Absolute Absorption Cross Sections Research Articles

Dye laser photoacoustic overtone spectroscopy of CHBr3

Spectra of the ν = 5 and ν = 6 levels of the CH stretch in CHBr3 have been recorded via in-cavity dye laser photoacoustic spectroscopy. Linewidths for these absorptions have been determined and are discussed in light of existing theory and results on the linewidths of the corresponding transitions in CHCl3. When data on the fundamental are combined with ν = 5 and ν = 6 data it is possible to determine values for the local mode harmonic frequency and anharmonicity. Absolute absorption cross sections for the ν = 5 and ν = 6 levels are reported.

Absorption cross-section and density measurement of dye vapors

The absolute absorption cross-section spectrum of dimethyl-POPOP vapor is measured and compared with solution spectra. The dye vapor density versus temperature is determined by absorption coefficient measurement. Calibration is achieved by evaporation of a fixed amount of dye.

Xe 2Cl and Kr 2F excited state (4 2Γ) absorption spectra: measurements of absolute cross sections

Absolute absorption cross sections for the lowest excited states (4 2Γ) of Xe 2Cl and Kr 2F have been measured at discrete wavelengths in the visible and ultraviolet (Xe 2Cl: 325 ⩽ λ ⩽ 495 nm; Kr 2F: 248 ⩽ λ ⩽ 570 nm). The wavelengths observed for the 9 2Γ ← 4 2Γ band peak in each spectrum confirm the predictions of theory. However, the spectral profile and magnitude of the Xe 2Cl and Kr 2F absorption cross sections are considerably different from those predicted theoretically for the corresponding rare gas dimer ions, Xe + 2 and Kr + 2.

Measurements of the UV absorption cross-sections for HO 2 and CH 3O 2 in the gas phase

Relative values of the gas phase absorption cross-sections σ for hydroperoxy (HO 2) and methylperoxy (CH 3 O 2) radicals were determined at 298 K over the wavelength λ ranges 210 – 250 nm and 210 – 280 nm respectively. These determinations were based on measurements of the initial optical density (OD) 0 of each radical vs. wavelength immediately following its flash photolytic generation. Chlorine atoms produced by the photolysis (λ ⩾ 300 nm) of Cl 2 were stoichiometrically converted into the desired peroxy radical via the reaction sequences ▪ or ▪ Cross-section ratios for the two radicals, determined in separate experiments at several wavelengths, agreed well with those calculated using data from the complete (OD) 0 vs. λ sets and were used to establish the relative intensities of the absorption spectral curves for the two radicals. These spectral data were converted to absolute cross-sections using measurements of the fractional photodissociation of Cl 2 per flash in conjunction with measurements of the initial methylperoxy radical OD as a function of Cl 2 concentration. The results are compared with existing data in the literature for both relative and absolute absorption cross-sections and the effect of these new values on the existing kinetics data base for both radicals in discussed.

The ultraviolet absorpton spectra of selected organometallic compounds used in the chemical vapor deposition of gallium arsenide

Ultraviolet absorption spectra and absolute absorption cross sections of trimethylgallium (CH 3) 3Ga, triethylgallium (C 2H 5) 3Ga, trimethylarsine (CH 3) 3As, and triethylarsine (C 2H 5) 3As have been recorded as a function of temperature. Room temperature peak absorption cross sections of 2.6×10 -17, ~1.7×10 -17, 5.6×10 -17, and 4.1×10 -17 cm 2, respectively, were obtained for the above compounds at 195.0, ~190.0, 201.0, and 208.5 nm. (CH 3) 3As and (C 2H 5) 3As were found to be thermally stable up to 350° C, whereas (CH 3) 3Ga and (C 2H 5) 3Ga began to show decomposition at approximately 300–350° C. This study resolves previous discrepancies in the absorption cross section for (CH 3) 3Ga, and provides data for photo-enhanced deposition studies and optical diagnostic measurements in conventional chemical vapor deposition.

The UV absorption spectra of the trichlorobenzenes and the higher chlorinated benzenes in the gas phase and in n-hexane solution

Absolute absorption cross sections are determined for the three isomeric trichlorobenzenes in the gas phase in the region from 33 000 to 60 000 cm −1, and relative absorption cross sections of the tetrachlorobenzenes, pentachlorobenzene, and hexachlorobenzene are presented in the region from 40 000 to 60 000 cm −1. Spectra in n-hexane solution are determined for comparison. The spectra in the gas phase are compared to the spectra in solution and to spectra of the analogous fluoro compounds. With respect to band shifts a comparison is made to theoretical considerations (developed for spectra in solution) and a revised set of parameters for gas-phase spectra is derived. Vapor pressures for the tetrachlorobenzenes and for pentachlorobenzene are estimated at room temperature from the observed absorption intensities.

Absorption cross sections for symmetric chlorine dioxide as a function of temperature in the wavelength range 240-480nm

The absorption spectrum of OClO has been measured by using a diode array spectrometer with a resolution of 0.25 nm at 204, 296, and 378 K in the wavelength range of 240-480 nm. Absolute absorption cross sections were determined by measuring OClO concentration using two independent methods. The general features of the spectrum at 396 K agree with previous observations. The individual bands of OClO get sharper and the peak cross sections increase as the temperature is lowered from 378 to 204 K.

Atmospheric water vapor absorption at 13 μm

Absolute absorption cross sections for water vapor and water vapor/air mixtures were measured in a frequency range encompassing that of the chemically pumped atomic iodine laser. Measurements were made with a temperature-controlled multipass absorption cell and a high-resolution Fourier transform spectrometer. The measurements covered a broad range of water vapor and air pressures. Several techniques of data analysis were used, and the absorption cross section of 2 kPa of water vapor in an atmosphere of air was determined to be 1.1 +/- 0.2 x 10(-24) cm(2) . In this paper, an expression is derived which allows estimation of the absorption cross section for any pressure of water vapor and air.

Absolute absorption cross-section measurements on NO3. Evaluation of the titration of NO3 by NO in the determination of absolute concentrations

The absorption cross-section of NO3 at λ= 662 nm and at room temperature was found to be (2.23 ± 0.35)× 10–17 cm2 molecule–1. Error limits are 95% confidence limits for random errors. A discharge flow apparatus was used to prepare NO3 by the reaction of F atoms with HNO3. The absolute concentration of NO3 in the absorption cell was determined by titration with NO. A stoicheiometric factor for the removal of NO3 was calculated using numerical integration of the kinetic equations derived from a simple reaction scheme. This factor is not always unity, and we indicate the consequences of erroneously assuming 1 : 1 stoicheiometry for NO3 removal by NO. The influence on the calculated value of σ of the initial concentrations of F atoms and HNO3 is discussed. The role of possible interfering reactions is also considered.

Absolute absorption cross sections of ozone in the 185‐ to 350‐nm wavelength range

The absorption cross sections of ozone have been measured in the wavelength range 185–350 nm and in the temperature range 225–298 K. The absolute ozone concentrations were established by measuring the pressure of pure gaseous samples in the 0.08‐ to 300‐torr range, and the UV spectra were recorded under conditions where less than 1% of the sample decomposed. The temperature dependence is significant for wavelengths longer than ∼280 nm. The absorption cross‐section values around 210 nm were found to be about 10% larger than the previously accepted values.

Absolute two-photon absorption and three-photon ionization cross sections for atomic oxygen

The technique of two-photon excited fluorescence has been used to measure the cross section for the 3p ${\mathrm{}}^{3}$${P}_{2}$,1,0\ensuremath{\leftarrow}2p ${\mathrm{}}^{3}$${\mathrm{P}}_{2}$ transition in atomic oxygen. Excitation was monitored by observing fluorescence at 845 nm from the 3p ${\mathrm{}}^{3}$${P}_{2}$,1,0\ensuremath{\rightarrow}3s ${\mathrm{}}^{3}$${\mathrm{S}}_{1}$ transition. Spontaneous Raman scattering from ${\mathrm{H}}_{2}$ was used to calibrate the fluorescence collection system. The spatial and temporal profiles of the exciting dye-laser pulses were carefully measured in a mildly focused excitation geometry, allowing the integrated two-photon absorption coefficient to be measured absolutely. The integrated absorption coefficient is the product of an atomic quantity, ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2), and the second-order correlation function of the laser field, ${G}^{(2)}$. The experimental value of this coefficient is ${\mathcal{J}}_{\mathrm{J}\mathcal{'}}$${\mathrm{\ensuremath{\sigma}}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2)${\mathrm{G}}^{(2)}$ =(2.66\ifmmode\pm\else\textpm\fi{}0.80)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{4}$. If chaotic photon statistics are assumed (i.e., ${G}^{(2)}$=2), excellent agreement with the calculations presented in the companion paper is obtained. The cross section for photoionization by a third, identical 226-nm photon has been determined by measuring the absolute number of ions produced per laser pulse. The excited-state photoionization cross section is ${\ensuremath{\sigma}}_{\mathrm{pi}=(5.3\ifmmode\pm\else\textpm\fi{}2.0)\ifmmode\times\else\texttimes\fi{}{10}^{\mathrm{\ensuremath{-}}19}}$ ${\mathrm{cm}}^{2}$.

Photoabsorption cross section of acetylene in the EUV region

Using synchrotron radiation as a continuum background, the absolute total absorption cross section of C2H2 has been measured using a double ionization chamber. The cross sections range from 2.5 to a maximum of 35 Mb in the 175–740 Å region. Two new Rydberg series have been identified and apparently converge to the B̃ 2Σ+u state of C2H+2 at 18.71 eV. The observed Rydberg states are tentatively assigned to (2σu)−1 nsσg 1Σ+u and (2σu)−1 ndσg 1Σ+u, respectively. The present cross section data have been used in the analysis of various sum rules including the TRK sum rule.

Photophysics and cis-trans isomerization of DCM

Cis and trans isomers have been identified in solutions of the dye DCM and the absolute absorption cross sections have been measured for both. Fluorescent lifetimes and quantum yields for fluorescence have been measured as a function of solvent and the results demonstrate the unique nature of the photophysics in this system.

Absorption spectra of thiophene between 225 and 246 nm at elevated temperatures

The u.v. absorption spectra of thiophene (C 4H 4S) between 225 and 246 nm were recorded at elevated temperatures. The absolute absorption cross sections for major features in this wavelength interval are reported for the temperatures 296, 323, 373, 423, 473, 573, 673 and 773 K. Vibrational structure was lost as the temperature approached 773 K. The thermal behavior of the spectra is compared to the expected thermal behavior based on previous band assignments.

Photoproperties of DCM

Cis and trans isomers have been identified in solutions of the dye DCM and the absolute absorption cross-sections have been measured for both. Fluorescent lifetimes and quantum yields for fluorescence have been measured as a function of solvent and the results demonstrate the unique nature of the photophysics in this system.

Photoacoustic absorption spectra of atmospheric gases near 7603 cm^−1

Absorption spectra of carbon monoxide, water vapor, memane, and ammonia are presented as part of an effort to determine absolute absorption cross sections for some atmospheric gases at the iodine laser wavelength.

Absolute OH absorption cross sections (for lidar measurements)

Absorption cross sections are important in quantitative optical measurements of OH. We have used existing experimental data to calculate the effective cross sections for a number of transitions in the A-X system of OH, taking into account such factors as temperature and the spectral shape and width of both the absorption line and the excitation source. Details of the calculations are presented to allow easy extension to other transitions.

New measurements of the absolute absorption cross-sections of ozone at 294 and 223 K in the 310-350 nm spectral range

Absolute measurements of the absorption cross-sections of ozone in the 310-350 nm range (Huggin's system) were carried out at 294 and 223 K with high resolution. Our results ― lower than all results known to date ― are, within a coefficient, in very close agreement with the (relative) values obtained recently by A.M. Bass and R.J. Paur with a resolution similar to that used in this work Determination de ces sections efficaces a haute resolution. Bon accord avec les donnees anterieures, en particulier avec les valeurs relatives de Bass et al.

Resonance Raman excitation profiles of bacteriorhodopsin

We have measured resonance Raman excitation profiles for 12 vibrational modes of light-adapted bacteriorhodopsin using 27 excitation wavelengths ranging from 472 to 752 nm. The excitation profiles are quite similar and have approximately the shape and width of the absorption spectrum. Calculations using a 29-mode harmonic sum over states with large amounts of inhomogeneous broadening (Gaussian standard deviation 750 cm−1) yield good fits to the absorption and excitation profile band shapes and to the absolute absorption and Raman cross sections. These calculations also give nearly correct intensities for most of the observed overtone and combination bands. The absolute Raman cross sections allow us to differentiate between the homogeneous and inhomogeneous contributions to the spectral breadth. Our upper limit of 250 cm−1 for the homogeneous linewidth, as well as the absence of strong low-frequency Raman lines, implies that the lack of resolution in the bacteriorhodopsin absorption is due primarily to inhomogeneous effects. Finally, the permissible contribution of torsional vibrations to the spectral diffuseness and the consistency of these results with an excited state isomerization are explored using wave packet propagation on a linear excited state torsional potential.

Absolute absorption cross-sections for Photosystem II and the minimum quantum requirement for photosynthesis in Chlorella vulgaris

Absolute absorption cross-sections for oxygen production ( σ O 2 ) were determined from the light-saturation behavior of oxygen flash yields from whole cells of Chlorella vulgaris illuminated with submicrosecond flashes of laser light. Light-saturation curves were well described by simple Poisson statistics with a single average cross-section per photosystem trap (RCII). The maximum variation about the average cross-section permitted by the data was a factor of 3. σ O 2 at the laser wavelength (596 nm) increased from 38 Å 2 for cells grown in high light to 115 Å 2 for cells grown in low light. The 3-fold variation in σ O 2 was accompanied by a 10-fold variation in total cell chlorophyll content. This behavior results, at least in part, from the partitioning of chlorophyll between Photosystem II (measured) and Photosystem I (unmeasured). The 596 nm in vivo absorption cross-section for chlorophyll in Chlorella ( σ Chl ) is 0.29 Å 2, independent of total cell chlorophyll content. The antenna size of RCII was calculated to range from 130 to 400 molecules of chlorophyll. At low flash energies, the relationship between the quantum requirement for oxygen production (QR), the maximum oxygen-flash yield or Emerson and Arnold number (PSU O 2 ) and our cross-sections is QR=( PSU O 2 )· ( (σ Chl ) (σ O 2 ) . QR, found to be independent of both total cell pigmentation and RCII antenna size, was constant at 10±1 photons absorbed per oxygen molecule evolved.