Photoionization and detection of ultracold Cs<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript> molecules through diffuse bands

We present the results of absorption measurements in a cesium vapor around 630 K, together with photoionization spectra through a resonance-enhanced two-photon absorption of ultracold cesium dimers created after photoassociation of ultracold cesium atoms. The maximum efficiency of the ultracold molecule ionization is found for wavelengths where absorption at thermal energies is the strongest, in agreement with our theoretical simulations of both processes, involving the so-called Cs₂ diffuse bands. This result will be helpful for further optimization of such a direct way of detection of ultracold molecules. Received 13 September 2001     

Formation of ultracold molecules via photoassociation with blue detuned laser light

We propose a new possibility to form ultracold molecules, via photoassociation of a pair of cold atoms into vibrational levels of the external well of an excited electronic state located at intermediate interatomic distance ( ≈ 20 Bohr radii), and embedded in the dissociation continuum above its dissociation limit. The existence of such a well is demonstrated by conventional free-free absorption spectroscopy at thermal energies. Estimation for cold atom photoassociation and cold molecule formation rates are obtained within a perturbative approach [Drag et al., IEEE J. Quant. Electr. 36, 1378 (2000)], and are found observable for usual conditions of photoassociation experiments. Received 1st March 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     

Blue satellite bands of KRb molecule: Intermediate long-range states

Three distinct satellite bands at 730-736 nm and a single shoulder at 755.5 nm that we assigned to KRb heteronuclear molecule are found in absorption measurements of hot K + Rb vapor. The interpretation of these bands is discussed in terms of recent ab initio calculations of the relevant potential curves. Semiclassical spectral simulations were performed with ab initio potentials and approximate transition dipole moment functions showing a good agreement with observations. The probabilities of cold molecule photoassociative formation into the external well of the double minimum (5)0⁺ state and decay to the ground state are discussed, and relative yields of molecular formation were estimated by using quantum mechanical calculations. Received 15 October 2001     

Steric and energetic properties of the Cl-–C6H6–Arn heterocluster

The dynamics of heteroclusters containing argon, benzene and chlorine has been investigated using a recently proposed potential energy functional that takes into account both the electrostatic and the non-electrostatic contributions to the overall noncovalent interaction. Related steric and energetic properties are compared with those homologous cationic clusters.     

Application of the stabilization method to interacting resonances

The stabilization method is applied to the case of interacting resonances in the photo-dissociation of van der Waals clusters composed by a rare gas atom bound to a dihalogen molecule. The study of an illustrative two-dimensional model consisting in a T-shaped NeI₂ molecule shows the adequacy of the method whenever the projection of the stabilization wave functions on the assumed prepared initial state is accounted for. The agreement of the fragmentation cross-sections with some previous results using the effective resolvent method and accurate close-coupling calculations is excellent. The method reveals its utility as a complementary tool since allows, through the analysis of the stabilization wave function in terms of zero-order levels, a precise characterization of the resonant states involved. Received 5 April 2002 / Received in final form 24 May 2002 Published online 19 July 2002     

Study of excitation transfer Li (3 D → 3 P ) occurring in optical collisions with rare gas atoms experimentally

By selective optical excitation of collision pairs and observation of the reemitted fluorescence information is obtained on the role of the molecular channels involved in inelelastic collisions. As an example case we have studied experimentally the Li( 3 D → 3 P) excitation transfer in Li(3D)X systems with X = Ne, Ar by means of the optical collision process Li (2 P ) + X + h ν→ LiX (3 D Λ) → Li (3 P , 3 D ) + X where LiX (3 D Λ) collision molecules dissociate into Li(3P, 3D) atoms following laser excitation h ν of Li (2 P ) + X pairs. For this purpose we measured the Li 3P/3D population ratio by the fluorescence from these levels as function of the laser detuning Δν from the Li(2P-3D) transition and the rare gas pressure, and determined from this the 3P/3D excitation ratio B (Δν) for single collision conditions. The experiments were performed using two step cw laser excitation of gaseous mixtures Li + X at temperatures around 600 K in the detuning range |Δν| ? 100 cm^-1. The B (Δν) profiles obtained display strong blue-red wing asymmetries both for Li ^* Ne and Li ^* Ar. This reflects different dissociation probabilities from the 3 D Σ or 3 D (Π,Δ) states that are initially prepared by blue wing or red wing excitation, respectively. The results are qualitatively discussed in terms of new ab initio potentials for the two systems. Received 23 February 2000 and Received in final form 5 July 2000     

Effective resolvent applied to interacting resonances

The wave operator theory of quantum dynamics is applied to characterize coupled metastable states. The theory extends to quasi bound states methods which are standard for dealing with bound levels. It is shown that interacting resonances can be investigated by means of small-dimensional effective Hamiltonians containing all the significant information of intrinsic properties which drive the various dynamical processes taking place. The study of the vibrational predissociation of I₂Ne, mediated by intramolecular energy redistribution, illustrates the power of the method. In addition to its accuracy to determine spectroscopic profiles and survival probabilities, it results more efficient than usual energy or time-domain numerical calculations. Received 25 January 2001 and Received in final form 11 May 2001     

Experimental determination of spin-rotation and spin-spin magnetic interactions in by nuclear spin conversion

When a gas sample of ¹³CH₃F is prepared with a population of isomers (ortho and para forms) far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This phenomenon called nuclear spin conversion is mainly governed by intramolecular spin-spin and spin-rotation interactions. In the quantum relaxation model [P.L. Chapovsky, Phys. Rev. A 43, 3624 (1991)], two pairs of ortho-para levels (J = 9, K = 3; J' = 11, K' = 1) and (J = 20, K = 3; J' = 21, K' = 1) are principally responsible for the conversion. The levels of the second pair are coupled by both spin-spin and spin-rotation interactions. The application of an electric field (up to 10 kV/cm) induces a crossing of the Stark components of this pair, which is observed for the first time. A specific experimental set-up based on an electric field of alternating triangular shape is used, which allows the determination of the strength of both interactions via the measurement of the spin conversion decay rates. This work yields the first experimental value for the electronic contribution to the spin-rotation interaction in ¹³CH₃F. Received 23 May 2002 / Received in final form 18 September 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: Patrice.Cacciani@univ-lille1.fr     

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     

Resonance interaction between one excited and one ground state atom

The nature of the resonance interaction between two isotropic atoms in an excited configuration is reinvestigated. The currently accepted oscillatory form for the long-range retarded resonance interaction is shown to be a subtle artefact that arises due to too drastic approximations. Formulation of the resonance interaction energy problem in terms of the interacting system leads to a form that it is ∝ r ^-4 in the retarded limit. We also demonstrate that the resonance interaction energy at any finite temperature goes over to purely classical long-range asymptote. This manifestation of the correspondence principle is due to thermal excitation of the electromagnetic field. We finally discuss why the textbook result for the F?rster energy transfer between two atoms is incorrect for the same reasons. Received 31 January 2002 / Received in final form 25 July 2002 Published online 29 October 2002 RID="a" ID="a"e-mail: mtb110@rsphysse.anu.edu.au     

Cold collisions between linear polar molecules

Cold collisions between electrostatically trapped linear polar molecules are theoretically investigated. It is consequently shown that the inelestic collision cross-section is determined by S-wave scattering alone, while the contribution of D-wave scattering to the elastic collision cross-section becomes significant when the electric field strength is high. It is also shown that as the temperature decreases, it becomes difficult to obtain the evaporative cooling effect without collision loss. Received 12 September 2002 / Received in final form 21 November 2002 Published online 29 April 2003 RID="a" ID="a"e-mail: kajita@crl.go.jp     

Collisions between electrostatically confined linear polar molecules

Collisions between linear polar molecules that were electrostatically trapped were investigated. The collisional transition from a low to a high field seeking state (inelastic collision) causes trap loss. The efficiency of evaporative cooling is improved at higher elastic collision rates. We calculated cross-sections of inelastic and elastic collision using a semi-classical treatment. Received 6 December 2001 / Received in final form 26 February 2002 Published online 28 June 2002     

Photofragment space distribution in the photodissociation of NaI in the spectral range 315–370 nm: the role of molecular axis rotation

The first determination of translational anisotropy parameters β in the photodissociation of NaI molecules in the spectral range 315–370 nm is reported. The anisotropy parameters were determined by the analysis of Doppler resolved absorption profiles of Na(²S_1/2) atoms produced in the photodissociation of NaI by linearly polarized light. The profiles were recorded for two orientations of the photolysis light: parallel and perpendicular to the direction of the probe beam. The value of the parameter β was obtained from a simultaneous fit of the profiles. The role of the rotation of the parent molecules on the branching ratio between parallel and perpendicular transitions in NaI during dissociation is discussed.     

Formation of Cs molecules via Feshbach resonances stabilized by spontaneous emission: theoretical treatment with the Fourier grid method

This paper proposes a general method to investigate Feshbach resonances in atomic collisions similar to Cs(6 s ) + Cs(6 p ) in the thermal or cold regime. In order to compute the predissociation widths of the C ¹ Π _u (6 s + 5 d ) bound vibrational levels of Cs₂, coupled both with the (2) ³ Σ ⁺ _u (6 s + 6 p ) continuum and with the (2) ³ Π _u (6 s + 5 d ) vibrational series, a Fourier grid method is implemented, with an optical potential. A convenient way of optimizing the latter is proposed. A large number of resonances are found and calculations of their cross-sections for stabilization into ground state molecules show that the rate may be important. This confirms the interpretation of Lintz and Bouchiat [Phys. Rev. Lett. 80, 2570 (1998)] who observed dimer formation in cell experiments. Possible generalization to the cold regime relies on the possibility to tune the position of a resonance to coincide with the maximum of the collisional energy distribution. Received 14 February 2002 Published online 28 June 2002     

Decay reactions of rare gas cluster ions: Kinetic energy release distributions and binding energies

We have carried out measurements on metastable fragmentation of mass selected argon cluster ions which are produced by electron impact ionization of a neutral argon cluster beam. From the shape of the fragment ion peaks (MIKE scan technique) one can deduce information about the distribution of kinetic energy that is released in the decay reaction. In this study, for Ar ₅ ⁺ to Ar ₁₅ ⁺, it is Gaussian and thus we can calculate from the peak width the mean kinetic energy release 〈KER〉 of the corresponding decay reactions. Using finite heat bath theory we calculate from these data the binding energies of the decaying cluster ions. Received 20 November 2000     

Instability of the tunneling destruction effect in a quasi-periodically driven two-level system

Here we consider the dynamics of a two-level system under an external time-dependent field. We show that in the case of a bichromatic field the dynamical localization effect is strongly sensitive with respect to the commensurability of the driving frequencies. Received 8 May 2002 / Received in final form 4 July 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: Sacchetti@unimo.it     

Laser cooling of molecules via single spontaneous emission

A general scheme for reducing the center-of-mass entropy is proposed. It is based on the repetition of a cycle, composed of three concepts: velocity selection, deceleration and irreversible accumulation. Well-known laser techniques are used to represent these concepts: Raman π-pulse for velocity selection, STIRAP for deceleration, and a single spontaneous emission for irreversible accumulation. No closed pumping cycle nor repeated spontaneous emissions are required, so the scheme is applicable to cool a molecular gas. The quantum dynamics are analytically modelled using the density matrix. It is shown that during the coherent processes the gas is translationally cooled. The internal states serve as an entropy sink, in addition to spontaneous emission. This scheme provides new possibilities to translationally laser-cool molecules for high precision molecular spectroscopy and interferometry. Received 25 June 2002 / Received in final form 28 September 2002 Published online 12 November 2002 RID="a" ID="a"e-mail: ooi@spock.physik.uni-konstanz.de RID="b" ID="b"e-mail: Peter.Marzlin@uni-konstanz.de RID="c" ID="c"e-mail: Juergen.Audretsch@uni-konstanz.de     

Stimulated Raman spectroscopic study on intermolecular vibrations of size-selected benzonitrile clusters

Low frequency intermolecular vibrations of benzonitrile-(H₂O)_{n = 1 - 3}, -(CH₃OH)_{n = 1 - 3}, and -(CHCl₃)_{n = 1 - 3} clusters were observed by stimulated Raman - UV double resonance spectroscopy combined with fluorescence detection. The Raman active vibrations, which appear in the region from 5 to 50 cm^-1, exhibit characteristic frequency shifts depending on the cluster structure and on the cluster size. In benzonitrile-(H₂O)_{n = 1 - 3} and -(CH₃OH)_{n = 1 - 3}, the lowest frequency band showed a lower frequency shift with an increase of the number of solvent molecules. On the other hand, that of benzonitrile-(CHCl₃)_{n = 1 - 3} showed a shift to the higher frequency side. Their characteristic shifts are discussed based on the structure of the clusters. Received 28 April 2002 / Received in final form 30 May 2002 Published online 13 September 2002     

Angular analysis of bremsstrahlung in α-decay

A new quantum electrodynamical method of calculations of bremsstrahlung spectra in the α-decay of heavy nuclei taking into account the angle between the directions of α-particle motion (or its tunneling) and photon emission is presented. The angular bremsstrahlung spectra for ²¹⁰Po have been obtained for the first time. According to calculations, the bremsstrahlung in the α-decay of this nucleus depends extremely weakly on the angle. Taking into account nuclear forces, such dependence is not changed visibly. An analytical formula of the angular dependence of the bremsstrahlung spectra is proposed and gives its harmonic behavior. The extremal values of the angle, at which the bremsstrahlung has maximal and minimal values, have been found. This paper is dedicated to the memory of Dr Ivan Egorovich Kashuba --a brilliant scientist with bright nature who worked in science till his last days.