M. Barat

Ultrafast deactivation mechanisms of protonated aromatic amino acids following UV excitation

Deactivation pathways of electronically excited states have been investigated in three protonated aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe). The protonated amino acids were generated by electrospray and excited with a 266 nm femtosecond laser, the subsequent decay of the excited states being monitored through fragmentation of the ions induced and/or enhanced by another femtosecond pulse at 800 nm. The excited state of TrpH+ decays in 380 fs and gives rise to two channels: hydrogen atom dissociation or internal conversion (IC). In TyrH, the decay is slowed down to 22.3 ps and the fragmentation efficiency of PheH+ is so low that the decay cannot be measured with the available laser. The variation of the excited state lifetime between TrpH+ and TyrH+ can be ascribed to energy differences between the dissociative pi sigma* state and the initially excited pi pi* state.

Photo-induced dissociation of protonated tryptophan TrpH+: A direct dissociation channel in the excited states controls the hydrogen atom loss

Protonated tryptophan ions (TrpH+) are generated by electrospray ionization and dissociated by irradiation with a UV laser. Different photo-fragments are observed among which a new photo-induced dissociation channel leading to the loss of a hydrogen atom that is not observed in conventional collision-induced dissociation. A tryptophan radical cation (Trp+) is produced in this process that subsequently leads to the m/z = 130 fragment through a Cα–Cβ bond cleavage, a typical fragmentation product of the Trp+ radical cation generated either by electron impact or by photo-ionization. These results can be understood considering the excited states of protonated tryptophan: UV excitation of TrpH+ produces a mixed ππ*/πσ* state, the ππ* state being mainly located on the indole chromophore while the πσ* is mainly on the protonated terminal amino group. This πσ* state is repulsive along the N–H bond coordinate and leads either to hydrogen atom detachment producing a Trp+ radical cation that undergoes further fragmentations or to internal conversion to the ground state of the protonated TrpH+ ion.

Comprehensive characterization of the photodissociation pathways of protonated tryptophan

The photofragmentation of protonated tryptophan has been investigated in a unique experimental setup, in which ion and neutral issued from the photofragmentation are detected in coincidence, in time and in position. From these data are extracted the kinetic energy, the number of neutral fragments associated with an ion, their masses, and the order of the fragmentation steps. Moreover, the fragmentation time scale ranging from tens of nanoseconds to milliseconds is obtained. From all these data, a comprehensive fragmentation mechanism is proposed.

Mechanisms of photoinduced CαCβ bond breakage in protonated aromatic amino acids

Photoexcitation of protonated aromatic amino acids leads to C(alpha)[Single Bond]C(beta) bond breakage among other channels. There are two pathways for the C(alpha)[Single Bond]C(beta) bond breakage, one is a slow process (microseconds) that occurs after hydrogen loss from the electronically excited ion, whereas the other is a fast process (nanoseconds). In this paper, a comparative study of the fragmentation of four molecules shows that the presence of the carboxylic acid group is necessary for this fast fragmentation channel to occur. We suggest a mechanism based on light-induced electron transfer from the aromatic ring to the carboxylic acid, followed by a fast internal proton transfer from the ammonium group to the negatively charged carboxylic acid group. The ion formed is a biradical since the aromatic ring is ionized and the carbon of the COOH group has an unpaired electron. Breakage of the weak C(alpha)[Single Bond]C(beta) bond gives two even-electron fragments and is expected to quickly occur. The present experimental results together with the ab initio calculations support the interpretation previously proposed.

Absolute detection efficiency of a microchannel plate detector for neutral atoms

The absolute detection efficiency (ADE) of microchannel plates for neutral sodium and potassium atoms is measured in the low keV energy range. It is shown that ADE is primarily a function of the particle energy. This result is compared to measurements made by other authors for ionic particles.

UV photoinduced dynamics in protonated aromatic amino acid

UV photoinduced fragmentation of protonated aromatic amino acids has emerged the last few years, coming from a situation where nothing was known to what we think a good understanding of the optical properties. We will mainly focus this review on the tryptophan case. Three groups have mostly done spectroscopic studies and one has mainly been involved in dynamics studies of the excited states in the femtosecond/picosecond range and also in the fragmentation kinetics from nanosecond to millisecond. All these data, along with high level ab initio calculations, have shed light on the role of the different electronic states of the protonated molecules upon the fragmentation mechanisms.

Mechanisms of UV photodissociation of small protonated peptides.

Photofragmentation of protonated dipeptides by 263 nm photons is investigated with an experimental technique based on the detection in coincidence of the ionic and neutral fragments. With this method, it is possible to determine whether the fragmentation takes place in one or several steps. The timing of these steps can also be evaluated. The interpretation of the various fragmentation pathways is tentatively developed along the same line as that previously proposed for tryptophan. The fragmentation can be explained by two types of mechanisms: internal conversions and direct fragmentations triggered by the migration of the photoactive electron on positive charged sites or on oxygen sites.

Photoinduced processes in protonated tryptamine

The electronic excited state dynamics of protonated tryptamine ions generated by an electrospray source have been studied by means of photoinduced dissociation technique on the femtosecond time scale. The result is that the initially excited state decays very quickly within 250 fs. The photoinduced dissociation channels observed can be sorted in two groups of fragments coming from two competing primary processes on the singlet electronic surface. The first one corresponds to a hydrogen-atom loss channel that creates a tryptamine radical cation. The radical cation subsequently fragments to smaller ions. The second process is internal conversion due to the H-atom recombination on the electronic ground state. Time-dependent density functional theory calculations show that an excited pisigma* state dissociative along the protonated amino N-H stretch crosses both the locally excited pipi* state and the electronic ground state S(0) and thus triggers the photofragmentation reactions. The two processes have equivalent quantum yields, approximately equal to 50% of the fragments coming from the H-atom loss reaction. The two primary reaction paths can clearly be distinguished by their femtosecond pump/probe dynamics recorded on the different fragmentation channels.

Collision induced fragmentation of small ionic sodium clusters: Competition between electronic and impulsive mechanisms

Collision induced fragmentation of small Nan+ (n=3–9) clusters with He atoms is investigated in the 100 eV center-of-mass collision energy range. The experiment is based on the determination of the velocity vectors of the fragments using a multicoincidence technique. The relative populations of the various fragmentation pathways are determined. Fragmentation mechanisms are discussed in detail. The most important pathways are primarily populated via momentum transfer in elastic binary collisions between the He atom and a Na+ core. Direct release of fast Na atoms is observed at variance with what is usually assumed at eV energies. However most of the fragmentation involves multistep dynamics with energy redistribution inside the cluster via Na–Na collisions. In contrast, production of Na+ fragments comes dominantly from electronic transitions towards repulsive potential energy surfaces of the cluster. The role of electron pairing is emphasized.

The dynamics of Cl− + H2 reactive collisions

Abstract A multicoincidence analysis of the crossed beam Cl − + H 2 system in the 5.6–12 eV energy range has shown the existence of four different product channels: reaction (R), reactive detachment (RD), simple detachment (SD) and dissociative detachment (DD). For the whole energy range both R and RD channels give rise to HCl molecules at a unique and common center-of-mass scattering angle whereas the vibrational excitation probability of HCl obeys completely different rules for each channel: v ≤3 in channel R and equal probability in all possible vibrational levels in RD. A Thomas-type collision model joined to curve crossing with an intermediate autodetaching HCl − state accounts well for all of the experimental findings.