A deuterium atom transfer mechanism has been studied in the excited state of perdeuterated phenol-(ND3)(n) clusters and compared to the hydrogen atom transfer process evidenced in phenol-(NH3)(n=1,4) cluster excited state. A strong H/D effect is observed implying a tunneling reaction process. In view of these results, the question of the competition between proton transfer and H transfer is raised. An alternative to the excited-state proton-transfer dynamics paradigm is proposed to explain the present and previous observations. Good agreement with experimental observations can be obtained with the following three propositions: excited-state H atom transfer occurs for n = 1 to 6; ground-state proton transfer takes place for n greater than or equal to 6 and direct excitation of ground-state proton transferred structures leads to fast evaporation/relaxation events in the excited state; excited-state proton transfer, although energetically favored for cluster sizes n 1 4, is not observed, probably because its rate is slow compared to the H transfer reaction rate.

Femtosecond pump–probe ionization experiments on small NaI–Sn=1–7 clusters, S:NH3,H2O, are reported. The excitation of the clusters by the pump laser induces the NaI bond breaking within the cluster as well as evaporation of solvent molecules both in the excited and ionic states. Depending on the detection scheme used, namely one-photon or resonant two-photon ionization, the resulting Na+–Sn time-dependent signals are not similar. This reflects the major role of the ionization process in the nature of the dynamics probed. Furthermore, a solvent-selective behavior evidenced on the Na+–Sn signals has been related to different cluster structures according to the solvent nature: NaI seems to be embedded within ammonia clusters while it sticks on the surface of water clusters, when they are larger than four molecules.

Femtosecond pump–probe ionization experiments on small NaI–Sn=1–7 clusters, S:NH3,H2O, are reported. The excitation of the clusters by the pump laser induces the NaI bond breaking within the cluster as well as evaporation of solvent molecules both in the excited and ionic states. Depending on the detection scheme used, namely one-photon or resonant two-photon ionization, the resulting Na+–Sn time-dependent signals are not similar. This reflects the major role of the ionization process in the nature of the dynamics probed. Furthermore, a solvent-selective behavior evidenced on the Na+–Sn signals has been related to different cluster structures according to the solvent nature: NaI seems to be embedded within ammonia clusters while it sticks on the surface of water clusters, when they are larger than four molecules.

Small phenol-(NH3)(n) clusters have been studied through two-color two-photon and one-photon VUV ionization, in order to disentangle the contributions of various dissociation or reaction paths in the excited and ionic states. The most striking result of the two-color experiment is that (NH4)(+)(NH3)(n=1,5) fragments are observed with large delays (up to a few hundred nanoseconds) between the excitation and ionization lasers, whereas these same fragments are not observed in the VUV one-photon ionization experiment. In order to account for these findings, a new deactivation channel in the excited state of phenol-(NH3)(n) clusters has to be introduced: the hydrogen atom transfer PhOH(S-1) - (NH3)(n)--> PhO. + (NH4)(NH3)(n-1). In this case, the delayed (NH4)(+)(NH3)(n-1) signals correspond to direct ionization of the (NH4)(NH3)(n-1) clusters produced in the excited state.

The femtosecond photodissociation of CsI in the CsI-CH3CN complex has been studied. Recurrences are observed in the detection of the Cs+-CH3CN ion complex. They are assigned to the first vibrations of the Cs-NCCH3 bond formed after the CsI dissociation. This result is in good agreement with calculations on the similar NaI-CH3CN system which predict a linear structure for the ground state complex.

The phenol−(NH3)n=1,2,3 (S1) lifetimes have been measured near their band origins by a picosecond pump/probe scheme. The very short lifetimes observed reveal that the excited clusters undergo a reactive process, namely the following dissociative hydrogen transfer reaction: PhOH*−(NH3)n → PhO• + NH4(NH3)n-1.

The picosecond evaporation dynamics of ammonia molecules from Na(NH3)n clusters produced via the dissociation of NaI within an ammonia cluster is studied experimentally. A time dependence of the mass selected ion signal is observed only for one cluster size and not for larger clusters nor for smaller ones. It will be shown that evaporation in the ionic state is responsible for the absence of an observation of the evaporation in the neutral state. The variation of the ionization cross section with cluster size is the key point for the interpretation of the results.

The intracluster proton transfer reaction in the methanol ionic dimer has been re-investigated using VUV ionization with synchrotron radiation. The energetic thresholds of the proton transfer issued from the methyl group or the hydroxy group have been obtained. These results are discussed in view of previous calculations and experiments.

The electronic spectrum of the silver–ammonia 1:1 complex AgNH3 has been observed for the first time. The complex was formed using laser ablation and cooled in a free jet expansion, and its spectrum was observed through resonantly enhanced multiphoton ionization (REMPI). The origin of the ÖX̃ band system of AgNH3 located at 467 nm is red-shifted from the corresponding Ag 5p2P1/2–5s2S transition by 8142 cm−1, indicating a significant stabilization upon electronic excitation. The vibrational frequency of the intermolecular stretching mode in the à state was determined to be 378 cm−1.

The study of the phenol–(NH3)3 cluster with two-color two-photon ionization shows that the main ion observed with delays between the lasers up to a few hundred nanoseconds is the (NH4)+(NH3)2 fragment, resulting from direct ionization of the (NH4)(NH3)2 product coming from the reaction: PhOH(S1)–(NH3)3→PhO•+(NH4)(NH3)2.

The proton-transfer reaction in the ground state of phenol–ammonia clusters [PhOH(NH3)n] has been investigated by single-photon ionization spectroscopy using synchrotron radiation and a photoelectron photoion coincidence detection technique. The proton abstraction by the base (PhOH⋯(NH3)n⇔PhO−⋯H+(NH3)n) depends on the proton affinity of ammonia clusters which increases with their size. The present experiment demonstrates that at least six ammonia molecules clustered with phenol are necessary to displace the equilibrium and induce the proton transfer in the ground state. The previous nanosecond and picosecond experiments on proton transfer occurring in the excited state of PhOH(NH3)n are discussed in the light of these new results.

The reactivity of vinyl chloride ionic clusters has been investigated by the Threshold PhotoElectron PhotoIon COincidences technique. In the case of the dimer, the competition between the three reactive channels (HCl, Cl⋅ and CH2Cl elimination) has been studied. The main reactive channel is HCl elimination which proceeds through a 0.2 eV barrier. This elimination reaction is still observed in the trimer but not in larger clusters. For these clusters, cooling by evaporation of neutral vinyl chloride monomers seems to be the favored channel that hinders the HCl elimination step.

The photodissociation of a chlorine molecule in the environment of a xenon cluster has been studied experimentally using the real time pump and probe technique through the formation of an XeCl reaction product. The photodissociating system is probed in such a way that the movement of a single chlorine atom in the xenon environment is detected. Various XenCl2 cluster sizes have been investigated leading to the distinction between uncapped, half-capped and doubly capped structures for these clusters. These structures have a profound influence on the photodissociation dynamics. Retrapping of one chlorine atomic fragment and stabilization of the XeCl reaction product is only observed for the half and doubly capped clusters. The experimental work is complemented by classical molecular dynamics calculations to get a full picture of the photodissociation.

The solvent effect on the appearance of a red-shifted twisted intramolecular charge-transfer (TICT) emission is studied in molecular clusters. Using a supersonic expansion, dimethylaminobenzomethyl ester-acetonitrile (DMABME-(CH3CN)(n)) clusters are studied by monitoring at the same time mass spectra and dispersed fluorescence spectra as well as by lifetime measurements. For DMABME-(CH3CN), clusters, a clear redshifted fluorescence is observed readily when the cluster contains one solvent molecule and the fluorescence decay becomes biexponential. Short-time evolution of the system has been monitored using a femtosecond pump/probe technique and picosecond photoelectron spectroscopy. The femtosecond dynamics is solvent dependent and is interpreted as a fast decay from the locally excited state to the TICT state reaching an equilibrium between these two states. The equilibrium shifts to the TICT states as the cluster size increases. The role of the triplet state in this process and in the biexponential character of the fluorescence is discussed.

The nuclear wave packet in the A state of NaI has been monitored in a pump-probe femtosecond experiment using a resonant two-photon ionization detection scheme. A strong asymmetry depending on the direction of propagation of the wave packet is observed in the \(\) ion signal. This phenomenon, well reproduced by a full quantum wave packet calculation, has been assigned to the combination of two effects: i) the motion of the wave packet in the intermediate state of the two-photon probe process, ii) the R-dependence of the photoionization efficiency of this intermediate state.

A study employing picosecond and subpicosecond excitation in a mass and photoelectron spectrometer is reported for the 7-azaindole (7-AI) dimers, reactive and unreactive. The 7-AI photoelectron spectrum is structured and has a sharp ionisation threshold at 8.17 eV above the neutral ground electronic state. The reactive dimer ionisation threshold was measured as 7.19 eV. The excited-state lifetime of the reactive dimer was measured by a technique that monitors the ionisation signal as a function of pulse duration. 1 + 1 resonance ionisation photoelectron spectra were recorded using 0.8 ps and 5 ps pulses. Our results indicate a lifetime substantially less than a picosecond, however, consistent with recent real time studies. Advantages of the method are discussed.

Nucleophilic substitution reactions have been studied in halobenzene cations with ammonia. In ionic 1-1 complexes, three behaviours can be observed, depending on the nature of the halogen atom X (F, Cl, Br or I): (i) no reactivity, (ii) X elimination: XBz+NH3 → BzNH3+ + X, (iii) HX elimination: XBz+NH3 → BzNH2+ + HX. We propose a reactive potential energy surface in which the two eliminations proceed through the same entrance channel: the formation of an addition σ complex after a barrier crossing. This should be the key point for interpreting the ensemble of previous results. The reaction time for this barrier crossing has been estimated using the RRK formalism, giving a set of consistent barriers for σ complex formation in each halogenated compound studied from a comparison of modeling and experimental results.