Near-complete chiral selection in rotational quantum states.

Controlling the internal quantum states of chiral molecules for a selected enantiomer has a wide range of fundamental applications from collision and reaction studies, quantum information to precision spectroscopy. Achieving full enantiomer-specific state transfer is a key requirement for such applications. Using tailored microwave fields, a chosen rotational state can be enriched for a selected enantiomer, even starting from a racemic mixture. This enables rapid switching between samples of different enantiomers in a given state, holding great promise, for instance, for measuring parity violation in chiral molecules. Although perfect state-specific enantiomeric enrichment is theoretically feasible, achieving the required experimental conditions seemed unrealistic. Here, we realize near-ideal conditions, overcoming both the limitations of thermal population and spatial degeneracy in rotational states. We achieve over 92% enantiomer-specific state transfer efficiency using enantiopure samples. This indicates that 96% state-specific enantiomeric purity can be obtained from a racemic mixture, in an approach that is universally applicable to all chiral molecules of C1 symmetry. Our work integrates the control over internal quantum states with molecular chirality, thus expanding the field of state-selective molecular beams studies to include chiral research.

Laser-Induced Ion Formation and Electron Emission from a Nanostructured Gold Surface at Laser Fluence below the Threshold for Plasma Formation.

The laser formation of positive and negative ions on a nanostructured metal surface is observed at laser fluence below the plasma formation threshold. The laser radiation energy dependences of the yield of positive and negative Au ions and charged clusters as well as electrons from the laser-induced nanostructures on the surface of gold are obtained at laser fluence below the plasma formation threshold using a pulsed laser with a wavelength of 355 nm and a pulse duration of 0.37 ns. It is shown that the ratio of the signals of positive and negative ions is constant over the entire range of the laser radiation energies, while the ion signal dependence on the laser radiation energy is described by a power function with an exponent of 9. The role of gold nanoparticles with a size of less than 5 nm in the formation of Au ions and charged Au clusters is discussed.

Quantitative Study of Enantiomer-Specific State Transfer.

We here report on a quantitative study of enantiomer-specific state transfer, performed in a pulsed, supersonic molecular beam. The chiral molecule 1-indanol is cooled to low rotational temperatures (1-2 K) and a selected rotational level in the electronic and vibrational ground state of the most abundant conformer is depleted via optical pumping on the S_{1}←S_{0} transition. Further downstream, three consecutive microwave pulses with mutually perpendicular polarizations and with a well-defined duration and phase are applied. The population in the originally depleted rotational level is subsequently monitored via laser-induced fluorescence detection. This scheme enables a quantitative comparison of experiment and theory for the transfer efficiency in what is the simplest enantiomer-specific state transfer triangle for any chiral molecule, that is, the one involving the absolute ground state level, |J_{K_{a}K_{c}}⟩=|0_{00}⟩. Moreover, this scheme improves the enantiomer enrichment by over an order of magnitude compared to previous works. Starting with a racemic mixture, a straightforward extension of this scheme allows one to create a molecular beam with an enantiomer-pure rotational level, holding great prospects for future spectroscopic and scattering studies.

Formation of Core-Shell Ethane-Silver Clusters in He Droplets.

Ethane core-silver shell clusters consisting of several thousand particles have been assembled in helium droplets upon capture of ethane molecules followed by Ag atoms. The composite clusters were studied via infrared laser spectroscopy in the range of the C-H stretching vibrations of ethane. The spectra reveal a splitting of the vibrational bands, which is ascribed to interaction with Ag. A rigorous analysis of band intensities for a varying number of trapped ethane molecules and Ag atoms indicates that the composite clusters consist of a core of ethane that is covered by relatively small Ag clusters. This metastable structure is stabilized due to fast dissipation in superfluid helium droplets of the cohesion energy of the clusters.

Formation of Large Ag Clusters with Shells of Methane, Ethylene, and Acetylene in He Droplets.

Helium droplets were used to assemble composite metal-molecular clusters. Produced clusters have several hundreds of silver atoms in the core, immersed in a shell consisting of methane, ethylene, or acetylene molecules. The structure of the clusters was studied via infrared spectra of the C-H stretches of the hydrocarbon molecules. The spectra of the clusters containing methane and acetylene show two distinct features due to molecules on the interface with silver core and those in the volume of the neat molecular part of the clusters. The relative intensities of the peaks are in good agreement with the estimates based on the number of the captured particles. Experiments also suggest that selection rules for infrared transitions for molecules adsorbed on metal surfaces are also valid for silver clusters as small as 300 atoms.

Stark Interference of Electric and Magnetic Dipole Transitions in the A-X Band of OH.

An experimental method is demonstrated that allows determination of the ratio between the electric (E1) and magnetic (M1) transition dipole moments in the A-X band of OH, including their relative sign. Although the transition strengths differ by more than 3 orders of magnitude, the measured M1-to-E1 ratio agrees with the ratio of the ab initio calculated values to within 3%. The relative sign is found to be negative, also in agreement with theory.

Internal Rotation of Methane Molecules in Large Clusters.

Methane is one of the very few substances that show rotation of individual molecules in the crystalline phase. Here we explore the evolution of the rotation spectrum of methane from single molecules to clusters containing up to about 4 × 10(3) molecules. The clusters were assembled in He droplets at T = 0.38 K and studied via infrared laser spectroscopy in the ν3 region of the methane molecules. Well-resolved rotational structure in the spectra was observed in clusters containing up to about 50 molecules. We have concluded that in distinction to the crystals molecular rotation in methane clusters is confined to the surface and is enabled by low coordination of the molecules. On the contrary the molecules in the cluster's interior are in amorphous state wherein the rotation is quenched. These results demonstrate that even at very low temperature the surface of the methane clusters remains fluxional due to quantum effects.

Rotation of methane molecules in dimers and small clusters.

This work reports on the study of the internal rotation of methane molecules in small clusters containing up to about five molecules. The clusters were assembled in helium droplets at T = 0.38 K by successive capture of single methane molecules and studied by infrared laser spectroscopy of the fundamental CH4 ν3 vibration around 3030 cm(-1). The spectra demonstrate well resolved structure due to internal rotation of the constituent molecules in the clusters. The most resolved spectrum for the dimers shows characteristic splitting of the lines due to anisotropic intermolecular interaction. The magnitude of the splitting is found to be in a good quantitative agreement with the recent theoretical anisotropic intermolecular potentials.

Infrared spectroscopy of carbonyl sulfide inside a pure 3He droplet.

The infrared spectrum of the ν(3) band of an OCS (carbonyl sulfide) molecule embedded inside pure (3)He droplets of about 12 × 10(3) atoms reported in 1998 [S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279, 2083 (1998)] is carefully evaluated. The spectrum, which consists of a broad central peak and a distinct shoulder at lower energy, was analyzed by assuming unresolved rotational line structure of either a linear or a symmetric top. In each case the spectrum was fitted using either Lorentzian or Gaussian peak shapes with a preassigned fixed temperature of 0.15 K or a best fit temperature. Many of the fits describe the spectra nearly equally well and indicate broad R(0), R(1), and P(1) peaks but no Q-branch, a moment of inertia which is about a factor six greater than for the free molecule, and a temperature of 0.07 ± 0.06 K which is significantly less than 0.15 K determined for mixed (3)He∕(4)He droplets. The increased moment of inertia is consistent with about 11 attached (3)He atoms which take part in the end-over-end rotations of the chromophore. The large line widths are attributed to creation of particle-hole pair excitations in the fermionic droplets.

Producing translationally cold, ground-state CO molecules.

Carbon monoxide molecules in their electronic, vibrational, and rotational ground state are highly attractive for trapping experiments. The optical or ac electric traps that can be envisioned for these molecules will be very shallow, however, with depths in the sub-milliKelvin range. Here, we outline that the required samples of translationally cold CO (X(1)Σ(+), v'' = 0, N'' = 0) molecules can be produced after Stark deceleration of a beam of laser-prepared metastable CO (a(3)Π(1)) molecules followed by optical transfer of the metastable species to the ground state via perturbed levels in the A(1)Π state. The optical transfer scheme is experimentally demonstrated and the radiative lifetimes and the electric dipole moments of the intermediate levels are determined.

Spectroscopic investigation of OCS (p-H(2))(n) (n=1-16) complexes inside helium droplets: Evidence for superfluid behavior.

Up to 16 parahydrogen and orthodeuterium molecules have been assembled around an OCS carbonyl sulfide chromophore molecule inside the pure (4)He and mixed (4)He(3)He droplets at temperatures of 0.38 and 0.15 K, respectively. The infrared spectra of the resulting complexes exhibit a sequence of rotationally resolved vibrational nu(3) bands in the vicinity of 2060 cm(-1), which are sufficiently separated to assign them to clusters with specific numbers of attached molecules for n=1-16. The present article contains the first complete analysis of the spectra for n=2-8 and a full documentation of the results for n=8-15 briefly described in a short report [Europhys. Lett. 83, 66008 (2008)]. Distinct rotational Q-branches are observed for all OCS-(o-D(2))(n) clusters at the He droplet temperatures of 0.38 K and 0.15 K, indicating that the (o-D(2))(n) shell rotates nearly freely about the molecular OCS axis. In the case of OCS-(p-H(2))(n) at 0.38 K, the Q-branch is seen for most n, with the exception of n=5, 6 and n=12. At 0.15 K, the Q-branch has disappeared for all n>or=11, indicating that the axial rotations are no longer active. Previously, the absence of a Q-branch for n=5 and 6 was explained by the high group symmetry of the bosonic p-H(2) rigid (donut) rings around the OCS molecule. This model, however, fails in explaining the disappearance of the Q-branch for n>or=11. In essential agreement with recent path-integral Monte Carlo calculations, the observed phenomenon is attributed to the onset of superfluidity in the multiring p-H(2) shell and the related permutations of bosonic p-H(2) molecules. A floppy shell model, which accounts for the effect of tunneling and exchange of molecules within the clusters, is able to explain the postulated superfluid behavior of the p-H(2) shell at low temperatures. Within this model the activation of states of low axial symmetry is responsible for the appearance of the Q-branch at higher temperatures.

Rotation of methane and silane molecules in He droplets.

This work studies the renormalization of the molecular moments of inertia I(G) in liquid helium. For this purpose we have measured the rotational-vibrational spectra of the nu(3) modes of a series of homologous light spherical top molecules such as CH(4), CD(4), SiH(4), and SiD(4) in He droplets. The spectra were fitted to an empirical gas phase Hamiltonian, yielding a set of spectroscopic constants. We found that the additional moment of inertia, DeltaI(He), scales approximately as square of I(G). This is in agreement with the theoretical model which assigns DeltaI(He) to coupling of molecular rotation with vibration of He in the molecular vicinity. Our results also indicate a large increase in the effective centrifugal distortion constants, which is another manifestation of the interaction of the molecular rotors with the He environment. Finally, the mechanism of the relaxation of rotational energy in liquid helium is discussed.

Intensity-resolved IR multiple photon ionization and fragmentation of C60.

The sequential absorption of multiple infrared (IR) photons by isolated gas-phase species can lead to their dissociation and/or ionization. Using the newly constructed "Free-Electron Laser for IntraCavity Experiments" (FELICE) beam line at the FELIX facility, neutral C(60) molecules have been exposed to an extremely high number (approximately 10(23)) of photons/cm(2) for a total time duration of up to 5 micros. At wavelengths around 20 microm, resonant with allowed IR transitions of C(60), ionization and extensive fragmentation of the fullerenes are observed. The resulting photofragment distributions are attributed to absorption in fragmentation products formed once C(60) is excited to internal energies at which fragmentation or ionization takes place within the duration of the laser pulse. The high IR intensities available combined with the large interaction volume permit spatially resolved detection of the ions inside the laser beam, thereby disentangling the contributions from different IR intensities. The use of spatial imaging reveals intensity dependent mass distributions that are substantially narrower than what has been observed previously, indicating rather narrow energy distributions. A simple rate-equation modeling of the excitation process supports the experimental observations.

Large enhancement of the vibration-rotation coupling of the nu1 and nu3 states of silane in helium droplets.

Silane molecules have been embedded in helium droplets and studied via infrared laser depletion spectroscopy in the range of 2190 cm(-1). We found that the R1 and Q2 lines of the nu(3) band have satellites shifted by about 2.3 cm(-1) towards low frequency and having similar intensity to the main lines. We assigned this perturbation in the spectrum to the coupling of the J=2 levels in nu(3) and close lying nu(1) vibration states. The strength of the coupling is a factor of about 50 larger in He droplets than in free molecules and have the same selection rules implied by the tetrahedral symmetry of SiH(4) molecules. The perturbation, which cannot be explained within the framework of a Hamiltonian of free molecules, is evidence of strong coupling of the molecule with some He excitations in the molecular vicinity.

Evolution of the vibrational spectrum of ammonia from single molecule to bulk.

Ammonia clusters (NH3)n (n=2-10(4)) have been assembled inside helium droplets and studied via infrared laser spectroscopy. The studied spectral range of 3100-3500 cm(-1) covers the nu1 and nu3 fundamental stretching bands as well as the 2nu4 overtone of the bend of ammonia molecules. The results show strong coupling of the 2nu4 overtone with the fundamental vibrations for all cluster sizes except dimers. The intensity of the nu3 band relative to the total intensity in the spectrum increases from about 30% to about 80% upon increase of the average cluster size from n=5 to n=10(4). We attributed this effect to the concomitant decrease in the fraction of the surface molecules. The results indicate that ammonia clusters obtained in He droplets have a compact structure and that inner molecules in the clusters have similar hydrogen-bonded coordination as in the crystalline form of ammonia. This surprising result is ascribed to a directionality of the hydrogen bond, which guides the low temperature growth of the cluster in He droplets.

Study of NH stretching vibrations in small ammonia clusters by infrared spectroscopy in He droplets and ab initio calculations.

Infrared spectra of the NH stretching vibrations of (NH3)n clusters (n = 2-4) have been obtained using the helium droplet isolation technique and first principles electronic structure anharmonic calculations. The measured spectra exhibit well-resolved bands, which have been assigned to the nu1, nu3, and 2nu4 modes of the ammonia fragments in the clusters. The formation of a hydrogen bond in ammonia dimers leads to an increase of the infrared intensity by about a factor of 4. In the larger clusters the infrared intensity per hydrogen bond is close to that found in dimers and approaches the value in the NH3 crystal. The intensity of the 2nu4 overtone band in the trimer and tetramer increases by a factor of 10 relative to that in the monomer and dimer, and is comparable to the intensity of the nu1 and nu3 fundamental bands in larger clusters. This indicates the onset of the strong anharmonic coupling of the 2nu4 and nu1 modes in larger clusters. The experimental assignments are compared to the ones obtained from first principles electronic structure anharmonic calculations for the dimer and trimer clusters. The anharmonic calculations were performed at the Møller-Plesset (MP2) level of electronic structure theory and were based on a second-order perturbative evaluation of rovibrational parameters and their effects on the vibrational spectra and average structures. In general, there is excellent (<20 cm(-1)) agreement between the experimentally measured band origins for the N-H stretching frequencies and the calculated anharmonic vibrational frequencies. However, the calculations were found to overestimate the infrared intensities in clusters by about a factor of 4.

Optical pumping of trapped neutral molecules by blackbody radiation.

Optical pumping by blackbody radiation is a feature shared by all polar molecules and fundamentally limits the time that these molecules can be kept in a single quantum state in a trap. To demonstrate and quantify this, we have monitored the optical pumping of electrostatically trapped OH and OD radicals by room-temperature blackbody radiation. Transfer of these molecules to rotationally excited states by blackbody radiation at 295 K limits the 1/e trapping time for OH and OD in the X(2)Pi(3/2), v" =0, J"=3/2(f) state to 2.8 and 7.1 s, respectively.

Infrared intensity in small ammonia and water clusters.

Helium droplet technique has been used in order to measure the strength of the infrared absorption in small ammonia and water clusters as a function of size. Hydrogen bonding in ammonia and water dimers causes an enhancement of the intensity of the hydrogen stretching bands by a factor of four and three, respectively. Two types of the hydrogen bonded clusters show different size dependence of the infrared intensity per hydrogen bond. In ammonia (NH3)2 and (NH3)3 it is close to the crystal value. In water clusters, it increases monotonically with cluster size being in tetramers, a factor of two smaller than in the ice. The measured infrared intensity in water clusters is found to be a factor of two to three smaller as compared to the results of numerical calculations.

Infrared multiphoton ionization of superhot C60: experiment and model calculations.

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9-19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500-800) infrared photons in the 450-1800 cm(-1) spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) is observed following the absorption of 1350-1400 cm(-1) as well as 1500-1530 cm(-1) photons while negligible fragmentation is observed when exciting around 520 cm(-1).

Fine structure of the (S(1)<--S(0)) band origins of phthalocyanine molecules in helium droplets.

The laser-induced fluorescence (LIF) excitation spectra of free base phthalocyanine (Pc), Mg-Pc, and Zn-Pc molecules in superfluid helium droplets at T=0.38 K have been studied. The spectra reveal the rich vibronic structure of the S(1)<--S(0) electronic transitions. The band origins of the transitions consist of zero phonon lines accompanied by phonon wings, which originate from simultaneous electronic excitation of the molecule and excitation of the collective modes of the helium surrounding it. The phonon wings have discrete structures suggesting localization of some helium atoms in the neighborhood of the molecules. Zero phonon lines of Mg-Pc and Zn-Pc molecules are split into three components, which are separated by 0.2-0.4 cm(-1). Possible mechanism of splitting involves static or dynamic Jahn-Teller interaction of metal-phthalocyanine molecules in the twofold degenerate S(1)((1)E(u)) state with the helium shell.