Absolute Absorption Cross Sections Research Articles

Frequency-domain gratings by simultaneous absorption of two photons

We study frequency-domain coherence gratings produced by interference in process of simultaneous absorption of two photons in an inhomogeneously broadened medium. In our experiment, we create a grating at a visible wavelength by illuminating an organic dye-doped polymer at low temperature with phase-locked pairs of 100-fs laser pulses at a near-infrared wavelength equal to two times the transition wavelength. We show that two-photon excited coherence can be detected either by measuring the spectrum of fluorescence or, after prolonged illumination, by observing spectrally modulated persistent spectral hole. We analyze the phase and contrast of the grating at different temperatures and obtain, for the first time to our knowledge, temperature dependence of Debye–Waller factor and phonon spectrum of two-photon transition of organic molecule in presence of large inhomogeneous broadening. We also study wavelength dependence of absolute two-photon absorption cross-section in a series of porphyrins and show that at some wavelengths in porphyrin B-band region the cross-section is resonantly enhanced by nearby one-photon allowed Q-transition. These experiments indicate that large organic molecules such as porphyrins are promising candidates for further investigation of multi-photon excitation of coherence.

The temperature dependence (203–293 K) of the absorption cross sections of O 3 in the 230–850 nm region measured by Fourier-transform spectroscopy

Absolute absorption cross sections of O 3 were measured in the 230–850 nm (11765–43478 cm −1) region at five different temperatures (203–293 K) using a Fourier-transform spectrometer, at a spectral resolution of 5.0 cm −1 (corresponding to about 0.027 nm at 230 nm and to about 0.36 nm at 850 nm). The spectral accuracy of the data is better than 0.1 cm −1 — about 0.5 pm at 230 nm and about 7.2 pm at 850 nm — validated by recording of I 2 absorption spectra in the visible using the same experimental set-up. O 3 absorption spectra at different concentrations were recorded at five different sample temperatures in the range 203–293 K, and at each temperature at two total pressures (100 and 1000 mbar) using O 2/N 2 mixtures as buffer gas. Within the limits of experimental uncertainties, no influence of total pressure on the O 3 spectrum was observed in the entire spectral region, as expected from the short lifetimes of the upper electronic states of O 3. The temperature dependence of the O 3 absorption cross sections is particularly strong in the Huggins bands between 310 and 380 nm, as observed in previous studies. An empirical formula is used to model the temperature dependence of the O 3 absorption cross sections between 236 and 362 nm, a spectral region that is particularly important for atmospheric remote-sensing and for photochemical modelling.

Laboratory spectra of cold gas phase polycyclic aromatic hydrocarbon cations, and their possible relation to the diffuse interstellar bands

A novel laboratory technique is described, combining the use of supersonic expansion, laser excitation and small aromatic-rare gas van der Waals (vdW) clusters properties, which was developed to access the electronic absorption spectra of the polycyclic aromatic hydrocarbon (PAH) cations in the visible. It consists in preparing vdW complexes of the PAH molecule with a rare gas in a molecular beam, to photoionize it by resonant selective two-photon ionization, then to photodissociate this ionic complex by means of a delayed laser pulse in a time-of-flight mass spectrometer. The method is illustrated by presenting the visible spectra of the Naphthalene, Phenanthrene, Fluorene and Phenylacetylene cations. Such spectra can be unambiguously compared to the astronomical spectra of reddened stars, which exhibit the so-called diffuse interstellar bands (DIBs) in absorption. An interesting feature of the technique is its ability to measure the absolute absorption cross-sections. The large values of the oscillator strengths of the transitions, which are derived, are discussed in the astrophysical context which consists in considering that the PAH cations could be carriers for some of the DIBs.

Absolute excited state absorption cross section measurements inEr3+:LiYF4 for laser applications around 2.8 µm and551 nm

Using a pump-probe technique, absolute excited state absorption cross sectionsare measured in various Er3+ doped laser crystals in a wavelength domainextending from 0.53 to 2.9 µm. The gain cross section around 2.8 µm andthe population ratio between the involved levels 4I11/2 and 4I13/2 are measured for different Er3+ concentrations and pumpingwavelengths. The cross sections for the transitions involved in the upconversionpumping processes of the characteristic green emission of Er:LiYF4 are alsoestimated and discussed. A comparison is made between these results and thoseobtained semi-theoretically by using the Judd-Ofelt formalism.

Sensitive measurement of absolute two-photon absorption cross sections

We develop a method for measuring absolute two-photon absorption cross sections (σ2) and employ it to determine the σ2 of Rhodamine-6G in methanol (16.2±2.4 GM at 806 nm). Our measurement calibrates the relative excitation spectrum previously reported for this chromophore. The method is based on our derivation of an analytical expression describing the transmission of Gaussian laser pulses through a two-photon absorbing medium. The expression is valid for arbitrary absorber thickness, at all distances from the focus. This generalizes the prevalent “z-scan” (translation of the sample along the beam direction) technique for measuring two-photon absorbance, removing the requirements of a “thin” (thickness ≪ Rayleigh range of the focused laser beam) sample and of placing the sample at the focus. This leads to an improvement of the sensitivity of the technique by over two orders of magnitude, enabling measurement of the two-photon absorption cross sections of even weakly absorbing specimens at moderate intensities. The results are significant for applications such as nonlinear microscopy, optical data storage and optical power limiting.

Measurements of High-, Room-, and Low-Temperature Photoabsorption Cross Sections of SO 2 in the 2080- to 2950-Å Region, with Application to Io

Using synchrotron radiation as a continuum light source, we have measured the absolute absorption cross sections of SO 2 from 2080 to 2950 Å with a resolution bandwidth (FWHM) of 0.5 Å and under temperature conditions of 400, 295, and 200 K. Significant temperature effects are clearly observable throughout the studied wavelength region. Many hot bands have been observed, including some in the 2080- to 2220-Å region, where no such transitions were previously known. It was also found that the apparent continuum cross section underlying the vibrational features increases by an order of magnitude when the gaseous SO 2 temperature increases from 200 to 400 K. The new laboratory cross section data should be useful in constraining both the temperature and abundance differences between the leading and the trailing sides of the Io's atmosphere.

UV absorption cross sections of nitrous acid

The UV absorption cross sections of nitrous acid (HONO) and its deuterated analogue (DONO) have been measured in a slow flow system. Nitrous acid was produced by a high purity source reaching concentrations up to 22 ppm with a purity of >95% and a long-term variation of ⩽2%. The concentrations were measured by ion chromatography. Differential absorption cross sections for a spectral resolution of 0.5 nm of (2.98±0.26)×10 −19 cm 2 and (3.04±0.26)×10 −19 cm 2 have been obtained for the HONO absorption maxima at 354.2 and 368.1 nm, respectively. In addition, differential absorption cross sections of DONO were measured for the first time. The values of (2.67±0.24)×10 −19 cm 2 at 353.3 nm and (3.20±0.26)×10 −19 cm 2 at 367.6 nm are comparable to those of HONO. Absolute absorption cross sections were also derived for these species. The values of both the differential and absolute absorption cross sections determined in this work are lower than those reported previously. Therefore, the concentrations in the atmosphere may be 30–50% higher than that reported in several publications in which DOAS spectrometers were used for the determination of the HONO concentration.

Electronic absorption spectrum of cold naphthalene cation in the gas phase by photodissociation of its van der Waals complexes

The electronic absorption spectrum of the naphthalene cation has been obtained in conditions relevant for comparison with the diffuse interstellar bands in astrophysics, i.e., cold species in the gas phase. The novel technique consisting to photodissociate a selectively R2P2CI-prepared PAH–argon van der Waals complex in a molecular beam [Ph. Bréchignac and T. Pino, Astron. Astrophys. 343, L49 (1999)] has been used. The various aspects of the method are described in detail. The whole visible range has been explored revealing two electronic transitions displaying 28 vibronic bands. Absolute absorption cross sections have also been measured, and found much larger than reported from rare gas matrices studies. The additional information on the matrix-induced or complex-induced shifts and widths, and on the intramolecular and intermolecular processes involved in these species, is discussed. No definite conclusion about the possible presence of the cation in space can be drawn so far.

Gas-phase absorption cross sections of 24 monocyclic aromatic hydrocarbons in the UV and IR spectral ranges

Absorption cross sections of 24 volatile and non-volatile derivatives of benzene in the ultraviolet (UV) and the infrared (IR) regions of the electromagnetic spectrum have been determined using a 1080 l quartz cell. For the UV a 0.5 m Czerny-Turner spectrometer coupled with a photodiode array detector (spectral resolution 0.15 nm) was used. IR spectra were recorded with an FT-IR spectrometer (Bruker IFS-88, spectral resolution 1 cm -1). Absolute absorption cross sections and the instrument function are given for the UV, while for the IR, absorption cross sections and integrated band intensities are reported. The study focused primarily on the atmospherically relevant methylated benzenes (benzene, toluene, o-xylene, m-xylene, p-xylene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, ethylbenzene, styrene) and their ring retaining oxidation products (benzaldehyde, o-tolualdehyde, m-tolualdehyde, p-tolualdehyde, phenol, o-cresol, m-cresol, p-cresol, 2,3-dimethylphenol, 2,4-dimethylphenol, 2,5-dimethylphenol, 2,6-dimethylphenol, 3,4-dimethylphenol, 3,5-dimethylphenol, 2,4,6-trimethylphenol and ( E,Z)- and ( E,E)-2,4-hexadienedial). The UV absorption cross sections reported here can be used for the evaluation of DOAS spectra (Differential Optical Absorption Spectroscopy) for measurements of the above compounds in the atmosphere and in reaction chambers, while the IR absorption cross sections will primarily be useful in laboratory studies on atmospheric chemistry, where FT-IR spectrometry is an important tool.

Low and room temperature photoabsorption cross sections of NH 3 in the UV region

Using synchrotron radiation as a continuum light source, we have measured the absolute absorption cross sections of NH 3 with a spectral bandwidth (FWHM) of 0.5 Å. The photoabsorption cross sections of NH 3 have been measured from 1750 to 2250 Å under temperature conditions of 295, 195, and 175 K. Significant temperature effects in the absorption threshold region which are mainly due to the presence of hot band absorption are observed. The cross section value at peaks and valleys for the vibrational progressions of the (0,0) to (4,0) bands vary between −80% and +40% as the temperature of NH 3 changes from 295 to 175 K. In contrast to this, the changes of cross section values, P c,T, are found to vary less than 20% for the ( v′, 0) vibrational progressions with v′ ⩾ 5. The measured separations between the doublet features of the (0,0), (1,0), and (2,0) bands are found to decrease as the temperature of NH 3 decreases. The shifts of peak positions of P c,T with respect to the corresponding room temperature absorption peaks show a sudden change at v′ = 3 which appears to agree with the trend observed in the homogeneous line widths of the vibrational bands of NH 3 ( Vaida et al., 1987; Ziegler, 1985; Ziegler, 1986). The unusual behavior of the line widths has been attributed to the à state potential surface which has a dissociation barrier.

Absolute Intensities of Nitric Acid Overtones

The absolute absorption cross sections for the Δv = 3 and Δv = 4 OH local mode transitions in room-temperature, gas-phase nitric acid have been measured and calculated ab initio. The measured [calculated at the QCISD/6-31+G(d,p) level] oscillator strengths are (2.9 ± 0.3) × 10-8 [2.78 × 10-8] and (2.7 ± 0.3) × 10-9 [2.14 × 10-9]. Scaling the calculated values by 1.15 (the average ratio of measured:calculated) yields predicted oscillator strengths for transitions to v = 5 and v = 6 of 2.9 × 10-10 and 4.0 × 10-11, respectively, corresponding to integrated absorption cross sections of 2.6 × 10-22 and 3.5 × 10-23 cm2 molecule-1 cm-1, respectively for these transitions. These values are almost 2 times larger than previously estimated.

Excited-State Reaction Dynamics of Chlorine Dioxide in Water from Absolute Resonance Raman Intensities

Resonance Raman spectra of aqueous chlorine dioxide (OClO) are measured for several excitation wavelengths spanning the photochemically relevant 2B1−2A2 optical transition. A mode-specific description of the optically prepared, 2A2 potential energy surface is derived by simultaneous analysis of the absolute resonance Raman and absorption cross sections. The resonance Raman spectra are dominated by transitions involving the symmetric stretch and bend coordinates demonstrating that excited-state structural evolution occurs predominately along these degrees of freedom. Scattering intensity is not observed for transitions involving the asymmetric stretch, demonstrating that excited-state structural evolution along this coordinate is modest. The limited evolution along the asymmetric stretch coordinate results in the preservation of C2v symmetry on the 2A2 surface. It is proposed that this preservation of symmetry is responsible for the increase in Cl photoproduct quantum yield in solution relative to the gas ...

Absorption Spectra Measurements for the Ozone Molecule in the 350-830 nm Region

Absolute absorption cross-sections of ozone have been measured at ambient (295 K) and low temperature (218 K) in the visible region corresponding to the Chappuis bands. The temperature effect has been studied and found to be very small. The minimum of the absorption between the Hartley and the Chappuis bands is observed for the first time at 377,5 nm.

State-specific lifetime determination of the a 3Π state in CO

Two different techniques were applied to measure the lifetime of the lowest rotational levels in the metastable a 3Π1(v=0) state of CO. First, measurement of the absolute absorption cross-section for several absorption lines of the a(v=0)←X(v=0) transition yields an Einstein coefficient of A0,0=97±3 s−1. In combination with the experimentally determined branching ratios for the a→X transition, the lifetime of each component of the a 3Π1(v=0,J=1) Λ-doublet is determined to be 3.67±0.20 ms. Second, detection of the spin-forbidden fluorescence at two positions in the molecular beam downstream from the excitation region, as a function of velocity of the molecules directly probes the exponential decay. With this technique the lifetime of the lower component of the same a 3Π1(v=0,J=1) Λ-doublet is determined to be 3.4±0.4 ms, while for the upper component a value of 3.8±0.5 ms is found.

Atmospheric radical production by excitation of vibrational overtones via absorption of visible light

We present calculations using a radiative transfer model which predict that in the lower stratosphere at high zenith angles, significant enhancements to the photodissociation rates of HNO3 and HNO4 can result from visible wavelength excitation of OH overtone vibrations containing sufficient energy to cleave the O‐O and N‐O bonds. The results indicate that atmospheric chromophores such as HONO2, HO2NO2 and H2O2, could make a potentially significant contribution to the production of HOx and NOx. Calculating the relative importance of their effect requires better knowledge of the absolute absorption cross sections, both for vibrational overtones and in the near UV. Stratospheric air masses in which this process could be important are those that experience lengthy exposure at high solar zenith angles: the outer regions of the polar winter vortex and the polar summer anticyclone. We note that the general mechanism may have application elsewhere, such as in the atmospheres of other planets and in generating the diffuse interstellar bands associated with molecular clouds.

Radiative forcing of climate change by CFC‐11 and possible CFC replacements

The infrared absorption cross sections of CFC‐11 and 16 other halogenated compounds have been measured. These spectra were used in detailed line‐by‐line calculations to derive radiative forcing values. The radiative forcing values for 14 of these gases have not, to our knowledge, been previously reported in the literature. The accuracy of a computationally inexpensive narrowband scheme, which included the effect of clouds and stratospheric adjustment, was investigated. Global warming potentials are presented where atmospheric lifetimes are available. In light of the substantial disagreement in values for the forcing due to CFC‐11 reported in the literature and its use as a standard to which other halogenated gases are often compared, we examined the sensitivity of CFC‐11 forcing to a number of assumptions. We find that the uncertainties in the calculated value of the radiative forcing caused by neglect of the temperature and pressure dependence of the IR absorption spectrum are much smaller than those resulting from uncertainties in the absolute absorption cross sections or the vertical profile of CFC‐11. Our best estimate is 0.285 W m−2 ppbv−1, which is 30% higher than the value adopted by the Intergovernmental Panel on Climate Change and is believed to be accurate to within about 10%. For the other gases represented here the lack of detailed knowledge of the likely vertical and horizontal distribution probably represents the most significant uncertainty in evaluating their radiative forcing.

S 1 singlet excited-state absorption of DODCI

excited-state absorption cross-section spectrum of DODCI in ethylene glycol is determined in the wavelength range from 500 nm to 750 nm. The S1 state is populated by amplified femtosecond dye laser pulses (wavelength 592 nm) and the S1-state absorption and emission behaviour is probed with a femtosecond light continuum. A general analysis procedure is developed to extract absolute excited-state absorption cross-sections from the measured transmissions. For DODCI in ethylene glycol the wavelength regions of saturable absorption (mode-locking application), reverse saturable absorption (optical limiter application), and effective stimulated emission (laser application) are determined.

Kinetics of the gas phase reaction OH + NO(+M)→HONO(+M) and the determination of the UV absorption cross sections of HONO

The reaction OH + NO(+M) → HONO(+M) with M = SF 6 as a third body has been employed as a clean source for recording the near-ultraviolet absorption spectrum of HONO without interference from other absorbing species. The reaction was initiated by the pulse radiolysis of SF 6/H 2O/NO mixtures with total pressures in the range 10–1000 mbar at 298 K. The pressure dependence of the rate coefficient was studied by time-resolved UV and IR spectroscopy. By analysis of the fall-off curve we have derived a value for the limiting low pressure rate constant k 0/[SF 6] = (1.5 ± 0.1) × 10 −30 cm 6molecule −2 s −1 at 298 K, using the values of k ∞ = (3.3±0.3)×10 −11 cm 3 molecule −1 s −1 and F cent = 0.81 reported by Troe and co-workers. The UV spectrum of HONO was recorded in the range 320–400 nm and an absolute absorption cross section of σ = (5.02 ± 0.76) × 10 −19 cm 2 molecule −1 has been determined for the strongest band of HONO located at 354.2 nm. Differential absorption cross sections to be used for field measurements of HONO were also investigated.

Gas Phase Absorption Spectrum and Cross Sections of Vinylperoxy (C2H3O2) Radical

The absorption spectrum and cross sections of vinylperoxy (C2H3O2, ethenylperoxy) radical have been determined, for the first time in the gas phase, in the spectral range 220−550 nm. The spectrum exhibits a relatively broad and intense absorption centered at about 232 nm, followed by at least two identifiable and weaker absorptions with maxima at about 340 and 420 nm. The absorption tail persists at longer wavelengths into the visible region. To discern competition between the stabilization of the vinylperoxy isomers and reaction, the effect of pressure on the absorption has been examined. Vinylperoxy radicals in these experiments were produced through photochemical production of vinyl radicals followed by reaction of vinyl radicals with molecular oxygen. Vinyl bromide (C2H3Br) photolyzed at 193 nm was used as the precursor of vinyl radicals, and methyl vinyl ketone (CH3COC2H3) photolyzed at 193 nm was used as a precursor of both methyl and vinyl radicals. In the latter system identical concentrations of methyl and vinyl peroxy radicals were produced, and by employing comparative methods and using the literature values for methylperoxy absorption cross sections, the absolute absorption cross sections for vinylperoxy were determined. Ab initio molecular orbital calculations of CH3O2, C2H3O2, C2H3O, and HCO have been employed to characterize the observed spectrum and to identify the species and assign transitions contributing to the spectrum. These calculations suggest that the observed spectrum can primarily be assigned to two stable isomers (conformations) of the vinylperoxy radical with the O−O bond in a cis or trans position relative to the C−C bond, with a minor contribution to the absorption spectrum from the vinoxy and formyl radicals. For unsaturated radicals and the weak bonds involved here, accurate geometries are difficult to calculate, but the geometry obtained by gradient optimization using the multiconfiguration self-consistent-field method yields excitation energies that most closely agree with experiment. The relative theoretical oscillator strengths of all relevant vinylperoxy and vinoxy transitions have been evaluated and assist the analysis of the pressure dependence of the absorption spectrum.

VUV optical-absorption and electron-energy-loss spectroscopy of formamide

Absolute absorption cross-sections for formamide have been measured using a synchrotron radiation source (5–11.2 eV; 250–110 nm), along with electron-energy-loss (EEL) spectra at (i) high incident electron energies and low scattering angles and (ii) near-threshold incident energies and large scattering angles. In the optical- and high-energy EEL data, the excitation energies of the historical amide absorption bands (W, R 1, V 1, R 2 and Q) are in agreement with expectation. Vibrational structure is assigned to the V 1 ( 1ππ ∗ ) band. It is proposed that the Q band (∼ 9.2 eV) arises from superposition of transitions to several Rydberg states, with second 1ππ ∗ state (V 2, once related to the Q band) lying at a higher energy; the V 2 state may be visible in the EEL data. A number of Rydberg series converging to the lowest ionisation potential are suggested and the possibility of using these Rydberg data to assign the two first (closely spaced) ionisation potentials is discussed. The near-threshold EEL data fail to resolve the low-lying 3ππ ∗ state (the V 1 state triplet) because of spectral congestion. Dissociative electron attachment occurs in formamide upon impact with electrons of energy around 6.3 eV.