Tunable Laser Spectrometer Research Articles

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.

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.

Absolute Line Intensities in the 2ν3Band of16O12C32S

The strengths of 100 lines in the 2ν3band of16O12C32S have been measured at high resolution in the spectral range 4069–4118 cm−1, using a tunable difference-frequency laser spectrometer. These intensities were obtained by fitting Voigt profiles to the measured shapes of the lines. The vibrational transition moment [(2.141 ± 0.020) × 10−2D] and the absolute intensity (16.19 ± 0.24 cm−2atm−1at 296 K) of the 2ν3band of16O12C32S are determined from these linestrength measurements.

N 2 and Ar broadening and line mixing in the P and R branches of the v3 band of CH 4

N 2- and Ar-broadened spectra of the allowed P- and R-branch manifolds for J ⩽ 10 in the v 3 band of CH 4, have been recorded from the Doppler limit to ∼ 67 kPa at T = 295 K using a tunable difference-frequency laser spectrometer. The broadening coefficients exhibit larger variations among the tetrahedral components in the R-branch manifolds than in the P branch, which in turn are very similar to the Q branch previously measured [A.S. Pine, J. Chem. Phys. 97, 773 (1992)]. The broadenings for Ar are uniformly 88(1)% of those for N 2. Line intensities, pressure shifts and Dicke narrowing coefficients are also obtained. Strong line mixing among the blended tetrahedral components is observed, and an analysis is presented restricting line coupling among transitions with the same nuclear spin A, E or F symmetry with the sum of the mixing coefficients for a given symmetry in a J manifold constrained to zero.

Pressure-Induced Lineshifts in the 2 ← 0 Band of CO Self-Perturbed and Perturbed by He, Kr, O2, and N2

The pressure-induced shifting coefficients for self-, He-, Kr-, O2-, and N2-broadened lines of CO in the first overtone band have been measured using a tunable difference-frequency laser spectrometer. The lineshift was directly obtained from the simultaneous record of the pressure-broadened line and the same line of pure CO at low pressure. Comparisons are made with results of theoretical calculations based on a semiclassical model and different interaction potentials. It is shown that the lineshifts mainly originate from vibrational dephasing effects. Although the finer details of theJdependence of the shifts are not accurately reproduced, their general trends are well predicted through the calculations.

The infrared spectrum of the Ar-CO complex Comprehensive analysis including van der Waals stretching and bending states

The infrared spectrum of the weakly-bound Ar-CO van der Waals complex in the 2150 cm-1 region of the CO stretch has been studied using a continuous supersonic slit jet expansion coupled with a tunable laser spectrometer, and also with a cooled (57 K) long-path cell coupled with a Fourier transform spectrometer. Seven new subbands were observed, and previously observed bands were observed in greater detail. Four substates were thus identified for the first time, including the excited van der Waals stretching state, v 3 = 1, and the excited bend, v 2 = 1, within the ground CO stretching state, v CO = 0. A comprehensive simultaneous analysis was made of all the existing infrared and microwave data on the complex for levels with K < 5, in order to obtain an accurate and self-consistent set of substate origins, rotational parameters, and centrifugal distortion parameters. A strong Coriolis interaction between K = 1 of the excited bend and K = 0 of the excited stretch was analysed in detail.

Lineshape Parameters of He- and Kr-Broadened HF Lines in the Fundamental Band

The spectral lineshapes for the P(6), P(8), P(9), R(7), and R(8) transitions of the fundamental band of HF in the presence of He and Kr buffer gases have been measured using a tunable difference-frequency laser spectrometer. For the light He perturber, the lineshapes are closely fitted by the diffusional Galatry (soft collision) and Rautian (hard collision) profiles incorporating the Dicke collisional narrowing of the Doppler distribution. For the heavy Kr perturber, the observed lineshape asymmetries have been modeled by modified Rautian or Galatry profiles which account for statistical correlation between velocity- and state-changing collisions. The speed dependence of collisional cross sections has also been considered through a generalized Voigt profile. Finally, the broadening, shifting, Dicke narrowing, and asymmetry coefficients have been derived from the best lineshape models used.

Rovibrationally resolved, Fourier-transform near infrared spectroscopy of the ν1 and ν2 vibrations of the HCl dimer in a supersonic jet

We have recorded the high-resolution (0.007 cm−1) Fourier-transform near infrared absorption spectrum of the symmetric 35Cl–35Cl, and the mixed 35Cl–37Cl isotopomers of the hydrogen chloride dimer in a supersonic jet. The spectrum was recorded under a low effective rotational temperature (≊20 K). Rovibrational analysis of the observed spectra has been performed for the K″a=0 subbands of the ν+1 and ν−1 perpendicular-type bands centered at 2890 and 2879 cm−1, respectively. The K″a=0 and 1 parallel-type subbands of the ν−2 at 2839 cm−1 and ν+2 at 2857 cm−1 have also been analyzed. Furthermore, we have observed and investigated the (H 35Cl)2ν+1, K″a=1, ΔK=−1 rovibrational transition centered near 2869 cm−1. The results are compared with spectra previously recorded using difference frequency tunable laser spectrometers both in a supersonic slit jet and in an equilibrium gas phase mixture to demonstrate the advantages and limitations of the FTIR technique.

Line shape asymmetries in Ar-broadened HF(v=1–0) in the Dicke-narrowing regime

Collisional line shapes have been measured for the fundamental band transitions of HF in an Ar buffer gas at T=296 K using a tunable difference-frequency laser spectrometer. The broadening and shift coefficients are in excellent agreement with recent close-coupling scattering cross section calculations [Green and Hutson, J. Chem. Phys. 100, 891 (1994)] based on a realistic intermolecular potential surface determined by high-resolution infrared and microwave spectroscopy of the Ar–HF van der Waals complex. Below atmospheric pressure, the line shapes exhibit strong collisional (Dicke) narrowing of the Doppler distribution and a slight asymmetry which we model with hard collision (Rautian) or soft collision (Galatry) profiles modified for partial correlation between velocity- and state-changing collisions.

Pressure broadening of a sub-mm transition of NO using tunable far-infrared spectroscopy

The nitrogen broadening coefficient γ, of the J=8.5←7.5 (F 1F 1) pure rotational absorption transition of NO at 851913.2(1) MHz has been measured using a tunable far-infrared laser spectrometer. The pressure broadening data was analysed by two independent methods, firstly, a non-linear least-squares routine which fitted the complete profile to a Voigt lineshape and secondly, the Olivero and Longbothum [J. Quantum Spectry. Radiative Transfer 17 (1977) 233] analytical formula for the linewidth. Both methods were found to be in reasonable agreement and the average values for γ were 2.15(4) and 1.93(4) MHz Torr −1, respectively.

Tunable far infrared laser spectrometers

The state of the art in far infrared (FIR) spectroscopy is reviewed. The development of tunable, coherent FIR radiation sources is discussed. Applications of tunable FIR laser spectrometers for measurement of rotational spectra and dipole moments of molecular ions and free radicals, vibration-rotation-tunneling (VRT) spectra of weakly bound complexes, and vibration-rotation spectra of linear carbon clusters are presented. A detailed description of the Berkeley tunable FIR laser spectrometers is presented in the following article.

Rotational predissociation, vibrational mixing, and van der Waals intermolecular potentials of NeDF

The near-infrared spectrum of NeDF formed in a slit free jet expansion is recorded with a high resolution, tunable laser spectrometer. Four bands, consisting of the DF stretching fundamental and three internal rotation and van der Waals stretch combination bands, are observed and analyzed for both the 20Ne and 22Ne isotopomers. All three combination bands reveal a sudden onset of rotational predissociation at modest J, which is modeled with effective one-dimensional potentials to determine the binding energy D0=34.7±0.8 cm−1 for 20NeDF (v=0) and D0=35.1±0.8 cm−1 for 20 NeDF (v=1). The experimental results are compared with predictions of a recently published ab initio anisotropic potential surface, and an improved potential is developed and tested. This refined potential has an absolute minimum of −86 cm−1 in the linear Ne–D–F geometry, a secondary minimum at −55 cm−1 in the inverted linear Ne–F–D geometry, and an intervening saddle point at −39 cm−1 near the perpendicular geometry. The lowest bound state lies ≊4 cm−1 above the saddle so internal DF rotation is only slightly hindered in this complex.

Tunable far-IR laser spectroscopy of jet-cooled carbon clusters: the nu 2 bending vibration of C3.

Seven rovibrational transitions of the (01(1)0) <-- (00(0)0) fundamental bending band of C3 have been measured with high precision with the use of a tunable far-infrared laser spectrometer. The C3 molecules were produced by laser vaporization of a graphite rod and cooled in a supersonic expansion. The astrophysically important nu 2 fundamental frequency is determined to be 63.416529(40) cm-1. These measurements provide the basis for studies of C3 in the interstellar medium with far-infrared astronomy.

Difference frequency laser spectroscopy of the ν3 fundamental band of NH+2

The ν3 band of NH+2 in the X̃ 3B1 ground electronic state was observed in direct absorption with a tunable difference frequency laser spectrometer in the 3 μ region, using velocity modulation detection. NH+2 and NH+3 ions were generated in an ac discharge of He and NH3, or of He, N2, and H2. Fifty-three rovibrational transitions were measured and fit to a triplet A-reduced Hamiltonian to determine rotational, centrifugal distortion, and spin–rotation constants. The band origin was found to be ν0=3359.932 cm−1, in excellent agreement with a recent calculation of Jensen, Bunker, and McLean. Indirect evidence from the spectrum suggested that NH+2 is quasilinear, but selection rules prevented a determination of the A rotational constant.

Frequency tuning and data processing of a high resolution colour-centre laser spectrometer using modern microcomputer systems

A broadly tunable CW colour-centre laser spectrometer is described. The tuning mechanism and the problems for broad-band continuous single-mode tuning (about 50 cm-1 in two hours) are discussed. Calibrating and scanning of a laser is obtained by a 68000-VME-Bus microcomputer system (Eltec, Eurocom 3), while the frequency calibration and evaluation is done by an 80286 personal computer (Hewlett Packard, Vectra). Some structures for comfortable data processing of spectral files from continuously tunable lasers are presented. Measurements of frequency stability and the linearity of the tuning behaviour show that the spectrometer can be used routinely for Doppler-limited spectroscopy (minimum step width 10 MHz) of molecular transitions.

Supersonic molecular beam and static gas phase spectroscopy of intermolecular hot bands associated with ν 116O 12C--- 1H 19F

Gas phase rovibrational analysis of the ν 1 (H stretching vibration) and its associated hot bands ν 1+ν 3−ν 3, ν 1 + ν 5 1−ν 5 1 and ν 1 + 2ν 5, −2ν 5 are investigated using a broadband tunable color center laser spectrometer. Rotational and distortion constants associated with the low frequency intermolecular stretching (ν 3) and bending (ν 5 1) states are determined to be for ν 3: B=0.100786(28) cm −1, D itJ=3.36(30)×10 −7 cm −1; and for ν 5 1: B=0.1029736(93) cm −1, D J =3.598(41)×10 −7 cm −1 q=0.000235(15) cm −1. The precisely evaluated band origin frequencies permit determination of two anharmonic cross terms associated with the hydrogen bond hypersurface x 13 = 8.634(20) and x 15 = 5.4099(6) cm −1.

Gas-phase rovibrational analysis of v1 HCN---H 35Cl

A continuously tunable single-frequency color-center laser spectrometer has been used to rovibrationally analyze the static gas-phase spectrum of the v 1 (C-H stretching) vibration in HCN---H 35Cl providing molecular parameters: v 1 = 3309.0286(3) cm −1, B 1 = 2007.37(36) MHz and D j 1 = 3.51(13) × 10 −3 MHz and a lower limit of 1 × 10 −9 s to the v 1 3 excited state lifetimes. Possible assignments for the hot band progressions ▪ and ▪ are also discussed.

Color center laser spectroscopy of ν2 HCN---HF

A continuously tunable single frequency color center laser spectrometer has been applied to analyze the rovibrational gas phase spectra of the fundamental ν2 (C–H stretching vibration) and its hot bands ν2+ν4−ν4 and ν2+ν17−ν17 in the linear dimer HCN---HF. The l-type doubling constants q″v and qv of the Π←Π band are both determined to be 12.4(8) MHz and anharmonic cross terms X024 and X027 are evaluated as −0.1656(17) and +0.1310(35), respectively from the available data. Excited state amplitude lifetimes in the ν2 band are demonstrated to be 1.3(4)×10−8 s.

Spectral intensities in the fundamental bands of HF and HCl

The transition intensities of the fundamental bands of natural isotopic HF and HCl vapors have been measured with Doppler-limited resolution using a tunable difference-frequency laser spectrometer. Precise values for the band intensities, vibrational moments and Herman-Wallis F factors have been obtained for H 19F, H 35Cl and H 37Cl.