Boris G. Sartakov

The rotational spectrum of single OCS molecules in liquid 4He droplets

Single OCS molecules have been embedded in large 4He droplets (N=1×103–8×103 atoms) and their infrared spectra in the vicinity of the ν3-fundamental at 2062 cm−1 have been studied using coaxial laser depletion spectroscopy. Sharp lines corresponding to the P- and R-branches with a linewidth of 160 MHz or greater are observed. From the line intensities a droplet temperature of 0.37±0.02 K is obtained and from the line positions the rotational constants B for the ground and excited states and an average centrifugal distortion constant D are determined. The former are about a factor of 2.8 smaller and the latter four orders-of-magnitude larger than for the free molecule. The decrease in B is attributed to an attached nonsuperfluid component which has the same effect as a ring of about six 4He atoms in an equatorial plane around the waist of the OCS molecule, which is carried along in the end-over-end rotation. The widths of the individual lines show an interesting asymmetry which is oppositely sloped for th...

Laboratory Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbon Molecules

Infrared spectroscopy of a variety of interstellar sources shows strong mid-IR emission bands, which are generally attributed to emission from highly vibrationally excited polycyclic aromatic hydrocarbon molecules (PAHs) in the neutral and, particularly, cationic states. Over the past decade, various experimental methods have been developed to record the infrared spectra of cationic PAHs in the laboratory. In this paper, we discuss available experimental spectra obtained with matrix isolation spectroscopy (MIS), infrared multiple-photon dissociation of trapped ions (MPD), dissociation spectroscopy of ionic PAH van der Waals clusters (VDW), and infrared emission (IRE). Moreover, we compare these experimental spectra to density functional theory (DFT) calculations. The main body of experimental data relies on MIS and MPD spectra, and we present a detailed comparison of results from these methods, providing linear and multiple-photon absorption data, respectively. The effects of multiple-photon absorption, as encountered in MPD, and multiple-photon emission, occurring in interstellar spectra, are carefully assessed with the use of mathematical models, which include the effects of vibrational anharmonicity. We show that an analysis of the multiple-photon and linear data can provide important information on the anharmonicity parameters, which is otherwise difficult to attain. This is illustrated with a detailed comparison of the linear and multiple-photon absorption spectra of the naphthalene cation, yielding experimental anharmonicity parameters for the IR-active modes in the 500-1700 cm-1 range.

High resolution infrared spectroscopy of single SF6 molecules in helium droplets. I. Size effects in 4He droplets

Single SF6 molecules have been embedded in large 4He droplets (N4≈103–104 atoms) in a molecular beam and studied via infrared laser depletion spectroscopy. The rotational fine structure of the ν3 transition is analyzed with an effective third order gas phase Hamiltonian of SF6 yielding seven spectroscopic parameters as a function of the measured mean droplet size. From the intensities of the rotational lines the rotational temperature in droplets with N4>103 atoms is determined to be Trot=0.38(1) K consistent with theoretical estimates. Quantitative information on the efficiency of evaporative cooling of the droplets could be obtained by increasing the droplet temperature up to Trot=0.55 K by many successive inelastic collisions with 4He atoms from the background gas. For small droplets the absorption maximum shows an increasing redshift with respect to the gas phase and only small downward shift with droplet sizes N4>2×103. This could not be explained quantitatively with the excluded volume model assu...

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 spectroscopy of gas-phase zirconium oxide clusters

Abstract Gas-phase zirconium oxide clusters are resonantly excited using IR radiation from a free electron laser. At sufficiently large excitation fluences, clusters with a composition ZrnO2n−1 are observed to thermally emit electrons. Scanning the IR wavelength while monitoring the ion yield gives IR spectra of the clusters. For all clusters, resonances are observed between 600 and 700 cm−1 and the spectra are almost independent of cluster size.

High resolution infrared spectroscopy of single SF6 molecules in helium droplets. II. The effect of small amounts of 4He in large 3He droplets

The rotationally resolved infrared spectra of single SF6 molecules embedded in large 3He droplets have been studied as a function of mean sizes between N3=2×103 and 105 atoms by the cluster beam depletion technique. The observed frequency shifts and line broadening of the dopant spectra are interpreted in terms of the small impurity of about 30–50 4He atoms, which because of their lower zero point energy completely surround the SF6 molecule. For the largest droplets, the observed rotational structure is similar to that observed in pure 4He droplets but the rotational temperature as determined from the relative line intensities is Trot=0.15(1) K, which is about a factor 2.5 lower than in pure 4He droplets. The lower temperature is expected on the basis of the lower heat of evaporation of the 3He atoms on the outside of the droplet. Mixed 3He/4He droplets produced by increasing the 4He concentration in the source gas to 1.2 and 4% of the 3He were found to have temperatures between that for the almost pure ...

Gas-phase infrared spectrum of the coronene cation

The gas-phase infrared spectrum of the coronene cation in the 700-1700 cm-1 range is presented. The spectrum is obtained via multiphoton dissociation spectroscopy of ionic coronene stored in a quadrupole ion trap using the intense and narrowband infrared radiation of a free electron laser. The spectrum shows main absorption peaks at 849, 1327, and 1533 cm-1 along with some weak and barely resolved features, in good agreement with density functional calculations if the effects of vibrational anharmonicity are accounted for. Relative line intensities show remarkable differences with respect to matrix isolation data. The novel experimental technique applied here leads in a natural way to an absorption spectrum of highly excited species. Hence, measured absorption spectra can be compared rather directly to interstellar emission spectra, negating to some extent the need for detailed model calculations.

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.

The structure of the OCS–H2 van der Waals complex embedded inside 4He/3He droplets

The rotational infrared spectra of complexes of OCS with single para-H2, HD, and ortho-D2 molecules inside cold (0.15 K) mixed 4He/3He droplets are used to determine the in-plane structure, in agreement with calculations for the free complex, as well as the out-of-plane amplitude imposed by the superfluid liquid 4He environment.

Spectroscopic investigation of OCS (p-H2)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.