Carlos E. Manzanares

Low temperature cell for cavity ring down absorption studies

Phase shift cavity ring down is a technique that due to its long optical path length is an ideal method to detect weak absorptions. Coupling the method to a custom fitted cryostat allows gas phase molecules to be studied at cryogenic temperatures in a thermally isolated vacuum chamber. A novel design is described to construct the complete instrument. With optical cavities of length 10⩽l⩽43cm, optical path lengths between 200m and 6km have been achieved. High vibrational overtones C–H (Δυ=5) are measured at 130K (methane), 150K (ethylene), and 155K (ethane). Oscillator strengths of each molecule calculated at different temperatures are in excellent agreement. The experimental setup can be used to study kinetics and spectroscopy of atmospheric molecules, planetary atmospheres, and molecular complexes in the gas phase. Low temperatures can be obtained using liquid He or liquid N2 as cryogens.

Vibrational spectroscopy of CH bonds of methane and tetramethylsilane in liquid argon solutions

Abstract The liquid phase spectra of the fundamental and first overtone (Δ v = 1, 2) around the CH stretch of methane and tetramethylsilane in liquid argon solutions have been measured using Fourier transform IR and near-IR techniques. The fourth overtone (Δ v = 5) of the CH stretch has been measured in methane pure at 116 K and in liquid argon solutions at 100 K using laser excitation with acoustic detection. The results indicate that absorption bands, which normally present extensive inhomogeneous congestion, could be simplified by studying the absorptions in diluted solutions at low temperatures with inert liquefied gases as solvents.

Cavity ring down absorption at low temperatures: C–H spectra (Δυ = 1–6) of CH3D and C–H overtones (Δυ = 5, 6) of CH2D2 and CH4

The spectra of the C–H stretch fundamental and overtones (Δυ = 1–6) of CH3D have been recorded. Absorptions in the visible were obtained with a phase shift cavity ring down technique where an optical cavity is inside a low temperature cryostat. Absorptions below 12,000 cm−1 were observed with a Fourier transform spectrophotometer. The local mode harmonic frequency and anharmonicity were obtained and used with the harmonically coupled anharmonic oscillator (HCAO) model to calculate energy levels and assign the absorption bands to particular transitions. Overtone absorptions (Δυ = 5 and 6) of CH4 and CH2D2 have also been obtained. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the band strength and the cross- section of the Δυ = 5 and 6 C–H transitions for each molecule. Cross–sections for CH4 agree within 10% with traditional absorption measurements with a multiple pass cell at high pressures. The importance of the new experimental technique is emphasized for laboratory studies of planetary atmospheres.

Overtone spectroscopy and thermal lens detection limit of methane in cryo-solutions

The overtone spectra of methane solutions from 5000 cm−1 to 17000 cm−1 in liquid nitrogen and liquid argon are presented. The region between 5000 and 15000 cm−1 has been measured using a Fourier transform spectrophotometer with methane concentrations between 0.41% and 37% in mole fraction. The thermal lens technique (TLS) was used to measure the fifth overtone, not observed with the Fourier transform spectrophotometer. For a fixed concentration, maximization of the TLS signal with low modulation frequency and high laser power of excitation is shown. Time-dependent signal profiles of single-beam thermal lens experiments are analysed with the parabolic and aberrant lens models. Fitting parameters are obtained from the time-dependent signals. The absorption coefficient of methane was calculated from the fitting parameters and thermo-optical properties of the solvent. The calculated absorption coefficient of the Δυ = 6 transition of methane is 1.7 × 10−3 cm−1, which is in good agreement with literature values. A limit of detection is calculated with measurements of the fifth overtone absorption of methane in liquid nitrogen for solutions of concentration from 0.012% to 0.36% in mole fraction. Calculated limits of detection are 54 ppm and 15 ppm according to two different definitions of limit of detection for the thermal lens technique.

Vibrational overtone spectroscopy of saturated hydrocarbons dissolved in liquefied Ar, Kr, Xe, and N2.

This article presents a collection of vibrational overtone spectra of hydrocarbons in cryogenic solutions. Vibrational overtone spectra of ethane and propane dissolved in liquid argon and n-butane and isobutane dissolved in liquid krypton were recorded between 5000 and 14,000 cm(-1). Spectral regions for the first four overtones were measured using a Fourier transform spectrophotometer. The fifth overtone (Deltaupsilon = 6) spectra were recorded with a double beam (pump-probe) thermal lens technique using concentrations as low as 10-3 mole fraction. We obtained the C-H (Deltaupsilon = 6) spectra of (a) liquid ethane at 100 K and ethane in solutions in liquid Ar at 92 K and liquid N2 at 85 K, (b) liquid propane at 148 K and propane in liquid Ar at 93 K, (c) n-butane in liquid Kr at 129 K, (d) n-pentane in liquid Xe at 160 K, and (e) isobutane liquid at 135 K and isobutane in liquid Kr at 130 K. Local-mode parameters were calculated for primary, secondary, and tertiary C-H oscillators in solution and compared with gas-phase local-mode parameters. The peak frequency shift (Deltaomega) from gas phase to solution is explained by the change in harmonic frequency and anharmonicity in solution with respect to the gas-phase values. The bandwidth (Deltaomega1/2) of the (Deltaupsilon = 6) C-H absorption bands of ethane in solution can be explained in terms of collisions with the solvent molecules.

Vibrational spectroscopy of nonequivalent CH bonds in liquid cis- and trans-3-hexene

Abstract The liquid phase vibrational fundamental and overtone spectra (Δν = 1–4) of nonequivalent CH bonds of cis -3-hexene and trans -3-hexene have been recorded using FTIR and near-IR spectroscopy. A technique which uses a piezoelectric detector, lock-in amplification, and a continuous wave dye laser modulated at 80 kHz with an acousto-optic modulator, has been used to detect the fourth (Δν = 5) and fifth (Δν = 6) overtones. The overtone bands have been computer deconvoluted to obtain information with respect to peak position and bandwidth of the overtone absorptions. Transitions corresponding to different CH bonds, olefinic (CH), primary (CH 3 ) and secondary (CH 2 ) are assigned using the local mode model. Other bands are assigned as local mode—normal mode combination bands. Local mode frequencies ω i and anharmonicities X ii are obtained for all the nonequivalent CH bonds. The results obtained for cis -3-hexene are used to interpret the Δν = 3 and 4 CH overtone spectra of cis -1,4-polybutadiene.

Vibrational Overtone Spectroscopy, Energy Levels, and Intensities of (CH3)3C—C≡C—H

The vibrational overtone spectra of the acetylenic (Δυ = 4, 5) and methyl (Δυ = 5, 6) C-H stretch transitions of tert-butyl acetylene [(CH(3))(3)C-C≡C-H] were obtained using the phase shift cavity ring down (PS-CRD) technique at 295 K. The C-H stretch fundamental and overtone absorptions of the acetylenic (Δυ = 2 and 3) and methyl (Δυ = 2-4) C-H bonds have been obtained using a Fourier transform infrared and near-infrared spectrophotometer. Harmonic frequency ω(ν(1)) and anharmonicities x(ν(1)) and x(ν(1), ν(24)) are reported for the acetylenic C-H bond. Molecular orbital calculations of geometry and vibrational frequencies were performed. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. Band strength values were obtained experimentally and compared with intensities calculated in terms of the HCAO model where only the C-H modes are considered. No adjustable parameters were used to get order of magnitude agreement with experimental intensities for all pure local mode C-H transitions.

Cis- and trans-3-hexene: infrared spectrum in liquid argon solution, ab initio calculations of equilibrium geometry, normal coordinate analysis, and vibrational assignments

Abstract The infrared spectra of cis -3-hexene and trans -3-hexene dissolved in liquid argon have been obtained at temperatures from 93 to 120 K. The absorptions were observed with a low-temperature cell and a Fourier transform infrared spectrophotometer. Ab initio molecular orbital calculations were performed to obtain the equilibrium geometry, vibrational frequencies, force fields, and infrared intensities. The calculations were done at the Hartree-Fock level using 6-31G basis set. The Cartesian force fields from ab initio calculations have been converted to the force field in symmetry coordinates. The scale factors of ab initio calculated force fields were determined. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. Vibrational normal modes calculated for the lowest energy rotamers of cis - and trans -3-hexene have been assigned to infrared absorption bands observed in liquid argon solution. The assignments were based on calculated frequencies and potential energy distributions. The equilibrium geometries of the two lowest energy rotamers (symmetry C 2 and C s ) of cis -3-hexene and of the three lowest energy rotamers (symmetry C i , C 2 , and C 1 ) of trans -3-hexene were calculated. Variable temperature studies of the infrared spectrum of cis - and trans -3-hexenes dissolved in liquid argon were done to obtain the ΔH of conversion between the rotamers C 2 and C s of cis -3-hexene and between the rotamers C i , C 2 , and C 1 of trans -3-hexene.

Cavity ring down and Fourier transform infrared spectroscopy at low temperatures (84-297 K): Fermi resonance and intensities of the C-H fundamental and overtone (Delta(upsilon) = 1-6) transitions of CHD(3).

The C-H stretch fundamental and overtone absorptions of CHD(3) have been obtained using Fourier transform infrared (FTIR), near-infrared, and phase shift and pulsed cavity ring down (CRD) techniques at temperatures between 84 and 297 K. The partially resolved rotational-vibrational spectra of CHD(3) that included the fundamental transition nu(1), the overtone transitions, and combination bands 5nu(1) and 4nu(1) + 2nu(5) and 6nu(1) and 5nu(1) + 2nu(5) were obtained and compared with the simulated spectra at the corresponding temperature. The strength of the Fermi resonance between levels upsilonnu(1) and (upsilon - 1)nu(1) + 2nu(5) with upsilon = 2-6 was calculated using the experimental perturbed energies and relative peak intensities. The integrated absorption was calculated as a function of the density of the gas samples and used to obtain the cross section and the band strength of the Deltaupsilon = 5 and 6 transitions. Ab initio molecular orbital calculations were done to obtain the dipole moment function (DMF) as a function of the C-H internuclear distance. Intensity calculations with the DMF correctly predict the order of magnitude of the experimental band strengths.

Phase shift cavity ring down and Fourier transform infrared measurements of C–H vibrational transitions, energy levels, and intensities of (CH3)3Si–C≡C–H

Phase shift cavity ring down and Fourier transform IR techniques have been used to observe the C-H stretch fundamental and overtone absorptions of the acetylenic (Δυ = 1-5) and methyl (Δυ = 1-6) C-H bonds of trimethyl-silyl-acetylene [(CH3)3CSi≡CH] at 295 K. Harmonic frequencies ω(ν1), ω(a), and ω(s) and anharmonicities x(ν1), ω(a)x(a), ω(s)x(s) were calculated for the acetylenic, methyl out-of-plane, and methyl in-plane C-H bonds, respectively. The harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. A hot band, assigned as υν1 + ν24 - ν24 is observed for transitions with Δυ = 1-5 in a region near the acetylenic stretch. The intensity of the hot band is reduced considerably at 240 K. The strength of a Fermi resonance between C-Ha transition (υν(a)) and the combination band ((υ-1)ν(a) + 2ν(bend)) with (υ = 3-6) was calculated using the experimental perturbed energies and relative intensities. The main bands are separated by computer deconvolution and are integrated at each level to get the experimental band strengths. For methyl absorptions, the dipole moment function is expanded as a function of two C-H stretching coordinates and the intensities are calculated in terms of the HCAO model where only the C-H modes are considered. Acetylenic intensities are derived with a one dimensional dipole moment function. The expansion coefficients are obtained from molecular orbital calculations. The intensities are calculated without using adjustable parameters and they are of the same order of magnitude of the experimental intensities for all C-H transitions.