No abstract

Although they are relatively different in band shape, infrared features around 3.4-3.5 m in the emission spectra of HD 97048 and Elias 1 and in the absorption spectra of various dense clouds have both been attributed to diamondoid molecules /particles. This assignment is based mainly on infrared spectra of hydrogenated diamond thin films and of diamond nanocrystals of known average size. Here we present an analysis of the astrophysical implications of recently reported solid-state 2.5-12.5 m spectra of individual diamondoid molecules, up to the size of hexamantane (C 26 H 30 ). These spectra provide the first experimental measurements of the infrared frequencies of this class of molecules. In addition, laboratory gas-phase infrared emission spectra of the three smallest members of the diamondoid family are reported, as well as theoretical spectra for some larger species. The present data set allows us to relate spectral signatures to the molecular size and structure. The spectra of tetrahedral diamondoids are found to be qualitatively different from those of lower symmetry species, which possibly explains the differences between the astrophysical emission and absorption spectra. Interestingly, the 3.53 m band is clearly observed in the spectra of these small molecular diamondoids, whereas previous studies on nanodiamond particles found this band only for species larger than %50 nm. Our results support the assignment of the 3.43 and 3.53 m emission features in HD 97048 and Elias 1 to diamondoids of a few nanometers in size as well as the suggestion that smaller diamondoid molecules contribute to the 3.47 m interstellar absorption band.

By combining results from a Doppler-free two-photon laser excitation study on several lines in the EF 1 ' þ g À X 1 ' þ g (0,0) band of H 2 with results from a Fourier-transform spectroscopic study on a low-pressure discharge in hydrogen, absolute level energies, with respect to the X 1 AE þ g , v ¼ 0, N ¼ 0 ground level, were determined for 547 rovibronically excited states in H 2 . While for some of the levels in the EF 1 AE þ g and B 1 AE þ u states the uncertainties are as low as 0.0001 cm À1 , the accuracy of other levels is lower. The general improvement in the accuracy for the comprehensive data set of level energies is by an order of magnitude with respect to previous measurements. An updated listing of transition wavelengths of the spectral lines in the Lyman and Werner bands is presented, based on combination differences between the presently obtained B 1 AE þ u and C 1 Å u level energies and those in the X 1 AE þ g ground state.

Imaging and time-resolved coincidence techniques are combined to determine ion-electron (v-->(i),v-->(e)) velocity correlations in dissociative photoionization of diatomic molecules induced by synchrotron linearly polarized light P-->. The (v-->(i),v-->(e), P-->) vector correlation yields the identification of each process, together with the ( straight theta(e), straight phi(e)) electron emission in the molecule frame for each orientation of the internuclear axis with respect to the polarization. Strong electron emission anisotropies are observed in the NO molecule frame for the parallel and the perpendicular transitions of the NO+hnu(22-25 eV)-->NO+(c(3) Pi)+e-->N+(3P)+O(3P)+e reaction.

We report in this paper the recording and analysis of the vibrational spectrum of naphthalene in the 1.6-200 microm (50-6000 cm(-1)) spectral range with a resolution of 0.005 cm(-1). The spectrum, recorded at room temperature, shows several complex structures in the Q branches of the c-type bands, which can be assigned to hot-band sequences as well as combination bands and overtones. To analyse the experimental data, we developed a model based on anharmonic calculations which predicts the transitions (positions and intensities) involving the vibrational levels populated at room temperature. This work permits us to estimate the validity and limitations of our calculations, which can be used to predict the band profiles of naphthalene (and larger PAHs) at various temperatures, with potential astrophysical applications.

Determination of the nitrogen isotopic ratios in different bodies of the solar system provides important information regarding the solar system's origin. We unambiguously identified emission lines in comets due to the 15 NH 2 radical produced by the photodissociation of 15 NH 3 . Analysis of our data has permitted us to measure the 14 N/ 15 N isotopic ratio in comets for a molecule carrying the amine (-NH) functional group. This ratio, within the error, appears similar to that measured in comets in the HCN molecule and the CN radical, and lower than the protosolar value, suggesting that N 2 and NH 3 result from the separation of nitrogen into two distinct reservoirs in the solar nebula. This ratio also appears similar to that measured in Titan's atmospheric N 2 , supporting the hypothesis that, if the latter is representative of its primordial value in NH 3 , these bodies were assembled from building blocks sharing a common formation location.

We describe a Fourier transform (FT) spectrometer designed to operate down to 60 nm (20 eV) on a synchrotron radiation beamline for high resolution absorption spectrometry. As far as we know, such an instrument is not available below 140 nm mainly because manufacturing accurate and efficient beam splitters remains a major problem at these wavelengths, especially if a wide bandwidth operation is desired. In order to overcome this difficulty, we developed an interferometer based on wave front division instead of amplitude division. It relies on a modified Fresnel bimirror configuration that requires only flat mirrors. The instrument provides path difference scanning through the translation of one reflector. During the scanning, the moving reflector is controlled by an optical system that keeps its direction constant within a tolerable value and provides an accurate interferometric measurement of the path difference variation. Therefore, a regular interferogram sampling is obtained, producing a nominal spectral impulse response and an accurate spectral calibration. The first results presented in this paper show a measured spectral resolution of delta(sigma)=0.33 cm-1 (interval between spectral samples). This was obtained with a sampling interval of 29 nm (path difference) and 512 K samples from a one-sided interferogram using a cosine FT. Such a sampling interval should allow the recording of large bandwidth spectra down to lambda=58 nm with an ultimate resolving power of 500,000 at this wavelength. In order to check the instrument performances, we first recorded an interferogram from a He-Ne stabilized laser. This provided the actual spectral impulse function, which was found to be fully satisfactory. The determination of the impulse response distortion and of the noise on the vacuum ultraviolet (VUV) spectral range provided accurate information in the sampling error profile over a typical scan. Finally, the instrument has been moved to the SU5 undulator-based synchrotron radiation beamline (Super-ACO facility, LURE, Orsay, France). A high resolution spectrum of O2 (the Schumann-Runge absorption bands, 185-200 nm) was computed from recorded interferograms using the beamline monochromator at the zeroth order to feed the instrument with an 11% relative bandwidth "white" beam (2003). These UV measurements are very close to those found in the literature, showing nominal performances of the FT spectrometer that should translate into an unprecedented resolving power at shortest VUV wavelengths. A recent upgrade (2007) and future developments will be discussed in light of the current installation of the upgraded FT spectrometer as a permanent endstation for ultrahigh resolution absorption spectrometry on the VUV beamline DESIRS at SOLEIL, the new French third generation synchrotron facility.

Two distinct high-accuracy laboratory spectroscopic investigations of the H 2 molecule are reported. Anchor lines in the EF 1 AE þ g À X 1 AE þ g system are calibrated by two-photon deep-UV Doppler-free spectroscopy, while independent Fourier-transform spectroscopic measurements are performed that yield accurate spacings in the B 1 AE þ u À EF 1 AE þ g and I 1 Å g À C 1 Å u systems. From combination differences accurate transition wavelengths for the B À X Lyman and the C À X Werner lines can be determined with accuracies better than $5 Â 10 À9 , representing a major improvement over existing values. This metrology provides a practically exact database to extract a possible variation of the proton-to-electron mass ratio based on H 2 lines in high-redshift objects. Moreover, it forms a rationale for equipping a future class of telescopes, carrying 30-40 m dishes, with novel spectrometers of higher resolving powers. DOI: 10.1103/PhysRevLett.101.223001 PACS numbers: 33.20.Àt, 06.20.Jr, 95.30.Dr, 98.80.Bp Fundamental physical constants may be subject to change on cosmological time scales. For the fine structure constant , evidence for a temporal drift with a 5 significance has been reported [1]. Recently, an indication of a possible decrease of the dimensionless proton-to-electron mass ratio ¼ m p =m e was reported at Á= ¼ ð2:45 AE 0:59Þ Â 10 À5 over a time interval of 12 Â 10 9 years, based on a comparison of spectra of molecular hydrogen [2,3]. The latter findings require three crucial input ingredients. First, a theory is required that relates possible changes in to observable shifts in the spectrum of H 2 . For this purpose sensitivity coefficients K i ¼ d ln i =d ln, which indicate how each line in the H 2 spectrum would drift as a result of a variation in the mass ratio , can be deduced either in a semiempirical fashion [3] or through quantum chemical ab initio calculations [4]. The second ingredient is the accurate determination of spectral line positions at high redshifts. Of the thousands of known quasar systems at redshifts z > 2, H 2 absorption features have only been observed in some 10 to 15 systems thus far. Of these, only Q0405-443 and Q0347-383 have high-quality and well-calibrated spectra containing many H 2 lines [5], which formed the basis of the finding on Á= [2]. Recently, HE0027-184 was established as another system with many resolved H 2 lines [6], and hence a potential source in deriving further constraints on Á. The final ingredient is a database comprising of high-precision laboratory measurements that represent present-day (z ¼ 0) H 2 spectra. The limited amount of available astrophysical data accentuates the need for a set of laboratory data that would not contribute to the uncertainties in estimating a possible drift in .The principle behind the novel determination of laboratory transition wavelengths in theWerner band systems is depicted in Fig. 1. Two entirely independent experiments are performed. First, the level energies of the lowest rotational states in. Schematic of the combi...