Real-time dynamics of vibronic wavepackets within Rydberg and ion-pair states of molecular iodine.

Real-time dynamics of vibrationally and electronically excited I2 molecules has been investigated experimentally using the pump-probe technique. A 2-photon excitation was performed either at 269 nm or 266 nm. An electronic and vibrational wavepacket was built as coherent superposition of a few (269 nm excitation) or many (266 nm excitation) Rydberg states of the and series partly coupled with ion-pair states. The probe operated by ionisation or photodetachement. The energy and angular distribution of the resulting photoelectrons, I+ photocations and I- photoanions were monitored. During the dynamics that is turned on by the pump excitation, the wavepacket splits and explores a variety of electronic states of Rydberg and ion-pair character. The experimental results were complemented by molecular dynamics calculations. This provided invaluable information to identify wavepacket motion along ion-pair potential energy curves.

Time-resolved photoelectron spectroscopy of 4-(dimethylamino)benzethyne - an experimental and computational study.

We investigated the excited-state dynamics of 4-(dimethylamino)benzethyne (4-DMABE) in a combined theoretical and experimental study using surface-hopping simulations and time-resolved ionisation experiments. The simulations predict a decay of the initially excited S2 state into the S1 state in only a few femtoseconds, inducing a subsequent partial twist of the dimethylamino group within ∼100 fs. This leads to drastically reduced Franck-Condon factors for the ionisation transition to the cationic ground state, thus inhibiting the effective ionisation of the molecule, which leads to a vanishing photoelectron signal on a similar timescale as observed in our time-resolved photoelectron spectra. From the phototoelectron spectra, an adiabatic ionisation energy of 7.17 ± 0.02 eV was determined. The experimental decays match the theoretical predictions very well and the combination of both reveals the electronic characteristics of the molecule, namely the role of intramolecular charge transfer (ICT) states in the deactivation pathway of electronically excited 4-DMABE.

Disentangling Spectral Phases of Interfering Autoionizing States from Attosecond Interferometric Measurements.

We have determined spectral phases of Ne autoionizing states from extreme ultraviolet and midinfrared attosecond interferometric measurements and ab initio full-electron time-dependent theoretical calculations in an energy interval where several of these states are coherently populated. The retrieved phases exhibit a complex behavior as a function of photon energy, which is the consequence of the interference between paths involving various resonances. In spite of this complexity, we show that phases for individual resonances can still be obtained from experiment by using an extension of the Fano model of atomic resonances. As simultaneous excitation of several resonances is a common scenario in many-electron systems, the present work paves the way to reconstruct electron wave packets coherently generated by attosecond pulses in systems larger than helium.

Oxygen-18 Isotopic Studies of HOOO and DOOO.

Owing to questions that still persist regarding the length of the O-H and central O-O bond, and large-amplitude torsional motion of trans hydridotrioxygen HOOO, a weakly bound complex between OH and O2, new 18O isotopic measurements of HOOO and DOOO were undertaken using Fourier transform microwave and microwave-millimeter-wave double resonance techniques. Rotational lines from three new 18O species of DOOO (D18OOO, DO18O18O, and D18O18O18O) were detected, along with the two singly substituted 18O isotopic species of HOOO (HO18OO and HOO18O) that were not measured in the previous isotopic investigation. From a least-squares fit, spectroscopic constants, including the three rotational constants, were precisely determined for all five species. The inertial defect of DOOO and its 18O species is uniformly negative: of order -0.04 amu Å2, regardless of the number or location of the 18O atoms, in contrast to that found for HOOO or its 18O isotopic species. A reanalysis of the molecular structure was performed using either normal HOOO and its four singly substituted isotopic species, the new DOOO data, or all the isotopic species (10 in total). The differences between the purely experimental (r0) structures are generally quite small, of order ±0.01 Å for the bond lengths and ±1° for the bond angle. The length of the O-H bond remains unrealistically short compared to free OH, and the central O-O bond length is consistently very close to 1.68 Å. On the basis of the effective O-H bond length derived from the experimental structure, the average displacement of the large amplitude torsional motion from planarity is estimated to be ∼22°.

Microwave detection of sulfoxylic acid (HOSOH).

Sulfoxylic acid (HOSOH), a chemical intermediate roughly midway along the path between highly reduced (H2S) and highly oxidized sulfur (H2SO4), has been detected using Fourier transform microwave spectroscopy and double resonance techniques, guided by new high-level CCSD(T) quantum-chemical calculations of its molecular structure. Rotational spectra of the two most stable isomers of HOSOH, the putative ground state with C2 symmetry and the low-lying C(s) rotamer, have been measured to high precision up to 71 GHz, allowing accurate spectroscopic parameters to be derived for both isomers. HOSOH may play a role in atmospheric and interstellar chemistry, and the present work provides the essential data to enable remote sensing and/or radioastronomical searches for these species. Spectroscopic characterization of HOSOH suggests that other transient intermediates in the oxidation of SO2 to H2SO4 may be amenable to laboratory detection as well.

Efficient table-top dual-wavelength beamline for ultrafast transient absorption spectroscopy in the soft X-ray region.

We present a table-top beamline providing a soft X-ray supercontinuum extending up to 370 eV from high-order harmonic generation with sub-13 fs 1300 nm driving pulses and simultaneous production of sub-5 fs pulses centered at 800 nm. Optimization of high harmonic generation in a long and dense gas medium yields a photon flux of  ~ 1.4 × 106 photons/s/1% bandwidth at 300 eV. The temporal resolution of X-ray transient absorption experiments with this beamline is measured to be 11 fs for 800 nm excitation. This dual-wavelength approach, combined with high flux and high spectral and temporal resolution soft X-ray absorption spectroscopy, is a new route to the study of ultrafast electronic dynamics in carbon-containing molecules and materials at the carbon K-edge.

Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy.

Conical intersections between electronic states often dictate the chemistry of photoexcited molecules. Recently developed sources of ultrashort extreme ultraviolet (XUV) pulses tuned to element-specific transitions in molecules allow for the unambiguous detection of electronic state-switching at a conical intersection. Here, the fragmentation of photoexcited iso-propyl iodide and tert-butyl iodide molecules (i-C3H7I and t-C4H9I) through a conical intersection between 3Q0/1Q1 spin-orbit states is revealed by ultrafast XUV transient absorption measuring iodine 4d core-to-valence transitions. The electronic state-sensitivity of the technique allows for a complete mapping of molecular dissociation from photoexcitation to photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited wave packet from the 3Q0 state into the 1Q1 state at the conical intersection is captured. The results show how differences in the electronic state-switching of the wave packet in i-C3H7I and t-C4H9I directly lead to differences in the photoproduct branching ratio of the two systems.

Evidence of depolarization and ellipticity of high harmonics driven by ultrashort bichromatic circularly polarized fields.

High harmonics generated by counter-rotating laser fields at the fundamental and second harmonic frequencies have raised important interest as a table-top source of circularly polarized ultrashort extreme-ultraviolet light. However, this emission has not yet been fully characterized: in particular it was assumed to be fully polarized, leading to an uncertainty on the effective harmonic ellipticity. Here we show, through simulations, that ultrashort driving fields and ultrafast medium ionization lead to a breaking of the dynamical symmetry of the interaction, and consequently to deviations from perfectly circular and fully polarized harmonics, already at the single-atom level. We perform the complete experimental characterization of the polarization state of high harmonics generated along that scheme, giving direct access to the ellipticity absolute value and sign, as well as the degree of polarization of individual harmonic orders. This study allows defining optimal generation conditions of fully circularly polarized harmonics for advanced studies of ultrafast dichroisms.