Weak Overtone Research Articles

Broadband laser enhanced dual-beam interferometry

We demonstrate a dual-beam, balanced detector approach, compatible with commercial Fourier transform infrared spectrometers that provide a single modulated output. Implemented with a near-IR mode-locked fiber laser source and an external broadband polarizing beamsplitter, the dual-beam method provides relative intensity noise reduction and real-time baseline drift cancellation. Noise levels within a factor of three above the shot noise limit (using 0.6 mW of optical power) are demonstrated for the weak second overtone of CO. The method should be particularly well suited for applications like broadband spectroscopy using a large fraction of the supercontinuum generated in a highly nonlinear fiber, and attenuated reflection spectroscopy, for which extreme pathlength enhancement is challenging.

Overtone spectroscopy of water molecules based on room temperature diode laser emitting in the NIR range

The measurement of very weak overtone spectroscopic line shape of water molecule is performed using wavelength modulation spectroscopy (WMS). In this work a high resolution near infrared diode laser spectrometer is used to record first derivative (1f) signal of buffer gas Helium and Carbon dioxide broadened water vapour transitions around 820 nm wavelength region. The effect of collision due to perturber pressure on signal to noise ratio (SNR) is discussed. The Lorentzian HWHM of observed 1f signal is calculated. The comparison is made between collisional broadening of water vapour transition broadened by inert gas Helium and high quadrupole moment active gas Carbon dioxide.

Cavity-enhanced resonant photoacoustic spectroscopy with optical feedback cw diode lasers: A novel technique for ultratrace gas analysis and high-resolution spectroscopy

Cavity-enhanced resonant photoacoustic spectroscopy with optical feedback cw diode lasers (OF-CERPAS) is introduced as a novel technique for ultratrace gas analysis and high-resolution spectroscopy. In the scheme, a single-mode cw diode laser (3 mW, 635 nm) is coupled into a high-finesse linear cavity and stabilized to the cavity by optical feedback. Inside the cavity, a build-up of laser power to at least 2.5 W occurs. Absorbing gas phase species inside the cavity are detected with high sensitivity by the photoacoustic effect using a microphone embedded in the cavity. To increase sensitivity further, coupling into the cavity is modulated at a frequency corresponding to a longitudinal resonance of an organ pipe acoustic resonator (f=1.35 kHz and Q approximately 10). The technique has been characterized by measuring very weak water overtone transitions near 635 nm. Normalized noise-equivalent absorption coefficients are determined as alpha approximately 4.4x10(-9) cm(-1) s(1/2) (1 s integration time) and 2.6x10(-11) cm(-1) s(1/2) W (1 s integration time and 1 W laser power). These sensitivities compare favorably with existing state-of-the-art techniques. As an advantage, OF-CERPAS is a "zero-background" method which increases selectivity and sensitivity, and its sensitivity scales with laser power.

IR spectroscopy of COmosphere dynamics with the CO ?rst overtone band

AbstractWe discuss observations of the weak ?rst overtone (Δν = 2) CO absorption band near 2300 nm with the U.S. National Solar Observatory Array Camera (NAC), a modern mid‐infrared detector. This molecular band provides a thermal diagnostic that forms lower in the atmosphere than the stronger fundamental band near 4600 nm. The observed center‐to‐limb increase in CO line width qualitatively agrees with the proposed higher temperature shocks or faster plasma motions higher in the COmosphere. The spatial extent of chromospheric shock waves is currently at or below the diffraction limit of the available CO lines at existing telescopes. Five minute period oscillations in line strength and measured Doppler shifts are consistent with the p‐mode excitation of the photospheric gas. We also show recent efforts at direct imaging at 4600 nm. We stress that future large‐aperture solar telescopes must be teamed with improved, dynamic mid‐infrared instruments, like the NAC, to capitalize on the features that motivate such facilities (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mid-infrared dual-comb spectroscopy with 2.4 μm Cr2+:ZnSe femtosecond lasers

The mid-infrared part of the electromagnetic spectrum is the so-called molecular fingerprint region because gases have tell-tale absorption features associated with molecular rovibrations. This region can be for instance exploited to detect small traces of environmental and toxic vapors in atmospheric and industrial applications. Novel Fourier-transform spectroscopy without moving parts, based on time-domain interferences between two comb sources, can in particular benefit optical diagnostics and precision spectroscopy. To date, high-resolution and -sensitivity proof-of-principle experiments have only been reported in the near-infrared region where frequency-comb oscillators are conveniently available. However, as most of the molecular transitions in this region are due to weak overtone bands, this spectral domain is not ideal for sensitive and rapid detection. Here we present a proof-of-principle experiment of frequency-comb Fourier-transform spectroscopy with two Cr2+:ZnSe femtosecond oscillators directly emitting in the 2.4 μm mid-infrared region. The acetylene absorption spectrum in the region of the \(\nu_{1}+\nu_{5}^{1}\) band, extending from 2370 to 2525 nm, could be recorded within a 10 μs acquisition time without averaging with 12 GHz resolution.

Infrared spectra and quantum chemical characterization of weakly bound clusters of the benzoyl cation with Ar and H2O

Weakly-bound clusters of the closed-shell benzoyl cation (C(6)H(5)CO(+), PhCO(+)) with Ar and H(2)O are investigated by infrared (IR) spectroscopy, mass spectrometry, and quantum chemical calculations in order to characterize the interaction of a closed shell aromatic cation with a nonpolar and a polar ligand. PhCO(+)-L dimers are produced by electron ionization of benzaldehyde in a supersonic plasma expansion. IR photodissociation (IRPD) spectra of PhCO(+)-L with L = Ar and H(2)O are analyzed in the C-O, C-H, and O-H stretch ranges (2000-3900 cm(-1)). The potential energy surface of the PhCO(+)-L dimers is characterized at the MP2/6-311++G(2df,2pd) level to locate the various minima and determine their energetic and vibrational properties. PhCO(+)-Ar prefers intermolecular π-bonding to the aromatic ring with a bond energy of D(0) = 6 kJ mol(-1). The weak interaction implies that the IRPD spectrum of PhCO(+)-Ar is very close to the IR spectrum of bare PhCO(+). The detection of the C-H and C-O stretch fundamentals provides valuable information about the C-O and C-H bond strengths in this prototypical aromatic acylium ion. Moreover, a variety of weak combination and overtone bands are assigned. The global minimum on the PhCO(+)-H(2)O potential has a planar charge-dipole configuration with D(0) = 41 kJ mol(-1) (with only the two H(2)O protons being out-of-plane), in which the lone pairs of H(2)O interact with the positive partial charges on the carbonyl carbon atom and the proton of the CH group in ortho position. The experimental IRPD spectra are in accord with the calculated global minima. The analysis of the charge distribution shows that the PhCO(+) cation is best represented as an oxocarbenium ion (Ph-C(+)[double bond, length as m-dash]O) with smaller contributions of the ketene structure (Ph(+)[double bond, length as m-dash]C[double bond, length as m-dash]O). This view is supported by the geometrical and vibrational properties of PhCO(+) as well as the shape of the intermolecular PhCO(+)-L dimer potentials.

V1647 ORIONIS: REINVIGORATED ACCRETION AND THE RE-APPEARANCE OF MCNEIL'S NEBULA

In late 2003, the young eruptive variable star V1647 Orionis optically brightened by over 5 magnitudes, stayed bright for around 26 months, and then decline to its pre-outburst level. In August 2008 the star was reported to have unexpectedly brightened yet again and we herein present the first detailed observations of this new outburst. Photometrically, the star is now as bright as it ever was following the 2003 eruption. Spectroscopically, a pronounced P Cygni profile is again seen in Halpha with an absorption trough extending to -700 km/s. In the near-infrared, the spectrum now possesses very weak CO overtone bandhead absorption in contrast to the strong bandhead emission seen soon after the 2003 event. Water vapor absorption is also much stronger than previously seen. We discuss the current outburst below and relate it to the earlier event.

Overtone spectroscopy of chlorine substituted benzene derivatives

The absorption spectra of di-, tri- and tetra-derivatives of chlorobenzene have been studied in their pure form in the spectral range 400–20,000 cm −1. A large number of bands associated with the fundamental, the overtones and the combination frequencies of C H stretching mode have been observed. Vibrational frequencies, anharmonicity constants and dissociation energies, for the C H stretch vibrations for the six molecules have been determined using local mode model. The C H stretch frequencies obtained from experiments are compared with the corresponding frequency determined theoretically using RHF and DFT methods with same 6–31 + G* basis set. This information has been used for the assignment of several combination bands as well as some weak overtone bands. Effect of hydrogen atom substitution by chlorine atom has been studied by measuring changes in the vibrational frequency of the C H stretching mode and the C H bond length. Frequency changes have been well correlated with the change in charge density on carbon as well as chlorine atoms.

Detection of vibrational overtone excitation in methane by laser‐induced grating spectroscopy

AbstractLaser‐induced grating spectroscopy is applied to measure weak overtone and combination bands of methane between 11 170 and 11 980 cm−1 at pressures ranging from 0.2 to 4 bar and ambient temperature. The background‐free method yields rotationally resolved spectra of transitions exhibiting cross‐sections as low as ≈ 10−26 cm2/molecule. The technique is suitable for clarifying the near‐infrared spectra of planetary objects and to provide data for the development and verification of new models describing the complex energy structure of highly vibrationally excited molecules. Copyright © 2008 John Wiley & Sons, Ltd.

A comparative Raman spectroscopic study of natural gas hydrates collected at different geological sites

Intact natural gas hydrates recovered on the West African margin in the South Atlantic Ocean (ZaiAngo and Neris II projects) and from the Norwegian Sea (Hakon Mosby Mud Volcano) are investigated by micro-Raman spectroscopy at ambient pressure and low temperature. The gas hydrates collected at different geological sites contain a high methane concentration relative to other minor components that are slightly dispersed in the samples. They crystallize in a type I cubic lattice structure as also confirmed by our preliminary synchrotron diffraction results obtained on the ZaiAngo specimen. However, detailed analysis of selected microscopic areas reveals a variation in the gas distribution among the different specimens. Trace amounts of CO 2 and H 2S can be identified by their characteristic vibrational signatures in the 1000–3800 cm − 1 spectral range. They are found to be co-clathrated with methane. Their presence produces a compositional effect on the relative cage occupancy of CH 4, as determined from the integrated band intensity ratio corresponding to the molecular stretching modes of methane in the hydrate. The comparative Raman analysis of synthetic hydrates of H 2S, CH 4 and CH 4-deuterohydrates allows the unambiguous assignment of weak band overtones of trapped methane and co-clathrated H 2S molecular vibrations.

Free surfactant synthesis of microcrystalline CdS by solvothermal reaction

Microcrystalline CdS was solvothermally synthesized by free surfactant reaction at 200 °C in a variety of solvents [benzene (BZ), toluene (TL), p-xylene (XL), cyclohexane (CHX) and tetrahydrofuran (THF)]. An X-ray diffractometer (XRD), a selected area electron diffraction (SAED) technique and an energy dispersive X-ray (EDX) analyzer show that the products are CdS (hexagonal structure) composed of Cd and S. The SAED patterns are also in accord with a simulation model. Strong fundamental and weak overtone modes were detected using a Raman spectrometer at 297.0 and 597.1 cm − 1, respectively. By using a transmission electron microscope (TEM), those synthesized in BZ, TL and XL are the mixtures of hexagonal and triangular plates. They are hexagonal and rod shapes in CHX and THF, respectively. Photoluminescent (PL) energies at the maximum intensities were detected over the range of 3.10–3.16 eV (392–400 nm).

The vibrational spectra of vinylphosphine revisited: Infrared and theoretical studies from CCSD(T) and DFT anharmonic potential

On the grounds of CCSD(T) and DFT anharmonic potential and the use of a variational method, the vibrational spectra of vinylphosphine and one of its isopotomer are calculated and the IR spectra in the gas phase revisited between 600 and 6300 cm −1. The agreement between calculated and observed frequencies is fairly good for all fundamentals. These joint experimental and theoretical analyses allow us to assign (very) weak combinations and overtones bands observed in the regions revisited.

Contribution of the asymmetric stretch, ν3B1, to the fundamental Raman spectrum of water

The OD-stretching overtone from liquid D2O, 2nu, and the fundamental OD stretch from dilute HDO, both display high-frequency depolarization ratio minima, but the fundamental OD stretch from neat D2O displays a maximum, at the equivalent position. The rhoL minima arises from the decreased depolarization ratio produced by the absence of B1 modes. The fundamentals of HDO are of A species, and the 2nu overtone of D2O only involves A1 species, e.g., 2nu3B1 has A1 species via B1 x B1 = A1. A and A1 modes display small rhoL values which produce minima in rhoL near 2665 cm(-1) for HDO, and near 5250 cm(-1) for the D2O overtone. These minima give way to a depolarization ratio maximum when the depolarized, rhoL = 34, nu3B1 fundamental, makes its appearance in D2O at 2650 cm(-1). Fundamental and overtone depolarization ratios were used to determine the nu3B1 contribution to the depolarization ratio of the fundamental OD stretch; a value of approximately 28% resulted at 2655 cm(-1). Liquid H2O displays completely analogous features; a value of approximately 20% resulted for it at 3660 cm(-1). Nonhydrogen-bonded nu3B1, and more strongly hydrogen-bonded nu3B1, modes are also indicated for D2O and H2O. A rigorous test of the current results can be accomplished by measuring the depolarization ratio of the extraordinarily weak second Raman overtone, 3nu, recently detected for D2O.

Infrared combination and difference bands of the NO dimer

The mid-infrared (1500–3800 cm −1) absorption spectrum of gaseous nitric oxide has been studied at low temperature (99 K) with a long absorption path (160 m) in order to observe weak combination, difference, and overtone bands of the NO dimer. About ten new bands were assigned with greater or lesser certainty. Combined with previous results, they lead to a set of 12 secure and 7 tentative vibrational term values for (NO) 2, essentially doubling our knowledge of NO dimer vibrational states. The strongest non-fundamental bands in this region, other than the ν 1 (symmetric N–O stretch) + ν 5 (asymmetric N–O stretch) overtone, involve combinations of ν 5 with ν 3 (intermolecular stretch). Excitation of ν 5 results in increased frequencies for the intermolecular modes ν 2, ν 3, and ν 4. A new value of 155.5 cm −1 was obtained for ν 4, the elusive infrared-inactive out-of-plane fundamental vibration.

Laser-locked, high-repetition-rate cavity ringdown spectrometer

We describe the design, construction, and initial performance evaluation of a high-repetition-rate cavity ringdown spectrometer. The spectrometer is based on the use of the Pound-Drever-Hall technique to lock the laser frequency to the maximum of a transmission fringe of the interferometer used as a sample cell. This results in continuous injection of light into the interferometer. The injection is repetitively interrupted with an acousto-optical modulator to generate ringdowns (exponential decays) at a typical rate of 10 kHz. Averaging of these large numbers of fitted ringdown times allows us to attain a minimum detectable absorption of 1.43×10−11 cm−1 Hz−1/2 short term and 9.0×10−11 cm−1 Hz−1/2 long term. In addition, the spectrometer has a continuous tuning capability of ∼1 cm−1, which allows the use of standard linearization and frequency calibration techniques for the spectrum. To illustrate the operation and sensitivity of the spectrometer, part of the Q-branch of a weak acetylene overtone has been recorded.

Fourier transform spectroscopy of N 2O weak overtone transitions in the 1–2 μm region

The absorption spectrum of the natural sample of nitrous oxide has been recorded at Doppler limited resolution with a Fourier-transform spectrometer in the spectral range 5000–10 000 cm −1. Ten cold bands (8Σ − Σ and 2Σ − Π), thirteen hot bands (11Π − Π, Σ − Σ, and Δ − Δ) of 14N 2 16O and the 3 ν 3 band of 14N 15N 16O have been newly detected. The uncertainty of the line position determination is estimated to be about 0.005 cm −1 for unblended lines. The assignment of the spectrum has been done with the help of the prediction performed within the framework of the polyad model of effective Hamiltonian. The spectroscopic parameters G v , B v , D v , H v , and q v have been determined for all newly detected bands. The line intensities of 13 weak bands have been measured. The uncertainty of the obtained line intensity values varies from 7 to 13%.

A simple scanning semiconductor diode laser source and its application in wavelength modulation spectroscopy around 825 nm

A very simple and inexpensive tunable semiconductor diode laser controller is designed for stable operation of the diode laser. The diode laser controller is stable within +/−8 μA and +/−10 mK, respectively. The noise spectrum of the current controller is studied by FFT analysis. We have used our home-made diode laser system in a tunable diode laser absorption spectrometer (TDLAS) to probe weak overtone transitions of water vapour molecule. The diode laser wavelength is coarsely tuned by changing the operating temperature to probe (2, 1, 1)←(0, 0, 0) band overtone transitions of water vapour within 818–835 nm. To demonstrate line shape study, seven transitions are scanned by ramping the drive current of the diode laser (at constant operating temperature) under different perturber (laboratory air) pressures within the sample cell. A balanced detector and a lock-in amplifier are used for phase sensitive detection purpose. Small current modulation amplitude, balanced detection and proper adjustment of the lock-in amplifier help to obtain a S/ N ratio ranging from ∼100 to ∼7 using a small sample path length of 1.5 m. Experimentally obtained derivative spectrum is numerically integrated to reveal the original line shape and fitted with a nonlinear least squares fitting program to extract air broadening coefficients and line strength parameters. The spectroscopic line parameters are compared with the results from HITRAN database.

A near-infrared spectroscopic study of hydroxyl in natural chondrodite

A chondrodite from a marble from Ambasamudram near the Archankovil shear zone, Kerala Khondalite Belt, India, was characterized by powder X-ray diffraction (XRD), electron microprobe and Fourier Transform Infrared (FTIR) spectroscopic techniques. FTIR measurements were carried out in the range 400-10000 cm - 1 , to record both fundamental and overtone modes. A set of absorption peaks at 3393, 3411, 3561, 3571, 3650, and 3685 cm - 1 were observed in the fundamental OH-stretching mode region. In the combination-mode region modes observed at 4103, 4328, and 4440 cm - 1 indicate that the majority of M sites in this chondrodite are occupied by Mg. The modes at 3650 and 4520 cm - 1 could be due to silanol (Si-OH) groups. A definite band was also observed at 5072 cm - 1 , whose origin presently is not clear. A weak overtone mode of the strongest OH stretching was observed at 6967 cm - 1 .

New observations of weak overtone transitions of N 2O by ICLAS-VeCSEL near 1.07 μm

We have completed our investigations of the weak overtone transitions of nitrous oxide near 1.05 μm by the study of the 9090–9560 cm −1 region by intracavity laser absorption spectroscopy (ICLAS) using a vertical external cavity surface emitting laser (VeCSEL). Eight bands were rotationally analyzed. Two hot bands are newly observed while the rotational analysis of the six cold bands has been improved. This Note includes the correction of a few misprints in the rovibrational parameters and the reconsideration of the analysis of an interpolyad Coriolis interaction system given in our previous contribution [J. Mol. Spectrosc. 220 (2003) 80].

High-resolution cavity-tuned ringdown spectrometer using a narrow-bandwidth pulsed laser source

A novel method of cavity ringdown spectroscopy is proposed to achieve high spectral resolution with tunable narrow bandwidth pulsed lasers. We demonstrate a cavity-tuned ringdown configuration in which only a single cavity mode is kept excited near the carrier frequency of a narrow bandwidth pulse laser. This is done simply by making a cavity resonance actively track the frequency reference served by the cw injection seed of the pulsed laser source. We present the servo mechanism used in the cavity resonance tracking, reliable procedures for transverse mode matching, and the evidence of single longitudinal mode excitation. The spectrometer performance is tested to record weak molecular overtone features of acetylene around the wavelength of 570 nm, showing cavity tracking stability within 5-MHz uncertainty which overcomes the bandwidth limit of pulsed laser sources itself.