Overtone Transitions Research Articles

Correlation between the intensities of vibrational overtone transitions and the repulsive branch of the molecular potential

Energy levels εn and n←0 vibrational overtone transition intensities for a distorted Morse potential (DMP) and a linear dipole moment function are calculated, and then are treated as “observed” quantities. The values of (εn−ε0)/n vs n fall on a straight line very closely. A linear least-squares fit provides an effective anharmonicity parameter (xe)eff which is then used to construct an “effective” Morse potential (EMP). The EMP closely follows the DMP near equilibrium but declines in the far repulsive and attractive regions. The intensities calculated for the EMP systematically overestimate or underestimate the DMP intensities, depending on whether the repulsive branch of the EMP above the dissociation limit runs over or under that of the DMP, respectively, and the discrepancies rapidly increase with the overtone number. This effect of the repulsive branch is further investigated quantitatively by comparing the steepness of the repulsive potential β calculated directly from the potential curve with its value found from the rate of the intensities falloff with the overtone energy. It is shown that these two β values coincide with each other in a wide range of parameters, as predicted by the quasiclassical theory. A semiempirical potential for hydroxyl radical is tested for the occurrence of the effect of the repulsive branch. The EMP and RKR potentials are simulated, and the latter is fitted with a kth-order polynomial, k=6–12. In all cases the energy levels and lower overtone intensities are reproduced with a high precision, but the higher overtone intensities are accurate only when the repulsive branch of the fitting function is close enough to that of the original semiempirical potential. An interpolation/extrapolation procedure commonly used to represent the RKR potential is also discussed.

Detection of H 2S at the ppm level using a telecommunication diode laser

Detection of traces of H 2S in air at the ppm level using a Distributed-Feed-Back (DFB) semiconductor diode laser spectrometer is reported. Two-tone frequency modulation spectroscopy is employed to investigate overtone transitions of H 2S near λ = 1580 nm and a sensitivity of 4 ppm·m for the 1577.32 nm component of the ν 1+ ν 2+ ν 3 band is achieved. This result improves the optical detection limit of H 2S in air by one order of magnitude. Self- and air-broadening coefficients at 25°C are determined with direct absorption spectroscopy.

Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy

We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. Their basic operating principles and corresponding performances are summarized and compared. We then present our latest results on the ultrasensitive detection of molecular overtone transitions to illustrate the principle and application of the cavity-enhanced frequency-modulation (FM) spectroscopy. An external cavity is used to enhance the molecular response to the light field, and an FM technique is applied for shot-noise-limited signal recovery. A perfect match between the FM sideband frequency and the cavity free spectral range makes the detection process insensitive to the laser-frequency noise relative to the cavity, and, at the same time, overcomes the cavity bandwidth limit. Working with a 1.064-μm Nd:YAG laser, we obtained sub-Doppler overtone resonances of C2HD,C2H2, and CO2 molecules. A detection sensitivity of 5×10-13 of integrated absorption (1×10-14/cm) over 1-s averaging time has been achieved.

Visible intracavity laser spectroscopy with a step-scan Fourier-transform interferometer

A Fourier-transform spectrometer has been used in a step-scan mode to make time-resolved measurements of the evolving laser pulse in intracavity laser spectroscopy (ILS) experiments. Spectra of broadband dye laser pulses at approximately 615 nm were recorded at relatively high spectral (0.5-cm(-1)) and temporal (as high as 5-mus) resolution. In the absence of an absorber, the height of the pulse is shown to be proportional to t(g)(0.57) (where t(g) is the generation time) for generation times as high as 500 mus. The system was constructed for feasibility studies of future use at infrared and near-infrared wavelengths where conventional ILS that uses diode arrays would be either expensive or simply not possible. The CH(4) overtone transition at 619.68 nm was used to test the linearity and sensitivity of the system. Comparable performance to conventional ILS systems was demonstrated, as were the advantages of the present system for studies of laser and absorption dynamics.

Reversible isomerization of methylcyclopentadienes directed by selective excitation of the methyl out-of-plane fourth overtone transition

Laser vibrational overtone excitation has been employed to study the reversible isomerization of 2-methylcyclopentadiene and 1-methylcyclopentadiene. Spectroscopy of the third through fifth overtones is reported for both isomers. The specific rate coefficients for the energies corresponding to the fourth overtone methyl out-of-plane CH stretch transitions have been derived and compared to those calculated from RRKM theory for the forward and reverse reactions. It is demonstrated that selective overtone excitation of one isomer in the 50:50 mixture enriches the mixture with another isomer.

Photodissociation of vibrationally excited CH 3Cl: modification of the dissociation dynamics

Vibrationally excited CH 3Cl is prepared by laser excitation in the fourth CH stretch overtone transition and photodissociated at fixed wavelengths near 238 nm which also probe the Cl( 2 P 3 2 ) and Cl( 2 P 1 2 ) photofragments through (2 + 1) resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. Jet cooling reveals two partially resolved bands, which are assigned as transitions to the local-mode [5,0,0] A 1,E and [3,0,0] + 4 ν 2 (A 1,E) levels. The ratio of Cl( 2 P 1 2 ) to Cl( 2 P 3 2 ) spin-orbit populations was found to be 1.08 ± 0.08 for 238 nm photolysis of vibrationally excited CH 3Cl, significantly larger than the spin-orbit population ratio (0.27 ± 0.01) in Cl fragments from the 235 nm photolysis of ground state CH 3Cl.

Energy and spectral characteristics of a supersonic electron-beam-sustained CO laser based on the first vibrational overtone and also operating in a selective regime

A numerical investigation is reported of the energy and spectral characteristics of the generation of the first vibrational overtone of the CO molecule in a high-power supersonic electron-beam-sustained CO laser operating under various conditions. It is shown that the electro-optical efficiency of this laser can exceed 30% for the first vibrational overtone. A study is also described of the efficiency of intracavity selection of the emission lines generated as a result of the fundamental and overtone transitions in the CO molecule. It is shown that the output power of the selected lines can be many times higher than the power of the corresponding lines generated by a free-running laser.

Excited-State Dynamics of Chlorine Dioxide in the Condensed Phase from Resonance Raman Intensities

Resonance Raman spectra of chlorine dioxide (OClO) dissolved in cyclohexane obtained with excitation throughout the 2B1−2A2 electronic transition are presented. Resonance Raman intensity corresponding to all vibrational degrees of freedom (the symmetric stretch, bend, and asymmetric stretch) is observed, demonstrating that excited-state structural evolution along all three coordinates occurs upon photoexcitation. The electronic absorption and absolute resonance Raman cross sections are reproduced employing the time-dependent formalism for Raman scattering using an anharmonic description of the 2A2, excited-state potential-energy surface. Analysis of the resonance Raman cross-sections demonstrates that both homogeneous and inhomogeneous broadening mechanisms are operative in cyclohexane. Comparison of the experimentally determined, gas-phase 2A2 surface to that in solution defined by the analysis presented here shows that although displacements along the symmetric stretch and bend are similar in both phases, evolution along the asymmetric stretch is dramatically altered in solution. Specifically, employing the gas-phase potential along this coordinate, the predicted intensity of the overtone transition is an order of magnitude larger than that observed. The analysis presented here demonstrates that the asymmetric stretch overtone intensity is consistent with a reduction in excited-state frequency along this coordinate from 1100 to 750 ± 100 cm-1. This comparison suggests that differences in evolution along the asymmetric stretch may be responsible for the phase-dependent reactivity of OClO. In particular, the absence of substantial evolution along the asymmetric stretch in solution results in the ground-state symmetry of OClO being maintained in the 2A2 excited state. The role of symmetry in defining the reaction coordinate and the nature of the solvent interaction responsible for modulation of the excited-state potential energy surface are discussed.

Mid-infrared DF laser pumped by a tunable and narrow-line Cr^4+: forsterite laser

We describe results of experiments on optically pumped HF and DF overtone transitions that use a specially developed tunable and narrow-line Cr4+: forsterite laser. The performance of this Cr4+:forsterite laser is first reported. Cascading laser transitions were generated in DF(3–2) and DF(2–1) bands between 3.64 and 3.85 µm. The experimental results are compared with the theoretical predictions of a computer model.

Dipole function for the C-H oscillator

Using local mode description for methyl chloride, the infrared intensities for the first four CH3 stretching overtone bands are used to calculate the dipole function for the C-H bond. Detailed expressions for overtone transitions for diatomic molecules derived by Bouanich are employed for the calculation of dipole moment derivatives.

Irregularity in the high resolution wagging band of methylamine

A number of transitions (8813) have been assigned in the n = 1 ← 0 wagging band range 680-900 cm −1 of the high resolution (0.0026 cm −1) spectrum of methylamine. The assigned transitions have been arranged into 44 complete branch sets (A type PQ, QQ, RQ and B type PP, QP, RP, PR, QR, RR) and 19 non-complete sets. Irregularities can be observed among the upper wagging levels relative to the periodic properties required by the Ohashi-Hougen formalism worked out for the description of the ground state and the first torsional state levels. Fifty-four torsional overtone transitions ( ν = 3 ← 0) to k = 0 ( σ = 0, s = 1) levels also have been found and assigned.

Observation of CO and CO_2 absorption near 157 µm with an external-cavity diode laser

Near-IR and visible room-temperature diode lasers in broadly tunable external-cavity configurations are becoming commercially available for gas-sensing applications. Near 1.57 mum, a coincidence of overtone and combination-band transitions from CO, CO(2), OH, and H(2)O is particularly interesting for combustion and combustor emissions monitoring. We report initial observations of the room-temperature absorption of CO and CO(2) made with a commercial external-cavity diode laser.

Vibrational Overtone Transitions of OCS in the Near Infrared

Vibrational overtone transitions of OCS in the near infrared, 10000–14000 cm−1, are observed for the first time by means of the photoacoustic technique. The spectra are analyzed using molecular parameters and Hamiltonions derived for energy levels below 5000 cm−1. It is demonstrated that the molecular parameters of OCS for the low vibrational states can be extended to higher vibrational states within experimental accuracy. A total of 14 transitions in three polyad groups were assigned. Strong perturbations were observed in these highly excited states of OCS.

Cavity ringdown spectroscopy: broad band absolute absorption measurements

Here we report further results in our recent extension of cavity ring down spectroscopy (CRDS) to CW single frequency lasers. We previously pointed out the excellent reproducibility of our spectra, in particular the baseline measurements obtained from the empty cavity. The ability of accurately measuring the zero absorption baseline is essential when studying very broad or congested absorption spectra or even continua. We demonstrate the performance of our CW-CRDS setup by obtaining the absolute absorption spectrum of a weak and broad overtone transition in CHF 3. We also discuss how the present results will apply to conventional pulsed-CRDS.

Ultrastable optical frequency reference at 1.064 μm using a C/sub 2/HD molecular overtone transition

Recently we have developed a new frequency modulation technique for detection of weak resonances of molecules located inside a high finesse cavity. We match the FM sideband frequency to the exact splitting of the cavity free spectral range. With this method we are now detecting saturated molecular overtone transitions with very high sensitivities. The resulting excellent signal-noise ratio for these weak but narrow overtone lines has opened the door for realizing high quality optical frequency references in many spectral regions in the visible and near infrared. Here we demonstrate its potential by presenting our recent frequency stabilization result of locking a diode-laser-pumped Nd:YAG laser to the P(5) line of the HCCD (/spl nu//sub 2/+3/spl nu//sub 3/) band at 1064 nm.

A Funnel Flow Origin for CO Bandhead Emission in Young Stellar Objects

Many young stellar objects show near-infrared CO overtone emission. The observed bandhead profiles are consistent with gas motion in the gravitational field of a central mass and have been interpreted as arising from the innermost regions of a circumstellar disk. However, there is evidence that many of these pre-main-sequence stars possess strong magnetic fields that disrupt the inner disk and channel the gas onto the stellar surface, thereby forming an accretion funnel. It is demonstrated that this funnel can naturally give rise to the observed CO emission. It is shown that gas free-falling along the field lines yields bandhead profiles in agreement with those observed, with the shape of the profile determined mainly by the viewer's inclination. At large inclinations (with respect to the stellar rotation axis) the bandhead has a blue shoulder and a redshifted peak relative to the rest wavelength of the bandhead, whereas at small inclinations the bandhead is narrow. Using stellar parameters appropriate to a low-mass young stellar object and an accretion rate typical of a classical T Tauri star (~10-7 M☉ yr-1), the computed luminosity of the v = 2-0 bandhead is near the low end of the observed range. Since the predicted bandhead luminosity is proportional to the accretion rate, it is concluded that only young stellar objects with large accretion rates should have detectable CO emission, which is consistent with the observations. The lower overtone transitions of CO (e.g., v = 2-0) are not strongly affected by the ambient radiation field since the densities in the funnel flow (nH ≈ 1012 cm-3) are large enough to ensure LTE. However, higher transitions (v ≥ 4) are more susceptible to IR and UV pumping. The calculations imply that radiative effects leading to deviations from LTE will be most noticeable for sources with large bolometric luminosities.

Vibrational overtone enhancement of methyl methacrylate polymerization initiated by benzoyl peroxide decomposition

Vibrational overtone initiated polymerization has been demonstrated using intracavity photolysis of a benzoyl peroxide/methyl methacrylate mixture. Excitation of the 6 ν CH overtone transition of the ground electronic state of benzoyl peroxide creates radicals which subsequently begin the polymerization process. Polymer yield was monitored by comparison of the 2 ν CH overtone absorptions for the methyl, methylenic and olefinic CH stretches at 5946 and 6170 cm −1, respectively. Plots of polymer yield versus time demonstrate an autoacceleration of the polymerization rate commencing many hours after the photolysis period. The delay before autoacceleration depends on the duration of the photolysis.

CW cavity ring down spectroscopy

Until now, applications of cavity ring down spectroscopy (CRDS) employed pulsed laser sources. Here, we demonstrate that a commercial single-frequency CW laser can also be conveniently employed, allowing to gain in spectral resolution, signal intensity and data acquisition rate. As a demonstration, we measured a section of the weak HCCH overtone transition near 570 nm, and compare to existing photoacoustic data. Our high quality and reproducible Doppler-limited spectra display a (rms) noise-equivalent absorption of 10 −9/cm or 5×10 −8 per pass through the sample. Most interesting applications of CW-CRDS include high resolution spectroscopy at low pressure, sub-Doppler absorption spectroscopy in a supersonic jet, and trace-gas detection using compact diode laser sources.

Fermi resonance structure and femtosecond quantum dynamics of a chiral molecule from the analysis of vibrational overtone spectra of CHBrClF

AbstractThe rovibrational spectra of bromochlorofluoromethane (CHBrClF) were measured at intermediate (0.1 cm−1) and high resolution (0.001 cm−1) between the far infrared (20 cm−1) and the visible range of the spectrum (14000 cm−1) by interferometric Fourier transform techniques and diode laser techniques in stationary samples and supersonic jets. All fundamentals and numerous combinations and overtone transitions could be assigned and their band positions and integrated strengths were determined. A quantitative interpretation of the complex multiple resonance structure of the CH chromophore spectra (about 50 assigned bands) is given in terms of an effective anharmonic CH‐stretching‐bending Hamiltonian and in terms of vibrational variational calculations on a 3‐dimensional grid using ab initio (MP2 and B3LYP) and empirically adjusted potential hypersurfaces for the CH chromophore normal coordinate subspace. The systematic analysis of anharmonic resonances of the alkyl CH chromophore in symmetric top (C3v) and asymmetric top (Cs) molecules is herewith extended to chiral molecules. We provide a detailed analysis of the chiral Cs symmetry breaking coupling in the effective Hamiltonian (coupling constant |k′sab| =(20 ± 10) cm−1) and in the potential (Csab and further terms) and their relationship. The implications for the femtosecond intramolecular dynamics for CHBrClF are discussed in terms of quantum dynamical results for time‐dependent populations, survival probabilities, wavepackets and also time‐dependent symmetries and their violations (formal a′ and a′′ species for vibrational Cs and l quantum number for vibrational C∞v symmetry). We have also developed an improved synthesis of CHBrClF, which is suitable for the preparation of large quantities (several mol) of the pure (racemic) compound, and have demonstrated enantiomer separation by gas chromatography.

High-Sensitivity Overtone Spectroscopy of Carbon Disulfide CS2

With opto-acoustic spectroscopy in a multipass cell, using a NaCl color center laser around 6500 cm−1, weak overtone transitions in the (3003) ← (0000) band of12C32S2could be detected. The analysis of the band, based on the extrapolation of molecular constants from the literature, revealed Fermi resonances between the levels of a Fermi polyad.