Doppler-free nonlinear absorption in ethylene by use of continuous-wave cavity ringdown spectroscopy.

We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator.

Rotational Line Strengths and Self-Pressure-Broadening Coefficients for the 1.27-microm, a (1)D(g)-X (3)?(g)(-), v = 0-0 Band of O(2).

We measured at 296 K the rotational line strengths and pressure-broadening coefficients for the 1.27-mum, a (1)D(g)-X (3)?(g)(-), v = 0-0 band of O(2) with a Fourier transform infrared spectrometer using an optical path length of 84 m, a spectral resolution of 0.01 cm(-1), and sample pressures between 13 and 104 kPa.

"Once is Enough" in Radiometric Calibrations.

The successful development of an Optical Technology Division quality system for optical radiation measurement services has provided the opportunity to reconsider the existing calibration procedures to improve quality and reduce costs. We have instituted procedures in our calibration programs to eliminate uninformative repetitive measurements by concentrating our efforts on controlling and understanding the measurement process. The first program in our calibration services to undergo these revisions is described in this paper.

The microwave spectrum of a two-top peptide mimetic: the N-acetyl alanine methyl ester molecule.

The rotational spectrum of N-acetyl alanine methyl ester, a derivative of the biomimetic, N-acetyl alanine N'-methyl amide or alanine dipeptide, has been measured using a mini Fourier transform spectrometer between 9 and 25 GHz as part of a project undertaken to determine the conformational structures of various peptide mimetics from the torsion-rotation parameters of low-barrier methyl tops. Torsion-rotation splittings from two of the three methyl tops capping the acetyl end of the -NH-C(=O)- and the methoxy end of -C(=O)-O- groups account for most of the observed lines. In addition to the AA state, two E states have been assigned and include an AE state having a torsional barrier of 396.

Magnetic-field-induced assemblies of cobalt nanoparticles.

Under the influence of a 0.05 T magnetic field, 15-nm diameter cobalt nanoparticles covered with surfactants in a colloidal solution assemble into highly constrained linear chains along the direction of the magnetic field. The magnetic-field-induced (MFI) chains become floppy after removal of the field, folding into three-dimensional (3D) coiled structures upon gentle agitation.

A Summary of Heat-Flux Sensor Calibration Data.

This paper presents a statistical evaluation of the responsivity data on a number of heat-flux sensors, calibrated using an electrical substitution radiometer as a transfer standard up to 5 W·cm(-2). The sensors, furnished by the customers, were of circular-foil or thermopile type. Comparison of the NIST and the customer measured responsivity values showed that the measurements agree within 3 % for more than half the number of sensors tested, so far.

The Ground and First Excited Torsional States of Acetic Acid.

A global fit of microwave and millimeter-wave rotational transitions in the ground and first excited torsional states (v(t) = 0 and 1) of acetic acid (CH(3)COOH) is reported, which combines older measurements from the literature with new measurements from Kharkov, Lille, and NIST. The fit uses a model developed initially for acetaldehyde and methanol-type internal rotor molecules. It requires 34 parameters to achieve a unitless weighted standard deviation of 0.

The Rotational Spectrum of Ar-SiH(4) and Ar-SiD(4).

Microwave spectra of Ar-(28)SiH(4), Ar-(29)SiH(4), Ar-(30)SiH(4), and Ar-(28)SiD(4) were recorded using a pulsed molecular beam Fourier transform microwave spectrometer. The K = 0 and K = 1 components of the J = 3 <-- 2 through the J = 7 <-- 6 transitions were measured and assigned in the 9-24 GHz region. For the primary (28)Si isotopic species, Ar-(28)SiH(4) and Ar-(28)SiD(4), a K = 0, A symmetry, a K = 0, F symmetry, a doubly degenerate K = 1, E symmetry, and an l/K-doubled, K = 1, F symmetry rotational progression are observed at the approximately 1 K rotational temperature of the supersonic expansion.

High-Resolution Microwave and Infrared Molecular-Beam Studies of the Conformers of 1,1,2,2-Tetrafluoroethane.

High-resolution microwave and infrared molecular-beam spectra have been measured for 1,1,2,2-tetrafluoroethane (HFC134). For the higher energy, polar, C2 symmetry, gauche conformer, microwave spectra have been recorded for the normal and mono-13C isotopomers and analyzed to determine a C-C bond length of 1.512(4) Å, in good agreement with a recent ab initio value (MP2/6-31G**) of 1.

Sub-Doppler Infrared Spectra and Torsion-Rotation Energy Manifold of Methanol in the CH-Stretch Fundamental Region.

The infrared spectrum of jet-cooled methanol in the CH-stretch fundamental region has been investigated by two sub-Doppler laser techniques: optothermally detected molecular-beam electric resonance and direct-absorption slit-jet spectroscopy. With the aid of microwave-infrared double resonance and ground state combination differences, 27 subbands in the frequency range 2967 to 3027 cm-1 have been assigned to the nu2 fundamental. Perturbation systems in the K' = 0 E, -1 E, and -2 E symmetry subbands have been analyzed to yield matrix elements of 0.

Infrared Diode-Laser Molecular-Beam Spectrum of the nu2 Band of Chlorine Nitrate at 1293 cm-1.

The nu2 band of chlorine nitrate (ClONO2 ) near 1293 cm-1 has been measured in a molecular beam with a diode-laser spectrometer. The low rotational temperature of the molecular beam, approximately 23 K, simplifies the spectrum allowing essentially complete assignment of the 35 Cl and 37 Cl lines. An a /b hybrid band is observed with the a -type transition moment being approximately a factor of 2 larger than the b -type transition moment.

Does ammonia hydrogen bond?

Spectroscopic characterizations of the stereochemistry of complexes of ammonia (NH(3)) have strongly confirmed some long-held ideas about the weak interactions of NH(3) while casting doubt on others. As expected, NH(3) is observed to be a nearly universal proton acceptor, accepting hydrogen bonds from even some of the weakest proton donors. Surprisingly, no evidence has been found to support the view that NH(3) acts as a proton donor through hydrogen bonding.