Integrated Line Strength Research Articles

Quantitative analysis of gas phase molecular constituents using frequency-modulated rotational spectroscopy.

Rotational spectroscopy has been used for decades for virtually unambiguous identification of gas phase molecular species, but it has rarely been used for the quantitative analysis of molecular concentrations. Challenges have included the nontrivial reconstruction of integrated line strengths from modulated spectra, the correlation of pressure-dependent line shape and strength with partial pressure, and the multiple standing wave interferences and modulation-induced line shape asymmetries that sensitively depend on source-chamber-detector alignment. Here, we introduce a quantitative analysis methodology that overcomes these challenges, reproducibly and accurately recovering gas molecule concentrations using a calibration procedure with a reference gas and a conversion based on calculated line strengths. The technique uses frequency-modulated rotational spectroscopy and recovers the integrated line strength from a Voigt line shape that spans the Doppler- and pressure-broadened regimes. Gas concentrations were accurately quantified to within the experimental error over more than three orders of magnitude, as confirmed by the cross calibration between CO and N2O and by the accurate recovery of the natural abundances of four N2O isotopologues. With this methodology, concentrations of hundreds of molecular species may be quantitatively measured down to the femtomolar regime using only a single calibration curve and the readily available libraries of calculated integrated line strengths, demonstrating the power of this technique for the quantitative gas-phase detection, identification, and quantification.

A MOLECULAR LINE SURVEY OF THE CARBON-RICH PROTOPLANETARY NEBULA AFGL 2688 IN THE 3 mm AND 1.3 mm WINDOWS

We present a spectral line survey of the proto-planetary nebula AFGL 2688 in the frequency ranges of 71-111 GHz, 157-160 GHz, and 218-267 GHz using the Arizona Radio Observatory 12m telescope and the Heinrich Hertz Submillimeter Telescope. A total of 143 individual spectral features associated with 32 different molecular species and isotopologues were identified. The molecules C3H, CH3CN, H2CO, H2CS, and HCO+ were detected for the first time in this object. By comparing the integrated line strengths of different transitions, we are able to determine the rotation temperatures, column densities, and fractional abundances of the detected molecules. The C, O, and N isotopic ratios in AFGL 2688 are compared with those in IRC+10216 and the Sun, and were found to be consistent with stellar nucleosynthesis theory. Through comparisons of molecular line strengths in asymptotic giant branch stars, proto-planetary nebulae, and planetary nebulae, we discuss the evolution in circumstellar chemistry in the late stages of evolution.

Faraday modulation spectrometry of nitric oxide addressing its electronic X^2Π−A^2Σ^+band: I theory

We give a simple two-transition model of Faraday modulation spectrometry (FAMOS) addressing the electronic X(2)Π(ν('') = 0) - A(2)Σ(+)(ν(') = 0) band in nitric oxide. The model is given in terms of the integrated line strength, S, and first Fourier coefficients for the magnetic-field-modulated dispersive line shape function. Although the two states addressed respond differently to the magnetic field (they adhere to the dissimilar Hund coupling cases), it is shown that the technique shares some properties with FAMOS when rotational-vibrational Q-transitions are targeted: the line shapes have a similar form and the signal strength has an analogous magnetic field and pressure dependence. The differences are that the maximum signal appears for larger magnetic field amplitudes and pressures, ∼1500 G and ∼200 Torr, respectively.

THE INTEGRATED RELATIVISTIC IRON LINE FROM ACTIVE GALACTIC NUCLEI: CHASING THE SPIN EVOLUTION OF SUPERMASSIVE BLACK HOLES

The spin of a supermassive black hole (SMBH) is directly related to the radiative efficiency of accretion on to the hole, and therefore impacts the amount of fuel required for the black hole to reach a certain mass. Thus, a knowledge of the SMBH spin distribution and evolution is necessary to develop a comprehensive theory of the growth of SMBHs and their impact on galaxy formation. Currently, the only direct measurement of SMBH spin is through fitting the broad Fe K line in AGNs. The evolution of spins could be determined by fitting the broad line in the integrated spectra of AGNs over different redshift intervals. The accuracy of these measurements will depend on the observed integrated line strength. Here, we present theoretical predictions of the integrated relativistic Fe K line strength as a function of redshift and AGN luminosity. The equivalent widths of the integrated lines are much less than 300 eV. Searches for the integrated line will be easiest for unobscured AGNs with 2-10 keV luminosities between 44 < log L_{X} <= 45. The total integrated line makes up less than 4% of the X-ray background, but its shape is sensitive to the average SMBH spin. By following these recommendations, future International X-ray Observatory surveys of broad Fe K lines should be able to determine the spin evolution of SMBHs.

Quantitative analysis of the in situ Fourier transform infrared absorption and emission spectrum of gas-phase SiO (Deltav = 1 and 2) produced in Si-N-O fiber growth.

The in situ Fourier transform infrared (FT-IR) spectrum of gasphase SiO produced in silicon oxynitride fiber growth has been quantitatively analyzed. Both absorption and emission FT-IR spectra at a spectral resolution of 0.5 cm(-1) were produced from the reaction zone at 1450 degrees C. The fundamental and hot bands were observed with vibrational levels up to v = 7. For the purposes of quantitative analysis the individual vibration-rotation integrated line strengths for the three main isotopes,( 28)SiO,( 29)SiO, and( 30)SiO, were calculated based on ab initio quantum chemical calculations of the electric dipole moment function and the transition moment. Vibrational anharmonicity and Hermann-Wallis correction factors were also incorporated. From the line strengths at specific temperatures and the known Dunham coefficients, the absorbance spectrum was simulated with best fits giving the averaged SiO concentration in the 400 mm reaction zone of 1.0 x 10(17) molecules/cm(3). Such quantitative measurements demonstrate the power of in situ infrared (IR) spectroscopy combined with quantum chemical calculations. The rapid determination of synthetic calibration datasets for chemometric analysis can thus lead to correlation of gas-phase species concentrations with fiber growth properties and subsequently to real-time process control.

Electronic transition moments for the Herzberg I bands of O2

Recently published extensive high-resolution measurements of absolute integrated photoabsorption cross sections for rotational lines of the (ν′ = 4–11, ν″ = 0) bands of the O2 Herzberg I system have been fitted using general rotational line-strength formulae for [Formula: see text] transitions. Good fits were obtained using only three independent electronic transition-moment parameters that accounted for transition strength borrowed from electric-dipole-allowed transitions through spin-orbit and orbit-rotation interactions involving both upper and lower states of the transition. Absolute values of transition-moment parameters have been obtained, corresponding to R-centroids from 1.29 to 1.32 Å (1 Å = 10−10 m). Band oscillator strengths derived from the calculated integrated line strengths are in good agreement with most experimental measurements. Principal electronic matrix elements have been estimated by assuming that strength is borrowed from only two electric-dipole-allowed transitions.

Kiloparsec-scale molecular gas excitation in spiral galaxies

We combine beam-matched ^13^CO, ^12^CO J = 3 - 2 and J = 2 - 1 line data to infer the molecular gas excitation conditions in the central 500 to 1600 pc diameters of a small sample of infrared-bright external galaxies: NGC 253, IC 342, M83, Maffei 2, and NGC 6946. We find that the central 170 to 530 pc diameter regions have typical molecular gas densities ranging from &lt;~ 10^4^ cm^-3^ (in M83) to &gt;~ 10^5^ cm^-3^ (in NGC 253) and that, outside of these regions, the densities are likely to be &lt;~ 10^4^ cm^-3^. The molecular clouds outside the inner 170-530 pc are at least as warm as the molecular clouds in our Galaxy. Column densities derived from integrated ^13^CO line strengths and Hα surface brightnesses suggest that the star formation rate is enhanced in the central 170-530 pc diameters by an order of magnitude over that inferred for the outer star-forming disks in spiral galaxies. The total luminosity of each sample galaxy over all CO rotational lines is ~10^5^-10^6^ L_sun_, which is within an order of magnitude of that of the important cooling line of [C II] at 158 microns. Additional observations of the J = 1 - 0 lines of C^18^O and ^13^CO suggest that emission from the central kiloparsec of spiral galaxies may be optically thick in the ^13^CO J = 1 - 0 line. If so, estimates of molecular gas mass using ^13^CO integrated line strengths could be too low. The sum of the molecular line observations imply the existence of two molecular gas components: a low-density [n(H_2_) &lt;~ 10^3^-10^4^ cm^-3^] component and a warm (T_K_ &gt;~ 50 K), high-density (n(H_2_) &gt;~ 10^4^-10^5^ cm^-3^] component. The warm, dense component is probably associated with star formation.

The dust and molecular gas cloud surrounding a planetary nebula - NGC 7027

We have searched for millimeter-wave emission from nine molecules in the planetary nebula NGC 7027: CN, CS, C/sub 2/H, HC/sub 3/N, SiS, C/sup 18/O, SiCC, /sup 13/CH/sub 3/OH, and H/sub 2/CO. A search for the J, K = (1,1) line of NH/sub 3/ and the J = 21..-->..20 transition of HC/sub 7/N was undertaken at 1.3 cm. Only the N = 1..-->..0 line of CN was positively detected. Limits on integrated line strengths, column densities, and relative abundances are calculated for most of the molecules. Abundances calculated for the planetary nebula are compared with those found for the evolved carbon star, IRC+10/sup 0/216. Differences between the apparent chemical compositions of the two objects may be explained by a lower maximum volume density in NGC 7027 and the effect of the radiation field of its central star. An analysis of the extinction in the circumstellar cloud shows that the abundance ratio, (dust)/(CO), has nearly the same value in NGC 7027, a number of highly evolved stars, and diffuse interstellar molecular clouds. This result is not easy to reconcile with abundance predictions made on the basis of purely gas-phase chemistry, and it is likely that gas-grain chemistry is important in many astronomicalmore » environments.« less

Concentration Measurements of Chlorine Atoms in A Plasma Reactor

AbstractInfrared absorption spectroscopy has been used to measure atomic chlorine concentrations over a range of plasma conditions in both Cl2 and CF3Cl discharges.These measurements were made utilizing the spin-orbit transitions in the ground state of atomic chlorine near 882 cm−1.The concentration studies were performed by passing light from a diode laser through a multi-pass (White) cell set in two opposed windows of a parallel plate plasma etching reactor.The plasma work was preceded by a laboratory measurement of the infrared absorption line strengths of the 2P1/2 ← 2P3/2 transition.This measurement was done in a known concentration of atomic chlorine produced in a low pressure discharge flow system by the reaction of Cl2 or HCl with excess fluorine atoms.These measurements resulted in an integrated line strength of 4.14 (±0.89) × 10−21 cm2-molecule−1-cm−1 for the strongest hyperfine component of the transition at 882.3626 cm−1.Measured atomic chlorine concentrations in Cl2 discharges varied between 0.2 and 8.0 × 1014 atoms/cm3, representing atomic chlorine fractions on the order of a few percent.The measured atomic chlorine concentrations increased approximately linearly with increasing power and pressure, and increased with increasing frequency above approximately 1 MHz.Below 1 MHz, the atomic chlorine concentration was relatively independent of frequency.

A measurement of the radiative lifetime of Cl(3p 52P [formula omitted

The radiative lifetime of the 2P 1 2 spin-orbit level of ground-state atomic chlorine has been determined by tunable diode laser absorption measurements of the 2P 1 2 ( F = 2) ← 2P 3 2 ( F = 3) transition at 882.36 cm −1. An integrated line strength of (4.14±0.89)×10 −21 cm 2 molecule −1 cm −1 (sum of the line strengths for natural abundance 35Cl and 37Cl) is measured for this transition at 297 K, corresponding to a 2P 1 2 radiative lifetime of 87.9±19 s. This result is in excellent agreement with calculations of the magnetic dipole transition probability.

The absorptive line shape of hadronic atoms

The exact line shape for all energies is derived analytically in the limit of negligible nuclear dimensions. The shape deviates from the Breit-Wigner form. The high-energy tail of the line has a universal shape, independent of the absorptive strength. The tails are different for different initial states, however. The integrated line strength diverges logarithmically. Renormalization of the hadron wavefunction by strong interactions leads to the usual shape near resonance and restores convergence for very large energies. The results resolve a logical inconsistency in the normal analysis of hadronic atoms and are of practical importance. It is shown that bound hadronic states (e.g., Y 0 ∗ ) give natural contributions in the high energy region. Numerical illustrations are given.