Integrated dual optical frequency comb source.

A monolithically integrated dual-channel optical frequency comb source is demonstrated in this paper. Three lasers are integrated on a single chip using a regrowth-free fabrication process in a master-slave-slave configuration. The master laser's power is split equally using a 1x2 multimode interference coupler and injection locks the two slave lasers. The slave lasers are gain-switched to produce dual optical frequency combs at 4.1 GHz and 5 GHz. To the best of our knowledge, this is the first demonstration of a dual optical frequency comb source with all light sources monolithically integrated in a photonic integrated circuit (PIC).

Off-Axis Cavity-Enhanced Absorption Spectroscopy of 14NH3 in Air Using a Gain-Switched Frequency Comb at 1.514 µm.

A custom-designed gain-switched frequency comb (GSFC) source was passively coupled to a medium finesse (F ≈ 522) cavity in off-axis configuration for the detection of ammonia (14NH3) in static dry air. The absorption of ammonia was detected in the near infrared spectral region between 6604 and 6607 cm-1 using a Fourier transform detection scheme. More than 30 lines of the GSFC output (free spectral range 2.5 GHz) overlapped with the strongest ro-vibrational ammonia absorption features in that spectral region. With the cavity in off-axis configuration, an NH3 detection limit of ∼3.7 ppmv in 20 s was accomplished in a laboratory environment. The experimental performance of the prototype spectrometer was characterized; advantages, drawbacks and the potential for future applications are discussed.

Time- and intensity-dependent broadband cavity-enhanced absorption spectroscopy with pulsed intra-cavity laser-induced plasmas.

A pulsed laser-induced plasma (LIP) was generated in ambient air inside a high-finesse (F≈ 5200) near-concentric optical cavity. The optical plasma emission was successfully trapped and sustained by the cavity, manifested by ring-down times in excess of 4 µs indicating effective mirror reflectivities of ∼0.9994. The light leaking from the cavity was used to measure broadband absorption spectra of gaseous azulene under ambient air conditions between 580 and 645 nm, employing (i) intensity-dependent cavity-enhanced, and (ii) time-dependent cavity-ring down methodologies. Minimum detectable absorption coefficients of 4.7 × 10-8 cm-1 and 7.4 × 10-8 cm-1 were achieved for the respective approaches. The two approaches were compared and implications of pulsed excitation for gated intensity-dependent measurements were discussed.

Laser-induced plasmas in ambient air for incoherent broadband cavity-enhanced absorption spectroscopy.

The emission from a laser-induced plasma in ambient air, generated by a high power femtosecond laser, was utilized as pulsed incoherent broadband light source in the center of a quasi-confocal high finesse cavity. The time dependent spectra of the light leaking from the cavity was compared with those of the laser-induced plasma emission without the cavity. It was found that the light emission was sustained by the cavity despite the initially large optical losses of the laser-induced plasma in the cavity. The light sustained by the cavity was used to measure part of the S(1) ← S(0) absorption spectrum of gaseous azulene at its vapour pressure at room temperature in ambient air as well as the strongly forbidden γ-band in molecular oxygen: b(1)Σ(g)(+)(ν'=2)←X(3)Σ(g)(-)(ν''=0).

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.

Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: part 2. 2-Hydroxy-1-naphthaldehyde.

The S(1)← S(0) fluorescence excitation spectrum of jet-cooled 2-hydroxy-1-naphthaldehyde (2H1N) with origin at 26,668 cm(-1) has been measured. Nine totally symmetric modes and three non-totally symmetric modes have been assigned in the excitation spectrum. Ab initio calculations indicate that 2H1N undergoes a planarity change upon excitation, which may account for the unusual intensity of non totally symmetric vibrational modes in the excitation spectrum. A number of low intensity features were observed on the low energy side of the origin which have been assigned to the 2H1N dimer rather than different ground state confomers of 2H1N. The origin of the S(1)← S(0) electronic transition of the dimer lies at ~26,401 cm(-1); combinations of two low frequency intermolecular modes of the dimer (59 cm(-1) and 17 cm(-1)) were also observed. The occurrence of excited state intramolecular proton transfer (ESIPT) in 2H1N cannot be proven on the basis of this work. A comparison of the (photo)physical properties of 2H1N with 1-hydroxy-2-naphthaldehyde (1H2N) [A. McCarthy and A.A. Ruth, PCCP, 2011, 13, 7485-7499 (Part 1)], however, indicate the plausibility of an ESIPT process in 2H1N. The strength of the intramolecular hydrogen bond (IMHB) in 2H1N was computed as ~10.6 kcal/mol, a value comparable to the IMHB strength of 1H2N. The establishment of a lower limit on the state lifetimes of 2H1N, of ~1.8 ps, indicates that any proposed ESIPT reaction in 2H1N may not proceed barrierlessly. Above an excess energy of ~1000 cm(-1), the intensity of the fluorescence excitation spectrum reduces significantly, indicating the onset of a non-radiative decay mechanism.

Fluorescence excitation and excited state intramolecular proton transfer of jet-cooled naphthol derivatives: Part 1. 1-Hydroxy-2-naphthaldehyde.

The S(0) → S(1) fluorescence excitation spectrum of jet-cooled 1H2N with origin at 25484 cm(-1) has been measured. Twelve totally symmetric modes and five non-totally symmetric modes have been assigned in the excitation spectrum. Theoretical calculations at DFT B3LYP/6-31G** and CIS/6-31G** levels indicate that the 1H2N molecular geometry is more planar in the S(1) state than in the ground state. The geometry of the naphthalene ring changes upon excitation and promotes a number of totally symmetric ring stretching modes, in the excitation spectrum. As a result of the geometry change upon excitation a number of non-totally symmetric modes gain intensity. Based on a rotational envelope fitting procedure the average excited rotational state lifetime was estimated to be between 7 and 16 ps for 0 ≤E≤ hc × 800 cm(-1) (E is excess energy above the S(1) origin). The decay rate coefficients, k, of the rotational S(1) states, are not constant over this range of excess energies. By applying a Golden Rule model, it was determined that internal conversion to S(0) is unlikely to be the sole non-radiative process contributing to the decay of the excited states. It was concluded that excited state intramolecular proton transfer (ESIPT) plays a role in the observed behaviour of the rate co-efficient with excess energy. The observation of (i) a sharp increase in rate coefficient, k, above an excess energy of ∼550 cm(-1), and (ii) a significant number of high intensity fluorescence excitation spectrum features above an excess energy of ∼700 cm(-1), may indicate the presence of an energy barrier of ∼550 cm(-1), between the enol and keto geometries in the S(1) state. This result supports the conclusions of S. De, S. Ash, S. Dalai and A. Misra, J. Mol. Struc. Theochem, 2007, 807, 33-41, who estimated a barrier to ESIPT of ∼750 cm(-1). It was concluded that ESIPT occurs in 1H2N, across an energy barrier with a rate constant, k(pt)≤ 10(11) s(-1). Hence, at low excess energies (≤ 550 cm(-1)), the observed emission band originates predominantly from the keto tautomer. Above an excess energy of ∼1600 cm(-1), 1H2N decays predominantly via a non-radiative mechanism.

Molecular iodine emission rates and photosynthetic performance of different thallus parts of Laminaria digitata (Phaeophyceae) during emersion.

The emission of molecular iodine (I(2)) from the stipe, the meristematic area and the distal blade of the brown macroalga Laminaria digitata (Hudson) Lamouroux (Phaeophyceae) was monitored under low light and dark conditions. Photosynthetic parameters were determined to investigate both the extent of stress experienced by different thallus parts and the effects of emersion on photosynthesis. Immediately after air exposure, intense I(2) emission was detectable from all thallus parts. I(2) emission declined continuously over a period of 180 min following the initial burst, but was not affected by the light regime. The total number of mole of I(2) emitted by stipes was approximately 10 times higher than those emitted from other thallus parts. Initial I(2) emission rates (measured within 30 min of exposure to air) were highest for stipes (median values: 2,999 and 5,222 pmol g(-1) dw min(-1) in low light and dark, respectively) and lower, by one order of magnitude, for meristematic regions and distal blades. After exposure to air for between 60 and 180 min, I(2) emission rates of all thallus parts were reduced by 70-80%. Air exposure resulted in a decrease of the maximum photosystem II (PSII) efficiency (F(v)/F(m)) by 3%, and in a 25-55% increase of the effective PSII quantum efficiency (F(v)/F'(m)); this was caused by a higher fraction of open reaction centres (q(P)), whereas the efficiency of the latter in capturing energy (F'(v)/F'(m)) remained constant. The results indicate the presence of an iodine pool which is easily volatilised and depleted due to air exposure, even under apparently low stress conditions.

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.

Incoherent broadband cavity-enhanced total internal reflection spectroscopy of surface-adsorbed metallo-porphyrins.

An improvement of conventional attenuated total reflection (ATR) spectroscopy is demonstrated by applying an incoherent broadband light source (short-arc Xe-lamp) in a cavity-enhanced evanescent-wave absorption method. With this novel approach the absorption spectra of several metallo-octaethyl porphyrins (palladium (PdOEP), platinum (PtOEP) and zinc (ZnOEP)) in thin acetone solution layers and on a fused silica (FS) surface are studied between approximately 390 and 625 nm. The time dependence of the evaporation process of the solution on the FS surface is described. The maximum sensitivity of the setup is estimated at approximately 2 x 10(-5) per pass, which translates into a minimal detectable surface density of less than 2 x 10(-3) monolayers for the porphyrins studied (based on the strong absorption in the Soret bands). Changes of surface and solution spectra are characterised and discussed on the basis of observed band broadenings and spectral shifts. For Pd- and PtOEP the changes of spectral feature can be interpreted with respect to J-aggregate formation supported by polarization dependent measurements. The reason for an observed blue-shift of ca. 10 nm for the Soret band in ZnOEP in combination with a large red-shift of the Q-bands (11 nm for Q1 and 18 nm for Q2) is discussed.

Incoherent broad-band cavity-enhanced absorption spectroscopy of the marine boundary layer species I2, IO and OIO.

The novel combination of incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS) and a discharge-flow tube for the study of three key atmospheric trace species, I(2), IO and OIO, is reported. Absorption measurements of I(2) and OIO at lambda=525-555 nm and IO at lambda=420-460 nm were made using a compact cavity-enhanced spectrometer employing a 150 W short-arc Xenon lamp. The use of a flow system allowed the monitoring of the chemically short-lived radical species IO and OIO to be conducted over timescales of several seconds. We report detection limits of approximately 26 pmol mol(-1) for I(2) (L=81 cm, acquisition time 60 s), approximately 45 pmol mol(-1) for OIO (L=42.5 cm, acquisition time 5 s) and approximately 210 pmol mol(-1) for IO (L=70 cm, acquisition time 60 s), demonstrating the usefulness of this approach for monitoring these important species in both laboratory studies and field campaigns.

Incoherent broadband cavity-enhanced absorption spectroscopy in the near-ultraviolet: application to HONO and NO2.

The first application of incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) in the near-ultraviolet for the simultaneous detection of two key atmospheric trace species, HONO and NO2, is reported. For both compounds the absorption is measured between 360 and 380 nm with a compact cavity-enhanced spectrometer employing a high power light-emitting diode. Detection limits of approximately 4 ppbv for HONO and approximately 14 ppbv for NO2 are reported for a static gas cell setup using a 20 s acquisition time. Based on an acquisition time of 10 min and an optical cavity length of 4.5 m detection limits of approximately 0.13 ppbv and approximately 0.38 ppbv were found for HONO and NO2 in a 4 m3 atmospheric simulation chamber, demonstrating the usefulness of this approach for in situ monitoring of these important species in laboratory studies or field campaigns.

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.

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 sensitivity in situ monitoring of NO3 in an atmospheric simulation chamber using incoherent broadband cavity-enhanced absorption spectroscopy.

We describe the application of incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) for the in situ detection of atmospheric trace gases and radicals (NO3, NO2, O3, H2O) in an atmospheric simulation chamber under realistic atmospheric conditions. The length of the optical cavity across the reaction chamber is 4.5 m, which is significantly longer than in previous studies that use high finesse optical cavities to achieve high absorption sensitivity. Using a straightforward spectrometer configuration, we show that detection limits corresponding to typical atmospheric concentrations can be achieved with a measurement time of seconds to a few minutes. In particular, with only moderate reflectivity mirrors, we report a measured sensitivity of 4 pptv to NO3 in a 1 min acquisition time. The high spatial and temporal resolution of the IBBCEAS method and its pptv sensitivity to NO3 makes it useful in laboratory studies of atmospheric processes as well as having obvious potential for field measurements.

Spin-lattice relaxation and rotational motion of aromatic triplet-state molecules in supercooled alkane solvents (part 1).

The phosphorescence of phenazine (PZ) and quinoxaline (QX) was investigated after pulsed laser excitation in the glass-transition range of several alkane solvents. Three relaxation processes of PZ and QX in the metastable triplet state, T1, were studied as a function of temperature: (1) the decay of the selective population of the strongly phosphorescent triplet substate T1x due to spin-lattice relaxation (SLR), (2) the time-dependent red shift of the phosphorescence spectrum due to the solvation of triplet-state molecules, and (3) the decay of the phosphorescence polarization due to orientational relaxation (OR). Various aspects and connections of the mechanisms governing the three relaxation phenomena are discussed. The relaxation dynamics were characterized at temperatures above the glass-transition temperature of the respective solvent, where the fundamental processes involved are strongly dependent upon the solvent viscosities. For the systems treated here, OR and solvation were satisfactorily described by a Vogel-Fulcher-Tammann temperature behavior. SLR also depends on properties of the alkane solvent above the glass transition. Upon cooling, SLR becomes independent of the specific solvent properties and is based on mechanisms that are typical for amorphous glasses or solids. (This particular aspect will be the subject of a subsequent publication, part 2).