Pressure Shift Coefficients Research Articles

Determination of self- and H 2-broadening and shift coefficients in the 2-0 band of [formula omitted] using a multispectrum fitting procedure

Self- and hydrogen-broadening coefficients and pressure-shift coefficients for the first overtone band transitions of 12 C 16 O at room temperature have been determined through analysis of nine high-resolution (0.0055 cm −1) absorption spectra. These spectra were recorded using the 1-m Fourier transform spectrometer (FTS) at the McMath-Pierce facility of the National Solar Observatory on Kitt Peak, Arizona. Because of the short path length of the sample cell (10.0 cm) , the volume mixing ratios of CO in hydrogen were relatively high, ∼18–22%, to achieve measurable absorption. These large volume-mixing ratios necessitated the simultaneous determination of the CO self-broadening and self-shift coefficients along with the hydrogen-broadening and hydrogen-induced shift coefficients. We have determined these coefficients at room temperature, along with line positions and absolute intensities, for the P(27) through R(27) 12 C 16 O 2-0 transitions by fitting the entire spectral interval from 4130 to 4345 cm −1 in all nine spectra simultaneously using our multispectrum nonlinear least-squares procedure. Our intensity measurements are consistently low (1–6%) compared with the HITRAN values but the majority of the intensities are within 4% of the HITRAN values. The values of self-broadening coefficients vary from 0.0452 to 0.0862 cm −1 atm −1 at 296 K and those of hydrogen-broadening coefficient range between 0.0475 and 0.0795 cm −1 atm −1 at 296 K . All of our measured self- and hydrogen-shift coefficients are negative and range from −0.002 to −0.008 cm −1 atm −1 . With the pressure and path length used in our study we did not find evidence of significant line mixing in either the self- or hydrogen-broadened spectra. This study represents the first high-resolution experimental determination of hydrogen-induced pressure broadening and pressure-shift coefficients in the 2-0 band of 12 C 16 O at room temperature.

Multispectrum analysis of self- and N 2-broadening, shifting and line mixing coefficients in the ν6 band of 12CH 3D

A multispectrum nonlinear least-squares fitting technique has been applied to determine accurate zero-pressure line center positions, Lorentz self- and N 2-broadening coefficients and self- and N 2-induced pressure shift coefficients of transitions in the ν 6 (E) perpendicular band of 12CH 3D between 1035 and 1270 cm −1 . Seventeen high-resolution (0.002– 0.006 cm −1 ) room temperature absorption spectra recorded with two Fourier transform spectrometers (FTS) were analyzed together. Self-broadening coefficients for over 700 transitions and self-shift coefficients for more than 600 transitions were determined. Measurements of nitrogen-broadening coefficients for more than 480 transitions and nitrogen pressure induced shift coefficients for nearly 430 transitions were also made. The measurements include transitions with rotational quantum numbers as high as J″=18 and K″=15 and several forbidden transitions. The measurements were made in all six sub-bands ( P P, P Q, P R, R P, R Q and RR). The measured self-broadening coefficients vary from 0.025– 0.096 cm −1 atm −1 at 296 K and the N 2-broadening coefficients from 0.02 to 0.08 cm −1 atm −1 at 296 K . Self-induced-shift coefficients range from −0.014 to +0.004 cm −1 atm −1 while the nitrogen-induced-shift coefficients vary from −0.013 to +0.008 cm −1 atm −1 . Very few of the measured pressure-shift coefficients were positive, and the positive shift coefficients were mostly associated with the J″= K″ transitions in the PQ sub-bands. The off-diagonal relaxation matrix element coefficients of a number of transitions with K″=3 doublets for both self- and N 2-broadening were also obtained. The results obtained with the two different broadening gases are compared and discussed. Comparisons with previous values from the literature are also made.

High resolution spectroscopy of a hydrocarbon with the triplet ground state, 2,2-dimethyl-2 H-dibenzo[ cd, k]fluoranthene and its closed-shell precursor in solid matrices

The triplet–triplet (T 1–T 0) fluorescence and excitation spectra of the title compound have been recorded at 8 K in a Shpol'skii matrix, n-decane. The ground-state vibronic frequencies and intensities were similar to those of a closed-shell model compound indicating that the four benzo rings have retained their aromatic character in the biradicaloid species. The broad feature in the excitation spectra up-shifted by 800 cm −1 with respect to the T 1–T 0 origin was assigned to the second purely electronic transition. The photochemical bond rupture in the cyclopropane precursor of the biradicaloid, 1,1-dimethyl-1a,11b-dihydro-1 H-benzo[ k]cyclopropa [4,5]cyclopent[1,2,3- c, d]fluoranthene was carried out by a pulsed dye laser at 420 nm. This leads to spectral hole burning with a quantum yield of 1.5±0.5% in frozen solvent glasses. Because of intrinsic low-temperature photochemistry, the compound is well suited for probing intermolecular interactions in glassy solids. In the second part the influence of hydrostatic pressure on spectral holes is reported. A linear dependence of the pressure shift coefficient d ν/d P on burning frequency ν was obtained. The slope of the plot was treated in terms of the isothermal compressibility of the matrix and the frequency at which there is no pressure shift was compared with the vacuum frequency of the nonsolvated chromophores. To account for the nontrivial relationships between these parameters a model was proposed basing on several microscopic solvent shift mechanisms with different interaction radii.

Measurements of air broadened width and air induced shift coefficients and line mixing in the ν5 band of 12CH 3D

In this study we report the first experimental measurements of air-broadening and air-induced pressure-shift coefficients for approximately 400 transitions in the ν 5(E) perpendicular band of 12CH 3D in the spectral region 1276– 1600 cm −1 . These results were obtained from analysis of 11 room temperature laboratory absorption spectra recorded at 0.005 cm −1 resolution with the McMath–Pierce Fourier transform spectrometer located on Kitt Peak National Solar Observatory. A multispectrum non-linear least-squares fitting algorithm was used in retrieving the results. The measured broadening coefficients vary from to 0.0265 to 0.0735 cm −1 atm −1 at 296 K for transitions up to J″=17 and K″=12, including 12 forbidden lines with ΔK=±2 (5 OQ and 7 SQ) . The corresponding pressure-induced shift coefficients range from +0.0003 and −0.0088 cm −1 atm −1 and the majority of these are negative. Subsets of air-broadened width coefficients vary somewhat smoothly as a function of the rotational quanta, and this behavior has been modeled to ±5% using empirical expressions. The off-diagonal relaxation matrix coefficients, W ij , in cm −1 atm −1 at 296 K which are associated with line mixing were determined for a few transitions with K″=3 ( A + A − splitting) in the P P and P Q sub-bands. The broadening and pressure-shift coefficients obtained from the analysis are compared with values previously determined by us for similar transitions in the ν 6(E) perpendicular band of 12CH 3D and with other values reported in the literature.

Measurements of air-broadened width and air-induced shift coefficients and line mixing in the ν6 band of 12CH 3D

Air-broadening and shift coefficients for more than 680 transitions in the 1040– 1410 cm −1 spectral region of the ν 6 perpendicular band of 12CH 3D have been measured. Values have been obtained from analysis of 11 room temperature absorption spectra recorded at 0.005 cm −1 resolution with the McMath–Pierce Fourier transform spectrometer located on Kitt Peak National Solar Observatory and retrieved using a multispectrum nonlinear least-squares fitting technique. The measured broadening coefficients range from 0.016 to 0.077 cm −1 atm −1 at 296 K for transitions up to J″=17 and K″=15, including 34 forbidden lines with 2≤Δ K≤4. The corresponding pressure-induced shift coefficients vary from −0.012 to +0.008 cm −1 atm −1 , and the majority of these are negative. Within each J series in the P P and R R sub-bands, the J″= K″ (and J′= K′) transition exhibited the smallest broadening coefficient. In the P Q sub-band, the J″= K″ transition in each J series of lines had the largest positive pressure-shift coefficient while the largest negative pressure-shift coefficients were associated with the J′= K′ lines in the R Q sub-band. Subsets of the air-broadened width coefficients vary somewhat smoothly as a function of the rotational quanta, and this behavior has been modeled to ±8% using empirical expressions. Finally, Line mixing effects were observed for the first time in several of the K″=3 ( A + A −) split components in the PP, PQ, PR, RP, RQ and R R sub-band transitions, and values for off-diagonal relaxation matrix coefficients were determined.

Pressure Shift Mechanisms of Spectral Holes in the Optical Spectra of Dyes in Polymer Host Matrices

The influence of hydrostatic pressure (P) up to 200 bar of gaseous He was investigated on holes burned over the inhomogeneous S1 ← S0 absorption bands of polycyclic hydrocarbons, a polymethine dye, and tetrapyrrolic compounds imbedded in polymer matrices. The pressure shift coefficients dν/dP show a linear dependence on hole burning frequency (ν) that can be extrapolated to the frequency ν0(P) where no pressure shift occurs. The ν0(P) values deviate significantly from the actual 0−0 origins of the nonsolvated chromophores. The dependence of dν/dP on ν can be considerably steeper than the 2-fold isothermal compressibility of the matrix 2βT, expected for the distance dependence of intermolecular potential r-6 (e.g. London forces). Other solvent shift mechanisms, such as linear and quadratic Stark effects in the matrix cavity field, yield lower slope values than 2βT (1/3βT and 2/3βT, respectively). Tentatively, these controversies are rationalized in terms of intermolecular repulsive interactions that have a...

Inhomogeneous broadening and pressure shifts of the optical spectra in organic glasses at low temperatures

Inhomogeneous bandwidths were measured at 6 K in glassy solutions of poly(methyl methacrylate) and ethanol for chromophores with comparatively weak electron–phonon coupling: polymethine dyes, polycyclic hydrocarbons and tetrapyrrolic compounds. The contributions of dispersive attraction, the internal electric field effects and the repulsive forces to the band broadening are considered. Spectral holes were burned in the inhomogeneous S 1←S 0 absorption bands and the pressure shift coefficients d ν/d P were determined using gaseous He as pressure transmitter. Pressure shift coefficients show a linear dependence on hole-burning frequency. The slope of this dependence generally differs from the value of 2 β T ( β T is the isothermal compressibility of the matrix), predicted from the dispersive solvent shift equation. The slopes smaller than 2 β T are accounted for in terms of other solvent shift mechanisms, such as linear and quadratic Stark effects in the cavity field. The slopes larger than 2 β T are ascribed to repulsive interactions.

Measurements of air broadening, pressure shifting and off diagonal relaxation matrix coefficients in the ν3 band of 12CH3D

A multispectrum nonlinear least-squares fitting technique has been applied to determine Lorentz air-broadening coefficients and air induced pressure-shift coefficients of 12CH3D for the ν3 fundamental band in the spectral region between 1154 and 1430cm−1. This analysis has utilized 11 absorption spectra recorded at an unapodized resolution of 0.005cm−1 with the McMath-Pierce Fourier transform spectrometer of the National Solar Observatory (NSO) on Kitt Peak. Low pressure spectra (with 1–2.99Torr of enriched 12CH3D samples) as well as lean mixtures (≈1%) of 12CH3D in dry air (total pressures of 100–400Torr) at room temperature have been measured to determine the pressure broadening coefficients of more than 360 ν3 transitions with quantum numbers as high as J″=17 and K″=17. The measured air-broadening coefficients range from 0.0155 to 0.0726cm−1atm−1 at 296K. The pressure-induced shift coefficients vary from about −0.0086 to +0.0058cm−1atm−1. The majority of the shifts are negative, and the positive shifts often involve transitions with J″=K″. Weak line mixing effects have also been observed in a few high J lines with K″=3 and approximate off diagonal relaxation matrix coefficients were determined for several A+A− (A1A2) split components. At low to medium values of J, the A+A− splitting is extremely small, and the two components are practically unresolved. Variations of the measured parameters with the A- and E- symmetry species and the rotational quantum numbers of the transitions involved are discussed. The present results are compared to the few available experimental values in the literature.

Pressure broadening and shift coefficients for H 2, He and N 2 in the 3 ν2 band of 12CH 3D retrieved by a multispectrum fitting technique

Improved determinations of CH 3D abundances, as well as of the deuterium to hydrogen ratio (D/H) in the atmospheric methane of Saturn, Uranus, Neptune and Titan are reachable with a complete set of spectroscopic parameters for the 3 ν 2 band of 12CH 3D which take into account the dominant gases in these atmospheres (i.e. H 2, He and N 2). Line intensities and self-broadening coefficients derived from a previous analysis made by our group for 255 lines in the region of the 3 ν 2 band of 12CH 3D at 1.55 μm have been recently reported. In this paper we present 12CH 3D line broadening coefficients and pressure-induced shifts by H 2, He and N 2. A set of Fourier transform spectra recorded at Kitt Peak National Observatory with pressures of 100, 250 and 500 Torr for H 2 and N 2, and 600 Torr for He, has been used for this study. Data reduction was performed using a non-linear least-squares fitting multispectrum program developed for this purpose. The broadening coefficients, averaged over K, range roughly between 0.0685 and 0.0569 cm -1 atm -1 for 12CH 3D-H 2, between 0.0434 and 0.0335 cm -1 atm -1 for 12CH 3D-He, and between 0.0665 and 0.0533 cm -1 atm -1 for 12CH 3D-N 2, up to a maximum J of 14. The uncertainties are estimated to about 1.5% in the H 2 and N 2 broadening cases, and 3% in the He case. The pressure shift coefficients averaged over both J and K are found to be −0.0067 cm -1 atm -1 for 12CH 3D-H 2, +0.0021 cm -1 atm -1 for 12CH 3D-He, and −0.0119 cm -1 atm -1 for 12CH 3D-N 2. We estimate an uncertainty in these average shift coefficients of 15% in the H 2 and N 2 broadening cases, and of 35% in the He case. Dependences in J and K for these parameters have also been investigated with empirical expressions.

PRESSURE BROADENING AND PRESSURE SHIFT COEFFICIENTS IN THE 2 ν02 AND ν1 BANDS OF 16O 13C 18O

In this study we report the first measurements of the pressure broadening and the pressure-induced line shift coefficients due to air and nitrogen for 93 individual rovibrational lines, P(46) to R(46) and P(50) to R(45), respectively, belonging to the 2 ν 0 2 and ν 1 bands of 16O 13C 18O. The results were obtained by analyzing ten long path, high-resolution laboratory absorption spectra using a multispectrum nonlinear least-squares technique. Broadening due to nitrogen was about 4% larger than by air. Similar to broadening coefficients, the pressure-shift coefficients were found to be transition dependent, but different for the P- and R-branch lines with the same rotational quantum number, J. Except for a few R-branch lines, the measured shift coefficients were negative. The shift coefficients varied from +4×10 -3 cm -1 atm -1 at 296 K to −5.6×10 -3 cm -1 atm -1 at 296 K. Comparisons of the broadening and shift coefficients between the two bands, between the P and R branches, and between the two broadening gases are reported. No significant difference between shift coefficients of the two bands or the two broadening gases was observed. The results obtained were compared with those reported in the literature for the more abundant 12C 16O 2 species.

Analysis of the Line Profiles of CH3CN for theJ= 5 ← 4 andJ= 6 ← 5 Rotational Transitions

The pressure broadening, pressure shift coefficients, and absolute intensities have been obtained for theJ= 6 ← 5 and theJ= 5 ← 4 absorption lines of acetonitrile CH3CN at 110 and 92 GHz, respectively. The absorption line shapes have been directly recorded modulating the radiation beam by an optical chopper. In addition to the self-effects, the foreign-broadening coefficients have also been measured for N2, O2, and Ar.

Air- and N 2-broadening coefficients and pressure-shift coefficients in the 12C 16O 2 laser bands

In this paper we report the pressure broadening and the pressure-induced line shift coefficients for 46 individual rovibrational lines in both the 12C 16O 2 00 01-(10 00–02 00) I and 00 01-(10 00–02 00) II laser bands (laser band I centered at 960.959 cm −1 and laser band II centered at 1063.735 cm −1) determined from spectra recorded with the McMath-Pierce Fourier transform spectrometer. The results were obtained from analysis of 10 long-path laboratory absorption spectra recorded at room temperature using a multispectrum non-linear least-squares technique. Pressure effects caused by both air and nitrogen have been investigated. The air-broadening coefficients determined in this study agree well with the values in the 1996 HITRAN database; ratios and standard deviations of the ratios of the present air-broadening measurements to the 1996 HITRAN values for the two laser bands are: 1.005(15) for laser band I and 1.005(14) for laser band II. Broadening by nitrogen is 3 to 4% larger than that of air. The pressure-induced line shift coefficients are found to be transition dependent and different for the P- and R-branch lines with same J′' value. No noticeable differences in the shift coefficients caused by air and nitrogen were found. The results obtained are compared with available values previously reported in the literature.

Measurements of H216O Linestrengths and Air-Induced Broadenings and Shifts in the 815-nm Spectral Region

The linestrengths for 40 absorption lines of H216O water vapor that were located between 813 and 820 nm were measured; most of these lines were selected for their potential usefulness in laser remote measurements of atmospheric humidity using the differential absorption lidar technique. The air-induced pressure-broadening coefficients were also measured for 32 of these lines and the air-induced pressure shift coefficients were measured for 29 lines. These spectroscopic parameters were derived from spectra obtained with an AlGaAs diode laser and two long-path absorption cells. Collisional narrowing effects were observed and were accurately described by a Galatry profile. Comparisons were made with previous experimental work or theoretical calculations as available.

Air-Broadening and Shift Coefficients of O3Lines in the ν2Band and Their Temperature Dependence

Room temperature measurements of self- and air-broadening coefficients are reported for over 370 transitions covering a range of 0 ≤J″ ≤ 45 and 1 ≤Ka″≤ 12 for the ν2ozone band in the 630 to 800 cm−1spectral region. In addition, the temperature dependence of air-broadened halfwidth and air-induced pressure shift coefficients have been determined for over 350 spectral lines. A total of 29 O3absorption spectra (0.005-cm−1resolution) recorded at various temperatures (29 to −63°C) with a Fourier transform spectrometer were used in the analysis. The spectral line parameters were deduced by analyzing all of the 29 spectra simultaneously using a nonlinear least-squares fitting technique. The results are compared with similar measurements obtained in the ozone ν1band.

Temperature dependence of Lorentz air-broadening and pressure-shift coefficients of 12CH 4 lines in the 2.3-μm spectral region

High-resolution (0.01 cm -1) absorption spectra of lean mixtures of CH 4 in dry air were recorded with the McMath-Pierce Fourier transform spectrometer (FTS) of the National Solar Observatory on Kitt Peak at various temperatures between 24 and -61°C. The spectra have been analysed to determine the values at room temperature of pressure-broadened widths and pressure-induced shifts of more than 740 transitions. The temperature dependence of air-broadened widths and pressure-induced shifts was deduced for approx. 370 transitions in the v 1 + v 4, v 3 + v 4, and v 2 + v 3 bands of 12CH 4 located between 4118 and 4615 cm -1. These results were obtained by analysing a total of 29 spectra simultaneously using a multi-spectral non-linear least-squares fitting technique. This new technique allowed the determination of correlated spectral line parameters (e.g. intensity and broadening coefficient) better than the procedure of averaging values obtained by fitting the spectra individually. This method also provided a direct determination of the uncertainties in the retrieved parameters due to random errors. For each band analysed in this study the dependence of the various spectral line parameters upon the tetrahedral symmetry species and the rotational quantum numbers of the transitions is also presented.

Far-Infrared Collisional Lineshapes of Lithium Hydride and Deuteride Perturbed by H 2 and D 2

We have measured the pressure-broadening and pressure-shift coefficients of several rotational transitions for 7LiH, 6LiH, and 7LiD in the presence of H 2 or D 2 as perturbing gases, using a tunable far-infrared spectrometer. Experimental data have been collected in the 1.3- to 3.1-THz frequency interval and are in good agreement with theoretical predictions, based on a dipole-quadrupole interaction scheme. Calculated values are presented also for self-broadening and shift coefficients for 7LiH. These measurements, reporting for the first time pressure-broadening and shift parameters of lithium hydride, provide accurate tests of theoretical models for very light molecules.

Line strengths (900–3600 cm^−1), self-broadened linewidths, and frequency shifts (1800–2360 cm^−1) of N_2O

Self-broadened linewidths and pressure-induced frequency shifts of N(2)O were measured from spectra obtained at high spectral resolution in the 1800-2630 cm(-1) region. From measurements of 136 lines, the average pressure-shift coefficient was found to be -0.0015 ± 0.0006 cm(-1)/atm. The shift coefficients were found to have a weak J dependence, in which the magnitude of the coefficients increases with increasing values of J. Line-strength measurements were derived from spectra obtained at low sample pressures, and these data were analyzed to determine vibrational band strengths, rotationless dipole-moment matrix elements, and F factor coefficients for several bands in the 900-3600 cm(-1) region. Measurements of over 15,000 selected vibration-rotation transitions, including eight N(2)O isotopic species, were used in the various analyses.

Frequency shifting of pulsed narrow-band laser light in a multipass Raman cell

A multipass cell is described which allows efficient stimulated Raman frequency shifting for low pump laser intensities and low gas pressures. The latter is important for Raman shifting of narrow-band Fourier-transform limited light pulses (Δ v=75 MHz). It is shown that frequency broadening of the Raman shifted light can be largely avoided in the Dicke narrowing regime at low pressures. For 75 MHz pump pulses and an H 2 density of 2.5 amagat we found a negligible broadening to 90 MHz of the stimulated Stokes light. This is far below the value of 250 MHz expected from spontaneous emission. The narrow-band Stokes pulses achieved in CO 2 enabled us to measure the pressure shift coefficient (-0.71×10 -2 cm -1/amagat) of this gas. It is demonstrated, for the example of benzene, that our technique provides a very practical light source for high resolution molecular spectroscopy.

Pressure induced shifts of CO2 lines: Measurements in the 003–000 band and theoretical analysis

Self-broadened and N2, Ar, He-broadened halfwidth and pressure shift coefficients of the rotational transitions in the 0003–0000 band of 12C16O2 have been measured from laboratory absorption recorded at room temperature with a Fourier transform spectrometer. Comparison is made with the results of a theoretical calculation based on a semiclassical model. Good agreement is obtained for all the IR vibrational bands for which data are available. It has been shown that the shifts mainly originate from vibrational dephasing. Infrared line shifts have been compared with results obtained from stimulated Raman spectroscopy and we show that they are not consistent. A number of possible explanations have been discussed to account for this discrepancy.

Measurements of Lorentz-broadening coefficients and pressure-induced line shift coefficients in the ν2 band of HD 16O

Abstract Room temperature Lorentz-broadening coefficients and pressure-induced line shift coefficients in air, nitrogen, and oxygen have been determined for 126 transitions in the ν 2 band of D 2 16 O from laboratory spectra recorded at 0.0053-cm −1 resolution with a Fourier transform spectrometer. Samples of D 2 O + HDO + H 2 O at low pressure (≤1 Torr) and lean mixtures of D 2 O + HDO + H 2 O broadened by nitrogen, oxygen, and air were used in the experiments. Transitions up to J ″ = 12 were measured using a nonlinear least-squares fitting procedure assuming a Voigt line profile. The broadening coefficients obtained for the three buffer gases generally decrease with increasing J ″. The ratio of the nitrogen-broadening coefficient to the corresponding oxygen-broadening coefficient is correlated with the magnitude of the oxygen-broadening coefficient (the narrower lines have the larger N 2 -to-O 2 broadening ratios). The pressure-shift coefficients are both positive and negative with values in the range −0.008 to +0.008 cm −1 atm −1 ; no significant correlation with the widths was found. The results are compared with previously reported measurements.