Difference Frequency Laser Research Articles

Atmospheric trace gas analysis with cavity ring‐down spectroscopy

AbstractCavity ring‐down spectroscopy (CRDS) is a highly sensitive laser absorption method. It can be used for quantitative analysis of molecular species at the sub‐ppb level. The absorption cell (cavity) is sealed by two high‐reflective mirrors on each side, which results in an effective absorption path‐length of some kilometers. Our experiments for atmospheric gas analysis have been carried out so far with an Excimer pumped dye laser in the UV‐VIS and a CO overtone sideband laser in the wavelength region around 3 μm. Experiments with an all solid‐state difference frequency laser system will follow. In the UV‐VIS region, we measured trace gas molecules like SO2, NO2, and CH2O. In the mid‐infrared, around 3 μm, we measured hydrocarbons like CH4, C2H6, and C2H4 with a detection limit of less than 1 ppb. The noise equivalent absorption coefficients in the MIR are in the order of 1.7·10−9 cm−1. Due to the high data acquisition rate and the high sensitivity, CRDS enables real‐time detection of trace gases in ambient air.

Photoacoustic monitoring of trace gases by use of a diode-based difference frequency laser source

We present a compact mid-infrared laser spectrometer for trace-gas monitoring. Difference frequency generation in periodically poled LiNbO(3) is used as laser source, yielding a tuning range 3.2-3.7mum at a linewidth of 154 MHz. The relatively high average power of 3 to 5 mW favors detection with a small resonant photoacoustic gas cell. Measurements of methane yield a detection limit in the low parts in 10(6) by volume concentration range.

Light modulation at molecular frequencies

It is well known that a carrier and two sidebands, depending on their relative phases, correspond to a frequency ~FM! or amplitude modulated ~AM! signal. Light is most often modulated by driving the dielectric constant of a material so as to produce phased sidebands with a frequency spacing of, at most, 150 GHz. In this Rapid Communication, we describe AM and FM light with a sideband spacing, and therefore modulation frequency, equal to the fundamental vibrational frequency of molecular deuterium (2994 cm '90 THz). We believe that this is a first step toward the synthesis of subfemtosecond pulses with prescribed temporal shape. As a light source for this experiment, we use a recently developed collinear Raman generator @1,2#. This light source produces a comb of ~at present! 17 sidebands spaced by about 2994 cm and extending from 2.94 mm in the infrared to 195 nm in the vacuum ultraviolet. The light source is based on the off-resonance adiabatic excitation of a Raman mode with a coherence that is sufficiently large that the importance of phase matching is negated and generation occurs collinearly. Although it was expected that sidebands obtained from such a source are mutually coherent, until now it was not clear if the relative phase among the sidebands remained the same across their temporal and spatial profiles. It is this uniform mutual coherence among the sidebands that allows the synthesis of AM and FM light, and in the future may lead to the generation of subfemtosecond pulses. In this work we separate out and use only three sidebands ~1.06 mm, 807 nm, and 650 nm! from the Raman comb. Frequencyor amplitude-modulated light is obtained by manually adjusting the relative phases of these sidebands. To detect this modulation, we make use of the fact that a Raman transition is driven by the near resonance component of the temporal envelope of the applied light intensity. These sidebands may be phased so that this component is either zero ~FM!, or maximized ~AM!. We note that it is essential that three sidebands be used in an experiment of this type. If two sidebands are used, as in coherent anti-Stokes Raman spectroscopy @3#, then the magnitude of the envelope is independent of phase. Our experimental setup is shown in Fig. 1. To construct the Raman generator, we use two transform-limited laser pulses at wavelengths of 1.0645 mm and 807.22 nm, such that the ~tunable! laser-frequency difference is approximately equal to the fundamental vibrational frequency in D2. The first laser is a Quanta-Ray GCR-290 Q-switched injectionseeded neodymium-doped yttrium aluminum garnet

Fine structure of the H2 5g–4f inter-Rydberg transition revealed by difference frequency laser spectroscopy

The spectrum of the 5g–4f inter-Rydberg band of H2 has been recorded with a difference frequency laser system and analyzed using multichannel quantum defect theory (MQDT). New transitions have been observed; in addition to the singlet–triplet splittings previously observed, the hyperfine structure of the ortho-hydrogen spectrum is partially resolved in the present experiment. MQDT is used to analyze the data in a two stage process. First, the ab initio MQDT predictions were refined by fitting the quantum defect functions over a range of internuclear separation R. Second, 4f singlet and triplet quantum defects are extracted from the para-hydrogen spectra, i.e., those lines without complicating hyperfine structure. This information was then used to calculate the fine structure of a sample ortho-hydrogen line, R3(2)v+=0. While the spectra are predominantly composed of absorption lines, some transitions from high vibrational levels of the 5g triplet manifold to 4f triplet levels are observed in stimulated emission.

Investigation on infrared laser absorption spectroscopy measurement of acetylene trace quantities

A laser-based infrared spectrometer was developed for use in high-resolution spectroscopic analysis of trace gases in the atmosphere. Continuous wave, broadly tunable coherent infrared radiation was generated from 8 to 19 μm in a gallium selenide crystal by laser difference-frequency mixing. Measurements of acetylene trace concentrations using laser absorption spectroscopy are reported. The measurements have been performed by using the P(21), Q(11), and R(9) lines of the ν 5 band, respectively, in order to investigate optimal detection conditions: a trade-off choice between higher line absorption strength for sensitive detection and better spectral discrimination from lines overlapping for open path trace-gas monitoring applications. Minimum detectable concentrations are compared under different experimental conditions. The ν 5 R(9) line seems to be close to optimum in terms of absorption strength and freedom from spectral interference for spectroscopic detection of acetylene trace concentration at atmospheric pressure.

Measurements of benzene concentration by difference-frequency laser absorption spectroscopy

Measurements of benzene concentration based on high-resolution laser absorption spectroscopy by use of the R(6) transition in the nu(4) fundamental vibrational band near 14.8 mum (676.6 cm(-1)) are reported. These measurements were performed with a tunable continuous-wave, mid-infrared spectroscopic light source that employs difference-frequency mixing of two Ti:sapphire lasers in a GaSe nonlinear optical crystal. A minimum benzene concentration detection of ~11.5 parts in 10(6) was realized at a reduced pressure of 40 mbars.

An approach for vibration measurement based on laser frequencysplitting technology

An approach for vibration measurement based on He-Ne laserfrequency splitting technology is presented and experiments have been performed.While a quartz crystal wedge, which is placed in the laser cavity, vibrateswith the object to be measured, the frequency difference of the laser ismodulated by the vibration signal. The vibration signal is restored bydemodulating the laser frequency difference signal. In the experiments a15 nm resolution and a ±1.6×10-4 reproducibility of thevibrational amplitude are obtained with the system. The frequency range ofvibration measurement is only up to the limit of mechanical structures. Themethod has a good potential prospect for application in high-resolution, lowor middle frequency vibration measurement.

Infrared absorption spectroscopy of D 3 : an investigation into the formation mechanisms of triatomic hydrogenic species

Infrared absorption spectra of D 3 are observed with a difference frequency laser system in the frequency ranges around 3600 cm -1 (3s 2 A ′ 1 ← 3p 2 E ′) and 3900 cm -1 (3d← 3p 2 E ′). The observed line shapes exhibit a non–Maxwellian velocity distribution, and the translational energy is derived to be ca .0.4 eV. From this finding, it is concluded that D 3 is formed through the dissociative recombination reaction D 5 + with electrons. The rotational dependence of the line shapes of the 3600 cm -1 band is brought about by a competition between the predissociation in the 3s 2 A ′ 1 state and the radiative decay in the 3p 2 E ′ state. The shorter lifetimes of the 3d complex make the line shape of the 3900 cm -1 band simpler, a superposition of two absorption profiles with different widths. The greater widths of the absorption lines of the 3900 cm -1 band are attributed to unresolved spin–splittings.

Submillimeter‐Wave Measurements and Analysis of the Ground and ν 2 = 1 States of Water

In order to facilitate further studies of water in the interstellar medium, the envelopes of late-type stars, jets, and shocked regions, the frequencies of 17 newly measured H_2 ^(16)O transitions between 0.841 and 1.575 THz are reported. A complete update of the available water line frequencies and a detailed calculation of unmeasured rotational transitions and transition intensities as a function of temperature are presented for the ground and ν_2 = 1 state levels below 3000 cm^(-1) of excitation energy. The new terahertz transitions were measured with a recently developed laser difference frequency spectrometer. Six of these transitions arise from the ν_2 = 1 state, and the other 11 are in the ground state; all have lower state energies from 700 to 1750 cm^(-1) and should be accessible to Stratospheric Observatory For Infrared Astronomy (SOFIA) through the atmosphere. The transitions near 0.850 THz are accessible from the ground with existing receivers. Observations of the newly measured ν_2 = 1 state transitions, which include the 1_(1, 1)-0_(0, 0) fundamental at 1.2057 THz and five other very low J transitions, should provide valuable insights into role played by the ν2 = 1 state in the cooling dynamics of jets, shocks, masers, and strongly infrared-pumped regions. The line list is presented to assist in the planning of observational campaigns with the Far-Infrared Space Telescope (FIRST) and other proposed space missions with which a full suite of water observations can be carried out.

The Intensities of Methane in the 3–5 μm Region Revisited

The analysis of the linestrengths of the infrared spectrum of methane (12 and 13) in the 3–5 μm region has been revisited on the basis of new measurements from Fourier transform spectra recorded at Kitt Peak under various optical densities. A simultaneous fit of these new data with previously reported tunable difference-frequency laser data has been done. An effective transition moment model in tensorial form up to the third order of approximation within the Pentad scheme has been used. The standard deviations achieved are very close to the experimental precision: 3 and 1.5%, respectively, for the two sets of data for the 12CH4 molecule, representing a substantial improvement with respect to earlier studies. The integrated bandstrengths obtained in the present work differ from previously reported values by factors ranging from −5 to +6%. The correction for the ν3 band, the strongest band of the Pentad system, is +2% with respect to the study of Hilico et al. [J. C. Hilico, J. P. Champion, S. Toumi, V. G. Tyuterev, and S. A. Tashkun, J. Mol. Spectrosc. 168, 455–476 (1994)].

Pressure Lineshift and Broadening Coefficients in the 2ν3 Band of 16O12C32S

In this study we report the first measurements of the pressure-induced lineshift coefficients due to Ar, He, O2, and N2 for 22 rovibrational lines from P(53) to R(53), belonging to the 2ν3 band of 16O12C32S at 4100 cm−1. The lineshift results were obtained from the simultaneous record of the pressure-broadened and pure low-pressure OCS lines, using a tunable difference-frequency laser spectrometer. For four lines of the 2ν3 band we also report Ar-, He-, O2-, and N2-broadening coefficients by fitting Voigt and Rautian profiles to the measured shapes of these lines. The broadening and shift coefficients are compared to the results of theoretical calculations based on the semiclassical Robert–Bonamy formalism and two different isotropic and anisotropic intermolecular potentials. For OCS–Ar we also consider the Smith–Giraud–Cooper model including all orders of the interaction within the peaking approximation. In all cases, the calculated broadening coefficients are in reasonable agreement with the experimental data. By considering adjustable parameters for the vibrational dependence of the isotropic potential, the general trends of the lineshifts with J can be roughly predicted, except at low J values where an asymmetry behavior for P and R branches is generally observed.

Speed-dependent broadening and line mixing in CH 4 perturbed by Ar and N 2 from multispectrum fits

Speed-dependent broadenings and shifts have been determined for the P and R branches of the ν 3 band of CH 4 perturbed by Ar and N 2 using a multispectrum fitting analysis of high-resolution tunable difference-frequency laser spectra recorded at pressures ≤67 kPa. For J≥3, the tetrahedral fine structure components in each J manifold are collisionally coupled and exhibit significant interference. The coupled lines are treated using a speed-dependent first-order line-mixing profile and are compared to a speed-independent full relaxation matrix inversion procedure with off-diagonal coupling elements calculated from an atom–atom Lennard–Jones potential model.

Lineshifts in the Fundamental Band of CO: Confirmation of Experimental Results for N2 and Comparison with Theory

We have used a three-channel version of a tunable difference frequency laser spectrometer to measure the collisionally induced lineshifts at room temperature for 26 lines of the fundamental band of CO perturbed by nitrogen. Each lineshift was obtained directly by comparing the line center positions of two simultaneous recordings, one for a pressure-shifted line, and the other for the same line in pure CO line at very low pressure. The experimental results are found to be in complete agreement with earlier measurements and confirm that shifts as small as 3 MHz may be measured in such a system. Our results are compared with theoretical calculations. The part of the shifting coefficient antisymmetric with respect to a change in sign of the line number m, is in disagreement with the calculations.

On-line multicomponent trace-gas analysis with a broadly tunable pulsed difference-frequency laser spectrometer

The design and application of a novel automated room-temperature laser spectrometer are reported. The compact instrument is based on difference-frequency generation in bulk LiNbO(3). The instrument employs a tunable cw external-cavity diode laser (795-825 nm) and a pulsed diode-pumped Nd:YAG laser (1064 nm). The generated mid-IR nanosecond pulses of 50-microW peak power and 6.5-kHz repetition rate, continuously tunable from 3.16 to 3.67 microm, are coupled into a 36-m multipass cell for spectroscopic studies. On-line measurements of methane are performed at concentrations between 200 ppb (parts in 10(9) by mole fraction) and approximately 1%, demonstrating a large dynamic range of 7 orders of magnitude. Furthermore computer-controlled multicomponent analysis of a mixture containing five trace gases and water vapor with an overall response time of 90 s at an averaging time of only approximately 30 s is reported. A minimum detectable absorption coefficient of 1.1 x 10(-7) cm(-1) has been achieved in an averaging time of 60 s, enabling detection limits in the ppb range for many important trace gases, such as CH(4), C(2)H(6), H(2)CO, NO(2), N(2)O, HCl, HBr, CO, and OCS.

Infrared Spectroscopy of H3O+: The ν1 Fundamental Band

The infrared spectrum of H3O+ in positive column discharges of H2/O2 gas mixtures has been studied by a difference frequency laser spectrometer. The ν1 fundamental band of H3O+ was identified in the region of the strong . Molecular constants were obtained by the least-squares fitting of the observed frequencies, and band origins of the and subbands were determined to be 3389.656(2) and 3491.170(2) cm−1, respectively. During this study, assignment of the ν3 fundamental band was extended to higher J, K transitions, which do not fit to the calculated pattern well, but have definitely been assigned by using the ground state combination differences and relative intensities. Vibration–rotation interactions between the ν1 and ν3 states have been considered, which explained some large discrepancies between observed and calculated frequencies and led to the identification of forbidden transitions. Energy differences in the ground state between the ΔK = 3 rotational levels were obtained from the combination differences of the forbidden and allowed transitions, which led to an accurate determination of C and DK. Equilibrium structure of H3O+ has been derived to be re = 0.974(1) Å and αe = 113.6(1)°.

Compact gas sensor using a pulsed difference-frequency laser spectrometer

We present a novel compact pulsed laser spectrometer based on difference-frequency mixing of a cw tunable external-cavity diode laser (795-825 nm) and a pulsed Nd:YAG laser (1064 nm) in bulk LiNbO(3) . The pulsed mid-IR source is continuously tunable from 3.16 to 3.67microm and exhibits a linewidth of only 154 MHz, a peak power of approximately 50microW , and a pulse duration of 6 ns at a 6.5-kHz repetition rate. Spectra of methane in room air and formaldehyde have been recorded at room-temperature operation in a multipass cell with deduced detection limits of 10 and 40 parts in 10(9) , respectively.

Line-mixing effects in He- and N2-broadened Σ↔Π infrared Q branches of N2O

Two Q branches of N2O near 579.3 and 2798 cm−1 belonging to the 2ν20e−ν21f and ν2+ν3 bands, respectively, of Σ←Π and Π←Σ symmetry, have been studied for He and N2 perturbers at pressures ranging from 0.1 to 2 atm, using a tunable diode laser and a difference-frequency laser spectrometer. To interpret the line-mixing effects in these spectra, we have applied a model based on the energy corrected sudden approximation whose parameters have been only derived from line-broadening data for N2O–He and also from the measured absorption by the Q branches for N2O–N2. This model provides a satisfactory agreement with experimental band shapes, whatever the band, the perturber and the pressure considered. Significantly larger line-mixing effects are shown for N2O–He with respect to N2O–N2. Finally, the assumption made in the calculations to treat separately the couplings in the even and odd j levels appears to have a negligible influence on the resulting band shapes.

Difference-frequency laser spectroscopy detection of acetylene trace constituent

sbandpectrum. Spectroscopic detection of 28 ppmm/Hz1/2 was performed. This corresponds to a minimum detectable mixing ratio of ∼1 ppb in conjunction with a 100-m White cell and balanced detection.

Compact laser difference-frequency spectrometer for multicomponent trace gas detection

O), carbon dioxide (CO2), and sulfur dioxide (SO2). Real-time detection of CO, N2O, and CO2 was performed in open air over a path length of 5 to 18 m. The feasibility of DFG spectroscopic measurement of the 13C/12C and 18O/17O/16O isotopic ratios in atmospheric carbon dioxide was also investigated. We report what to our knowledge is the first simultaneous spectroscopic measurement of all three isotopes of oxygen in ambient CO2.

Optical parametric oscillator based difference frequency laser source for photoacoustic trace gas spectroscopy in the 3 μm mid-IR range

We report on the extension of the emission wavelength range of a nanosecond pulsed commercial optical parametric oscillator (OPO) towards the infrared. By difference frequency mixing the idler of the OPO with the fundamental of its Nd:YAG pump laser the wavelength range from 2.5–4.5 μm is covered at considerable pulse energies. This laser source is then applied for the first time to photoacoustic trace gas spectroscopy in the fundamental CH stretch band located around 3.3 μm. Spectra of single- and multi-component gas mixtures of volatile organic compounds are recorded and analyzed with respect to composition and concentration. The detection limits for the individual substances within the gas mixtures are found in the lower parts per million (ppm) range.