to model the experiment. Our study reveals that the shift between photoionization and high harmonic emission is due to several effects: the directivity of the recombining electrons and emitted polarization, and the shape of the recolliding electron wavepacket.Comment: 13 page

Time-resolved predissociation of the vibrationless level of the B state of CH3I

The predissociation dynamics of the vibrationless level of the first Rydberg state 6s (B 2E) state of CH3I has been studied by femtosecond-resolved velocity map imaging of both the CH3 and I photofragments. The kinetic energy distributions of the two fragments have been recorded as a function of the pump-probe delay, and as a function of excitation within the umbrella and stretching vibrational modes of the CH3 fragment. These observations are made by using (2+1) Resonant Enhanced MultiPhoton Ionization (REMPI) via the 3pz 2A2" state of CH3 to detect specific vibrational levels of CH3. The vibrational branching fractions of the CH3 are recovered by using the individual vibrationally state-selected CH3 distributions to fit the kinetic energy distribution obtained by using nonresonant multiphoton ionization of either the I or CH3 fragment. The angular distributions and rise times of the two fragments differ significantly. These observations can be rationalized through a consideration of the alignment of the CH3 fragment and the effect of this alignment on its detection efficiency. Two extra dissociation channels are detected: one associated with Rydberg states near 9.2 eV that were observed previously in photoelectron studies, and one associated with photodissociation of the parent cation around 15 eV.Comment: submitted Physical Chemistry Chemical Physics (2011

Controlling the polarization and vortex charge of attosecond high-harmonic beams via simultaneous spin–orbit momentum conservation

[EN]Optical interactions are governed by both spin and angular momentum conservation laws, which serve as a tool for controlling light–matter interactions or elucidating electron dynamics and structure of complex systems. Here, we uncover a form of simultaneous spin and orbital angular momentum conservation and show, theoretically and experimentally, that this phenomenon allows for unprecedented control over the divergence and polarization of extreme-ultraviolet vortex beams. High harmonics with spin and orbital angular momenta are produced, opening a novel regime of angular momentum conservation that allows for manipulation of the polarization of attosecond pulses—from linear to circular—and for the generation of circularly polarized vortices with tailored orbital angular momentum, including harmonic vortices with the same topological charge as the driving laser beam. Our work paves the way to ultrafast studies of chiral systems using high-harmonic beams with designer spin and orbital angular momentum.The authors are thankful for useful and productive conversations with E. Pisanty, C. Durfee, D. Hickstein, S. Alperin and M. Siemens. H.C.K. and M.M.M. graciously acknowledge support from the Department of Energy BES Award No. DE-FG02–99ER14982 for the experimental implementation, as well as a MURI grant from the Air Force Office of Scientific Research under Award No. FA9550–16–1–0121 for the theory. J.L.E., N.J.B. and Q.L.N. acknowledge support from National Science Foundation Graduate Research Fellowships (Grant No. DGE-1144083). C.H.-G., J.S.R. and L.P. acknowledge support from Junta de Castilla y León (SA046U16) and Ministerio de Economía y Competitividad (FIS2013–44174-P, FIS2016–75652-P). C.H.-G. acknowledges support from a 2017 Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation. L.R. acknowledges support from Ministerio de Educación, Cultura y Deporte (FPU16/02591). A.P. acknowledges support from the Marie Sklodowska-Curie Grant, Agreement No. 702565. We thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (RES-AECT-2014–2–0085). This research made use of the high-performance computingresources of the Castilla y León Supercomputing Center (SCAYLE, www.scayle.es),financed by the European Regional Development Fund (ERDF). Certain commercial instruments are identified to specify the experimental study adequately. This does not imply endorsement by the National Institute of Standards and Technology (NIST) or that the instruments are the best available for the purpose

Polarization control of isolated high-harmonic pulses

High-harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, thus far, the shortest isolated attosecond pulses have only been produced with linear polarization, which limits the range of physics that can be explored. Here, we demonstrate robust polarization control of isolated extreme-ultraviolet pulses by exploiting non-collinear high-harmonic generation driven by two counter-rotating few-cycle laser beams. The circularly polarized supercontinuum is produced at a central photon energy of 33 eV with a transform limit of 190 as and a predicted linear chirp of 330 as. By adjusting the ellipticity of the two counter-rotating driving pulses simultaneously, we control the polarization state of isolated extreme-ultraviolet pulses—from circular through elliptical to linear polarization—without sacrificing conversion efficiency. Access to the purely circularly polarized supercontinuum, combined with full helicity and ellipticity control, paves the way towards attosecond metrology of circular dichroism.The experimental work was carried out at National Tsing Hua University, Institute of Photonics Technologies, supported by the Ministry of Science and Technology, Taiwan (grants 105-2112-M-007-030-MY3, 105-2112-M-001-030 and 104-2112-M-007-012-MY3). The concept of isolated circularly polarized attosecond pulses was developed by C.H.-G., D.D.H., M.M.M., C.G.D., H.C.K., A.B. and A.J.-B.. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under Research Executive Agency grant agreement no. 328334. C.H.-G. and L.P. acknowledge support from Junta de Castilla y León (SA046U16) and the Ministerio de Economía y Competitividad (FIS2013-44174-P, FIS2016-75652-P). C.H.-G. acknowledges support from a 2017 Leonardo Grant for Researchers and Cultural Creators (BBVA Foundation). M.M.M. and H.C.K. acknowledge support from the Department of Energy Basic Energy Sciences (award no. DE-FG02-99ER14982) for the concepts and experimental set-up. For part of the theory, A.B., A.J.-B., C.G.D., M.M.M. and H.C.K. acknowledge support from a Multidisciplinary University Research Initiatives grant from the Air Force Office of Scientific Research (award no. FA9550-16-1-0121). A.J.-B. also acknowledges support from the US National Science Foundation (grant no. PHY-1734006). This work utilized the Janus supercomputer, which is supported by the US National Science Foundation (grant no. CNS-0821794) and the University of Colorado, Boulder. This research made use of the high-performance computing resources of the Castilla y León Supercomputing Center (SCAYLE, www.scayle.es), financed by the European Regional Development Fund (ERDF). J.L.E. acknowledges support from the National Science Foundation Graduate Research Fellowship (DGE-1144083). L.R. acknowledges support from the Ministerio de Educación, Cultura y Deporte (FPU16/02591)

Des états super-excités vers une empreinte Rydberg

Les expériences pompe-sonde de photoionisation sont examinées dans le cas d’une ionisation à deux photons avec une résonance à un photon au voisinage du seuil d’ionisation. Les trois systèmes explorés sont C10H8, CH3I et NO2. De manière surprenante ces trois photoionisations présentent une empreinte Rydberg plus ou moins importante, soit une transition sonde vers un état doublement excité qui relaxe pendant la durée de l’impulsion sonde vers des états de Rydberg. Le second photon sonde photoionise alors ces états de Rydberg dans le cas de C10H8 et CH3I ou encore dissocie et photoionise dans le cas de NO2

The optical spectrum of HCSi

A new emission band system has been observed in the gas phase at around 850 nm. This system correlates with absorption bands previously measured in a neon matrix and assigned to a triplet electronic transition of SiC. However, the gas phase bands display a clear doublet structure. Preliminary molecular parameters resulting from the rotational analysis of the (000) - (000) band coincide with the expected values obtained by a recently published ab initio calculation carried out on the HCSi radical. We conclude that both the gas phase and the neon matrix spectra originate from the HCSi radical which is here identified for the first time

The photodissociation dynamics of NO2 at 308 nm and of NO2 and N2O4 at 226 nm.

Velocity-map ion imaging has been applied to the photodissociation of NO(2) via the first absorption band at 308 nm using (2 + 1) resonantly enhanced multiphoton ionization detection of the atomic O((3)P(J)) products. The resulting ion images have been analyzed to provide information about the speed distribution of the O((3)P(J)) products, the translational anisotropy, and the electronic angular momentum alignment. The atomic speed distributions were used to provide information about the internal quantum-state distribution in the NO coproducts. The data were found to be consistent with an inverted NO vibrational quantum-state distribution, and thereby point to a dynamical, as opposed to a statistical dissociation mechanism subsequent to photodissociation at 308 nm. Surprisingly, at this wavelength the O-atom electronic angular momentum alignment was found to be small. Probe-only ion images obtained under a variety of molecular-beam backing-pressure conditions, and corresponding to O atoms generated in the photodissociation of either the monomer, NO(2), or the dimer, N(2)O(4), at 226 nm, are also reported. For the monomer, where 226 nm corresponds to excitation into the second absorption band, the kinetic-energy release distributions are also found to indicate a strong population inversion in the NO cofragment, and are shown to be remarkably similar to those previously observed in the wavelength range of 193-248 nm. Mechanistic implications of this result are discussed. At 226 nm it has also been possible to observe directly O atoms from the photodissociation of the dimer. The O-atom velocity distribution has been analyzed to provide information about its production mechanism

Molecular photofragment orientation in the photodissociation of H2O2 at 193 nm and 248 nm.

Angular momentum orientation has been observed in the OH(X(2)Π, v = 0) fragments generated by circularly polarized photodissociation of H(2)O(2) at 193 nm and 248 nm. The magnitude and sign of the orientation are strongly dependent on the OH(X) photofragment rotational state. In addition to conventional laser induced fluorescence methods, Zeeman quantum beat spectroscopy has also been used as a complementary tool to probe the angular momentum orientation parameters. The measured orientation at 193 nm is attributed solely to photodissociation via the Ã(1)A state, even though at this wavelength H(2)O(2) is excited near equally to both the Ã(1)A and B(1)B electronic states. This observation is confirmed by measurements of the photofragment orientation at 248 nm, where access to the Ã(1)A state dominates. Several possible mechanisms are discussed to explain the observed photofragment orientation, and a simple physical model is developed, which includes the effects of the polarization of the parent molecular rotation upon absorption of circularly polarized light. Good agreement between the experimental and simulation results is obtained, lending support to the validity of the model. It is proposed that photofragment orientation arises mainly from the coupling of the parent rotational angular momentum with that induced during photofragmentation