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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.

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.

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.

In this paper we report on the structure of van der Waals anthracene–argonn (n=1–6) clusters, which were interrogated by a complete set of experimental diagnoses based upon mass-selective, fragmentation-free, two-color, resonant two-photon ionization and laser induced fluorescence as well as a theoretical modeling of the ionization energy. We were able to provide a global assignment of the spectral features which rests on several independent sources of information: the spectral shifts δν and linewidths of the S1←S0 transition, the ionization energy shifts δI.E. and threshold widths and their comparison with the theoretical simulations, the hole-burning spectra, the structure-specific predissociation dynamics. The existence of two cluster ensembles, which present qualitatively different spectral features and significant relative intensity, has been evidenced for each cluster size. The first ensemble exhibits spectral features, typical of cold, rigid-like clusters. Only one prominent stable structure is found for each cluster size of this ensemble: (1|0) for n=1, (2|0) for n=2, (2|1) for n=3, (2|2) for n=4, (3|2) for n=5, (3|3) for n=6, where the notation (n-p|p) refers to the distribution of the argon atoms between the two molecular sides. The second ensemble has been assigned to more or less `hot' clusters having probably the same (n-p|p) arrangement as the first ones, but excited in high van der Waals vibrational levels through an intracluster vibrational relaxation of anthracene molecules. Results on similar heteroclusters reported in the literature are discussed on the light of our new assignment of the spectral features of small Ant–Arn clusters to structures differing by their internal energy content and not by their configuration.

The present study combines both experiment and molecular dynamics simulations in order to document the ionization behavior of the C6H6−H2O and C6H6−D2O complexes close to the ionization threshold, in particular its nonadiabatic character. Using the two-color two-photon resonant ionization laser technique, the ionization thresholds of these species have been measured together with the threshold for dissociative ionization. A binding energy has been deduced for the neutral species: D0(C6H6−H2O) = 106 ± 4 meV and D0(C6H6−D2O) = 116 ± 5 meV, which significantly increases the precision compared to literature. Using a semiempirical potential model, the minimum energy structures of the neutral and ionic species have been determined, and the potential energy surfaces have been analyzed using a two-dimensional approach. As a result, the formation of a stable C6H6+−H2O complex close to the threshold is found to be controlled by a pure quantum effect and is ascribed to the classically forbidden region of the neutra...

Two-color two-photon femtosecond ionization experiments have been performed on NaI. The wave packet evolution of the A excited state has been followed by detecting photoions and photoelectrons. The results indicate that the Na+ ions are formed when the wave packet is located at the outer turning point of the excited state. Surprisingly, the NaI+ ions are also observed to be in phase with the Na+ signal. Photoelectron spectra show that high kinetic energy electrons are produced when ionizing around the outer turning point, in agreement with the NaI+ formation. The absence of signal corresponding to ionization from the covalent part of the excited state potential can only be understood if the absolute ionization cross section is much smaller in the covalent region of the A state (where the molecule can be considered as a van der Waals complex) than in the ionic Na+···I- part of the A state potential (where the interatomic distance is such that the ionization process may be considered as a photodetachment of...

In this paper, new experimental results using UV spectroscopy of the small benzene–Arn clusters are presented. We have found evidence for the vibrational predissociation of the S1 state of some of these species on a nanosecond time scale and we propose a new assignment for the UV spectral features of this system. This assignment is consistent with other experimental data and it accounts additionally for the previously reported spectral anomalies, in particular the ionization potential measurements of these species. The two-color R2PI spectra performed can thus be considered as nanosecond pump–probe experiments that allow us to estimate the relaxation rate of the S1state of these clusters. The vibrational relaxation is found to be strongly size and structure dependent: The two isomers [(1/1) and (2/0)] of the n=2 species exhibit different lifetimes differing by at least one order of magnitude. The size dependence of the relaxation process within the one-sided isomer series appears also paradoxical since th...

We report the study of several higher excited Sn(n > 2) valence states of chlorobenzene observed by double resonance multiphoton ionization photoelectron spectroscopy in the 56000–67000 cm−1 region. The simultaneous measurement of the kinetic energy of the electrons and of the corresponding absorption spectrum enabled to determine the respective vibrationless level of these states (55120, 56600, 61100 and 62210 cm−1) and to distinguish direct ionization for Rydberg states from ionization after IVR of the initially excited level for valence states. The onset of IVR mechanism, at about 1000 cm−1 internal energy, was determined from double resonance absorption spectroscopy.

The intensity distribution of benzene+-Arn cluster ions formed by laser ionization of neutral clusters has been investigated: two main intensity anomalies (magic numbers atn=20 and 45) have been observed in the 15–60 size range. The evaporation dynamics of these species in the 2–50 microsecond time window following ionization has been studied using the electrostatic mirror of a reflectron time-of-flight mass spectrometer as a kinetic energy analyser capable to distinguish parent and daughter ions. The magic numbers are interpreted in terms of size dependent evaporation behaviors: beyondn=20, a sudden decrease of the evaporation energy is observed; in then=45–47 size range, the magic number is accounted for by the specific dynamics of then=46 and 47 clusters, in particular the possible loss of two argon atoms forn=47 within the experimental time window. These results and their implications on the cluster structure are discussed in the light of the evaporative ensemble model and compared to the evaporation characteristics of similar species, in particular the neat rare gas clusters.

In this first application of double resonance multiphoton ionization photoelectron spectroscopy to study the higher excited states of a polyatomic molecule, we have obtained the double resonance absorption spectrum of chlorobenzene in the energy region 56000 to 66700 cm −1 . Three new band systems are identified as members of Rydberg series converging to the first ionization energy. These systems with the 0 0 0 bands at 63000, 64690 and 66390 cm − are tentatively assigned to the 3d, 4p and 4d Rydberg states, respectively.

International audience; In light of a recently published study on the IR spectroscopy of guanine in He droplets (Choi, M. Y.; Miller, R. E. J. Am. Chem. Soc. 2006, 128, 7320), the present letter proposes a new interpretation of the resonant two-photon ionization (R2PI) experiments on gas phase guanine, which is supported by quantum chemistry calculations. Whereas He droplet experiments detect the most stable forms, only one of these forms is observed (very marginally) in the R2PI spectrum, which is actually dominated by three less stable “rare” tautomers, whose stabilities lie in the 3-7 kcal/mol range. The absence of the most stable forms in the R2PI spectrum suggests that a tautomer-dependent ultrafast relaxation process takes place in the excited state of these stable tautomers. The present reinterpretation modifies qualitatively the picture of the excited state of guanine tautomers and should contribute to the understanding of the deactivation mechanisms taking place in the excited state of DNA bases.

International audience; We propose a very simple and efficient way to stabilize ions issued from a collision complex through a femtosecond coherently controlled pump/probe process. Starting from a van der Waals complex, one can initiate a collision at a well-defined time and with a restricted impact parameter. Formation of stable ionic complex can be achieved by ionizing the collision complex at the "right time." We present in this paper its application to the NaI–(CH3CN)1–2 system. Na+–CH3CN ion formation is coherently controlled by ionization of colliding Na atom on CH3CN molecules issued from the dissociation of NaI within NaI–(CH3CN)1–2. Classical mechanic calculations using simple ionization/dissociation conditions can reproduce the experimental data and give an insight into the control of such a reaction.

The knowledge of the spectroscopy of the vibrational modes of the Van der Waals complexes is a key for the theoretical understanding and modelling of various physical processes occurring in these systems, e.g. intramolecular vibrational relaxation, unimolecular reaction dynamics leading to fragmentation, intersystem crossing, etc...

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.

Two-color, two-photon resonance-enhanced ionization spectra have been obtained for aniline, phenol and phenotole near and above the ionization threshold in an effusive and supersonic beam. Precise adiabatic ionization energies have been measured. The electric field dependence of these energies is found to be consistent with a field-ionization red shift of the thresholds. The two-color photoionization technique has been successfully applied to determine a set of vibrational frequencies of the excited neutral and ground ionic states of these molecules.

Laser-induced dissociation of phenetole ions has been carried out. The ions have been prepared in a well-defined excited state by resonance-enhanced two-photon, two-color ionization (R2PI/2C). Appearance energies of 2.431 and 2.82 eV have been determined for the first dissociation pathways leading to C 6 H 6 O + and C 5 H 6+ ionic fragment. The dissociation spectrum as well as detailed dissociation rate constants versus internal energy have been obtained. The dissociation rates are compared with those expected from RRKM theory. A complex reaction scheme has been assumed in order to explain the observed discrepancy.

The velocity and angular distributions of photofragments in well-defined internal states have been determined using a combination of the resonance-enhanced multiphoton ionization (REMPI) method with time-of-flight (TOP) mass spectrometry. This technique has been applied to probe the NO X, v = 0,1 photofragments in the dissociation of NO 2 with an excess energy of up to 2700 cm −1 above the threshold. The high kinetic resolution obtained (δν = 260 m s for a 30 amu mass) is compared with previous measurements.

Ionisation of argon atoms in high Rydberg states (HR) by collisions with CCl4 and CCl3 F has been investigated. Absolute cross sections for the dissociative electron attachment are reported, together with the identities of the negatively and positively charged species produced in coincidence in the collision. The dependence of the cross sections on the principal quantum number n and the HR argon velocity alows us to determine the limits of the theoretical "free electron" model and the role of the core Ar+ in the outcome of the reaction.

Measurements of absolute ionizing reaction cross sections in collisions between argon atoms in high Rydberg states (HR) and CCl4, CCl3F, CH3I, SF6, C6F6, CF4, and CO are reported together with their dependence on the HR argon velocity (570–2100 m/sec). The identity of the positively and negatively charged reaction products has been investigated with a coincidence TOF mass spectrometer. A comparison of the present data with results of other bound and free electron experiments and theoretical predictions is given.

Photoionization of organic molecules with tunable UV laser light is studied by time of flight mass spectrometry (TOF Ms). The possibilities of the apparatus are described. In the present work, the total ion current is used for the study of aniline, benzylamine and benzylchloride ions. Two and three photon ionization have been observed.

The well‐resolved absorption spectrum of a large non‐radiative aromatic ion like chlorobenzene has been obtained by the method of resonance‐enhanced multiphoton dissociation (REMPDI). The cations are prepared in the ground electronic state with well‐defined rovibrational energy excess by resonace‐enhanced two‐photon ionization (R2PI). The light of a second laser excites the ions through a (1+1) process into predissociative states which dynamics has been first studied. The wavelength dependence of the fragment ion yield reflects the spectroscopy of the intermediate ionic state reached by the first photon. In this work, we present the completely vibrationally resolved spectrum of the B←X transition of φC1+. For the first time the rotational contour of the vibronic bands of R2PI‐prepared ions has been calculated. The comparison with the experimental band shapes has brought additional arguments for the assignment of the spectral features.

Two-color, two-photon resonance enhanced ionization (R2PI-2C) is shown to be a powerful tool for the determination of adiabatic ionization potentials and vibrational frequencies of ground ionic states of polyatomic cations. Results on the laser induced dissociation of phenetole cations prepared by R2PI-2C will be presented.