Robert Beuc

Diffuse bands in the visible absorption spectra of dense alkali vapours

The authors report the results of extensive absorption measurements in dense alkali vapours with the main emphasis on the diffuse bands, located close to the short wavelength end of the B1 Pi u-X1 Sigma g+ alkali-band system. At extremely high pressures they succeeded in observing Li2 and Na2 diffuse bands at about 422.5 and 436.5 nm, respectively. Detailed measurements of the absorption coefficient of potassium diffuse band at 575 nm show square dependence on the atom density and independence on the temperature. The relevant data for dense rubidium and caesium vapours have been remeasured. The authors believe that diffuse bands originate in a transition from the ground triplet state towards the 23 Pi g state, which suffer peculiar configuration interaction with the first excited ionic state of the same symmetry.

Cesium dimer spectroscopy on helium droplets

Visible absorption spectra of cesium-doped helium nanodroplets between 14,500 and 17,600 cm(-1) were probed by laser-induced fluorescence. A strong absorption band peaking around 16,700 cm(-1) is identified as Cs2 1(a) 3Sigmau+-3 3Sigmag+ transition. A broad unstructured band near 17,520 cm(-1) is assigned as the Cs2 1(X) 1Sigmag+-2 1Sigmau+ transition. Explanations of the observations are discussed on the basis of ab initio potential curves calculated by Spies and Meyer [(unpublished)]. All spectra have been modeled using narrow Frank-Condon windows around the equilibrium internuclear distance of the lowest singlet and triplet states. Many observed absorption peaks of smaller intensities could be identified, some of which may be due to transitions of cesium trimers formed on the droplets.

Population and deactivation of lowest lying barium levels by collisions with He, Ar, Xe and Ba ground state atoms

Excitation transfer between the barium low lying excited states 6s6p3P10, 6s5d1D2 and 6s5d3DJ by collisions with He,Ar,Xe and Ba has been investigated. The population densities in all levels involved were probed by absorption or by fluorescence usingcw lasers. The depopulation cross sections of the Ba3P10 state by collisions with noble gases were found to be σHe(3P10)=5.5·10−16 cm2, σAr(3P10)=4.6·10−16 cm2, and σXe(3P10)=1.7·10−16 cm2. For Ar, the collisional depopulation of the3P10 level is exclusively due to the transition to the1D2 state. Under the assumption that the3DJ metastable states are populated collisionally by1D2 →3DJ transfer only, we have deduced the upper limit for the corresponding cross section σ13Ar=1.5·10−18 cm2. From the Ba1D2 and Ba3DJ steady-state diffusion distributions, collisional relaxation rates of the1D2 and3DJ levels were evaluated. The collisional relaxation rates by Ar and Ba yielded total cross sections for the depopulation of metastable levels: σAr(1D2)=1.5·10−17 cm2, σBa(1D2)℞1·10−13 cm2, σAr(3DJ)=7·10−21 cm2, and σBa(3DJ)=1·10−15 cm2. Furthermore, it was found that the main contribution of the collisional depopulation of the1D2 state by Ar is related to back transfer to the3PJ0 state, whereas the deactivation of the3DJ metastable state is due to back transfer to the1D2 state. Taking into account other cross sections reported in literature we can conclude that collisional deactivation of both metastable levels by Ba ground state atoms can be attributed to their mutual collisional mixing.

New diffuse bands in the KRb molecule

New diffuse bands in the KRb molecule at 597 nm, 586.7 nm and 569 nm were discovered by means of absorption and laser-induced fluorescence observations. The qualitative model developed for diffuse bands of homonuclear alkali dimers has been extended and applied to KRb diffuse bands.

Predictions for the observation of KRb spectra under cold conditions

Quantum-mechanical simulations of the excitation spectra of KRb from the lowest vibrational level of the lowest triplet and singlet electronic states have been performed using recently calculated interaction potential curves and corresponding transition dipole moments. The obtained spectra can be used for a comparison with experimental absorption spectra of KRb molecules produced in their vibronic ground state or attached to cold helium droplets. In addition, we compare the semiclassically simulated spectra with absorption measurements in dense K–Rb vapour at high temperatures, which helped us to identify three diffuse bands as 1 3 � + –3 3 � ,1 3 � + –4 3 � and 1 1 � + –4 1 � + transitions. The first may be observable in an excitation spectrum of KRb dimer formed on cold helium droplets.

Photoassociation of cesium atoms into the double minimum Cs231Σu+ state

Abstract We have observed the transition from the free colliding pair of cesium atoms into the outer well of the double minimum Cs 2 3 1 Σ u + state. This photoassociation process takes place in a cesium vapor consisting predominantly of atoms, in which dimers have been about 85% thermally dissociated. Excellent agreement with quasiclassical simulations was found. We discuss the possibility of using a double minimum Cs 2 3 1 Σ u + state for the formation of ultracold Cs 2 X 1 Σ g + molecules.

Pure long-range ion-pair Cs2 molecules

Abstract We performed absorption measurements in dense cesium vapor, around the Cs second principal series lines (6s→7p). Three satellite bands peaking at 454.3, 459.1 and 471.6 nm are found. New ab initio Cs2 potential curves calculated with the inclusion of the spin–orbit interaction are used for the identification and simulation of the observed satellite bands. The 454.3 and 459.1 nm satellite bands stem from avoided crossings of long-range ion-pair potential curves with long-range covalent potential curves associated with various asymptotes. Semiclassical spectral simulations of these ion-pair satellite bands show satisfactory agreement with measured spectra.

The non-Lorentzian wings of alkali resonance lines: the determination of the atom number density in pure and mixed alkali vapours

The improved method for the determination of the atom number densities in pure and mixed alkali vapours has been proposed. In order to obtain the correct results one has to go beyond the simple Lorentzian shape of the quasistatic absorption profile. The method has been tested by the white-light absorption measurements in the pure K, Rb and Cs vapours. The statistical accuracy of the obtained data for the atom number density is +or-3%. The method has been applied for determining the atom number densities in the vapour over the Rb-Cs mixture. The obtained values have been used in the evaluation of the effective C6 constants for Rb*-Cs interaction: C6eff=(4.9+or-0.3)* 10-30 cm6 s-1 for the D2 line. The theory predicts C6eff=4.95*10-30 cm6 s-1 for the D1 line and C6eff=2.37*10-30 cm6 s(-1) for the D2 line.

The investigation of the satellite rainbow in the spectra of diatomic molecules

A refinement of the theoretical model for the description of the shape of a satellite rainbow in the spectra of diatomic molecules, based on the uniform Airy approximation, is presented. The satellite rainbow in the red wing of the caesium 852.1 nm line has been measured by means of classical absorption spectroscopy and excellent agreement with the calculated shape is found. The method for the quasistatical singularity position determination from the position of the minimum of the first derivative of the reduced absorption coefficient is proposed. The first derivative of the experimentally determined reduced absorption coefficient with respect to wavelength gives the position of the quasistatical singularity of the satellite rainbow to be 857.32+or-0.02 nm, while theoretical calculations predict its position at 857.36 nm.

Superheating in the heat-pipe oven

We present a device which modifies the heat-pipe oven in such a way that the molecular number density can be drastically reduced. The device is essentially a small chamber with an independent internal heater inserted into the center of the ordinary linear heat-pipe oven which, in turn, serves as a reservoir of the liquid metal. The advantage of the present design of superheating within the heat-pipe oven, where temperature differences of several hundreds degrees centigrade were obtained, is shown in the case of far blue wings of the first self-broadened potassium resonance lines.