Difference Frequency Laser System Research Articles

Trace-gas sensing in the 3.3-μm region using a diode-based difference-frequency laser photoacoustic system

A compact, diode-based difference-frequency laser system combined with a photoacoustic detection scheme is presented for trace-gas sensing. It features a broad, continuous tuning range (3.2–3.7 μm), a narrow line width (154 MHz), and room-temperature operation, and thus allows numerous gas species to be measured both isolated and in mixtures of different gases. Several trace-gas species of environmental interest were detected, and gas mixtures were analysed. The detection limits are in the low-ppmV range, e.g. 1.3 ppmV for methane, 1.8 ppmV for ethane, and 1.2 ppmV for hydrogen chloride.

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.

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.

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.

Difference frequency laser spectroscopy of the ν1 fundamental band of CH 3CNH +

The first spectroscopic identification of CH 3CNH + was made in a hollow cathode discharge of CH 3CN (∼ 10 mTorr) and H 2 (∼ 500 mTorr) by observing the ν 1 (NH stretch) fundamental band ( ν 0 = 3527.2887 cm −1) with a difference frequency laser system. The band was found to be perturbed and some of the K structure was not resolved. The spectroscopic constants were determined through a least-squares fit of the unperturbed lines. The nature of the perturbation was elucidated in part in an analysis of the perturbed K = 1 lines.

The observation of the ν1 fundamental and ν1 + ν7 − ν7 hot bands of HNCCN +

The ν 1 (NH stretch) fundamental and ν 1 + ν 7 − ν 7 hot band of HNCCN + have been observed with the difference-frequency laser system in the hollow cathode discharge. The ion was observed in a dry-ice cooled discharge of (CN) 2 and H 2, modulated at 10 kHz. The constants obtained for the fundamental were ν 0 = 3448.2701(1) cm −1, B″ = 4438.22(8) MHz, D″ = 602(25) Hz, B′ = 4430.37(9) MHz, and D′ = 598(26) Hz. For the hot band we obtained ν 0 = 3448.2281(1) cm −1, B″ = 4456.22(9) MHz, D″ = 739(61) Hz, B′ = 4448.30(10) MHz, and D′ = 741(66) Hz. The values for the fundamental band agree extremely well with the theory.

A study of the structure and dynamics of the hydronium ion by high resolution infrared laser spectroscopy. III. The ν3 band of D3O+

294 transitions were measured by velocity modulation spectroscopy using a difference frequency laser system with a D2/O2 discharge and assigned to the ν3 (antisymmetric stretch) band of D3O+. A simultaneous analysis of this data with the ν2 band spectra reported by Sears, et al. [J. Chem. Phys. 83, 2676 (1985)] yielded a refined set of parameters for both the ground and the excited states of both bands. The ν3 bandcenters are 2629.6512(39) and 2624.2376(47) cm−1 for the s–s and a–a inversion components, respectively.

The observation of the ν1 fundamental bands of HOSi+ and DOSi+

The ν1 (H–O) stretches of HOSi+ and DOSi+ have been observed with the difference frequency laser system in the hollow cathode discharge. The ions were observed either by discharge of (CH3)3SiOH or (CH3)3SiOD in a buffer containing H2 (or D2) and He, or by discharging SiH4 and N2O in a buffer of H2 or H2 and He. Discharging SiH4 and H2O in buffers of H2 and H2 and He was not successful in producing HOSi+. The constants obtained were: ν0=3662.364 67(15) cm−1, B000=18 260.75(22) MHz, D000=20.44(31) kHz, B100=18 195.05(23) MHz, and D100=19.91(34) kHz for HOSi+. For DOSi+ we obtained: ν0=2716.558 13 (12) cm−1, B000=16 231.05(24) MHz, D000=15.69(38) kHz, B100=16 154.41(22) MHz, and D100=15.60(33) kHz. These values agree well with the theory. The combination of the two B0 values results in r0(H–O)=0.940 Å and r0(O–Si) =1.537 Å. The differences between these values and the calculated equilibrium values are similar to the corresponding results for HOC+.

Difference frequency laser spectroscopy of SH3+: A simultaneous analysis of the ν1 and ν3 fundamental bands

The ν1 and ν3 fundamental bands of SH3+ have been observed in absorption by using a difference frequency laser system in a hollow cathode discharge cell. Observed spectra show complications due to vibration-rotation interactions between the v1 = 1 and the v3 = 1 states. The molecular constants are determined by a least-squares analysis. Observations of “forbidden” transitions enhanced by higher-order vibration-rotation interactions allow us to determine the rotational constants about the molecular axis. The major molecular constants determined are (in cm−1) Ground statev1=1 statev3=1 stateν02521.04816(40)2525.74710(32)B4.89464187(83)4.843738(25)4.845719(18)C4.228258(36)4.188702(42)4.207978(31)(Cζ)30.092352(66)q3(0)-0.010910(41)Z13(0)-0.01018(55)

Diode laser spectroscopy of the ν4 band of benzene

Sections of the ν 4 band of benzene near 674 cm −1 have been recorded under conditions of Doppler-limited resolution using a diode laser spectrometer. In contrast to earlier measurements on this parallel band both the J and the K structures were resolved and this has enabled a number of band parameters to be determined accurately for the first time. The set of ground state constants determined from these data are compared with those determined by J. Pliva and A. S. Pine ( J. Mol. Spectrosc. 93, 209–236 (1981)) using a difference frequency laser system.

Infrared spectroscopy of carbo-ions. III. ν3 band of methyl cation CH+3

The infrared spectrum of the degenerate ν3 vibration–rotation band of methyl cation CH+3, one of the most fundamental carbo-ions, has been observed and analyzed. The spectrum was observed in the frequency range of 3240–2960 cm−1 using the difference frequency laser system as the tunable coherent infrared source. A gas mixture of He:H2:CH4∼700:20:1 with a total pressure of ∼7 Torr was used for the ac discharge in an air-cooled and a water-cooled multiple inlet–outlet discharge tube. The velocity modulation method together with noise subtraction and the unidirectional multiple passing of the beam was used for high sensitivity. The spectral lines of CH+3 appeared amid many other lines of C2H+2 and C2H+3 but their widths and their response to chemical conditions gave a good initial clue for the identification. Over 200 absorption lines have been assigned to CH+3 and the isotopic species 13CH+3. The spectral pattern clearly shows that the equilibrium molecular structure is a planar, equilateral triangle as expected from Herzberg’s Rydberg spectrum of CH3 and from ab initio theory. The spectrum has been analyzed by using the symmetric rotor Hamiltonian in a degenerate vibrational state with various A1–A2 splittings and l resonance. The spectral lines have been least-squares fit with a standard deviation of 0.0058 cm−1, and led to 21 vibration–rotation constants of CH+3. Several analytical expressions have been derived among these constants to determine the structure and potential constants. Some perturbation has been noticed and discussed. Also quite a few lines which are definitely due to carbocations with one carbon atom have been left unassigned. The prominence of the CH+3 spectrum in hydrocarbon discharges confirms the importance of this species in the ion–molecule reaction scheme in the laboratory and in space which has already been noted by many people in the field. A simple discussion is given about the formation and destruction of CH+3 as well as other carbocations.

High-resolution infrared identification of HOCS+ with difference frequency laser spectroscopy

The v 1 fundamental band of HOCS+ has been observed in absorption in a hollow cathode discharge with a difference frequency laser system. The precise spectroscopic parameters have been determined through a least squares fit of more than 550 lines to an effective vibration-rotation hamiltonian. It has been concluded that the species is HOCS+, since the vibrational frequency (v 0 = 3435·1636 cm-1) is very likely to be the O-H stretch, and not an S-H or C-H stretch. Rotational transition frequencies for Ka ⩽ 2 are calculated to assist the search for the species in interstellar space and in the laboratory. An isomer, HSCO+, has been searched for in the range of 2450–2600 cm-1 (expected S-H stretch frequency region) without success.

Dicke narrowing and pressure broadening in the infrared fundamental band of HCl perturbed by Ar

Dicke narrowing due to state preserving but velocity changing collision has been observed in several high- J vibration-rotation transitions of the fundamental band of HCl with Ar as the collision partner. Because of the long wavelength of radiation and the small diffusion coefficient, the narrowing occurs at a relatively low pressure of ∼200 Torr. A difference frequency laser system was employed for the studies of linewidths and lineshapes over the temperature range of 297–420 K. The results were analyzed using the diffusion model.

The observation of the high-resolution infrared spectrum of SH +3

The v 3 fundamental band of SH + 3 has been observed in absorption in a hollow cathode discharge with a difference frequency laser system. The molecular constants of the ground state have been determined through a fit of the ground-state combination differences (from the observed transition frequencies). The band is perturbed and a preliminary analysis is made to obtain the molecular constants for the excited state. The lowest rotational transition frequency useful for astrophysical searches is predicted to be 293464(3) MHz.

Difference-frequency laser spectroscopy of the ν4 fundamental band of NH3D+

The high-resolution spectrum of the ν4 fundamental band of NH3D+ has been observed in absorption in a hollow-cathode discharge with a difference-frequency laser system. The molecular constants have been determined through a least squares fit of the observed transition wave numbers to an effective Hamiltonian. The equilibrium rotational constant is estimated to be 4.438 ± 0.027 cm−1, from which the equilibrium N—H bond length is calculated to be 1.021 ± 0.003 Å. Some low-lying rotational transition frequencies in the ground state are calculated to assist in the search for pure rotational transitions in the laboratory and in interstellar space.

Infrared spectrum of the fundamental vibration–rotation band of OD−

The fundamental v=1←0 vibration–rotation band of OD− has been observed using the tunable infrared radiation from a difference frequency laser system and the velocity modulation technique for detection. The band origin is determined to be 2625.332(3) cm−1. The rotational constant B and the centrifugal distortion constant D have been determined for both the ground state and the first excited state. A remarkable similarity between molecular constants of OD− and OD has been noticed and utilized to estimate equilibrium vibration–rotation constants. These vibration–rotation constants were used to estimate the equilibrium bond length and the quadratic, cubic, and quartic force constants.

Spectroscopic detection of protonated N 2O

The high-resolution infrared spectrum ofprotonated N 2O has been observed with a difference frequency laser system. Protonated N 2O is generated in a hollow cathode discharge through a mixture of N 2O (≈ 15 mTorr) and H 2, (≈ 250 mTorr). It is found to be a bent molecule similar to HN 3, but it is not clear from this spectroscopic observation which isomer, O-protonated or N-protonated, has been detected. The molecular constants have been obtained for the ground state. The band origin of the V 1 fundamental is 3330.91070 cm −1. Only the effective molecular constants have been given for the excited state due to a perturbation.

Difference frequency laser spectroscopy of HCNH +: Observation of the isotopic species and the hot bands

The infrared spectra of the isotopic species of HCNH +, HCND + and H 13CNH +, have been observed with a difference frequency laser system. The r s structure of the ion has been determined by combining the rotational constants of the normal species and DCNH + previously obtained. Two hot bands of the normal species, presumably ν 1 + ν 4 ← ν 4 and ν 1 + ν 5 ← ν 5, have also been observed and the spectroscopic constants have been determined.

Observation and analysis of the ν2 and ν3 fundamental bands of the H2D+ ion

The observation of the high-resolution absorption spectrum of the H2D+ molecular ion in the region 2010–2610 cm−1 in discharges through mixtures of H2 and D2 gases is reported. Two types of tunable monochromatic sources are employed, either a diode laser (in Ottawa) or a difference-frequency laser system (in Chicago), and the sensitivity is improved by using either discharge modulation or Doppler velocity modulation techniques. A total of 66 new lines of H2D+ have been measured and assigned, mostly on the basis of ground-state combination differences, to specific rotational transitions of the ν2 and ν3 bands. These data, as well as the two known microwave lines, are fitted by means of two theoretical models, either an effective Hamiltonian model including a Padé representation of a conventional A-reduced centrifugal Hamiltonian for each vibrational level together with Coriolis and higher rotational interactions between ν2 and ν3, or a supermatrix model in which the matrix of the untransformed Hamiltonian is set up in a large vibration-rotation basis and diagonalized directly. In the supermatrix model most of the vibrationally off-diagonal matrix elements are constrained to values derived from Carney’s ab initio calculations, while the ν1 parameters are fitted to the observed lines of Amano. Because of the large number of parameters required in the effective Hamiltonian, the less flexible supermatrix model was valuable as a check of the assignments. The results of these fits make it possible to assign seven of the nine lines reported by Shy, Farley, and Wing in 1981. The observed band origins, ν2=2205.87 cm−1 and ν3=2335.45 cm−1, as well as the rotational constants, are in good agreement with ab initio predictions. With the use of one calculated term value to relate the stacks of levels with even and odd values of K″a, a table of observed term values of the lower rotational levels of the ground state and the ν2 and ν3 states is constructed.

Pressure broadening, lineshapes, and intensity measurements in the 2 ← 0 band of NO

Lineshape and intensity measurements were made on the overtone band ( v = 2 ← 0) of nitric oxide (NO) using a tunable difference-frequency laser system. Self- and N 2-pressure broadening coefficients were obtained at 296 K, and a small amount of collisional, or Dicke, narrowing (which reduces the Doppler width by ∼9% at 50 Torr) was also evident. The pressure broadening observed for the 2 Π 3 2 transitions was larger than that of the 2 Π 1 2 for corresponding J by about 7%, so an empirical scaling law model was fit to the broadening coefficients to determine the role of interstate (spin-flipping) collisions. A small collision-induced asymmetry in the Λ-doublet transitions of the 2 Π 1 2 level was also observed. The measured line intensities yield an integrated band intensity of 1.878( −40 +90) cm −2 atm −1 at 296 K and a Herman-Wallis F factor which determines the relative signs of terms in the dipole moment expansion.