S. Skowronek

Ultrafast dynamics and energetics of the intracluster harpooning reaction in Ba⋯FCH3

Applying the pump–probe technique with femtosecond laser pulses we have determined an intracluster reaction time of (270±30) fs in Ba⋯FCH3 after resonant excitation of the complex at 618 nm. The BaF reaction product is formed within this time in an electronically excited state whose energy has been estimated from the energy balance of the reaction.

Spectroscopy and dynamics of van der Waals reactions: the Ba⋯FCH3∗ photofragmentation channels

Abstract The simultaneous measurement of both the photodepletion action spectrum of the Ba⋯FCH 3 complex and the action spectra of the reactive BaF and non-reactive Ba photofragmentation channels, over the energy range 17795–18250 cm −1 allowed direct determination of the reaction probabilities of a van der Waals reaction as a function of excitation energy. The energy dependence of the Ba⋯FCH 3 + hν →(Ba⋯FCH 3 ) ∗ →BaF ∗ +CH 3 and Ba ∗ +CH 3 reaction probabilities showed an oscillatory behaviour with opposite phase, which could be due to quantal interferences between the two photofragmentation channels. In addition the energy spacing of the oscillations can be related to the internal motions of the transition state.

Resonances in the Ba…FCH3+hν→BaF+CH3 reaction probability

By measuring both the Ba…FCH3 photodepletion and the BaF action spectra, the Ba…FCH3+hν→BaF+CH3 reaction probability, PR(E), has been determined over the 16 065–16 340 cm−1 energy range. PR(E) shows a peak structure with an energy spacing of 10.9 cm−1 that may be related to the internal motion of the transition state of the reaction.

Molecular beam depletion by homogeneous and oscillating electric fields

Abstract As was suggested by a theoretical study made by one of us [An. Fis. 93 (1997) 105] the attenuation of a N 2 O supersonic beam was observed when molecules interact with both a homogeneous electric field and a radiofrequency resonant with the Stark N 2 O(1,±1)→(1,0) transition. The experimental observation of the new phenomenon opens interesting possibilities not only for fundamental research but also for technological applications, as for example the development of new analytical techniques based on such a molecular beam depletion.

Isotopic and internal CX3 (X=D,H) rotational motion effects in the Ba⋯FCX3+hν→BaF+CX3 intracluster reactions

Photodepletion and action spectra of the laser-induced Ba...FCD3 fragmentation have been measured over the 16 075-16 380 cm(-1) range. The observed band and peak structures allowed us to estimate the vibrational and rotational structures of the excited complex at the transition state configuration. The relative reaction probability P(R)(E) for the intracluster Ba...FCD3 + h nu --> BaF + CD3 reaction has been determined over the cited energy range. P(R)(E) shows a peak structure with an energy spacing of 8.9 cm(-1) which was attributed to an internal rotation of the CD3 group in the intermediate state. A comparison with previous Ba...FCH3 photofragmentation spectra reveals the dynamical role of the internal CX3 (X = H,D) motion which is manifested by the presence of rotational resonances in the laser-induced intracluster reaction.

SPECTROSCOPY AND DYNAMICS OF THE LASER INDUCED INTRACLUSTER (Ba··FCH3)* -> BaF* + CH3 AND Ba* + FCH3 REACTION

An overview of the spectroscopy and dynamics of the intracluster Ba··FCH3 + hv → BaF* (Ba*) + CH3 (CH3F) bimolecular reaction is presented. The Ba··FCH3 weakly bound complex is produced by laser vaporisation of a solid Ba sample followed by supersonic expansion of the Ba vapour with the solvent gas. Typical mass spectra obtained by the laser ionisation technique and time-of-flight mass spectrometry are presented. The Ba··FCH3+ ionisation potential was found to be 4·55 ± 0·03 eV. Laser induced photodepletion of the Ba··FCH3 complex was observed at distinct wavelengths of the excitation laser, together with two open channels for complex photofragmentation: a reactive channel, giving BaF as a product, and a non-reactive channel where Ba is formed. The photodepletion spectrum was measured in the 545–630 nm region, displaying two distinct regions that were interpreted as two different electronic states of the complex. The lifetime of the excited A-state was estimated by applying the inverse Fourier transform to the measured spectrum, giving a lifetime of about 250 fs. This result was confirmed in a pump and probe femtosecond experiment, where a lifetime of about 270 ± 30 fs was measured. In addition to the photodepletion spectrum, product action spectra were measured. This allowed the direct determination of the reaction probability as a function of excitation energy. In the A-state, the BaF action spectrum has been measured over the 16065–16340 cm-1 energy range. The reaction probability, P R (E), displays a peak structure with an energy spacing of 10·9 cm-1 that may be related to the internal motion of the transition state of the reaction. For the B-state, product action spectra for both the reactive and non-reactive channel are presented for the 17795–18250 cm-1 energy range. Here, the reaction probabilities show an oscillatory behaviour with opposite phase, which could be due to quantal interferences between the two photofragmentation channels. In addition, the energy spacing of the oscillations can be related to the internal motions of the transition state. All these features are discussed taking into account the interplay between the spectroscopy and dynamics associated with each electronically excited state of the complex that governs the photoinitiated harpooning reaction.

Laser-Induced Charge Transfer Under Crossed-Beam Conditions: Application to the Na

A crossed-beam charge transfer study was carried out for the Na(32P3/2,1/2) + I2 → Na+ + I 2 − system using laser excitation of the Na atom. The excitation functions were measured for the title reaction, searching for spin orbit effects. A common energy threshold of E0≃0.23eV was found for both processes. In addition, similar post-threshold laws for each spin-orbit state indicate the absence of such effects over the low collision energy range.

The (Ba · ·FCH3) Photofragmentation Channels: Dynamics of the Laser Induced Intrachister (Ba · ·FCH3) → + BaF + CH3 and Ba + FCH3 Reaction

The spectroscopy and dynamics of the intermediate species between reac-tives and products of chemical reactions, i.e. the transition state region [1], have been the subject of several studies in recent years [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. There are different experimental approaches to gain insight into this part of the potential energy surface: Zewail’s group has probed these short-lived states in real time using femtosecond lasers [4]. Another approach pioneered by Soep and co-workers [5, 6, 7, 8, 9, 10] and by Wittig and co-workers [11, 12] is to study photoinduced chemical reactions in van der Waals molecules, were the reaction surface is accessed directly by laser excitation of the complex. The reactants are oriented and the set of impact parameters is limited by the complex geometry. Soep and co-workers studied the Hg··Cl2 and the Ca··HX (X = halogen) complexes, monitoring the yield of the electronically excited product, while Wittig and co-workers substituted the metal by a molecule as for example in the HI··OCO complex. Unfortunately, the reaction dynamics information obtained from these specific van der Waals approaches where product action spectra are measured, is somewhat blurred by the (normally) unknown spectroscopic factors controlling the laser excitation from the ground (non-reactive) up to the excited (reactive) van der Waals potential.

Research Activity Using Molecular Beam and Laser Techniques: A Progress Report

We shall briefly describe the main research areas, now in progress in our molecular beam and laser group. The current activity includes our traditional molecular beam work as well as other experiments carried out in the new labs of our Institute. Essentially we present here the current progress on Crossed-beam and Beam-gas Chemilumine scence, Laser-induced Beam Surface Reaction, Laser Analytical Chemistry, Laser-initiated Reactions Studies by Chemiluminescence and Laser-induced Fluorescence, Spectroscopy and Structure of van der Waals molecules. For each section recent results are presented. Due to the basic scope of the present progress report the emphasis is made on the new experimental developments as well as on the capabilities of both basic and applied research techniques now operative in our group.