Wavenumber Scale Research Articles

Fourier transform measurement of NO2 absorption cross-section in the visible range at room temperature

New laboratory measurements of NO2 absorption cross-section were performed using a Fourier transform spectrometer at 2 and 16 cm-1 (0.03 and 0.26 nm at 400 nm) in the visible range (380–830 nm) and at room temperature. The use of a Fourier transform spectrometer leads to a very accurate wavenumber scale (0.005 cm-1, 8×10-5 nm at 400 nm). The uncertainty on the new measurements is better than 4%. Absolute and differential cross-sections are compared with published data, giving an agreement ranging from 2 to 5% for the absolute values. The discrepancies in the differential cross-sections can however reach 18%. The influence of the cross-sections on the ground-based measurement of the stratospheric NO2 total amount is also investigated.

Log‐Normal Description of Fluorescence Spectra of Organic Fluorophores

AbstractThe smooth fluorescence bands of various organic fluorophores of different classes (e. g. indole, tryptophan, tyrosine, phthalimides, quinine sulfate, aminopyridines, acetylanthracenes) in different ionization states of their substituents and dissolved in various solvents can be accurately fitted on the frequency (wavenumber) scale by the four‐parametric log‐normal distribution curves up to the far spectral wings. This fact suggests that the log‐normal function is a good analytical description for smooth emission spectra of so‐called complex molecules. Examination of log‐normal parameters of 126 spectra of several tryptophan derivatives measured in different ionization states in various solvents revealed a straight‐linear relation between three shape parameters of the log‐normal function, namely, the positions of spectral maxima and of the two half‐maximal amplitudes. This fact indicates that only one parameter is needed to describe the band shape for the set of tryptophan derivatives. A similar linearity was also observed for the series of spectra of 1‐ and 2‐acetylanthracenes or 3‐amino‐N‐methyl‐phthalimides when varying the solvent. As a result, the spectral maximum position can entirely determine the shape of emission spectra of a series of a fluorophore derivatives in any environmental condition. Such a simplified mathematical representation significantly facilitates the problem of analytical resolution of composite fluorescence spectra, for instance, those of proteins.

Line strengths for Δ v3=1 transitions in isotopic CS 2 species with a stabilized tunable diode laser

We describe a spherical Fabry Perot etalon which is utilized as an internally coupled Fabry Perot Interferometer (icFPI). This icFPI uses piezoceramic translators for high frequency internal pathlength modulation and for controlled scanning of the icFPI cavity length. A tunable diode laser (TDL) is phase locked to the pathlength modulated icFPI, reducing laser phase noise and improving the precision of the wavenumber scale (in cm -1). This stabilized laser system produces spectra with normal line shapes instead of first or second derivative line shapes normally associated with systems in which the laser emission wavelength is modulated in order to achieve stabilization. Line strengths of the 13C 32S 2, 13C 33S 32S and 13C 34S 32S isotopes of CS 2 were measured in the Δ v 3=1 fundamental region using the stabilized TDL system.

Performance of an étalon-stabilized tunable diode laser system

A wavelength-stabilized tunable diode laser and a reflective optical system can produce fringe-free spectra with a noise level of 10(-5) absorbance units and a precision of 10(-5) cm(-1) in the wave-number scale.

X- and Y-Polarized Spectra of Chlorophyll a and Pheophytin a in the Red Region: Resolution Enhancement and Gaussian Deconvolution

The red region of the X- and Y-polarized spectra obtained from the measurement of linear dichroism of chlorophyll a and pheophytin a oriented in a liquid crystal-1 has been reexamined. A combination of resolution enhancement of the spectra involving their Fourier transformation and manipulation in the Fourier domain, and deconvolution into Gaussian bands on the wavenumber scale has produced results conforming to previous conclusions that place the origin of the Qx transition in chlorophyll a some 800-900 cm-1 to the blue relative to the origin of the Qy transition. In pheophytin a, however, the separation between the Qx and QY bands is much larger, about 3600 cm-l. The results also confirm previous reports of an angle near 70� between the transition moment directions of the Qy transitions and the molecular X-axis for both chlorophyll a and pheophytin a. The resolved vibronic bands in the case of the Qy transition of chlorophyll a, at least, have their counterparts in both solution spectra and high-resolution low-temperature fluorescence excitation spectra.

Precision Fe i and Fe ii wavelengths in the ultraviolet spectrum of the iron–neon hollow-cathode lamp

The wave numbers of 167 Fe i lines between 26 000 and 34 100 cm−1 (385–293 nm), 146 Fe i lines between 33 700 and 51 400 cm−1 (297–195 nm), and 221 Fe ii lines between 35 900 and 54 600 cm−1 (279–183 nm) are measured with a relative precision of 30 parts in 109 by Fourier-transform spectrometry. Bridging techniques are used to place the measurements on the same Ar ii–based absolute wave-number scale as previously measured visible spectra. The uncertainty in the absolute wavelengths is limited by small source shifts and the accuracy of the available standards and is estimated to be 0.002 cm−1 (0.008 pm at 200 nm). This is an order of magnitude better than that of the majority of current UV standards.

High-resolution OCDR in dispersive waveguides

In the analysis of integrated optical waveguide structures by coherence-domain reflectometry, spatial resolution may be seriously impaired by second order dispersion of the waveguides under test and by the presence of structures in the source spectrum. An algorithm for evaluation of measured interferograms is presented to correct for these effects. It is based on Fourier and wavenumber scale transformations.

Photocurrents of cone photoreceptors of the golden-mantled ground squirrel.

1. Visual transduction in photoreceptors of the ground squirrel, Citellus lateralis, was studied by recording membrane current from individual cones in small pieces of retina. 2. Brief flashes of light produced transient reductions of the dark current; saturating response amplitudes were up to 67 pA. A flash strength of about 11,000 photons microns-2 at lambda max was required to give a half-saturating response. The stimulus-response relation was well fitted by an exponential saturation curve. Responses below 20% of maximum behaved linearly. 3. The response to a dim flash in most cells had a time to peak of 20-30 ms and resembled the impulse response of a series of five low-pass filters. 4. The variance of the dim-flash response amplitude put an upper limit of 80 fA on the size of the single photon response. Estimates based on the effective collecting area suggest the single photon response to be of the order of 10 fA. 5. Flash responses of squirrel cones usually lacked the undershoot observed in primate cones, although in about 1/3 of the cells a small undershoot developed during recording. 6. Background lights slightly shortened the time to peak of the flash response and reduced the integration time. 7. Spectral sensitivity measurements showed two classes of cones with peak sensitivities at about 520 and 435 nm. Rod sensitivity peaked near 500 nm. Spectral univariance was obeyed by all three classes of cells. 8. The shapes of the spectral sensitivity curves of the rod and both types of cones were similar to each other when plotted on a log wave number scale, but differed significantly from similar plots of monkey and human cone spectra. 9. The kinetics and sensitivity of flash responses of the blue- and green-sensitive cones were indistinguishable.

Spectral sensitivity of cones of the monkey Macaca fascicularis.

1. Spectral sensitivities of cones in the retina of cynomolgus monkeys were determined by recording photocurrents from single outer segments with a suction electrode. 2. The amplitude and shape of the response to a flash depended upon the number of photons absorbed but not the wave-length, so that the 'Principle of Univariance' was obeyed. 3. Spectra were obtained from five 'blue', twenty 'green', and sixteen 'red' cones. The wave-lengths of maximum sensitivity were approximately 430, 531 and 561 nm, respectively. 4. The spectra of the three types of cones had similar shapes when plotted on a log wave number scale, and were fitted by an empirical expression. 5. There was no evidence for the existence of subclasses of cones with different spectral sensitivities. Within a class, the positions of the individual spectra on the wave-length axis showed a standard deviation of less than 1.5 nm. 6. Psychophysical results on human colour matching (Stiles & Burch, 1955; Stiles & Burch, 1959) were well predicted from the spectral sensitivities of the monkey cones. After correction for pre-retinal absorption and pigment self-screening, the spectra of the red and green cones matched the respective pi 5 and pi 4 mechanisms of Stiles (1953, 1959).

Measurements of electron density structure in striated barium clouds

Results are presented of the first measurements of detailed structure of electron density within striated vapor releases in the ionospheric F‐region. Six rocket probes penetrated ion clouds in three separate barium release events (two in each case) and showed considerable enhancement in F‐region electron density over the normal background ionization. In four traversais the electron density exceeded 10 cm−3 with the maximum of 5 × 106cm−3 observed in one case. Two showed dramatic structure in the electron density profiles associated with the passage through striated portions of the cloud. These structures had spatial extent as measured by the rocket probe normal to the terrestrial magnetic field of hundreds of meters with the density changing factors in the range from 2 to 10 as the probe passed into and out of the structures. The change of density on some of the features had particularly fast drop off corresponding to less than 20 meters travel normal to the magnetic field. The spectral power distribution of the irregularity amplitudes versus wavenumber scale size could be roughly described as a power law with a spectral index of from −2 to −3 within the striated regions. These results are in good agreement with predictions of the gradient drift instability mechanism as depicted by the simulation studies of Scannapieco, et al. [1976] in the late time nonlinear regime.

Wavenumber Scale Shift in Fourier Transform Infrared Spectrometers Due to Vignetting

Absorbance subtraction spectra, a common application of Fourier transform infrared (FT-ir) spectrometers, typically requires wavenumber scale repeatabilities of the order of 10−3 cm−1. Such repeatabilities are possible only if both spectra are taken with the same or no vignetting, as the following analysis shows.

Infrared Multiplexed Studies of Transient Species

Fourier transform techniques in spectroscopy have been shown to offer significant improvements over classical dispersive methods in the study of infrared spectral features both in astronomical measurements and in laboratory studies. These improvements result from now well known principles including spectral multiplexing, high etendue or throughput, and a significantly improved wave number scale.

Determination of Wavelength Scale Repeatabilities in Computerized Spectrometers

Current practice in wavenumber scale repeatability determinations involves repetitious measurements of peak centers for narrow absorption bands. Shifts as small as 1/10 bandwidth can readily be detected.

The Computer Conversion of a Sine Drive Spectrometer to Linear Wavenumber Drive

For various reasons, we have had only sine drive spectrometers in our Raman laboratory, in spite of the advantages of working with spectra plotted linearly with respect to frequency. Since the inception of a research program in resonance Raman spectroscopy, it became more desirable to record spectra on a linear wavenumber scale, in order to facilitate comparison of spectra obtained with different excitation wavelengths. We have interfaced one of our spectromters to a minicomputer based data acquisition system, which can record data on magnetic tape for further processing at our Computation Centre; it is, of course, straightforward to replot these data on a wavenumber scale. However, for much of our work there is no other reason to acquire and store these spectra, and it is thus inconvenient to obtain a wavenumber plot in this manner. Therefore, it was decided to write a computer program for the system which would drive the sine drive spectrometer in a constant wavenumber per unit time mode, such that we could use normal detection methods to obtain a chart output which was linear in wavenumber.

High resolution far infrared fourier transform spectrometry using Michelson interferometers with and without collimation

A two-beam interferometer of the Michelson type has been used to measure the absorption spectra of carbon monoxide, nitrous oxide and nitric oxide in the spectral region 15–40 cm −1, at a nominal resolution of 0.05 cm −1. The interferometer was used in both a collimated and uncollimated mode, and a detailed comparison of measured wavenumbers was made for carbon monoxide. The wavenumber contractions in the collimated interferometer are as expected from finite aperture considerations, and the larger contractions observed with the uncollimated interferometer can be related to the geometry of the collecting optics. Once the wavenumber scale given by the collimated interferometer is calibrated in terms of calculated absorption positions for CO. measurements made on NO and N 2O show agreement with expected positions better than 0.003 cm 1. Distortions of the spectral profiles are observed in both cases; they are more marked for the uncollimated interferometer. The theories of Parshin and Steel are discussed, and attempts made to relate these to our results.

An On-Line Spectrophotometer for Collection and Manipulation of Absorbance Spectra

A uv-visible recording spectrophotometer has been developed which uses an on-line general purpose digital computer and the optical train of a conventional double beam recording spectrophotometer. The computer controls all functions of the instrument and permits the collection and manipulation of high quality absorption spectra without the use of electronic or optical correction devices. Spectra are represented by a series of up to 500 individual data points. Spectra can be manipulated in a variety of ways to meet particular experimental situations and can be displayed or plotted on an arbitrary absorbance scale. A spectrum can be added to, or substracted from, any other spectrum, differentiated, converted to log absorbance, or multiplied by an arbitrary factor. Data can be preserved on paper tape or presented graphically as a high quality labeled plot of variable size on either a wave-length or a wave number scale. The complete system can scan a full spectrum at a maximum rate of 30 nm/sec over any part of a range from 230 to 700 nm. Details of the system are presented with examples of its performance.

Time impulse response and time frequency response of optical pupils.:Experimental confirmations and applications

Time impulse response and time frequency response of optical pupils.:Experimental confirmations and applications

An On-Line Spectrofluorimeter System for Rapid Collection of Absolute Luminescence Spectra

A spectrofluorimeter system has been developed which uses an on-line general purpose digital computer to both collect and correct fluorescence and luminescence spectra; no servosystems or complex analog circuitry are employed. Spectra are represented by a series of up to 500 individual data points, each of which is, in turn, the average of 16 samplings of the signal photomultiplier. The signal produced by a reference photomultiplier is also collected and used to make corrections appropriate to the type of spectra being collected. Wavelength drive is accomplished with a reversible stepping motor driven by commands from the computer; thus the wavelength position can be automatically and precisely controlled by the computer without recourse to cumbersome mechanical components. Corrections to collected spectra are made in real time on the basis of correction factors derived from fluorescence standards or a standard lamp. The complete system can collect a full spectrum (500 points) in 8 sec, average the results of any number of automatically repeated scans, and furnish the operator with both corrected and uncorrected spectra on either a wavelength or a wavenumber scale. In addition, the data can be retained on punched paper tape or presented graphically in a format acceptable for publication. Details of the system are presented with examples of its performance.

Spectroscopic properties of porphyropsins

The difference spectra of porphyropsins bleached at various pH as well as in the presence of hydroxylamine and sodium borohydride are described, and their relationships with absorption spectra are discussed. Since bleaching in NaBH 4 results in the formation of 3-dehydro N-retinylopsin, with an absorption band well-separated from those of the porphyropsins, difference spectra obtained in the presence of this reagent are very close to the corresponding absorption spectra over a considerable wavelength range. These difference spectra, together with the absorption spectrum of a porphyropsin extract still containing some impurities, are used to obtain an approximation to the absorption spectrum of pure porphyropsin. Bleaching in the presence of hydroxylamine also gives difference spectra which are quite close to the absorption spectrum, but over a more restricted range. Plotted against a wavenumber scale, instead of wavelength, porphyropsin difference spectra (hydroxylamine) ranging in λ max from 521 to 543 nm are identical in shape. Since we are considering only those regions unaffected by photoproduct absorption, this is applicable to the absorption spectra as well. These observations are used as a basis for tabulating a standard porphyropsin spectrum. Tables arc included which enable easy calculation of the Amax of any porphyropsin, given its difference spectrum: conversely, they may be used to predict the spectrum of any unknown porphyropsin, given its λ max. A different, narrower standard must be used in the case of rhodopsins. The spectroscopic properties of porphyropsins differ from those of rhodopsins with respect to absorption maxima, extinction coefficients and spectrum shape. Since corresponding differences are found between compounds of the retinol and 3-dehydroretinol series, it is concluded that similar features of chromophoric structure influence light-absorption in both the photopigments and these substances.

Response of Flexible Panel to Turbulence

Using a relatively simple functional representation of space-time correlation of the wall pressure fluctuation and by the use of the Lyon-Dyer method, the motion of a simple supported panel can be predicted, and the results agree reasonably well with the author's experimental results. The most striking feature of the excitation mechanism is called coincidence. This is a condition under which the wavenumber and frequency spectrum coincide with the turbulence and panel spectra. The matching and mismatching in wavenumber and frequency scale clearly indicate that the mean-square displacement peaks corresponding to the various panel modes coincide with those of a pressure at a given frequency. The frequencies were chosen so that 2πf/K1* corresponds to the convection velocity, K1* being the wavenumber at the spectrum peak. If, for certain frequencies and wavenumbers, mismatch between panel spectrum and turbulence spectrum should occur, a reduction in panel-displacement amplitude is observed. Such a mismatch is also reflected in the acoustic power radiated where similar reduction is obtained.