Control of bond-selective photochemistry in CH BrCl using adaptive femtosecond pulse shaping

Adaptive femtosecond laser pulse shaping is employed to achieve bond selective photodissociation/photoionization of CH₂ClBr in the gas phase. The photoproduct signal measured in a reflectron time-of-flight mass spectrometer is used as feedback to improve iteratively the spectral phases of the laser pulse via an evolutionary algorithm. We observe an increase of the fission of the stronger versus the weaker carbon halogen bond by 100%. Single parameter control schemes proved unable to achieve bond-selectivity. The complexity of the control problem is addressed by mapping it onto the well-known problem of maximizing second-harmonic generation (SHG). Further spectroscopic results indicate that the control involves manipulation of wave-packet dynamics on the neutral surfaces. Received 21 December 2001 / Received in final form 2 April 2002 Published online 28 June 2002     

Analysis and control of ultrafast photodissociation processes in organometallic molecules

In this paper we characterize the ultrafast fragmentation in electronically excited Fe(CO)₂(NO)₂ and CpMn(CO)₃ by means of femtosecond time-resolved spectroscopy combined with mass spectrometry. From the transient two-color multi-photon ionization data, it was possible to record the transients of the parent molecule ions and their photofragment ions. The experimentally observed decay times indicated an ultrafast loss of the first ligands (sub-100 fs decay times). Further we performed a feedback control experiment on the photofragmenting CpMn(CO)₃ molecular system in order to maximize the yield of desired ionic products through pulse modulation. The shape of the pulses obtained from optimization reflect well the intrinsic molecular dynamics during photofragmentation and the change of the CpMn(CO)⁺/CpMn(CO)₃ ⁺ ratio shows a clear evidence for the capability of the optimization method to find tailor-made system-specific pulses. Received 9 January 2001     

Time-dependent fragment distributions detected via pump-probe ionisation: a theoretical approach

We show how in molecular predissociation a method combining ultrafast pump-probe techniques with a measurement of the relative recoil velocity can map time-dependent neutral fragment distributions into the ionic continuum. With an appropriate probe pulse exciting a resonant transition (such as (1+1) Resonance Enhanced Multiphoton Ionisation, or excitation of ZEKE states), the temporal evolution of fragment distributions can in principle be measured. Numerical simulations on NaI predissociation are compared to a simple approximate mapping interpretation. The results are discussed in terms of the interplay between temporal and energetic resolution with respect to current experimental limitations. Received 13 October 2000 and Received in final form 8 December 2000     

Laser pulse control of ultrafast electron transfer reactions

The optimal control (OC) scheme for molecular dynamics is applied to the study of ultrafast bridge mediated electron transfer (ET). Utilizing the methods of dissipative quantum dynamics in combination with the OC approach the guided charge motion in a donor-bridge-acceptor system including a single active vibrational coordinate is studied. The control field drives the optical transition from the electronic ground-state of the ET system into the donor-level and can be used to prepare special electronic and vibrational states. In particular, it is demonstrated that charge localization becomes possible at the acceptor or bridge molecule as well as in the electronic ground-state of the ET system. Received 30 August 2000 and Received in final form 25 October 2000     

Spectroscopy of calcium deposited on large argon clusters

Spectroscopic experiments have been performed, providing emission and excitation spectra of calcium atoms trapped on argon clusters of average size 2 000. The two experimental spectra fall in the vicinity of the calcium resonance line ¹P ₁ → ¹S₀ at 422.9 nm. The excitation spectrum consists in two bands located on each side of the resonance line of the free calcium. In addition, Monte Carlo calculations, coupled to Diatomics-In-Molecule potentials are employed to simulate the absorption spectrum of a single calcium atom in the environment of a large argon cluster of average size 300. The theoretical absorption spectrum confirms the existence of two bands, and shows that these bands are characteristic of a calcium atom located at the surface of the argon cluster and correspond to the excited 4p orbital of calcium either perpendicular or parallel to the cluster surface. The precise comparison between the shape of the absorption spectrum and that of the fluorescence excitation spectrum shows different intensity ratios. This could suggest the existence of a non adiabatic energy transfer that quenches partly the fluorescence of trapped calcium. Another explanation, although less likely, could be a substantial dependence of the calcium oscillator strength according to the alignment of the calcium excited orbital with respect to the cluster surface. The emission spectrum always shows a band in the red of the resonance line which is assigned to the emission of calcium remaining trapped on the cluster. When exciting the blue band of the excitation spectrum, the emission spectrum shows a second, weak, component that is assigned to calcium atoms ejected from the argon clusters, indicating a competition between ejection and solvation. Received 7 May 2002 Published online 1st October 2002 RID="a" ID="a"e-mail: jmm@drecam.saclay.cea.fr RID="b" ID="b"URA 2453 du CNRS RID="c" ID="c"UMR 5626 du CNRS     

Photodissociation dynamics of CF3I investigated by two-color femtosecond laser pulses

We report an experimental study of the multiphoton dissociation dynamics of CF₃I performed on a home-built femtosecond laser pump-probe system, with time-of-flight mass spectrometer. The first repulsive A band and the 5pπ³7sσ υ₂=1 Rydberg state of CF₃I were accessed by one- and two-photon transitions at 267 nm, respectively, with the latter two-photon absorption followed by a further two-photon probe transition at 401 nm to the state of the parent ion. The observed signals from the CF₃ ⁺ and I⁺ fragments show similar multi-component exponential decay patterns but the former is 4 times stronger than the latter. However, the parent CF₃I⁺ signal was observed to evolve in a very different manner, decreasing sharply when probed in the first 289 fs following excitation and subsequently rising again after 860 fs to a constant level below that measured at negative pump-probe delay times when the pump and probe pulses exchange roles. This dip observed in the parent ion profile, is very different from that previously reported at shorter pump wavelengths of 264 nm or 265 nm, and is interpreted as the competition between two different ionization channels. One from the vibrationally excited υ₂=1 of the irradiated Rydberg state and the other from the dissociative vibrational origin of the same electronic state which is populated by internal vibrational relaxation.     

Anharmonic effects in large-amplitude vibrations of metal clusters

Two types of extreme collective motion, large-amplitude many-phonon vibration of the ionic core and rotation of the cluster with high angular momenta, are considered. The interplay between vibration and collective motion towards fission is discussed. A new mechanism of formation and rupture of the neck is proposed which is based on the Franck-Condon principle, and accounts for the interplay between vibration and fission. Under rotation, the change of the shape of the cluster and a phase transition from axially symmetric to triaxial ellipsoid are predicted. For studying the effects, vibrational motion can be induced by laser radiation. Rotational motion may arise in collisions of clusters. Received 26 April 2001 and Received in final form 15 October 2001     

Ionic recoil energies in the Coulomb explosion of metal clusters

The photoionization of metal clusters in intense femtosecond laser fields has been studied. In contrast to an experiment on atoms, the interaction in this case leads to a very efficient and high charging of the particle where tens of electrons per atom are ejected from the cluster. The recoil energy distribution of the atomic fragment ions was measured which in the case of lead clusters exceeds 180 keV. Enhanced charging efficiency which we observed earlier for specific pulse conditions is not reflected in the recoil energy spectra. Both the average and the maximum energies decrease with increasing laser pulse width. This is in good agreement with molecular dynamics calculations. Received 20 December 2000     

Stability of charged metal clusters in the vicinity of an ionic charge

Stability of highly charged metal clusters in the electric field of an external ion is investigated with the classical liquid drop model. We study the optimum shape of the cluster which has a local minimum of the total energy, taking account of the effects of the surface charge polarization on the Coulomb energy and the cluster deformation on the surface energy. We find that the cluster deformation greatly affects the total energy of the system and that a cluster with a fissility larger than some critical value 0.7-0.8 can become unstable against deformation. We investigate the local competition between the Coulomb force and the surface tension at the cluster surface and show that the surface charge polarization which is induced by the external electric field significantly affects the shape of the cluster and its stability. Received 5 November 2002 / Received in final form 27 January 2003 Published online 11 March 2003 RID="a" ID="a"e-mail: hamada@konan-u.ac.jp     

Hydrogen atom transfer in indole clusters: formation dynamics of , n = 1-6, fragments

The H atom transfer reaction in electronically excited indole(NH ₃ ) _n clusters is studied in pump-probe experiments with femtosecond laser pulses. By applying different probe photon energies we are able to detect the dissociation products (NH ₃ ) _{n - 1} NH ₄ for n = 1-6. Furthermore we show that the analysis of the corresponding ion signals is not distorted by contributions from larger cluster ions due to evaporation of NH ₃ molecules. The formation times of the products are ca. 140ps for n = 2-4 and about 80ps for n = 5, 6. Received 30 April 2002 / Received in final form 29 May 2002 Published online 13 September 2002     

Control of island morphology by dynamic coalescence of soft-landed clusters

The deposition of preformed clusters on surfaces has been established as a new way for growing nano-suctures on surfaces. It has been shown that supported island morphology relies on the dynamics of clusters, during the growth, giving rise to shapes from compact to ramified types. This paper identifies and discusses, in the case of antimony cluster deposits, several processes responsible for the non-equilibrium island shapes: limited kinetic cluster aggregation, size dependent coalescence, “wetting-like behavior” of antimony clusters on antimony islands. Using successive predetermined cluster sizes during the deposition process to synthesize polymorphic structure involves the interplay of those mechanisms. Received 1st December 2000     

Landau fragmentation and deformation effects in dipoSe response of sodium clusters

Recent experimental data on the dipole plasmon in axial sodium clusters Na _N ⁺ with 11 ≤ N ≤ 57 are analyzed within a self-consistent separable random-phase approximation (SRPA) based on the deformed Konh-Sham functional. Good agreement with the data is achieved. The calculations show that, while in light clusters plasmon properties (gross structure and width) are determined mainly by deformation splitting, in medium clusters with N τ 50 the Landau fragmentation becomes decisive. Moreover, in medium clusters shape isomers come to play with contributions to the plasmon comparable with the ground state one. As a result, commonly used methods of the experimental analysis of cluster deformation become useless and correct treatment of cluster shape requires microscopic calculations.     

Structure determination of mixed clusters by surface scattering

In this paper, we investigate the global structure of mixed clusters created by coexpansion. To determine the relative dopant sites within the mixed clusters, we take advantage of the strong dependence of the cluster/surface collision dynamics on the incident mixed cluster structure. Using both experiments and molecular dynamic simulations, we show that the coexpansion process leads to the most stable cluster structure for Ar _<880> Kr _<120> clusters. This structure corresponds to an annealed structure and can be characterized as a nearly homogeneous mixture throughout the cluster with a thin argon coating. Received 4 October 2000     

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters

Neutral ammonia clusters (NH₃)_m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH₃)_m-2NH₄(3s)NH₂ with the subunit NH₄ in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH₃)_m-2NH₄(3p)NH₂ states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation. Received 22 September 2001 and Received in final form 31 January 2002     

Effect of melting on ionization potential of sodium clusters

The effect of melting transition on the ionization potential has been studied for sodium clusters with 40, 55, 142, and 147 atoms, using ab initio and classical molecular dynamics. Classical and ab initio simulations were performed to determine the ionization potential of Na₁₄₂ and Na₁₄₇ for solid, partly melted, and liquid structures. The results reveal no correlation between the vertical ionization potential and the degree of surface disorder, melting, or the total energy of the cluster obtained with the ab initio method. However, in the case of 40 and 55 atom clusters, the ionization potential seems to decrease when the cluster melts. Received 1st November 2002 Published online 24 April 2003 RID="a" ID="a"e-mail: ar@phys.jyu.fi     

Intracluster electron transfer from a metal atom/cluster followed by anionic oligomerization of vinyl molecules

Intracluster electron transfer and oligomerization reaction were investigated by mass spectrometry of clusters of alkali metal atom (M) with acrylonitrile (AN; CH₂=CHCN). In the photoionization mass spectra of M(AN)_n, magic numbers were clearly observed at n = 3k (k = 1-4 for M = Na and K, k = 1 for M = Li). The results of photodissociation of neutral K(AN)_n indicate that the n = 3 cluster has an anomalous stability relative to other sizes of clusters. The C=C bond in vinyl molecules is also found to be necessary to form the magic numbers by measuring the photoionization mass spectrum of K atom with propionitrile. These results strongly support the intracluster anionic oligomerization reaction initiated by electron transfer from the alkali atom. The quantum chemical calculations have revealed that the evaporation induced by excess energy generated by intracluster oligomerization is important to form the magic numbers in the present clusters. Received 29 November 2000     

Electron impact ionization of thymine clusters embedded in superfluid helium droplets

Embedding molecules in helium clusters has become a powerful technique for the preparation of cold targets for spectroscopy experiments, as well as for the assembly of complex, fragile molecular species. We have recently developed a helium cluster source and a pick-up cell to produce neutral beams of doped helium droplets, to be used as targets in studies on electron collisions with molecules of biological relevance. In the present work we present the results of a series of experiments on electron-impact ionization of helium clusters doped with thymine and 1-methylthymine, where several interesting phenomena were observed, i.e., (i) electron impact ionization of molecular clusters inside the helium droplets leads predominantly to protonated clusters; (ii) the appearance energies are close to the ionization threshold of the helium atom but ionization efficiency curves in addition extend down by several eV; (iii) ionized molecular clusters can undergo metastable decay via the loss of one neutral monomer.     

Metastability of gold-carbonyl cluster complexes, Au N(CO) M -

Smaller gold-cluster anions, typified by Au ₇ ^- , adsorb multiple CO molecules in a high-pressure, room-temperature flow-reactor, tending toward previously unknown saturation compositions, Au₇(CO) ₄ ^- . The weakness of the gold-carbonyl adsorption bond is evidenced indirectly by the high CO partial pressure required and more directly by the high probability of fragmentation in the field-free flight region of the reflectron-type time-of-flight mass spectrometer. The analysis of this metastability reveals that the actual distribution f_N,M of products Au₇(CO) _M ^- in the reactor may be highly non-statistical, e.g. with only even-M species present. Received 17 April 2001     

DFT studies of ionic vibrations in Na clusters

We use time-dependent density functional theory coupled to molecular dynamics for ionic motion to compute the spectra of ionic vibrations in small Na clusters. Comparison with results from the distance dependent tight-binding approach shows good agreement between these two very different methods. We discuss the evolution of the spectra with cluster size and charge and the impact of ionic vibrations on the optical response. Received 23 July 2001 / Received in final form 5 July 2002 Published online 8 October 2002 RID="a" ID="a"e-mail: suraud@irsamc.ups-tlse.fr     

Growth of tellurium clusters in presence of metal impurities

The growth of small tellurium clusters in helium and the influence of a metal impurity (dysprosium atoms) on the cluster size distribution are investigated in a double laser vaporization source. A model describing the role of the carrier gas as collision partner is presented, emphasizing the crucial influence of the gas pressure on cluster formation. Changes in cluster reactivity due to dysprosium addition are discussed in terms of ionic structures Dy ^{3 +}(Te _N)^{3 -} containing a radical electron. Received 28 November 2000