Source Frequency Modulation Research Articles

Local Thermal Resistance Extraction in Monolithic Microwave Integrated Circuits

The thermal resistance of a high electron mobility transistor (HEMT) forming part of a monolithic microwave integrated circuit (MMIC) is noninvasively extracted under real working conditions (electrical and thermal) by infrared thermal imaging. The HEMT thermal resistance considers the device local maximum temperature and dissipated power. An experimental approach to this end is currently not available, as the HEMTs thermal interaction does not allow extracting its individual heat generation. Thanks to thermal field confinement offered by heat source frequency modulation, the power dissipation in each device is inferred, making feasible its individual thermal resistance extraction. As a result, reasonable values of the local thermal resistance of each individual HEMT integrated in the MMIC (i.e., 57.8 ± 3.4 °C/W and 24.8 ± 1.4 °C/W) are obtained in agreement with studies on discrete devices available in the literature.

Self-, N 2- and O 2-broadening of pure rotational transitions of HFC-134a

Because of their lower depleting effect on the ozone in the atmosphere, there has been much recent interest in the fluorinated hydrocarbons as replacements for chlorofluorocarbons and hydrochlorofluorocarbons. Since data on the broadening of rotational lines are required for quantitative monitoring of trace compounds in the atmosphere (especially broadening by collision with N 2 and O 2), self- and pressure-broadening of rotational transitions of CF 3CFH 2 have been studied and the self-, N 2- and O 2-broadening coefficients evaluated. In particular, we have analyzed the line shape of the following b type transitions: J=10 0,10←9 1,9 and J=10 1,10←9 0,9 degenerate doublet, J=15 1,15←14 0,14 and J=20 0,20←19 1,19. The source frequency modulation technique has been employed to record the spectra of the rotational transitions investigated.

Near-infrared band of the nitrate radical NO3 observed by diode laser spectroscopy

We have analyzed the near-infrared band of NO3 observed at 7602 cm−1 by using diode laser spectroscopy. Most of the spectral lines were recorded using source-frequency modulation. Zeeman modulation was found useful in selectively detecting some Q branch lines, which provided us with a clue to the assignment of the observed spectra. The band satisfied selection rules for a parallel band and was thus ascribed to a 2A1″–2A2′ vibronic component associated with the 2E′′–X̃ 2A2′ electronic transition, namely, to a transition from the ground vibronic state to the A1″ vibronic state resulting from excitation of the degenerate in-plane bending mode in the 2E′′ electronically excited state manifold. The band was almost free of perturbations, except for some K=6 lines. The least-squares analysis of 581 assigned lines led to molecular parameters of the upper state, where ground-state parameters were fixed to those obtained from the infrared study previously reported [K. Kawaguchi, E. Hirota, T. Ishiwata, and I. Tanaka, J. Chem. Phys. 93, 951 (1990)]. The upper-state B rotational constant gave the effective N–O distance of 1.271 Å, which is to be compared with 1.240 Å in the ground vibronic state. The εbb spin–rotation interaction constant of the upper state was close in magnitude to that in the ground vibronic state, but of opposite sign. This observation indicates that the spin–rotation interaction is primarily caused by that between the 2E′′ excited and the ground electronic states.

Two highly sensitive microwave cavity spectrometers

A description is presented for two unconventional highly sensitive spectrometers operating in the decimeter-wave region (90–450 MHz) and millimeter-wave region (90–150 GHz), respectively. In both cases a resonant cavity is used as an absorption cell. Either design combines high sensitivity with high resolution and wide spectral range. The radio frequency spectrometer allows measuring of extremely low absorption quantity 3×10−12 cm−1 in combination with high spectral resolution of order 1 kHz with the radio frequency–microwave double resonance method. The gas cell is a coaxial cavity (Q≊103) with a diameter of 40 cm and length about a wavelength. The millimeter-wave spectrometer utilizes an orotron oscillator as the tunable, coherent source of radiation from 90 to 150 GHz. A gas cell is placed into a high quality (Q≊104) Fabry–Pérot resonator of the orotron, and the absorption signal is detected by variation of the orotron electron current in the collector circuit. The sensitivity limit of (2–5)×10−10 cm−1 is achieved with Stark modulation as well as source frequency modulation. Some results of successful application of both designs in molecular spectroscopy are presented, and potential use of the millimeter-wave cavity spectrometer as a gas analyzer with a detection limit better than 1 ppm is briefly discussed.

Infrared diode laser spectroscopy of the LiO radical

The fundamental vibrational band of the 7LiO radical in the ground electronic state X 2Πi was observed in a region from 720 to 850 cm−1 using a source frequency modulation infrared diode laser spectrometer. Radicals were generated in a high-temperature cell by the reaction of lithium metal vapor with nitrous oxide. The observed spectrum was analyzed together with the radio-frequency and microwave spectra already reported. It was found that the vibration-rotation Hamiltonian employed in a previous paper was insufficient to fit all of the observed spectra simultaneously. The Hamiltonian was thus extended to include higher-order corrections for the centrifugal distortion and Λ-type doubling terms, and was used to derive molecular parameters.

A passive homodyne fibre vibrometer using a three-phase detector array

A non-contact, optical fibre-based vibrometer is described with output in the form of a set of spatial fringes. The fringe system is sampled at three different phases using a non-multiplexed linear detector array. No source frequency modulation is necessary, allowing convenient use of a wide variety of lasers, and surface motion is measured free from directional ambiguity. Fringe interpolation and multiple-fringe unwrapping are effected using a digital fringe processor. With a received power of 4 nW, a minimum noise-equivalent phase sensitivity of 150 mu Rad Hz-1/2 is demonstrated.

Highly sensitive millimetre-wave spectrometer based on an orotron

The design and operation of a gas millimetre-wave spectrometer based on a tunable generator orotron are presented. The absorption cell occupies a part of the high quality (Q approximately=104) orotron Fabry-Perot cavity. The limit of sensitivity gamma min approximately=(3-5)*10-10 cm-1 has been achieved with source frequency modulation at the output band of a receiver of 1 Hz. The narrow spectral width of the orotron radiation (10-15 kHz without locking) provides the Doppler resolution of spectral lines. An absorption signal of gas under investigation is detected by variations of the collector current of the orotron. New results on rotational transition of SiH4 in a vibrational dyad v2/v4 with the absorption coefficient gamma approximately=10-8 cm-1 are briefly given as an illustration of the sensitivity of this spectrometer.

Chaos in an optical bistable system subjected to source frequency modulation

The existence of chaos is demonstrated in a passive all-optical bistable system when the input field is frequency modulated. Bifurcation diagrams, return maps, attractors and state diagrams in a phase plane have been investigated experimentally. The experimental results are fairly well reproduced by numerical simulations on a plane wave single mode model.

The microwave spectrum of cyclopropane-1,1- d2 molecular structure of cyclopropane

The microwave spectrum of cyclopropane-1,1- d 2 has been observed in the frequency region from 320 to 400 GHz using a source-frequency modulation microwave spectrometer. Sixty-four observed a-type R-branch transitions were analyzed by the least-squares method to determine the rotational and centrifugal distortion constants to be A = 18 835.662(18), B = 16 370.2703(70), C = 11 409.2285(67), Δ J = 0.011246(12), Δ JK = 0.005087(35), Δ K = 0.00706(12), δ J = 0.0030280(79), and δ K = 0.005561(27), in MHz with three standard errors in parentheses. The observed rotational constants of the 1,1- d 2 species and the B 0 and C 0 constants of C 3H 6 and the B 0 constant of C 3D 6 available in the literatures were corrected for harmonic terms to derive an average structure: r z (CH) = 1.0797(34) A ̊ , r z (CC) = 1.5157(23) A ̊ , θ z (HCH) = 115.47(38)°, and r z (CH) − r z (CD) = 0.00029(16) A ̊ with standard deviation of the fit in parentheses. Using the vibration-rotation constants reported, the equilibrium structure was also estimated, where only two “diagonal” third-order anharmonic terms were taken into account.

Millimeter-wave spectrum and internal rotation of 1-silylpropyne, CH 3CCSiH 3

Rotational transitions of CH 3CCSiH 3 have been observed in the millimeter-wave region using a computer-controlled source-frequency modulation spectrometer with a 1.8-m-long free space absorption cell. The observed spectrum clearly showed the effect of internal rotation with a small potential barrier. It has been analyzed by calculating the torsion-rotation energies on the basis of torsional wave functions obtained by diagonalizing the torsional part of the Hamiltonian. The least-squares analysis has yielded the rotational constant B = 2068.2817(4) MHz and a few centrifugal distortion constants. The barrier height to internal rotation has been determined to be 3.77(70) cm −1 from the contour map of the standard deviation. Also, the A rotational constant of the silyl group around the symmetry axis has been estimated by fixing the A constant of the methyl group to the value of CH 3CCH.

Feasibility of a heterodyne all-fibre optical gyroscope

The paper discusses the feasibility of a novel heterodyne approach to the design of an all-fibre optical gyroscope. The IF is produced by source frequency modulation which also serves to reduce the effective source coherence. Such a technique offers distinct advantages in that the optical medium is completely passive, making it ideal for an all-fibre approach.

The microwave spectrum of the chloromethyl radical, CH2Cl

The gas phase pure rotational spectrum of the chloromethyl radical, CH2Cl, has been observed for the first time in the millimetre wave region using a source frequency modulation microwave spectrometer equipped with a 1 m long free space absorption cell. The radical was generated by the reaction of CH3Cl with 2450-MHz microwave discharge products of CF4. The a-type R-branch transitions have been observed with resolved fine and hyperfine components for both the 35Cl and 37Cl isotopic species in the ground vibrational state. The small positive inertial defect. Δ0 = 0.0333 amu Å2, calculated from the rotational constants obtained for the 35Cl species indicates that the radical is planar in the ground vibronic state. The observed fine and hyperfine interaction constants are consistent with 2B1 symmetry, i.e., with the unpaired electron occupying a pπ orbital extending perpendicular to the molecular plane.

Microwave spectrum and internal rotation of 2-butyne-1, 1, 1-d3 (dimethylacetylene), CH3C≡CCD3

The rotational transitions of CH3C≡CCD3 have been observed for J=13←12, 15←14, 18←17, 19←18, 20←19, 22←21, 23←22, and 26←25 using a source-frequency modulation microwave spectrometer with a 3.7 m long free space absorption cell maintained at −50 to −60 °C. The observed spectrum clearly shows the effect of internal rotation with a small potential barrier. The expression for the rotational transition frequency derived by Kirchhoff and Lide [J. Chem. Phys. 43, 2303 (1965)] using a second-order perturbation theory for the contributions of the internal rotation was employed in analyzing the observed spectrum, and the least-squares analysis has yielded the rotational constant B=2982.4980±0.0011 MHz and a few centrifugal distortion constants. The barrier height to internal rotation has been estimated to be 5.62±0.16 cm−1.

Analytical Description of Tunable Diode Laser Derivative Spectrometry

The instrument functions are investigated, and a quantitative description is given of how a second-derivative spectrometer employing a tunable diode laser attains the power to suppress spectral noise. As a result, it is found that the power can be improved by modifying the source-frequency modulation profile. The essence of this approach is to make the profile at its extreme as sharp as possible, and a hyperbolic sine profile with suitable parameters is proposed as the optimal profile that can be attained in practice.

The microwave spectrum of the N 35Cl radical in the X 3Σ− state

The microwave spectrum of the N 35Cl radical in the ground vibronic state has been observed by using a source frequency modulation spectrometer with a 3.5 m long free space absorption cell. The NCl radical was generated directly in the cell by a dc discharge in a mixture of N2 and Cl2 flowing through the cell. The electric-dipole allowed rotational transitions of N up to 4 ← 3, each being resolved into a few spin components, were observed in the frequency region 41.5–155 GHz. Each transition was found to consist of many hyperfine components. To analyze the observed spectrum, the matrix elements of magnetic hyperfine and electric quadrupole hyperfine Hamiltonians were derived for a diatomic molecule in the triplet state of which both nuclei have nonzero spins. The rotational, spin–spin interaction, and spin-rotation interaction constants were determined to be B=19 383.4655(42) MHz, λ=56 390.850(16) MHz, and γ=−208.6306(96) MHz, with three standard errors in parentheses. The hyperfine parameters b, c, and eQq for both the nitrogen and chlorine atoms were precisely determined together with the nuclear-spin rotation interaction constant for the chlorine atom. The spin density of the unpaired electrons was estimated to be 76% and 22%, respectively, on the nitrogen and chlorine atoms from the observed hyperfine coupling constants.

The microwave spectrum of the fluoromethyl radical, CH2F

The microwave spectrum of the fluoromethyl radical, CH2F, has been observed by using a source-frequency modulation spectrometer with a 1 m long free space absorption cell. The CH2F radical was generated directly in the cell by the reaction of methyl fluoride with 2450 MHz discharge products of CF4. The a-type R-branch transitions, N=1 ← 0, 2 ← 1, and 3 ← 2 for the ground vibrational state and N=2 ← 1 and 3 ← 2 for the first excited state of the out-of-plane bending mode (ν4), have been observed; all of them were found to be split into fine and hyperfine components, and individual lines were measured precisely. The hyperfine structure observed for the ground state is consistent with electronic symmetry of B1 and with odd parity with respect to the reflection on the ‘‘molecular plane.’’ A relative intensity measurement shows that the v4=1 state of even parity is located 300 (30) cm−1 above the ground state. These observations suggest that the CH2F molecule is essentially planar, although the presence of a small hump at the planar configuration cannot be excluded.

Microwave spectroscopy of the NCO radical in thev2= 12Σ state

The microwave spectra of the NCO radical in the (0, 1, 0) μ 2Σ and (0, 1, 0) κ2Σ states have been observed by using a source-frequency modulation spectrometer. The radical was generated by the reaction of HNCO with microwave discharge products of CF4. The observed spectra were analysed using a hamiltonian that included the interactions of the (0, 1, 0) state with (1, 1, 0), (0, 1, 1) and (0, 3, 0), and the rotational, spin-rotation, centrifugal distortion, rovibronic interaction and hyperfine coupling constants were determined precisely for the (0, 1, 0) μ 2Σ and (0, 1, 0) κ2Σ states. From the rovibronic interaction constants two anharmonic potential constants W 1 and W 2 were calculated to be 29·5 (1) and 3·3 (1) cm-1, respectively, with three standard errors in parentheses. The effective B rotational constant in the (0, 1, 0) 2Σ state was found to differ by 1·99 (12) MHz from that in the (0, 1, 0) 2Δ state previously reported. This difference was accounted for by higher order rovibronic interactions; an algebraic expression was derived for the difference. This formula was applied also to BO2, CNC, CCC and CO2 +. For BO2 and CNC the calculated values agreed well with the observed values, whereas discrepancies remained for CCC and CO2 +.

The microwave spectrum of the CF radical

Rotational transitions of the CF radical in both the 2Π1/2 and 2Π3/2 ground electronic states were observed in the region up to 300 GHz by using a source-frequency modulation microwave spectrometer. The CF radical was generated directly in a free-space absorption cell by a dc discharge in carbon tetrafluoride containing a small amount of methyl fluoride. The rotational constant, the centrifugal distortion constant, and the Λ-type doubling constants, as well as all four hyperfine coupling constants a, b, c, and d of the fluorine nucleus, were determined with good precision. From the observed hyperfine coupling constants the unpaired-electron spin and spatial (orbital) distributions 〈1/r3〉s, 〈(3 cos2 χ−1)/r3〉s, 〈sin2 χ/r3〉s, and 〈1/r3〉0 were calculated and discussed in terms of a simple molecular-orbital picture.

The microwave spectrum of the PH 2 radical

Three rotational transitions, 1 10 ← 1 01, 2 20 ← 2 11, and 3 30 ← 3 21, of the PH 2 radical in the ground vibronic state were observed in the mm-wave region, by using a source-frequency modulation spectrometer with a 1-m-long free space cell. The PH 2 radical was generated directly in the cell by glow discharge in a mixture of phosphine and oxygen. Thirty four fine and hyperfine components were measured for the three transitions. An analysis of the observed spectra, combined with far-infrared and mid-infrared laser magnetic resonance spectra and laser-induced fluorescence spectra obtained by other works, improved the rotational and spin-rotation interaction constants in precision. The observed hyperfine structure gave the magnetic hyperfine coupling constants for both P and H nuclei and also the P nuclear spin-rotation interaction constants.

The microwave spectrum of the trifluoromethyl radical

The microwave spectrum of the trifluoromethyl radical in the ground 2A1 state has been observed by using a source frequency modulation spectrometer. The CF3 radical was generated directly in the absorption cell by a glow discharge in trifluoroacetic acid anhydride (CF3CO)2O. The observed transitions included N = 4 ← 3, 5 ← 4, and 6 ← 5, all of which are resolved into K as well as fine and hyperfine structure components. A magnetic hyperfine Hamiltonian was derived for three identical nuclei with a spin of 1/2 (the fluorine nuclei) in a doublet symmetric-top molecule, in order to analyze the observed spectrum. Most of the hyperfine structure could be explained by considering the total nuclear spin operator I0 = 𝒥iIi, except for the K = 1 transitions; some of them were clearly resolved into doublets and the observed splittings allowed us to determine the additional dipole–dipole hyperfine interaction constant ‖ Taa−Tbb ‖. The rotational constant B0 was determined to be 10 900.9118(52) MHz, with 2.5 times the standard error given in parentheses, and the unpaired π-electron spin density on each fluorine atom was estimated to be 12.3% from the observed hyperfine coupling constants.