Albert A. Ruth

Incoherent broad-band cavity-enhanced absorption spectroscopy

Abstract A new highly sensitive method for incoherent broad-band cavity-enhanced absorption measurements of gaseous samples, using a white-light source is demonstrated. The light of a short-arc Xe-lamp transmitted by an optically stable cavity is dispersed with a grating monochromator and detected by a sensitive photodiode-array. The technique is a development of the previously published principle of cavity-enhanced absorption spectroscopy. The method is characterized by great experimental simplicity, high sensitivity, and high temporal resolution. These features are demonstrated on basis of the b 1 Σ g + (v ′ =2)← X 3 Σ g − (v″=0) absorption transition of molecular oxygen and the S1,ν′←S0 absorption spectrum of gaseous azulene at its vapour pressure at room temperature.

Long optical cavities for open-path monitoring of atmospheric trace gases and aerosol extinction

An incoherent broadband cavity-enhanced absorption spectroscopy setup employing a 20 m long optical cavity is described for sensitive in situ measurements of light extinction between 630 and 690 nm. The setup was installed at the SAPHIR atmospheric simulation chamber during an intercomparison of instruments for nitrate (NO(3)) radical detection. The long cavity was stable for the entire duration of the two week campaign. A detection limit of approximately 2 pptv for NO(3) in an acquisition time of 5 s was established during that time. In addition to monitoring NO(3), nitrogen dioxide (NO(2)) concentrations were simultaneously retrieved and compared against concurrent measurements by a chemiluminescence detector. Some results from the campaign are presented to demonstrate the performance of the instrument in an atmosphere containing water vapor and inorganic aerosol. The spectral analysis of NO(3) and NO(2), the concentration dependence of the water absorption cross sections, and the retrieval of aerosol extinction are discussed. The first deployment of the setup in the field is also briefly described.

Fourier-transform cavity-enhanced absorption spectroscopy using an incoherent broadband light source

A cavity-enhanced absorption setup employing an incoherent broadband light source was used in combination with a Fourier-transform spectrometer to measure the spin-forbidden B-band of gaseous oxygen at approximately 688 nm and several weak absorption transitions of water vapor in the same spectral region at room temperature in ambient air. The experiments demonstrate that the sensitivity of a Fourier-transform spectrometer can be significantly improved by increasing the effective path length, while retaining a rather small sample volume. In comparison with a single-pass absorption measurement, we report a path-length enhancement factor of 200 and an improvement of the signal-to-noise ratio of approximately 6 in the present cavity-enhanced absorption experiment. The practical advantages and limitations of this novel approach are outlined and potential applications are briefly discussed.

High-resolution Fourier-transform cavity-enhanced absorption spectroscopy in the near-infrared using an incoherent broad-band light source.

An incoherent broad-band cavity-enhanced absorption (IBB-CEA) set-up was used in combination with a Fourier-transform (FT) spectrometer in order to explore the potential of this technique for high-resolution molecular spectroscopy in the near-infrared region. Absorption spectra of overtone bands of CO2, OCS, and HD18O were measured between 5800 and 7000 cm(-1) using a small sampling volume (1100 cm3, based on a 90 cm cavity length). The quality of the spectra in this study is comparable to that obtained with Fourier transform spectrometers employing standard multi-pass reflection cells, which require substantially larger sampling volumes. High-resolution methods such as FT-IBB-CEAS also provide an elegant way to determine effective mirror reflectivities (R(eff), i.e. a measure of the inherent overall cavity loss) by using a calibration gas with well-known line strengths. For narrow absorption features and non-congested spectra this approach does not even require a zero-absorption measurement with the empty cavity. Absolute cross-sections or line strengths of a target species can also be determined in one single measurement, if gas mixtures with known partial pressures are used. This feature of FT-IBB-CEAS reduces systematic errors significantly; it is illustrated based on CO2 as calibration gas.

The S0 → S1 cavity ring-down absorption spectrum of jet-cooled azulene: dependence of internal conversion on the excess energy

The S0→S1 cavity ring-down (CRD) absorption spectrum of jet-cooled azulene is reported in the spectral region between 14275 and 17300 cm-1. The excess energy dependence of the fast internal conversion rate kic (S1S0) was investigated in the frequency domain by evaluating the linewidths of vibronic states up to excess energies of ∽hc×2500 cm-1. A value for the S1,0 lifetime of ≈2.6±0.4 ps was found. New experimental evidence for a conical intersection of the S0 and S1 potentials at ≈2100±100 cm-1 was found. The possibility of azulene being one of the carriers of the diffuse interstellar bands (DIB) is briefly discussed.

Photophysical triplet state processes of 4-H-1-benzopyrane-4-thione in a perfluoroalkane. Part 2.—Delayed S2→ S0 fluorescence due to triplet–triplet annihilation

With 4-H-1-benzopyrane-4-thione (BPT) in perfluoro-1,3-dimethylcyclohexane a delayed monomer fluorescence S2→ S0 due to triplet–triplet annihilation (TTA) is observable. The probability of generating an S2 state by the annihilation of two triplet states is ca. 0.03. The generation of S2 by TTA can be interpreted as a special case of Forster energy transfer. There is no indication of a delayed fluorescence from an upper excimer state.

Anti-Smoluchowski time dependence of the delayed fluorescence from anthracene in viscous solution due to triplet-triplet annihilation : effect of Förster energy transfer S1 + T1 → S0 + Tn on the initial spatial distribution of molecules in T1

Abstract The delayed S 1 → S 0 fluorescence from the first electronically excited singlet state S 1 of an aromatic compound in liquid solution is caused by diffusion-controlled triplet-triplet annihilation (TTA) T 1 + t 1 → S 1 + S 0 . For a random spatial distribution of triplet state molecules at time t = 0, Smoluchowskis theory for a diffusion-controlled reaction predicts a time-dependent rate constant k d ( t ) of TTA with k d (0) ⪢ k d (∞). If the triplet state is populated by optical excitation S 0 →S 1 and subsequent intersystem crossing S 1 → T 1 , it is principally impossible to generate a random distribution of triplet state molecules. Since the pair S 1 ⋯ T 1 is an intermediate during the creation of a triplet pair T 1 ⋯ T 1 , Forster energy transfer S 1 + T 1 → S 0 + T n may compete with the generation of T 1 ⋯T 1 at short intermolecular distances. As a consequence, one expects an anti-Smoluchowski behavior of TTA with k d (0)⪡ k d (∞), or with respect to the intensity of the delayed fluorescence, a strong initial rise. The anti-Smoluchowski behavior of a delayed fluorescence has been observed for the first time (with anthracene in a viscous alkane mixture as solvent). The anti-Smoluchowski behavior can be quantitatively described with a simple kinetic model, which contains only four parameters: the diffusion coefficient of molecules in T 1 , the Forster radius for the S 1 -T 1 energy transfer, and two parameters specifying an exponential distance dependence of the annihilation rate constant for a triplet pair.

Molecular iodine (I2) emission from two Laminaria species (Phaeophyceae) and impact of irradiance and temperature on I2 emission into air and iodide release into seawater from Laminaria digitata.

Kelps of the genus Laminaria accumulate iodine at high concentrations, but the iodine retaining capacity can be affected by emersion and physiological stress. In this study, I2 emission into the atmosphere from Laminaria digitata and Laminaria hyperborea was compared under controlled low irradiances and temperatures. The two species exhibited different I2 emission rates as blades of L. digitata emitted I2 at rates five times higher than those from newly-grown blades (current growth season) of L. hyperborea. I2 emission was not detectable from old blades (previous growth season) of L. hyperborea. Additionally, effects of irradiance and temperature on both I2 emission into air and net I(-) release into seawater where assessed for L. digitata while monitoring photo-physiological parameters as stress indicators. Irradiances between 30 and 120 μmol photons m(-2) s(-1) had only marginal effects on both I2 emission and I(-) release rates, but physiological stress, indicated by photoinhibition, was observed. The results suggest that the irradiances applied here were not stressful enough to impact on the iodine release. By contrast, at elevated temperatures (20 °C), photoinhibition was accompanied by an increase in I2 emission rates, but net I(-) release rates remained similar at 10-20 °C. High I2 emission rates into air and I(-) release into seawater observed from L. digitata underpin the fundamental function of this kelp as mediator of coastal iodine fluxes.

Solvent effects on the optical limiting action of C60 solutions

Effects of the solvent on the optical limiting action of C60 solutions are investigated under conditions of sequential two-photon absorption. C60 transmission measurements are reported both at constant wavelength and at constant linear ground state absorbance for a set of solvents using visible nanosecond laser radiation. The limiting action is shown to depend not only on the ground state absorbance of the solute but also on the local field created by the solvent.

TEMPERATURE-DEPENDENCE OF VISCOSITY AND DENSITY OF GLASS-FORMING ALKANES

The viscosity ( ) and the density of 3-methylpentane (3MP), 2,3-dimethylpentane (2,3DMP), methylcyclohexane (MCH), methylcyclopentane (MCP), and of the 1:1 mixtures MCH/MCP and 3MP/Isopentane were measured from 293 down to either the freezing point or to the temperature at which » 106 mPa s. For the glass-forming solvents 3MP, 2.3DMP, MCH/MCP, and 3MP/Isopentane ( ) can be represented by the sum of two Arrhenius terms and one Vogel-Tammann-Fulcher term: ( )& exp(Ei/RT) + A2exp(E2/RT) + A3exp[E3/R(T-T0)]. For the smaller viscosity ranges of the two crystallizing alkanes MCH and MCP, two Arrhenius terms are sufficient for the description of ( ).