Valérie Blanchet

Polyatomic molecules in strong laser fields: Nonadiabatic multielectron dynamics

We report the observation and characterization of a new nonresonant strong field ionization mechanism in polyatomic molecules: Nonadiabatic multi-electron (NME) dynamics. The strong field response of a given molecule depends on important properties such as molecular geometry and bonding, the path length of delocalized electrons and/or ionization potential as well as on basic laser pulse parameters such as wavelength and intensity. Popular quasi-static tunnelling models of strong field molecular ionization, based upon the adiabatic response of a single active electron, are demonstrated to be inadequate when electron delocalization is important. The NME ionization mechanism greatly affects molecular ionization, its fragmentation and its energetics. In addition, multi-electron effects are shown to be present even in the adiabatic long wavelength limit.

Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy

Coincident vibrational and electronic rearrangements in a photoexcited molecule are tracked in fine detail. Conical intersections play a crucial role in the chemistry of most polyatomic molecules, ranging from the simplest bimolecular reactions to the photostability of DNA. The real-time study of the associated electronic dynamics poses a major challenge to the latest techniques of ultrafast measurement. We show that high-harmonic spectroscopy reveals oscillations in the electronic character that occur in nitrogen dioxide when a photoexcited wave packet crosses a conical intersection. At longer delays, we observe the onset of statistical dissociation dynamics. The present results demonstrate that high-harmonic spectroscopy could become a powerful tool to highlight electronic dynamics occurring along nonadiabatic chemical reaction pathways.

Temporal coherent control in the photoionization of Cs2: Theory and experiment

Two identical femtosecond pulses are used to create a coherent superposition of two vibrational wave packets in a bound electronic state of cesium dimers. The oscillations of these two wave packets are further detected after photoionization of the system. Quantum interferences between the two wave packets result in a temporal coherent control of the ionization probability. The interferogram exhibits the following features as a function of the time delay between the two laser pulses: high-frequency oscillation corresponding to Ramsey fringes (at the Bohr frequency of the transition) modulated by a slow envelope corresponding to the oscillations of vibrational wave packets (vibrational recurrences). Here the control parameter is the time delay between the two laser pulses which can be used to control the preparation of a wave packet in a quantum system and monitor its evolution. The detailed theory of this experiment is presented and compared with the pump-probe experiment. The temporal coherent control exp...

One-color coherent control in Cs2. Observation of 2.7 fs beats in the ionization signal

Abstract We present the theory of one-color coherent control with two identical time-delayed laser pulses and the experimental observation of the resulting wave packet interferences in the B 1 Π u state of Cs 2 . The B state population is detected by two-photon ionization. The wave packet interference produces beats in the Cs 2 + ion signal at the optical frequency, i.e. with a period of 2.7 fs which are resolved for the first time. These beats are modulated by the vibrational recurrences and allow a determination of the vibrational period. Furthermore, we show that such interferences can be observed even when the probe step involves an electronic state parallel to the excited state in which the wave packet oscillates.

Observation of Coherent Transients in Ultrashort Chirped Excitation of an Undamped Two-Level System

The effects of coherent excitation of a two-level system with a linearly chirped pulse are studied theoretically and experimentally [in Rb (5s-5p)] in the low field regime. The coherent transients are measured directly on the excited state population on an ultrashort time scale. A sharp step corresponds to the passage through resonance. It is followed by oscillations resulting from interferences between off-resonant and resonant contributions. We finally show the equivalence between this experiment and Fresnel diffraction by a sharp edge.

Nonadiabatic dynamics in polyatomic systems studied by femtosecond time-resolved photoelectron spectroscopy

We investigate the use of time-resolved photoelectron spectroscopy for studying nonadiabatic polyatomic dissociation dynamics. In particular, we emphasize the importance of the electronic structure of the ionization continuum in interpreting the results and provide an experimental example of these effects in the dissociation dynamics of the NO dimer.

Electronic continua in time-resolved photoelectron spectroscopy. I. Complementary ionization correlations

We examine the role of electronic continua in time-resolved photoelectron spectroscopy studies of polyatomic nonadiabatic dynamics. We have investigated the two limiting cases for such studies. We consider here the limiting case of complementary ionization correlations where the two nonadiabatically coupled excited electronic states (S2 and S1) correlate (in the Koopmans’ picture) to different cation electronic states. We show, using an example of ultrafast internal conversion a linear polyene, that this favorable case allows for disentangling of the electronic population dynamics from the coupled vibrational dynamics. In the following paper, we investigate the unfavorable case of corresponding ionization correlations.

On the dissociation of the naphthalene radical cation: new iPEPICO and tandem mass spectrometry results.

The dissociation of the naphthalene radical cation has been reinvestigated here by a combination of tandem mass spectrometry and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). Six reactions were explored: (R1) C(10)H(8)(•+) → C(10)H(7)(+) + H (m/z = 127); (R2) C(10)H(8)(•+) → C(8)H(6)(•+) + C(2)H(2) (m/z = 102); (R3) C(10)H(8)(•+) → C(6)H(6)(•+) + C(4)H(2) (m/z = 78); (R4) C(10)H(8)(•+) → C(10)H(6)(•+) + H(2) (m/z = 126); (R5) C(10)H(7)(+) → C(6)H(5)(+) + C(4)H(2) (m/z = 77); (R6) C(10)H(7)(+) → C(10)H(6)(•+) + H (m/z = 126). The E(0) activation energies for the reactions deduced from the present measurements are (in eV) 4.20 ± 0.04 (R1), 4.12 ± 0.05 (R2), 4.27 ± 0.07 (R3), 4.72 ± 0.06 (R4), 3.69 ± 0.26 (R5), and 3.20 ± 0.13 (R6). The corresponding entropies of activation, ΔS(‡)(1000K), derived in the present study are (in J K(-1) mol(-1)) 2 ± 2 (R1), 0 ± 2 (R2), 4 ± 4 (R3), 11 ± 4 (R4), 5 ± 15 (R5), and -19 ± 11 (R6). The derived E(0) value, combined with the previously reported IE of naphthalene (8.1442 eV) results in an enthalpy of formation for the naphthyl cation, Δ(f)H°(0K) = 1148 ± 14 kJ mol(-1)/Δ(f)H°(298K) = 1123 ± 14 kJ mol(-1) (site of dehydrogenation unspecified), slightly lower than the previous estimate by Gotkis and co-workers. The derived E(0) for the second H-loss leads to a Δ(f)H° for ion 7, the cycloprop[a]indene radical cation, of Δ(f)H°(0K) =1457 ± 27 kJ mol(-1)/Δ(f)H°(298K)(C(10)H(6)(+)) = 1432 ± 27 kJ mol(-1). Detailed comparisons are provided with values (experimental and theoretical) available in the literature.

UV photodissociation of methyl bromide and methyl bromide cation studied by velocity map imaging

We employ the velocity map imaging technique to measure kinetic energy and angular distributions of state selected CH(3) (v(2)=0,1,2,3) and Br ((2)P(3/2), (2)P(1/2)) photofragments produced by methyl bromide photolysis at 215.9 nm. These results show unambiguously that the Br and Br( *) forming channels result in different vibrational excitations of the umbrella mode of the methyl fragment. Low energy structured features appear on the images, which arise from CH(3)Br(+) photodissociation near 330 nm. The excess energy of the probe laser photon is channeled into CH(3) (+) vibrational excitation, most probably in the nu(4) degenerate bend.

Inhomogeneous high harmonic generation in krypton clusters.

High order harmonic generation from clusters is a controversial topic: conflicting theories exist, with different explanations for similar experimental observations. From an experimental point of view, separating the contributions from monomers and clusters is challenging. By performing a spectrally and spatially resolved study in a controlled mixture of clusters and monomers, we are able to isolate a region of the spectrum where the emission purely originates from clusters. Surprisingly, the emission from clusters is depolarized, which is the signature of statistical inhomogeneous emission from a low-density source. The harmonic response to laser ellipticity shows that this generation is produced by a new recollisional mechanism, which opens the way to future theoretical studies.