Ultrahigh-resolution laser spectroscopy of the S1B2u ← S0Ag transition of perylene

A rotationally resolved ultrahigh-resolution fluorescence excitation spectrum of the S₁ ← S₀ transition of perylene has been observed using a collimated supersonic jet technique in conjunction with a single-mode UV laser. We assigned 1568 rotational lines of the band, and accurately determined the rotational constants. The obtained value of inertial defect was positive, accordingly, the perylene molecule is considered to be planar with D_2h symmetry. We determined the geometrical structure in the S₀ state by ab initio theoretical calculation at the RHF/6-311+G(d,p) level, which yielded rotational constant values approximately identical to those obtained experimentally. Zeeman broadening of each rotational line with the external magnetic field was negligibly small, and the mixing with the triplet state was shown to be very small. This evidence indicates that intersystem crossing (ISC) in the S₁¹B_2u state is very slow. The rate of internal conversion (IC) is also inferred to be small because the fluorescence quantum yield is high. The rotational constants of the S₁¹B_2u state were very similar to those of the S₀¹A_g state. The slow internal conversion (IC) at the S₁ zero-vibrational level is attributed to a small structural change upon electronic transition.     

Laser spectroscopy of desorbing molecules

In this article the application of tunable dye lasers to desorption phenomena is illuminated. These lasers provide radiation continuously tunable from 105 nm in the vacuum ultraviolet to about 10 m in the mid-IR. By employing either laser induced fluorescence (LIF) or resonance enhanced multiphoton ionization (REMPI) spectroscopy almost all diatomic and many polyatomic molecules can be probed with the sensitivity required to detect desorbing molecules under UHV conditions. The spectral resolution of the lasers is sufficiently high that rotational state selectivity is achieved. Recent developments permit in addition the velocity distributions of molecules to be determined with internal quantum state resolution. Therefore very detailed information about the molecular dynamics has been obtained. In most experiments so far reactive recombinations off surfaces have been investigated. In this paper special emphasis will be given to the recombination of hydrogen on copper and palladium surfaces. For these systems very detailed data about the internal state populations at various surface temperatures have been obtained. The rotational cooling previously observed in molecular beam scattering has also been established for desorption. Strong vibrational excitation has been observed, which in the case of desorption from copper may be associated with the recombination dynamics, whereas for desorption of D₂ from Pd(100) a molecular precursor state might be responsible. By measuring the velocity distribution in each quantum state, the complete energetics of the desorbing molecules has been determined. Some first experiments on laser induced desorption with state selective detection of the desorbing molecules will also be discussed. Finally, making use of the polarization analysis of the signal, alignment effects in the desorption can be observed, permitting observation of molecular dynamics with a magnifying glass.Heisenberg fellow of the Deutsche Forschungsgemeinschaft     

UV laser desorption of nitric oxide from semiconducting C<Subscript>60</Subscript>/Cu(111)

The desorption of NO from a well-characterized, epitaxially grown semiconducting C₆₀ surface is reported. Two different channels are identified in the laser desorption. Both channels yield a comparably high desorption cross section of σ₁=7.0×10^-17 cm² and σ₂=5.5×10^-17 cm² for the first and second channel, respectively. The laser desorbed NO molecules are detected with rovibrational state selectivity by (1+1) REMPI in the -bands. In the first channel the desorbing molecules are highly excited with an average kinetic energy of 〈E_kin〉=174 meV. The rotational population distribution can be fitted by a rotational temperature of T_rot=800 K. A rotational–translational coupling is observed, with velocities ranging from 1000 m/s for low to 1300 m/s for high rotational states. The vibrationally excited population is estimated to be less than 1% of the ground state. The second channel yields less excited molecules and an almost Boltzmann distributed rotational population with a temperature of T_rot=280 K. The apparent velocity distribution derived from the pump-probe delay yields molecules much too slow to be explained by even a thermal desorption. This desorption is probably caused by a long-lived electronic excitation in the substrate for which a lifetime of τ≈160 μs is estimated. PACS 42.62.Fi; 34.50.Dy; 68.49.Df; 68.43.Tj; 79.20.La     

Efficiently generated rotational and vibrational stimulated raman emission by means of a two-color laser beam

When a two-color laser beam is introduced into pressurized hydrogen, about 40 laser emission lines are generated from the ultraviolet to the visible regions. This phenomenon is ascribed to the stimulated Raman effect due to a combination ofJ=1 J=3 rotational andv=0 v=1 vibrational transitions. By introducing the two-color laser beam, the threshold for generation of the rotational line is substantially reduced. The present phenomenon is attributed to four-wave mixing, which allows efficient generation of higher-order rotational and vibrational Raman lines.     

Narrow-line diode laser system for laser cooling of strontium atoms on the intercombination transition

We report a diode laser system developed for narrow-line cooling and trapping on the ¹S₀–³P₁ intercombination transition of neutral strontium atoms. Doppler cooling on this spin-forbidden transition with a line width of /2=7.1 kHz enables us to achieve sub-K temperatures in a two-step cooling process. The required reduction of the laser line width to the kHz level was achieved by locking the laser to a tunable Fabry–Pérot cavity. The long-term drift (>0.1 s) of the reference cavity was compensated by employing the saturated absorption signal obtained from Sr vapor in a heat pipe of novel design. We demonstrate the potential of the system by performing spectroscopy of Sr atoms confined to the Lamb–Dicke regime in a one-dimensional optical lattice. PACS 32.80.Pj; 39.30.+w; 42.55.Px     

Highly selective and efficient multiphoton dissociation of polyatomic molecules in multiple-frequency IR-laser fields

An ir multiphoton dissociation (MPD) process in multiple-frequency ir-laser fields has been experimentally realized. A selectivity ofS=10⁴ was obtained in separating¹³C/¹²C isotopes upon multiple-frequency ir multiphoton dissociation (MFMPD) of the CF₂HCl molecule, the dissociation yield¹³ for the¹³CF₂HCl molecule amounting to around 1%. A yield of¹³ was reached at a selectivity ofS=10², and the total laser fluence required for the process was reduced. A new mechanism—sticking of molecules on the lower discrete vibrational levels—responsible for the low MPD yields observed for some molecules is discussed and a technique to eliminate it and thus maximize the dissociation yields is proposed. Ways to improve the selectivity by MFMPD are analyzed and a simple method for obtaining from a single TEA CO₂ laser a multiple-frequency radiation suitable for experimental realization of MFMPD is suggested.     

Spectroscopic evidence for superfluidity in liquid He droplets

A beam of liquid He _N droplets (N≈5000) is prepared by a cryogenic free jet expansion, doped with single molecules and studied via laser spectroscopy. The spectra consist of sharp lines (Δν?0.003–0.1 cm^?1) and show rotational structure (for SF₆), providing the first information on the droplet temperature, which is 0.37 K for⁴He _N and 0.14 K for³He _N . The rotational constants give information on the microscopic interaction of the molecule with the He environment. Each of the sharp vibronic lines of the elecronic S₁←S₀ transition in glyoxal (C₂O₂H₂) is accompanied by a phonon sideband, showing a distinct gap followed by a sharp peak at the roton energy (8.1 K) and a second weaker maxon peak followed by a broad multiphonon wing. These provide the first evidence for superfluidity in⁴He _N predicted theoretically.     

Electronic circular dichroism of some double-helical alkynyl cyclophanes with 1,1′-binaphthyl auxiliaries investigated using time-dependent density functional calculations

For several double-helical molecules, the dependence of electronic circular dichroism (ECD) spectra on the molecular and electronic structures is investigated on the basis of TDDFT computations with the B3LYP hybrid functional. The calculations of a model molecule reveal that an acute-angled rotational dihedral angle other than an obtuse-angled rotational dihedral angle between two naphthyls implies Davydov splitting of exciton coupling. For the R enantiomers, the acute-angled rotational dihedral angle implies a negative lowest-energy Cotton effect, which is induced by electron density transfer from binaphthyls to phenyethynyls in the S_0?→?S₄ excitations. Due to the large positive rotational strength of the S_0?→?S₁ transition, the lowest-energy Cotton effect of the simplest cyclophane R-1 is positive, where the electron density transfer from phenyethynyls to binaphthyls plays an important role.     

Photoassociation experiments with ultracold metastable helium

The present article gives a review of various photoassociation (PA) experiments performed at ENS with a gas of ultracold atoms of metastable helium in the 2³S₁ state, using a PA laser beam red-detuned from the 2³S₁-2³P transitions. Molecular spectra close to the D₂ atomic line (2³S P₂) are presented. All the measured lines are identified as a signature of molecular bound states having a strong (if not pure) quintet spin character at short interatomic distance. Close to the D₀ atomic line (2³S P₀), giant helium dimers can be produced [see Phys. Rev. Lett. 91, 073203 (2003)]. A laser set-up improved recently allows us to measure very accurately the binding energy of the ro-vibrational ground state of the 0 _u ⁺ purely long-range potential and the agreement with the theory published previously is excellent. Finally, preliminary results on 2 photon PA spectroscopy are given.Received: 1 July 2004, Published online: 26 October 2004PACS: 32.80.Pj Optical cooling of atoms trapping - 33.20.Kf Visible molecular spectra - 34.50.Gb Electronic excitation and ionization of molecules - 34.20.Cf Interatomic potentials and forcesJ. Léonard: Present address: Van der Waals-Zeeman Institute, Universiteit van Amsterdam, The Netherlands.M. Walhout: Permanent address: Calvin College, Grand Rapids, MI, USA.     

Quenching of perylene fluorescence by Co2+ ions in dipalmitoylphosphatidylcholine (DPPC) vesicles

We report the fluorescence quenching of perylene by CoCl₂·6H₂O in small unilamellar DPPC vesicles via energy transfer. At the probe-to-lipid ratio of 1200 and quencher to lipid ratios of 12.51, donor-donor energy transfer between clustered perylene molecules was observed as well as energy transfer from the perylene molecules to cobalt ions bothabove andbelow the main phase transition temperature of the lipid. The fluorescence quenching of perylene by CoCl₂·6H₂O in the lipid gel state is shown to be well described by Förster long-range energy transfer when both donor-donor and donor-acceptor energy transfer are considered. In the liquid crystalline phase diffusion of the molecules is described as well as energy transfer. The interaction radiusR ₀ for energy transfer from perylene to Co²⁺ is found to be 13.4±1.1 Å in the gel phase at 303 K, in good agreement with the theoretical value forR ₀ of 13.9 Å. In the liquid crystalline phase at 323 K the lower value obtained forR ₀, 11.3±1.6 Å, is attributed to saturation of the Co²⁺ ions at the interfacial region of the bilayer. A diffusion coefficient of (1.06±0.15)×10^–6 cm² s^–1 is obtained for perylene-cobalt diffusion in the liquid crystalline phase at 323K.     

Resonance lonization mass spectroscopy with a pulsed thermal atomic beam

Resonance ionization mass spectroscopy (RIMS) and pulsed-laser induced desorption (PLID) have been combined for ultrasensitive detection and spectroscopy of very small samples of refractive elements. The method has been tested and applied to laser spectroscopy of 5×10⁹ atoms (1.5 pg) of¹⁹⁵Au (T _1/2= 183d) implanted at the ISOLDE online mass separator with 60 keV into graphite. A pulsed thermal atomic beam was formed by laser desorption with a 10 ns NdYag laser pulse. Subsequently the atoms were photoionized in a three-colour, three-step resonant excitation to an autoionizing state. The selectivity was enhanced by a time-of-flight measurement of the photo ions. In resonance, one ion was detected per 10⁵ atoms implanted resulting in a gain in detection efficiency by three orders of magnitude in comparison to the use of a continuous atomic beam. In the course of the experiments several unknown autoionizing states were found, and the lifetime of the 6d ² D ^3/2 state of gold was determined to be=10.7(6) ns.     

Application of the Symmetric Top Approximation to Spectroscopy of High-Temperature Water Vapor

A new model for calculating the energies of pure rotational levels of water vapor molecules treated as a symmetric top is suggested for large values of the quantum number K _a J/2 >> 1. The effective rotational constants of model representation (using the Padé form) of the ground vibrational state of the H₂O molecule are determined. The results of calculations of the rotational line intensities and positions of line centers for the suggested model are preliminarily tested with the help of the information system HOTGAS. The contribution of transitions between high rotational states to the water vapor absorption at temperatures between 1000 and 10000 K is estimated for the 8–12 m wavelength range.     

Loading rate of Yb<Superscript>+</Superscript> loaded through photoionization in radiofrequency ion trap

We investigate the loading rate of Yb⁺ ions loaded through photoionization in a radiofrequency trap. The absolute or relative number of the loaded trapped ions is measured by use of an electric resonance of the secular motion. This method is applicable even in the presence of anharmonicity. In two-color photoionization, where the first-excitation laser drives the ¹S₀–¹P₁ transition in the Yb atom and the second one ionizes the atom from the ¹P₁ state, the loading rate is at its highest by the excitation of the ionization potential. A similar loading rate is observed at the second-laser wavelength around 369.5 nm, which is the wavelength for the cooling transition of Yb⁺. We estimate the loading cross section to be 40(15) Mb for the two-color excitation of the ionization potential. The excitation of the Yb atoms in the Rydberg states is detected by the enhancement of the loading rate. By irradiation with only the first-excitation laser, Yb⁺ is produced at a rate three orders of magnitude smaller than that when the non-resonant two-photon absorption from the ¹P₁ state is the dominant process. We also measure the charge-exchange rate between Yb⁺ and Yb, and discuss its effect on isotope-selective photoionization loading.     

Electric field effects on persistent spectral holes: Perylene in the polar polymer polyvinylbutyral

We investigated the effects of an electric field on a spectral hole burned in the inhomogeneously broadened S ₀–S ₁ transition of perylene in different samples of the polar polymer polyvinylbutyral (PVB) and in cellulose nitrate. The spectral hole is broadened and reduced in depth by the electric field. It was checked experimentally for perylene in PVB that the hole area remains constant when an electric field is applied. We determined the effective matrix-induced electric dipole moment differences * for perylene in different PVB samples and in cellulose nitrate. Within experimental accuracy the value of * is approximately independent of the composition of PVB and its water content. For perylene in cellulose nitrate the value of * is larger by a factor of 1.5 than in PVB. The results are discussed on the basis of a simple model for the electric field effect.     

Deuterium separation by multiphoton dissociation of dichlorofluoromethane

Absorption and dissociation probabilities of CHCl₂F and CDCl₂F were investigated by a pulsed CO₂ laser in the wavenumber region of largest selectivity for deuterium. The absorption of CHCl₂F, which is a difference band, can largely be suppressed by cooling. At 200 K and at 920 cm^–1 absorption selectivities up to 4000 were found by extrapolation. In the presence of buffer gas, CDCl₂F can be multiphoton excited nearly like a linear absorber (harmonic oscillator). This is interpreted by a nearly resonant collisional relaxation v₇ to v₂ and by the smallness of the cross anharmonicity x₂₇. The dissociation selectivityS was 24,000 at natural abundance. Such large values were measured by a chromatographic method.S depends only onp _D, the partial pressure of the deuterated species. This dependence is approximately ln Sp _D ^–1 . It can be rationalized by considering only the average energy transferred to the nonresonant molecules by collisions with CDCl₂F. The above functional shape is related to an Arrhenius type law. Comparison with trifluoromethane for D separation shows that CHCl₂F has primarily two advantages: its rapid H-D exchange with water and the less stringent requirements of laser energy and pulse length.A preliminary account of this work has appeared in Quantum Electr.2, 13 (1985) (in Chinese)     

Analysis of the Electronic Vibrational Spectra of Bisanthene by Calculating Normal Modes

At 4.2 K, the quasiline fluorescence and fluorescence-excitation spectra (in the region of the S ₁ S ₀- and S ₂ S ₀ transitions) of bisanthene in n-hexane have been obtained. Using the MO/M8ST method, the calculation of the A _g normal modes in the S ₀, S ₁, and S ₂ electronic states was carried out for the bisanthene molecule; the relative intensities of the transitions in the corresponding vibronic spectra were also calculated. The analysis of the results of calculations and experiment made it possible to draw conclusions on the form of the modes for a number of normal vibrations which are active in the bisanthene vibronic spectra.     

Wave mixing and amplified spontaneous emission in pure potassium and mixed sodium-potassium vapors

We report observations of several wave-mixing and stimulated processes in pure potassium and mixed sodium-potassium vapors which are excited by a pulsed laser operating in the range 680–800 nm. When the laser is tuned to the potassium two-photon 4S6S transition, we observe stimulated emission on the various cascade transitions as well as four- and six-wave mixing. When the laser is tuned over the range 747 to 753 nm, which is well away from all atomic transitions, we observe strong forward and weak backward emission at the potassium 3D _3/24P _1/2 transition wavelength (1.17 m). However, this latter emission is only observed in the mixed sodium-potassium vapor. We present data on the excitation spectrum, forward to backward asymmetry, temporal dependence, and the laser power dependence of this emission. We speculate that twophoton photodissociation of the NaK molecule is responsible for this emission.     

On quantum systems in thermal contact

It is shown that under rather general conditions two K.M.S. states ₁ and ₂ of systemsS ₁ andS ₂, respectively, can be simultaneously extended to a K.M.S. state of a system composed ofS ₁ andS ₂, provided both systems have equal temperatures. This result gives further support to the conjecture that K.M.S. states are equilibrium states. In the second part, a model of thermal coupling is constructed which satisfies the assumptions of the first part, thereby showing that the result is also valid in the interesting case of systemsS ₁ andS ₂ in thermal contact.     

Photophysical Processes in the Gas Phase at High Levels of Vibrational Excitation of Triplet Molecules of Benzophenone and Fluorenone

By the delayed fluorescence activated by direct multiphoton excitation of triplet molecules by CO₂–laser radiation we have studied the prevailing deactivation pathways of triplet molecules with a high store of vibrational energy E _vib. The dependences of the kinetic characteristics of delayed fluorescence on the presence of vapors and foreign gases have been used to estimate the rates and efficiencies of intermolecular vibrational relaxation in the vibrational quasi–continuum of the triplet state T ₁. By the changes in the intensities and decay rates over a wide range of vibrational energies we have established the E _vib dependences of reversible intercombination conversion between the states T ₁ and S ₁ and interconversion from T ₁ to the ground electronic state S ₀ for both the case of isolated excited molecules and at a steady vibrational temperature. It is shown that at high vibrational temperatures the radiationless transition from the T ₁ state to S ₀ has an activation character and is accomplished through the energy barrier. The conditions for going to an exponential dependence have been determined. It has been found that the obtained dependences are in good agreement with the known experimental results. The influence of molecular and environmental characteristics on the decay rate of triplet molecules is compared.