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The present work reports a combined experimental and theoretical study of the 9-methylguanine (9MG) nucleobase and its monohydrate in the gas phase. Laser spectroscopy provides optical evidence for the presence of only one tautomer of 9MG, among the three forms expected from an energetic basis. Comparison with the calculated vibrational frequencies enables us to assign with confidence this form to the anti rotamer of the 9MG enol tautomer. The same approach allows us to conclude that the monomer conformation is retained in the monohydrate, with the water molecule bridging the OH proton donor and the N7 acceptor sites of 9MG. A complete comparison between all IR absorption data known for the enol tautomers of guanine and related compounds in the gas phase and the present calculations enables us to propose a consistent assignment for all of these enol tautomers in terms of rotamers. Finally, the fact that one does not observe either the most stable (and biologically relevant) keto form or the other enol rotamer (very close in energy) raises the question of the efficiency of UV spectroscopy to detect properly these missing species.

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.

A microscopic approach to the in vacuo energetics of the binding of a water molecule to the side chain of tryptophan, modeled by an indole molecule, is presented. Two binding sites have been studied, the most bound one, which corresponds to the conventional NH−OH2 hydrogen bond and a slightly less bound, so-called π-type hydrogen bond in which the hydrogen atoms interact with the π aromatic ring of indole. The structure of these two complexation sites as well as the potential energy surface has been obtained by a semiempirical model coupled with efficient procedures for the exploration of the surface. The H-bonded complex was observed in the supersonic expansion, and its binding energy (4.84 ± 0.23 kcal/mol) was measured using a laser two-color photofragmentation technique. The nonstandard H-bonded complex, not observed with indole, was observed with 1-methylindole, a substituted indole, in which the formation of the conventional bond is hindered. Its binding energy, measured with a similar accuracy (4.10...

The proton-transfer reaction in the ground state of phenol–ammonia clusters [PhOH(NH 3 ) 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⋯(NH 3 ) n ⇔PhO − ⋯H + (NH 3 ) 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(NH 3 ) n are discussed in the light of these new results.

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.

Exciplex and excimer formation have been probed in several jet cooled complexes using mass selective two-photon ionisation and fluorescence excitation spectroscopy as well as ground state depletion spectroscopy (hole burning): (i) In the anthracene—dimethyl-ortho-toluidine system, it has been found that the ionisation step takes place with a much higher efficiency from the charge transfer state responsible for the exciplex emission than from the locally excited state giving rise to the resonant fluorescence. (ii) The dimer, trimer, and higher clusters of anthracene all show only excimer emission. When compared to the dimer, the trimer exhibits a peculiar behaviour (structured fluorescence excitation and hole burning spectra, short lifetime and low ionisation efficiency) which has been related to a significant locally excited character of the initially prepared state of the species excited state. (iii) The influence of an intermolecular hydrogen bond on the electron transfer process has been studied in the 2,2,2-trifluoro-1-(9-anthryl)ethanoldimethylaniline system. A threshold for exciplex formation higher than in the case of the anthracene—dimethylaniline complex is observed. PACS numbers: 31.50.-1-w, 32.30.—r, 33.15.—e, 33.20.—t

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 CH 2 Cl 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.

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 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)n clusters, a clear red-shifted 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 charac...

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...

The present study combines both experiment and molecular modeling to describe the photoionization behavior of the gas-phase hydrogen-bonded complexes of phenol with water, methanol, and dimethyl ether, in particular the occurrence of fragmentation following ionization. Using the two-color two-photon resonant ionization laser technique, the threshold for dissociative ionization of these species has been measured. For the first time, precise binding energies have been deduced for the neutral species: D0(phenol−H2O) = 243 ± 5 meV and D0(phenol−CH3OH) = 265 ± 8 meV. Using a semiempirical potential model, the minimum energy structures of both neutral and ionic species have been determined. This theoretical study has emphasized the role of the dispersive interactions in the geometry of these neutral complexes, in particular the interactions between the alkyl group of the solvent molecule (CH3 in the case of methanol or dimethyl ether) and the π-cloud of the aromatic molecule. In addition, the comparison between...

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...

The chemical reactivity of benzene cations in medium-sized (n ∼ 25) (H2O)n water clusters has been investigated. After formation in a supersonic jet, the neutral Bz-(H2O)n species are ionized by resonance-enhanced two-photon ionization. The existence of magic numbers in the mass spectra of these cluster ions has been revealed by reflectron time-of-flight mass spectrometry. These magic numbers are assigned to size-dependent evaporation processes following the ionization. The similarities observed for the present magic numbers compared to those of protonated water clusters allow us to assign them to the formation of a protonated water cluster without ejection of the benzenoid product from the cluster ion. With this diagnosis, we present evidence for the existence of a size-dependent chemical reactivity of benzene cations: a reaction is observed in the medium size range (n ∼ 20−30), whereas small clusters are known not to react. This result is compared to the chemical reactivity of toluene+ in water cluster...

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 + NH 3 → BzNH 3 + + X, (iii) HX elimination: XBz + NH 3 → BzNH 2 + + 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.

The ionization of allyl bromide-ammonia clusters produced in a supersonic expansion leads to an intracluster nucleophilic substitution reaction: C3H5Br+ − NH3 → C3H5NH3+ + Br. Larger clusters also react leading to ammonia solvated C3H5NH3+ clusters. The geometries of the complexes and the energetics of the different reactive channels have been computed through ab initio calculations. A barrier to the reaction is evidenced.

The present work has addressed the question of the dissociation time of four alkyl nitrites upon photoexcitation in the S1 state (methyl nitrite, n‐ , and t‐butyl nitrites and i‐amyl nitrite). The time resolved Laser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiated at 351 nm by 150 fs pump pulses, and has been probed using a two‐photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two‐photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A←X transition. Conversely, the two‐photon process is resonant or quasiresonant for detection of the excited nitrite molecule in the S1 level before it dissociates. This leads to an enhanced detection efficiency of the non‐dissociated excited molecule versus that of the NO fragment. A simple ...

Substitution reactions in small ionic fluorochlorobenzene-(NH 3 ) n clusters have been studied through threshold photoelectron-photoion coincidence techniques using synchroton radiation. In this experiment the internal energy of the cluster ion is well defined and therefore the change of the reactive pathways with the internal energy content of the parent ion can be evidenced.

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.

We present here the first application of a “magnetic bottle” photoelectron spectrometer associated with a laser ionization source for studying the vibrational spectroscopy of large molecular ions. The instrument combines good resolution, very high and constant collection efficiency of electrons with up to 2 meV kinetic energy and ease of use. Photoelectron spectra of chlorobenzene- h 5 and chlorobenzene- d 5 , after one- or two-color, two-photon ionization via various vibrational levels of the first excited neutral state 1 B 2 , have been investigated. This has enabled the assignment of 15 vibrational frequencies of the 2 B 1 chlorobenzene- h 5 ion and one of its deuterated homologues. The vibrational state distribution of the generated ions has been determined very precisely providing new data for further spectroscopic studies.

The photoionization efficiency spectra of chlorobenzene- h 5 and chlorobenzene- d 5 . have been obtained by two-color laser photoionization spectroscopy. The spectra consist of direct ionization steps and display Franck-Condon-like behavior. The adiabatic ionization energies of the two species have been determined to be almost identical (73176±10 cm −1 ) indicating similar stability of the molecular structure of the ground neutral and ion states. In addition, some precise frequencies of vibrational modes of the ground state chlorobenzene cation have been measured.

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.

International audience; The electronic spectrum of laser-desorbed, jet-cooled guanine has been obtained by resonance enhanced two-photon ionization, laser induced fluorescence and hole-burning spectroscopy. Three tautomers of the molecule have been found, characterized by well-defined vibronic spectra and different excited state dynamics (fluorescence decay times of 12, 22 and 360 ns). For one of the tautomers the origin transition of the lowest electronically excited state, proposed to be of nπ*-type, was found to be located at 32 803 cm−1, very near to the origin of the first intense ππ* transition located at 32 864 cm−1. The ππ*-type origin transitions of the other tautomers lie at 33 269 and 33 491 cm−1. Three different forms have also been found for the guanine–water complex whose ππ*-type origin transitions are shifted to higher energy and are located at 185, 352 and 435 cm−1 above the origin of the lowest energy tautomer of guanine.

Van der Waals complexes formed between chiral molecules in the isolated gas phase were studied by combining supersonic expansion techniques with laser spectroscopy. The weakly bound diastereoisomers formed between a chiral secondary alcohol, butan-2-ol, and a chiral aromatic derivative such as 2-naphthyl-1-ethanol or 1-phenylethanol used as a resolving agent were discriminated on the basis of the spectral shifts of the UV S(0)-S(1) transition of the chromophore. Ground-state depletion spectroscopy (hole burning) has shown that, while only one structure was detected for the 1-phenylethanol/butan-2-ol homochiral complex, the heterochiral complex is trapped in the jet under two different conformations. Two isomers have also been shown for each diastereoisomeric pair of the 2-naphthyl-1-ethanol/butan-2-ol complexes. Using a semiempirical potential model, these isomeric forms were related to calculated structures which exhibit a folded or extended geometry depending on the solvent conformation (anti or gauche). The relative binding energy of the complexes involving R-1-phenylethanol and R- or S-butan-2-ol were obtained from fragmentation threshold measurements following two-color photoionization. Comparison of the diastereoisomers exhibiting a similar spectral signature shows that the homochiral pair is more stable than the heterochiral one by about 0.7 kcal/mol. The fragmentation threshold has been shown to depend on the jet-cooled isomer and this result addresses the role of conformational control in enantioselective interactions.

The present work has addressed the question of the dissociation time of four alkyl nitrites upon photoexcitation in the S1 state (methyl nitrite, n‐ , and t‐butyl nitrites and i‐amyl nitrite). The time resolved Laser Induced Fluorescence technique has been used in the femtosecond regime under bulk conditions. The photodissociation has been initiated at 351 nm by 150 fs pump pulses, and has been probed using a two‐photon process at 467 nm by 200 fs pulses. The LIF signal has qualitatively the same shape for the four nitrites: it passes through a transient peak before reaching a plateau. The two‐photon process that induces the detected fluorescence is nonresonant for detection of the dissociation product NO through the A←X transition. Conversely, the two‐photon process is resonant or quasiresonant for detection of the excited nitrite molecule in the S1 level before it dissociates. This leads to an enhanced detection efficiency of the non‐dissociated excited molecule versus that of the NO fragment. A simple ...

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.

Three liquids, CH3F, CH3Cl and CHF3, have been submitted to systematic investigation in the microwave and far infra-red frequency domains. The studies have been extended over a gradual variation of temperature and density scanning the whole liquid range beneath the critical region. Accordingly, the relative location in time of absorption processes were observed to evolve towards each other as the liquids expand when approaching their critical region. We discuss the dielectric relaxation processes and their characteristic times, together with some very important features from the very intense absorption in the far infra-red. The correlative dispersion is fitted by a distribution of resonances model. The occurrence of specific alignment effects are shown to be responsible for local enhancement of the dipole moments, resulting in excess of integrated intensities. Throughout this report a direct presentation of the experimental features displays already a number of characteristics of molecular dynamics. Their...

For the three liquid methylhalides, CHF3, CH3F, and CH3Cl, we describe a correlation function analysis of the rotational motion, extended over all the liquid-gas coexistent states. These orientational correlation functions are exponential at long times and damped oscillatory at short times. Experimental memory functions are computed, and a comparison with the rotational velocity correlation functions show generally acceptable agreement. The nature of the microscopic motion in the frame of either Cole-Glarum or Fatuzzo-Mason theories is compared, indicating generally weak deviations. Some examples are presented of the amplitudes of mean squared torques and their variations under orthobaric conditions. The physical situation in fluoroform is further analysed with the help of τ1/τ2 ratios, indicating that in this liquid a satisfactory description is gained with the itinerant oscillator model. Finally, the validity of the collisional models under their various formulations is shown to be, at the least, dubious.

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.

L'interaction des rayonnements α, β ou γ avec la matiere conduit a l'ionisation et l'excitation des molecules du milieu. La photoionisation a l'aide du rayonnement synchrotron ou de lasers permet l'etude detaillee des mecanismes primaires de production et de dissociation d'ions. Des etudes, a l'aide de lasers, de spectroscopie, d'energie des etats et de cinetique de fragmentation des ions sont decrites pour le phenol et le phenetole. Le rayonnement synchrotron permet d'etudier l'excitation et l'ionisation en couche interne dans le domaine d'energie entre 20 et 1 000 eV. Une predissociation rapide de HBr et CH3 Br excitees en couche 3d a ete observee ainsi qu'une dissociation selective de l'ion doublement charge CH3 Br++ .

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.

Well-resolved absorption spectra of the B←X forbidden transition of the chlorobenzene cation have been obtained for the first time by two-photon induced dissociation of the ground state ion. The φ + fragment ions are detected with a time-of-flight mass spectrometer. The wavelength dependence of the fragment ion yield reflects the spectroscopy of the φCl + B intermediate state reached by the first photon. Chlorobenzene cations are formed by resonance-enhanced two-photon ionization (R2PI) process. B←X vibronic transitions are observed due to vibronic interactions between the nearby B and C states. A preliminary analysis of the spectra is proposed.

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.