Lionel Poisson

VUV photoionization of gas phase adenine and cytosine: A comparison between oven and aerosol vaporization

We studied the single photon ionization of gas phase adenine and cytosine by means of vacuum ultraviolet synchrotron radiation coupled to a velocity map imaging electron∕ion coincidence spectrometer. Both in-vacuum temperature-controlled oven and aerosol thermodesorption were successfully applied to promote the intact neutral biological species into the gas phase. The photoion yields are consistent with previous measurements. In addition, we deduced the threshold photoelectron spectra and the slow photoelectron spectra for both species, where the close to zero kinetic energy photoelectrons and the corresponding photoions are measured in coincidence. The photoionization close and above the ionization energies are found to occur mainly via direct processes. Both vaporization techniques lead to similar electronic spectra for the two molecules, which consist of broadbands due to the complex electronic structure of the cationic species and to the possible contribution of several neutral tautomers for cytosine prior to ionization. Accurate ionization energies are measured for adenine and cytosine at, respectively, 8.267 ± 0.005 eV and 8.66 ± 0.01 eV, and we deduce precise thermochemical data for the adenine radical cation. Finally, we performed an evaluation and a comparison of the two vaporization techniques addressing the following criteria: measurement precision, thermal fragmentation, sensitivity, and sample consumption. The aerosol thermodesorption technique appears as a promising alternative to vaporize large thermolabile biological compounds, where extended thermal decomposition or low sensitivity could be encountered when using a simple oven vaporization technique.

Spectroscopy and Dynamics of K Atoms on Argon Clusters.

We present a combined experimental and simulation study of the 4s → 4p photoexcitation of the K atom trapped at the surface of ArN clusters made of a few hundred Ar atoms. Our experimental method based on photoelectron spectroscopy allows us to firmly establish that one single K atom is trapped at the surface of the cluster. The absorption spectrum is characterized by the splitting of the atomic absorption line into two broad bands, a Π band associated with p orbitals parallel to the cluster surface and a Σ band associated with the perpendicular orientation. The spectrum is consistent with observations reported for K atoms trapped on lighter inert gas clusters, but the splitting between the Π and Σ bands is significantly larger. We show that a large amount of K atoms are transiently stuck and eventually lost by the Ar cluster, in contrast with previous observations reported for alkaline earth metal systems. The excitation in the Σ band leads systematically to the ejection of the K atom from the Ar cluster. On the contrary, excitation in the Π band leads to the formation of a bound state. In this case, the analysis of the experimental photoelectron spectrum by means of nonadiabatic molecular dynamics simulation shows that the relaxation drives the system toward a basin where the coordination of the K atom is 2.2 Ar atoms on the average, in a poorly structured surface.

Femtosecond dynamics of the 2-methylallyl radical: A computational and experimental study

We investigate the photodynamics of the 2-methylallyl radical by femtosecond time-resolved photoelectron imaging. The experiments are accompanied by field-induced surface hopping dynamics calculations and the simulation of time-resolved photoelectron intensities and anisotropies, giving insight into the photochemistry and nonradiative relaxation of the radical. 2-methylallyl is excited at 236 nm, 238 nm, and 240.6 nm into a 3p Rydberg state, and the subsequent dynamics is probed by multiphoton ionization using photons of 800 nm. The photoelectron image exhibits a prominent band with considerable anisotropy, which is compatible with the result of theory. The simulations show that the initially excited 3p state is rapidly depopulated to a 3s Rydberg state, from which photoelectrons of high anisotropy are produced. The 3s state then decays within several 100 fs to the D1 (nπ) state, followed by the deactivation of the D1 to the electronic ground state on the ps time scale.

Ultrafast Photoelectron imaging of the electronic relaxation of a molecule deposited at the surface of an argon cluster

Molecules rarely reemit from the initially excited level after the absorption of a photon. If the molecule is sufficiently complex, light emission is not the main decay channel for the excited state. The electronic energy is rapidly degraded and thermalized within the ground electronic state of these systems, preventing most of the photochemistry by dissipation of the energy in the medium. These relaxation properties arise from the coupling among the electronic configurations of the molecular systems accessible by optical excitation and the other energetically accessible configurations. Ultrafast Photoelectron imaging is a direct mechanism that can allow an extremely fast relaxation in molecules. This chapter emphasizes the fact that photoelectron spectroscopy can be applied to the investigation of cluster dynamics. On large clusters, photoelectron spectroscopy proves to be very sensitive. Deposition of large molecule on the argon cluster maintains important information relative to the electronic dynamics occurring in electronic relaxation of the molecules embedded in a medium.

Multipronged mapping to the dynamics of a barium atom deposited on argon clusters

The dynamics of an electronically excited barium atom deposited at the surface of an Ar≈500 cluster was explored in a multipronged approach which associates information from frequency-resolved nanosecond experiments and information from femtosecond time-resolved experiments. In both types of experiments, the dynamics is monitored by photoelectron and photoion spectroscopy.

Ultrafast Electronic Relaxation of Excited State of Biomimetic Metalloporphyrins in the Gas Phase

Biomimetic molecules are systems that provide a direct and simpler reproduction of a molecular process essential to the living world, i.e. they mimic a function or a structure of a biomolecular edifice. The idea is to isolate this process and allow a selective study of a specific function by physicochemical methods, in a reductionist way. In that respect, gas phase studies of biomolecules are ideal, since they allow the application of a variety of highly sensitive and selective tools to the study of these model systems: mass spectrometry, photoelectron spectroscopy, for example, provided that the molecular system has been vaporised. In addition, the system is studied in absence of solvent, which greatly simplifies the interpretation of the fundamental effects and consequently allows to infer the effects of the medium since, in the gas phase solvents can be introduced in a gradual manner via clusters. Also gas phase measurements allow a direct comparison with results from quantum chemistry calculations. Great progresses have been fuelled in this direction by the developments of mass spectrometry. The studies in the gas phase have been mostly dedicated to the elucidation of the basic structure of the building blocks of proteins, i.e. small peptides (Simons, 2004). In turn, other biological functions have been less studied by the gas phase biomimetic approach. Hematoproteins hosting hemoporphyrins (the heme, see fig 1) are ideal systems to examine in this way, since they are at the heart of the oxygen and small molecules transport within blood and that their active site is localised very precisely on the metal atom embedded in their central porphyrin. Thus, if one studies the active porphyrinic site of myoglobin at short time scales (in the subpicosecond time domain), the interaction with the environing protein (globin myoglobin) can be minimised. In hemoproteins, the attachment of oxygen to the iron of the heme is a complex process involving a potential barrier. This barrier results from the crossing of two surfaces of

Characterization of a seeded pulsed molecular beam using the velocity map imaging technique

An experimental study has been performed to characterize the density and the velocity distribution in a pulsed molecular beam generated by a source associating a pulsed valve and an oven placed just downstream. In its operating mode, the flow is alternatively in a supersonic and effusive regime. The Velocity Map Imaging (VMI) technique associated with laser ionization allows measuring the velocity distribution and the density of molecules as a function of time during the expansion. It gives us a very precise insight into the structure of the molecule bunch, and therefore into the nature of the expansion from which the molecular beam is extracted.

Femtosecond Photoelectron Imaging as a Probe for Non-Adiabatic Evolution

Excited state electronic relaxation is ubiquitous in large molecules. Femtosecond electron imaging represents an ideal tool to investigate this process, probing with high efficiency the relevant electronic configuration changes in molecules and molecular clusters, surprisingly.