Multi-channel electronic and vibrational dynamics in polyatomic resonant high-order harmonic generation.

High-order harmonic generation in polyatomic molecules generally involves multiple channels of ionization. Their relative contribution can be strongly influenced by the presence of resonances, whose assignment remains a major challenge for high-harmonic spectroscopy. Here we present a multi-modal approach for the investigation of unaligned polyatomic molecules, using SF6 as an example. We combine methods from extreme-ultraviolet spectroscopy, above-threshold ionization and attosecond metrology. Fragment-resolved above-threshold ionization measurements reveal that strong-field ionization opens at least three channels. A shape resonance in one of them is found to dominate the signal in the 20-26 eV range. This resonance induces a phase jump in the harmonic emission, a switch in the polarization state and different dynamical responses to molecular vibrations. This study demonstrates a method for extending high-harmonic spectroscopy to polyatomic molecules, where complex attosecond dynamics are expected.

High harmonic spectroscopy of the Cooper minimum in molecules.

The Cooper minimum (CM) has been studied using high harmonic generation solely in atoms. Here, we present detailed experimental and theoretical studies on the CM in molecules probed by high harmonic generation using a range of near-infrared light pulses from λ=1.3 to 1.8 µm. We demonstrate the CM to occur in CS(2) and CCl(4) at ~42 and ~40 eV, respectively, by comparing the high harmonic spectra with the known partial photoionization cross sections of different molecular orbitals, confirmed by theoretical calculations of harmonic spectra. We use CM to probe electron localization in Cl-containing molecules (CCl(4), CH(2)Cl(2), and trans-C(2)H(2)Cl(2)) and show that the position of the minimum is influenced by the molecular environment.

Gas-phase electronic spectrum of the C14 ring.

The electronic spectrum of a cyclic C(14) in the visible range has been detected in the gas phase by a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Absorption is localized in the 19 000 to 20 000 cm(-1) region and appears as a dozen of 3-7 cm(-1) narrow peaks belonging to one or two close-lying electronic states. Bands have structures which for the narrowest ones is likely to be the rotational profile contour. The spectrum is attributed to a cyclic form of C(14) based on time-dependent density-functional calculations and reactivity with H(2). The spectral pattern differs from that previously seen in the larger C(4n+2) member rings, C(18) and C(22), indicating some sort of a structural crossover.

Gas phase electronic spectra of the carbon chains C5, C6, C8, and C9.

Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 micros. Hole-burning experiments on the 2 (3)Sigma(u)- <-- X (3)Sigma(g)- transition of C6, C8, and (1)Sigma(u)+ <-- X (1)Sigma(g)+ of C9 confirm the predissociative nature of the excited electronic states.

Gas-phase electronic spectra of C18 and C22 rings.

The electronic spectra of C(18) and C(22) in the 15 150-36 900 cm(-1) range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D(9 h) symmetry for C(18) and D(11 h) for C(22), based on time-dependent-density-functional calculations and reactivity with respect to H(2). The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the A(1)A(2) (")<--X(1)A(1) (') transition of C(18) at 592.89 nm shows a pair of 1 cm(-1) broadbands spaced by 1.5 cm(-1). The next electronic transitions exhibited much broader, approximately 30 (in the visible) to 200 cm(-1) (in ultraviolet range), features. The spectrum of C(22) exhibits an absorption pattern similar to C(18), except that the narrow features to the red are missing; the oscillator strength of the A<--X transition is predicted to be low.

Gas phase electronic spectra of two C5H5 radical isomers.

Gas phase electronic spectra of two C5H5 radical isomers have been observed in the 440-470 nm spectral region. The technique was a mass selective resonant two-color two-photon ionization coupled to a supersonic plasma source. Structures, relative energies and vertical electronic transition energies of six isomers of C5H5 have been calculated. Based on an analysis of the rotational profiles of the observed bands and theoretical calculations, the spectra are assigned as the A 2A" <-- X 2A" electronic transition of isomer 1 and A 2A2 X 2B1 of 6 with origin band at 461.8 nm and 456.1 nm, respectively. Isomer 1, 1,3-vinylpropargyl, has Cs symmetry, while 6, a planar zig-zig chain with one hydrogen on each carbon, has C2v symmetry.