Resonant two-photon ionization spectroscopy of Al atoms and dimers solvated in helium nanodroplets.

Resonant two-photon ionization (R2PI) spectroscopy has been applied to investigate the solvation of Al atoms in helium droplets. The R2PI spectra reveal vibrational progressions that can be attributed to Al-He(n) vibrations. It is found that small helium droplets have very little chance to pick up an aluminum atom after collision. However, the pick-up probability increases with the size of the helium droplets. The absorption band that is measured by monitoring the ions on the mass of the Al dimer is found to be very little shifted with respect to the Al monomer band (∼400 cm(-1)). However, using the same laser wavelength, we were unable to detect any Al(n) photoion with n larger than two.

Reactivity of iron atoms at low temperature.

We have studied the reactions of iron atoms and clusters with oxygen, acetylene, and water molecules in superfluid He droplets at T = 0.37 K. For all systems, the formation of weakly bound adducts was found, but the insertion reaction of iron into existing molecular bonds could not be observed. The formation of FeOH2 and FeC2H2 complexes was evidenced by mass spectrometry. However, it was found that the reaction of iron atoms with oxygen molecules under similar conditions leads to the stabilization of an intermediate reaction product, the weakly bound linear FeOO adduct, which undergoes complete dissociation upon electron impact ionization. All reactions observed are not expected to proceed in the gas phase. The R2PI spectrum of the y5D4 0 ← a5D4 atomic transition of Fe solvated in helium droplets was recorded. A relatively small blue shift of ∼ 120 cm(-1) with respect to the gas phase position was found.

Ultra-low-temperature reactions of C(³P0) atoms with benzene molecules in helium droplets.

The reaction of carbon atoms with benzene has been investigated in liquid helium droplets at T = 0.37 K. We found an addition of the carbon atom to form an initial intermediate complex followed by a ring opening and the formation of a seven-membered ring. In contrast to a previous gas phase study, the reaction is frozen after these steps and the loss of hydrogen does not occur. A calorimetric technique was applied to monitor the energy balance of the reaction. It was found that more than 267 kJ mol(-1) were released in this reaction. This estimation is in line with quantum chemical calculations of the formation energy of a seven-membered carbon ring. It is suggested that reactions of this kind could be responsible for the low abundance of small polycyclic aromatic hydrocarbon molecules in the interstellar medium. We also found the formation of weakly bonded water-carbon adducts, in which the carbon atom is linked to the oxygen atom of the water molecule with a binding energy of about 33.4 kJ mol(-1).

Measurement of vibrational modes in single SiO2 nanoparticles using a tunable metal resonator with optical subwavelength dimensions.

Using a tunable optical subwavelength microcavity, we demonstrate controlled modification of the vibronic relaxation dynamics in a single SiO(2) nanoparticle. By varying the distance between the cavity mirrors we change the electromagnetic field mode structure around a single nanoparticle and the radiative transition probability from the lowest vibronic level of the electronically excited state to the progression of phonon levels in the electronic ground state. We demonstrate redistribution of the photoluminescence spectrum between zero-phonon and phonon-assisted bands and modification of the excited state lifetime of the same individual SiO(2) particle measured at different cavity lengths. By comparing the experimental data with a theoretical model, we extract the quantum yield of a single SiO(2) nanoparticle.

Spectroscopy of dibenzorubicene: experimental data for a search in interstellar spectra.

We report on the characterization of dibenzo[cde,opq]rubicene (C(30)H(14)). The molecule was studied in solution at room temperature with absorption spectroscopy in the visible (vis) and ultraviolet (UV) wavelength ranges, and with emission spectroscopy. The infrared (IR), visible, ultraviolet, and vacuum ultraviolet (VUV) absorption spectra of a thin film were measured also at room temperature. In addition, the UV/vis absorption spectrum was measured at cryogenic temperatures using the matrix isolation spectroscopy technique. The interpretation of spectra was supported by theoretical calculations based on semiempirical and ab initio models, as well as on density functional theory. Finally, the results of the laboratory study were compared with interstellar spectra.

Low-temperature chemistry in helium droplets: reactions of aluminum atoms with O2 and H2O.

The doping of He droplets by Al atoms and their reactions with H(2)O and O(2) at T = 0.37 K was investigated. It was found that at high doping concentrations, the incorporated Al atoms do not aggregate to form clusters. They rather remain as separated atoms inside of the He droplets. Mass spectrometry and the recently developed depletion method have been applied to study the reactions. It was found that single Al atoms react with single O(2) molecules. The dominant product of this reaction occurring inside of the He droplets is AlO(2). The reaction between Al and O(2) clusters has also been detected. The Al clusters react with single H(2)O molecules or clusters. While single Al atoms react with H(2)O clusters, no reaction of single Al atoms with a single water molecule was found.

Ultra-low-temperature reactions of Mg atoms with O2 molecules in helium droplets.

The reaction of magnesium atoms and clusters with oxygen molecules has been investigated in liquid helium droplets at T = 0.37 K. The energy released during the reaction predominantly leads to the ejection of the reaction products out of the He droplets. The product molecules Mg(2)O, Mg(2)O(2), Mg(3)O, Mg(3)O(2), and Mg(4)O(2) were detected by mass spectrometry. With more than one Mg atom picked up per He droplet, chemiluminescence (CL) light could be detected. This CL results from electronically and vibrationally excited reaction products (Mg(n)O(2), n > or = 2) which left the He droplets. For large He droplets with diameter d > 13 nm, the intensity of the CL light decreases linearly with increasing He droplet diameter. No evidence for chemical reaction was found for d > 50 nm. A considerable time delay between chemical reaction and light emission was observed. It was found that the chemical reaction is unexpectedly fast and has a first-order reaction rate larger than 5 x 10(4) s(-1).

Oxidative reactions of silicon atoms and clusters at ultralow temperature in helium droplets.

The reaction between Si and O(2) was studied in liquid He droplets at low temperature (T = 0.37 K) by monitoring the energy release during the reaction. Additionally, the reactions of Si atoms and clusters with the oxidation agents H(2)O and O(2) have been studied by mass spectrometry. It was found that Si atoms react fast with O(2) molecules. On the other hand, Si atoms and clusters do not react with H(2)O molecules. The energy released during the chemical reaction leads to the ejection of the products from small He droplets. In contrast, large He droplets (N(He) > 20000) are capable of keeping part of the reaction products in their interior. The observation of SiO(2) products with the mass spectrometer reveals that the He droplet can stabilize intermediate products in the exit channel.

Imaging and spectroscopy of defect luminescence and electron-phonon coupling in single SiO2 nanoparticles.

Silicon nanocrystals were synthesized by CO(2) laser pyrolysis of SiH(4). The fresh silicon nanopowder was oxidized in water to obtain SiO(2) nanoparticles (NPs) exhibiting strong red-orange photoluminescence. Samples of SiO(2) NPs embedded in low concentration in a thin polymer layer were prepared by spin-coating a dedicated solution on quartz cover slides. Using an argon ion laser at 488 nm with higher-order laser modes (azimuthally and radially polarized doughnut modes) for excitation, the three-dimensional orientation of the nanoparticles' transition dipole moment was investigated in a confocal microscope. The linear transition dipole moment was found to be rather stable and randomly oriented. However, dynamical effects such as fluorescence intermittency and transition dipole moment flipping could also be observed. The spectral analysis of single SiO(2) NPs revealed double-peak spectra consisting of a narrow zero-phonon line and a broader phonon band being associated with the excitation of longitudinal optical phonons in the SiO(2) NP.

Cavity ring-down laser absorption spectroscopy of jet-cooled L-tryptophan.

The jet-cooled ultraviolet direct absorption spectrum of the amino acid tryptophan is reported. The spectrum measured by cavity ring-down laser absorption spectroscopy covers the region where the origin bands of the S(1) <-- S(0) transitions of six conformers (A to F) are located. Tryptophan was transferred into the gas phase by two different methods, namely, thermal heating and laser vaporization. The latter technique allowed us to obtain higher densities of tryptophan in the jet at the time when it was probed for spectroscopy. It also avoided thermal decomposition of the sample. On the other hand, a higher signal-to-noise ratio was obtained with thermal heating. Measurements were carried out by laser-induced fluorescence as well. The comparison of the absorption and excitation spectra reveals a higher fluorescence yield and a shorter radiative lifetime for the S(1) state of conformer A relative to the other conformers. Moreover, the comparison of our spectra with each other and with literature data led us to assign a band to a new conformer, which we named G. Finally, the theoretical structure and vibrational frequencies obtained from density functional theory based calculations confirm that the progression observed in the S(1) <-- S(0) spectrum of conformer A is consistent with a torsional motion of the amino acid side chain relative to the indole chromophore.

UV/visible spectroscopy of matrix-isolated hexa-peri-hexabenzocoronene: Interacting electronic states and astrophysical context.

Absorption spectra of hexa-peri-hexabenzocoronene isolated in rare-gas matrices are reported for the wavelength range between 200 and 500 nm. Measurements were carried out in neon and in argon at 5.8 and 12.0 K, respectively. Calculations based on semiempirical models and on density-functional theory were performed to assign the observed features. The electronically excited states involved in Clar's alpha- and p-bands are identified as S(1)(B(2u)) and S(2)(B(1u)), respectively. Although the upper state associated with the beta-band is found to be a (1)E(1u) state, it remains undetermined whether it is S(3) or S(4). Structures in the beta-band are interpreted as resulting from the interaction between the (1)E(1u) state and the e(2g) vibrational manifold of S(2)(B(1u)). The new measurements are used to narrow down the wavelength ranges where the bands of hexa-peri-hexabenzocoronene should be found in the gas phase. A previous estimate of the interstellar abundance of this polycyclic aromatic hydrocarbon is discussed.

IR, Raman, and UV/Vis spectra of corannulene for use in possible interstellar identification.

The spectroscopic characterization of corannulene (C(20)H(10)) is carried out by several techniques. The high purity of the material synthesized for this study was confirmed by gas chromatography-mass spectrometry (GC-MS). During a high-performance liquid chromatography (HPLC) process, the absorption spectrum of corannulene in the ultraviolet (UV) and visible (vis) ranges is obtained. The infrared (IR) absorption spectrum is measured in CsI pellets, and the Raman scattering spectrum is recorded for pure crystal grains. In addition to room temperature measurements, absorption spectroscopy in an argon matrix at 12 K is also performed in the IR and UV/Vis ranges. The experimental spectra are compared with theoretical Raman and IR spectra and with calculated electronic transitions. All calculations are based on the density functional theory (DFT), either normal or time-dependent (TDDFT). Our results are discussed in view of their possible application in the search for corannulene in the interstellar medium.

Quantitative Structure-Retention Relationships for Polycyclic Aromatic Hydrocarbons and their Oligoalkynyl-Substituted Derivatives.

Reversed-phase high-performance liquid chromatography (RP-HPLC) has been carried out for a series of unsubstituted polycyclic aromatic hydrocarbons (PAHs) and the corresponding ethynyl, 1,3-butadiynyl, and 1,3,5-hexatriynyl derivatives. Theoretical values of the isotropic polarizability and several polarity descriptors have been computed for each compound by using semiempirical models and density functional theory (DFT), with the aim of evaluating linear functions as quantitative structure-retention relationships (QSRRs). The polarity has been described by using either the permanent electric dipole moment, the subpolarity, or a topological electronic index. Three types of partial atomic charges have been used to calculate the subpolarity and a topological index. The choice of the theoretical model, of the polarity descriptor, and of the partial atomic charges is discussed and the resulting QSRRs are compared. Calculating the retention times from the polarizability and the topological electronic index (AM1, PM3, or DFT-B3LYP/6-31+G(d,p)) gives the best agreement with the experimental values.

Electron-phonon coupling and localization of excitons in single silicon nanocrystals.

We report a detailed photoluminescence (PL) study on single silicon nanocrystals produced by laser pyrolysis. The PL spectra reveal nearly homogeneously broadened zero-phonon lines coupled to Si-O-Si phonon transitions in the SiO2 shell. A systematic investigation of electron-phonon coupling is reported on the basis of single nanocrystals. The stepwise localization of electron and hole at the Si-SiO2 interface for nanocrystals smaller than d approximately 2.7 nm is driven by electron-phonon coupling. From the localization energies the effective Bohr radii of the (localized) electron and hole are estimated to be in the range of 1-2 bond lengths of Si-O and Si-Si.

Ultraviolet spectroscopy of pyrene in a supersonic jet and in liquid helium droplets.

In a series of experiments devoted to the study of polycyclic aromatic hydrocarbons for astrophysical applications, the S(2)<--S(0) transition of jet-cooled pyrene (C(16)H(10)) at 321 nm has been studied by an absorption technique for the first time. The spectra observed by cavity ring-down spectroscopy closely resemble the excitation spectra obtained earlier by laser-induced fluorescence (LIF) and show the same band clusters arising from the vibronic interaction of S(2) with S(1). We have also investigated pyrene when it was incorporated into 380 mK cold helium droplets. These spectra which were recorded employing LIF and molecular beam depletion spectroscopy are broadened and redshifted by 0.94 nm but retain the essential features of the gas phase spectra.