Absorption Intensity Measurements Research Articles

A new instrumentation for simultaneous terahertz and mid-infrared spectroscopy in corrosive gaseous mixtures.

The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200-350K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200-350K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42m and by a THz radiation beam (82.5GHz to 1.1THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.

Carrier dynamics in β-Ga2O3 nanowires

Carrier dynamics have been investigated in β-Ga2O3 nanowires (NWs) grown by the vapor-liquid-solid mechanism, using ultrashort transient absorption spectroscopy in conjunction with time-correlating single photon counting photoluminescence. UV femtosecond pulse excitation has been utilized to generate nonequilibrium carrier distributions near the band edge of the NWs and nondegenerate pump-probe techniques have been employed to follow carrier relaxation through the defect related states located within the band gap of the NW semiconductor. Relaxation of the photogenerated carriers through these states appears to be biexponential with a fast component on the order of 3–5 ps and the slower component around 40–90 ps depending on the states being probed. Transient absorption intensity measurements reveal that recombination mechanisms such as Auger and bimolecular become contributing factors to the relaxation dynamics for absorbed fluences larger than 90 μJ/cm2. In the low fluence regime, time-correlated single photon counting photoluminescence measurements revealed a nonradiative relaxation mechanism with time constants ranging from 0.6–1 ns and a radiative relaxation with a time constant of approximately 4 ns.

Quantitative measurements of O 2 b ← X(2,1,0←0) bands by using cavity ring-down spectroscopy

Quantitative measurements of individual rotational line intensities of molecular oxygen are made for the (0←0), (1←0), (2←0) bands of the b 1 Σ g +← X 3 Σ g − transition using highly sensitive cavity ring-down spectroscopy (CRDS). The absorption profiles of six rotational lines of the three bands of O 2 are measured under pressures up to 1 atm. Corrections have been made to eliminate the effect of laser bandwidth on the absorption intensity measurements. The gas detection limits in our measurement for the three bands are 0.04, 0.80 and 7.00 Torr, respectively. The line strengths in the three bands of O 2 have been determined by CRDS and are in good agreement with previous results using a multiple pass absorption cell.

Intensities of Hot-Band Transitions: HCN Hot Bands

A simple vibrational Hönl-London-type formula for hot-band intensities is tested by measuring the intensities of a number of vibrational transitions, including many hot bands, for HCN. This vibrational intensity formula is based on one- and two-dimensional harmonic oscillator functions and a nonlinear electric dipole function that is expanded in a Taylor series with respect to the normal coordinates. It is shown that, when this formula is included, the observed transition dipoles for the bending hot bands differ by only a few percent from the transition dipoles for the same quantum number changes from the ground state. Infrared absorption intensity measurements are given for the transition dipoles for the transitions 01 10-0000, 0200-0000, 0310-0000, 0400-0000, 1000-0110, 1200-0000, and 2000-0000 and the accompanying hot bands involving the lower states ν2 = 1, 2, and 3. This simple model is limited to well-behaved systems, but would be useful for estimating the intensities of some high-temperature spectra. For HCN the Herman-Wallis constants that are quadratic in J(or m) are shown to be determined principally by the effect of l-type resonance.

Absorption intensity measurements of the second overtone band of NO

The absolute intensity A3–0 of the second overtone band of NO has been measured using helium and argon as pressure broadening gases at mixture densities up to 360 amagat. The absolute intensity, obtained from the integrated absorption coefficients of NO–He and NO–Ar gaseous mixtures at high pressures, is found to be (4.08 ± 0.21) × 10−2 cm−2 amagat−1. From the line intensity measurement data of Meyer, Haeusler, and Barchewitz and the absolute intensity obtained with the NO–He gaseous mixtures, the rotationless matrix element R3–0 (0) of the 3–0 band of NO is calculated and its value is 7.75 × 10−4 D. This value of R3–0 (0) is combined with previous results on the NO 1-0 and 2-0 bands to obtain a dipole moment expression accurate to M3. The following expansion for the dipole moment function in the ground state of NO is determined: M (x) = −0.166 + 2.56x−1.94x−1.20x3 D, where x is the relative internuclear separation (r−re)/re.

Absolute infrared transition moments for open shell diatomics from J dependence of transition intensities: Application to OH

A general approach to the determination of the dipole moment function and of the absolute vibrational transition moments for diatomic molecules is presented. This method utilizes the variation of intensity with J within a vibrational transition, together with permanent dipole moment information, to extract the absolute transition moments. An essential feature of the model is its use of algebraic expressions for calculating vibration–rotation line intensities. These expressions can be rapidly evaluated in a least squares fit which determines the dipole moment function. This approach is general in that it is not limited to 1Σ state molecules, nor to the simplest of Hund’s case couplings of spin, orbital and mechanical angular momentum. It is also not limited to molecules with essentially linear dipole moment functions. The model is successfully applied to the OH molecule which violates each of these restrictions. In the accompanying work we report experimental measurements of relative infrared absorption intensity measurements for OH v=1←0 transitions and the extraction of an experimental μ(r) using the approach presented here.

IR spectra and conformational behavior of N-acetyl-(glycine, L-alanine, L-leucine)-N'-methylamides in chloroform.

AbstractNH stretching bands of N‐acetyl‐(glycine, L‐alanine, L‐leucine)‐N′‐methylamides in dilute chloroform solution have shown that these dipeptides are present as a mixture of intramolecularly hydrogen‐bonded five‐membered ring species and nonhydrogen bonded species. Integrated absorption intensity measurements revealed that the concentration of the intramolecularly hydrogen bonded species decreased from 62% in glycine to 35% in the L‐leucine derivatives.

Spectroscopic evidence of vibrational dipole–dipole coupling between nitrosyl groups in some nitroprussides

The nature and magnitude of the intermolecular vibrational coupling between nitrosyl groups in Sr[Fe(CN)5NO]⋅4H2O and Ba[Fe(CN)5NO]⋅2H2O have been studied quantitatively by IR spectroscopy. The isotopic dilution technique has been used by incorporating in the first compound up to 40% of 18O as N18O ligand in the crystal. The results can be interpreted by postulating a dynamic electric dipole–dipole interaction which operates between the polar NO stretching vibrations. The main features of the fine structure displayed by the NO stretching band in the infrared spectra of polycrystalline Sr[Fe(CN)5N(16O/18O)]⋅4H2O, and in single crystals of Ba[Fe(CN)5NO]⋅2H2O can be explained in terms of a linear-chain model of nitrosyls vibrationally coupled through transition dipole–dipole forces, with interactions between nearest neighbors. The value ‖(∂m/∂S)0‖=13 D/Å calculated on the basis of this model for the NO dipole-moment derivative agrees satisfactorily with the value reported from integrated IR absorption intensity measurements. The coupling force constants between neighboring nitrosyls f1=+0.172 mdyn/Å obtained for barium nitroprusside, and the average f̄1=+0.189 mdyn/Å value obtained for the strontium salt, show that these interactions are the strongest thus far reported for ionic solids.

5320. Absorption intensity measurements of the first overtone band of CO: G Chandraiah and G R Hébert,Can J Phys,59 (10), 1981, 1367–1372

5320. Absorption intensity measurements of the first overtone band of CO: G Chandraiah and G R Hébert,Can J Phys,59 (10), 1981, 1367–1372

Absorption intensity measurements of the first overtone band of CO

The absolute intensity A2–0 of the 2–0 band of carbon monoxide has been measured with helium and argon as pressure broadening gases at pressures up to 600 amagat. A separate band intensity value has been derived from the measurements of several P-branch line intensity data and the Herman–Wallis formula. The best value obtained is A2–0 = (2.11 ± 0.08) cm−2 amagat−1. The square of the rotationless matrix element, [Formula: see text] has been found to be (4.39 ± 0.02)10−5 D2, as estimated from the measured line intensity values.

Doppler-limited spectra of the CH stretching fundamentals of formaldehyde

The Doppler-limited spectrum of H 2CO in the 2700 to 3000 cm −1 region has been recorded using a tunable difference-frequency laser system. This region encompasses the ν 1 and ν 5 fundamental CH stretching bands of formaldehyde as well as a number of strongly interacting combination bands. The tunable laser spectrometer affords complete spectral coverage with a calibration precision better than 10 −3 cm −1 for the transition frequencies and with absolute absorption intensity measurements better than 2%. The band centers for the ν 1 and ν 5 vibrations are measured to be 2782.4572 ± 0.0010 cm −1 and 2843.3254 ± 0.0015 cm −1 respectively, independent of the upper-state rotational constants.

Associated Complex Formation in a Binary Solution and the Concentration Fluctuation: The Chloroform–Diethyl Ether System

Abstract The relations between the associated complex formation and the concentration fluctuation in a binary solution were derived. The concentration fluctuation of a chloroform–diethyl ether system was determined by observing light-scattering spectra at various concentrations. The theoretical values of the concentration fluctuation of this system were calculated from the proposed relation by the use of the shape and the number population of the associated complex, as determined from the infrared absorption intensity measurements. The observed and calculated concentration fluctuations agreed quite well.

Determination of transition moments from power saturation

Abstract The electric (or magnetic) dipole transition moment may be calculated from the power broadening of saturation spectra if the ratio (T2/T1) is known or can be assumed as unity. The transition moment of the (43-0) P(13) R(15) lines of I2 coincident with the 5145 A Ar+ laser line is calculated from literature data on the power broadening of the saturation spectrum and compared to that obtained from absorption intensity measurements.

Spectra of C3 in Solidified Gases at 4° and 20°K

Wavelength, linewidth, and intensity measurements have been made from 3000 to 4500 Å of the spectra of triatomic carbon molecules trapped in solid matrices of Ar, Kr, Xe, O2, N2, CO2, and SF6 at 4° and 20°K. The carbon source was a Knudsen cell, operated at temperatures up to 2900°K, from which effused a molecular beam with known densities of C3. Absorption transitions are assigned to vibrational levels of both upper and lower states of the 1Πu—X 1Σg+ 4050-Å group of C3. The fundamental vibrational frequencies ν1 and ν2 for the upper 1Πu state, ν2 for the ground 1Σg+ state, and Renner constants for ν2 of the 1Πu state in these matrices are given. Measurements of absorption intensities give an oscillator strength, averaged for all matrices, equal to 6×10−2 for the C3 group. Little or no matrix effect on the intensities is found. However, experimentally determined corrections to the intensities are necessary for effects caused by apparent diffusion of carbon in the matrices to form nonabsorbing carbon aggregates. C3 linewidths vary with matrix from 40 to 500 cm−1, and C3 line shifts vary from −410 to 1065 cm−1. All matrix shifts are to the red except in the case of the CO2 matrix. No definite effect of temperature on linewidths or shifts is found.

Theoretical Evaluation of Atomic Polarizations of Diatomic Molecules

The atomic polarizations, PA (theor.), of five diatomic molecules, HCl, DCl, HBr, NO, and CO, have been calculated from an equation derived from dispersion theory (Van Vleck, 1927), using recent data obtained from infrared dispersion and absorption intensity measurements. These PA (theor.) values are compared with the PA (exp.) values obtained from dielectric constant and optical refractive index measurements. The greater accuracy of the PA (theor.) values is emphasized. The PA (theor.) values have been used to recalculate the dipole moments, μ, of the diatomic molecules considered. A qualitative relationship is discussed between the series of dμ/dr and μ values for some of the diatomic molecules. The PA (theor.) are calculated approximately for the gaseous alkali halides.

The ultraviolet absorption spectra of desoxypentose nucleic acid

The intensity of the ultraviolet absorption of sodium desoxypentose nucleate (SDN) in aqueous solution is lowered by the addition of salts, such as alkali halides and phosphates. SDN absorbance in sodium chloride solutions has been carefully measured and appears to vary linearly with the log of the sodium chloride concentration over a 10 5 change in salt concentration. In the absence of salt, the ultraviolet absorption of SDN solutions show strong pH dependence between pH 3 and pH 11. Aqueous SDN at pH 6.7 in the absence of added salt has an absorptivity ( k) of 25.5 and A 1cm 1% of 258 at 259 mμ (when measurements are made on concentration less than 0.01%). These values are considerably higher than previously reported. Aqueous SDN solutions in the pH range 2 to 10 have an isosbestic point at 233 m/gm. Addition of lysozyme and bovine plasma albumin to SDN solutions also result in a decrease in the ultraviolet absorption due to SDN. On the other hand, the addition of non-ionic substances such as polyvinyl alcohol, sucrose, or glycerol to aqueous SDN solutions, even in high concentrations, does not decrease its ultraviolet absorption. An explanation of these data in terms of the association of the component purine and pyrimidine groups is suggested. These results indicate that caution must be exercised in using measurements of absorption intensities in the ultraviolet for the determination of the purity of nucleic acid preparations and for the determination of the nucleic acid content of tissues and cells unless the ionic strength, pH, and Beer's Law deviation are known.

The Application of Infrared Spectrophotometry to Quantitative Measurements: Statistical Studies of Absorption Intensity Reproducibility

Because of its imporatnce in quantitative measurements, the degree of reproducibility of infrared absorption intensity measurements during one day, from day to day, and over an extended time interval of several months, has been studied. Polystyrene films and wire screens were proved to have satisfactory stability for use as standard samples in this study. Absorbance values used in this study were collected on a custom built double beam instrument as well as on two Perkin-Elmer double beam instruments. The data have been treated statistically. Comparable absorbance reproducibility was obtained with all three instruments but absorbance level shifts were observed. Instrumental, operational and sample variables which contribute to lack of absorbance reproducibility have been considered. Recommendations are included which should interest instrument users as well as instrument producers.

Intensities in the Harmonic Band of Hydrogen Chloride

Measurement of absorption intensities.---The intensities of the first nine lines of the negative branch of the first harmonic absorption band of HCl at 1.76\ensuremath{\mu} have been measured with a prism spectrograph, using a galvanometer and thermo-relay to observe thermopile voltages. The positive branch, not being resolvable, was measured as a whole. The areas under the lines in a plot of percent absorption against frequency were obtained for 5 different tube lengths, namely 74.5, 25.0, 14.96, 9.90 and 4.88 cm and the integrated absorption coefficient of each line was calculated from the slope of the curve of area against tube length at the origin by methods developed by Bourgin. The probable error of the intensity measurements for the stronger lines was 6 percent.Correlation with theory.---A calculation is made of the intensity ratio of the band as a whole to that of the fundamental based on a solution of Schr\"odinger's equation obtained by Morse for an anharmonic oscillator, and the value so obtained is within 6 percent of the experimental value. A second order perturbation calculation based on harmonic oscillator functions did not give concordant results. An unpublished analysis of the effect of rotation on the relative intensities of the lines in a band by Kemble is given and the relative intensities calculated for this band, giving agreement with experiment within 5 percent.