Soft X-Ray Second Harmonic Generation as an Interfacial Probe.

Nonlinear optical processes at soft x-ray wavelengths have remained largely unexplored due to the lack of available light sources with the requisite intensity and coherence. Here we report the observation of soft x-ray second harmonic generation near the carbon K edge (∼284 eV) in graphite thin films generated by high intensity, coherent soft x-ray pulses at the FERMI free electron laser. Our experimental results and accompanying first-principles theoretical analysis highlight the effect of resonant enhancement above the carbon K edge and show the technique to be interfacially sensitive in a centrosymmetric sample with second harmonic intensity arising primarily from the first atomic layer at the open surface. This technique and the associated theoretical framework demonstrate the ability to selectively probe interfaces, including those that are buried, with elemental specificity, providing a new tool for a range of scientific problems.

Recombination-pumped triatomic hydrogen infrared lasers.

Mid-infrared laser lines observed in hydrogen/rare gas discharges are assigned to three-body recombination processes involving an electron, a rare gas (He or Ne) atom, and the triatomic hydrogen ion (H(3)(+)). Calculations of radiative transitions between neutral H(3) Rydberg states support this interpretation, and link it to recent results for hydrogenic∕rare gas afterglow plasmas. A mechanism for the population inversion is proposed, and the potential generality and astrophysical implications of such molecular recombination laser systems are briefly discussed.

Measurement of quantum tunneling between chiral isomers of the cyclic water trimer.

A detailed experimental study of the water trimer is reported. A vibration-rotation-tunneling band of the perdeuterated cluster has been measured near 89.6 wave numbers by tunable far infrared laser absorption spectroscopy. The data indicate a chiral six-membered ring structure with rapid quantum tunneling occurring between the enantiomers. The observed vibration involves torsional motion of the water subunits about each hydrogen bond axis. It is speculated that larger water clusters would also exhibit transient chirality and that similar effects may be found in liquid water.

Molecular interactions and hydrogen bond tunneling dynamics: some new perspectives.

The recent development of tunable far-infrared lasers and other high-resolution spectroscopic probes of weakly bound clusters is having a significant impact on our understanding of intermolecular forces and on the complex quantum tunneling dynamics that occur in hydrogen-bonded systems. Far-infrared studies of a variety of interactions are discussed, including several prototypical water-hydrophobe complexes, the water trimer, and the ammonia dimer. Particular attention is paid to the inversion of spectroscopic data to yield detailed intermolecular potential energy surfaces. Investigations of nonpairwise additivity are also described.

The C7 cluster: structure and infrared frequencies.

Observation and characterization of the C7 cluster are reported. Carbon clusters are produced by laser vaporization of a graphite target followed by supersonic expansion of the vaporized material within a gas dynamically focused argon jet. Thirty-six sequential rovibrational lines of the nu 4 antisymmetric stretch fundamental of C7 are probed by gated detection of diode laser absorption. The observed spectrum is characteristic of a symmetrical linear molecule. Analysis of the spectrum indicates an effective average bond length of 1.2736(4) angstroms and a vibrational frequency of 2138.1951(10) reciprocal centimeters, in excellent agreement with ab initio calculations. This work will facilitate the astrophysical detection of this cluster.

Spectroscopic determination of the water pair potential.

A polarizable water pair potential was determined by fitting a potential form to microwave, terahertz, and mid-infrared (D2O)2 spectra through a rigorous calculation of the water dimer eigenstates. It accurately reproduces most ground state vibration-rotation-tunneling spectra and yields excellent second viral coefficients. The calculated dimer structure and dipole moment are very close to those determined from microwave spectroscopy and high-level ab initio calculations. The dimer binding energy and acceptor switching and donor-acceptor interchange tunneling barriers are in excellent agreement with recent ab initio theory, as are cyclic water trimer and tetramer structures and binding energies.

Tunable far-IR laser spectroscopy of jet-cooled carbon clusters: the nu 2 bending vibration of C3.

Seven rovibrational transitions of the (01(1)0) <-- (00(0)0) fundamental bending band of C3 have been measured with high precision with the use of a tunable far-infrared laser spectrometer. The C3 molecules were produced by laser vaporization of a graphite rod and cooled in a supersonic expansion. The astrophysically important nu 2 fundamental frequency is determined to be 63.416529(40) cm-1. These measurements provide the basis for studies of C3 in the interstellar medium with far-infrared astronomy.

Infrared laser spectroscopy of the linear C13 carbon cluster.

The infrared absorption spectrum of a linear, 13-atom carbon cluster (C13) has been observed by using a supersonic cluster beam-diode laser spectrometer. Seventy-six rovibrational transitions were measured near 1809 wave numbers and assigned to an antisymmetric stretching fundamental in the 1 sigma g+ ground state of C13. This definitive structural characterization of a carbon cluster in the intermediate size range between C10 and C20 is in apparent conflict with theoretical calculations, which predict that clusters of this size should exist as planar monocyclic rings.

The unidentified interstellar infrared bands: PAHs as carriers?

Infrared emission spectra of gas-phase naphthalene and pyrene have been measured in the range of 3 to 7.5 micrometers with ultraviolet laser desorption-excitation and a spectroscopic technique featuring single-photon counting in the infrared. The spectra were compared with the unidentified infrared emission bands that are observed in many astronomical objects. Marked discrepancies between those observations and the laboratory emission spectra in the wavelengths and relative intensities of principal spectral features led to the conclusion that small neutral unsubstituted polycyclic aromatic hydrocarbons cannot be the carriers of the unidentified infrared emission bands.

Quantifying hydrogen bond cooperativity in water: VRT spectroscopy of the water tetramer.

Measurement of the far-infrared vibration-rotation tunneling spectrum of the perdeuterated water tetramer is described. Precisely determined rotational constants and relative intensity measurements indicate a cyclic quasi-planar minimum energy structure, which is in agreement with recent ab initio calculations. The O-O separation deduced from the data indicates a rapid exponential convergence to the ordered bulk value with increasing cluster size. Observed quantum tunneling splittings are interpreted in terms of hydrogen bond rearrangements connecting two degenerate structures.

Diode-laser absorption spectroscopy of supersonic carbon cluster beams: the nu 3 spectrum of C5.

A new spectroscopic experiment has been developed in which rovibrational transitions of supersonically cooled carbon clusters, which were produced by laser vaporization of graphite, have been measured by direct-absorption diode-laser spectroscopy. Thirty-six sequential rovibrational lines of the nu 3 band of the C5 carbon cluster have been measured with Doppler-limited resolution. The absorption spectrum is characteristic of a linear molecule with a center of symmetry. Least-squares analysis of the spectrum indicates an effective carbon-carbon bond length of 1.283 angstroms, in good agreement with ab initio quantum chemical calculations. This work confirms the detection of C5 in IRC + 10216 reported in the accompanying paper.

X-ray absorption spectroscopy of liquid methanol microjets: bulk electronic structure and hydrogen bonding network.

We have measured the X-ray absorption (XA) spectrum of liquid (298 K) methanol at the oxygen and carbon K edges. The 4a(1) orbital at the O K edge exhibits a pronounced sensitivity to the formation of intermolecular hydrogen bonds, with significant differences observed between the vapor and bulk spectra, whereas the C K edge reveals only subtle corresponding spectral changes. Comparison with DFT computed spectra of model methanol clusters indicates that the bulk liquid comprises long chains (n > 6) and rings of hydrogen-bonded monomers.

Local hydration environments of amino acids and dipeptides studied by X-ray spectroscopy of liquid microjets.

The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an "acceptor-only" configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s --> sigma(NH) preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s --> pi(CN) resonance at 401.2 eV and a broad shoulder resonance at 403 eV, also suggesting the presence of an acceptor-only species. Although this assignment is not as unambiguous as for glycine, it implies that the nitrogen terminus of most proteins is capable of existing in an acceptor-only conformation at high pH. The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwitterionic glycine, and leading us to conclude that the alpha amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.

pH dependence of the electronic structure of glycine.

The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s --> pi*(C=O) transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine, have been reassigned as transitions to sigma* bound states. The observation of these peaks indicates that the amine moiety is in an acceptor-only hydrogen bond configuration at high pH. At low pH, the oxygen 1s --> pi*(C=O) transition exhibits a 0.25-eV red shift due to the protonation of the carboxylic acid terminus. These spectral differences indicate that the variations in electronic structure observed in the NEXAFS spectra are determined by the internal charge state and hydration environment of the molecule in solution.

Detection of the linear carbon cluster C10: rotationally resolved diode-laser spectroscopy.

Detected in interstellar space and as intermediates in soot formation, molecules of pure carbon in the form of linear chains or ring structures have interested researchers for several decades, who attempt to elucidate their physical properties and the processes govering their formation. A high-resolution infrared spectrometer housing a tunable diode laser and combined with an effective laser ablation source for the cluster production has been used to study the molecular properties of small carbon clusters; reported herein is the first gas-phase spectrum of linear C10.

Characterization of silicon-carbon clusters by infrared laser spectroscopy. The nu 1 band of SiC4.

The nu 1 fundamental vibration of linear SiC4 has been observed by infrared diode laser spectroscopy of a supersonic cluster beam. Twenty-four rovibrational transitions were measured in the spectral region of 2094.6 to 2097.1 cm-1, the rotational temperature was 10 K. A combined least-squares fit of these transitions with previously reported microwave data yielded the following molecular constants: nu 1 = 2095.45806(37) cm-1, B" = 0.051161131(52) cm-1, and B' = 0.0509157(96) cm-1. These results are compared to vibrational spectroscopy measurements of SiC4 trapped in a solid Ar matrix and to ab initio calculations.

Close coupling results for inelastic collisions of NH3 and Ar. A stringent test of a spectroscopic potential.

We have calculated state-to-state total cross sections for rotational excitation and inversion of NH3 by collisions with Ar using the close coupling method. The Ar-NH3 interaction potential has been obtained from a fit to the spectrum of this van der Waals molecule. The calculated cross sections agree to within about 30% with the measured values; the estimated error in the latter is 10% to 20%.

Absorption spectroscopy: technique provides extremely high sensitivity.

NASA: Technology associated with cavity ringdown laser absorption spectroscopy is reviewed. The technique is used to study general trace analysis, free radicals in flames and chemical reactors, molecular ions in electrical discharges, biological molecules and water clusters in supersonic jets, and vibrational overtones of stable molecules. Its specific enough to detect about 1-ppm fractional absorption by a gaseous sample in about 10 microseconds. The use of mirrors in ringdown sepctroscopy is explained. Other topics include the generation of pulsed infrared rays and the adaptation of ringdown spectroscopy for use with narrow-bandwidth continuous-wave lasers.^ieng