Detection of Sulfur Dioxide by Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS)

Sulfur dioxide (SO2) is an important precursor for formation of atmospheric sulfate aerosol and acid rain. We present an instrument using Broad Band Cavity Enhanced Absorption Spectroscopy (BBCEAS) for the measurement of SO2 with a minimum limit of detection of 0.6 ppbv using the spectral range 305.5–312 nm and an averaging time of 60 seconds. The instrument consists of high reflectivity mirrors (0.9984 at 310 nm) and a deep UV light source. The effective absorption path length of the instrument is 610 m in a 0.957 m base length. Published reference absorption cross-sections were used to fit and retrieve the SO2 concentrations and were compared to a diluted standard for SO2. The comparison was well correlated, R2 = 0.9985 with a correlation slope of 1.01.

FLASH early science – discovery of an intervening H i 21-cm absorber from an ASKAP survey of the GAMA 23 field

ABSTRACT We present early science results from the First Large Absorption Survey in H i (FLASH), a spectroscopically blind survey for 21-cm absorption lines in cold hydrogen (H i) gas at cosmological distances using the Australian Square Kilometre Array Pathfinder (ASKAP). We have searched for H i absorption towards 1253 radio sources in the GAMA 23 field, covering redshifts between z = 0.34 and 0.79 over a sky area of approximately 50 deg2. In a purely blind search, we did not obtain any detections of 21-cm absorbers above our reliability threshold. Assuming a fiducial value for the H i spin temperature of Tspin = 100 K and source covering fraction cf = 1, the total comoving absorption path-length sensitive to all Damped Lyman α Absorbers (DLAs; NH i ≥ 2 × 1020 cm−2) is ΔX = 6.6 ± 0.3 (Δz = 3.7 ± 0.2) and super-DLAs (NH i ≥ 2 × 1021 cm−2) is ΔX = 111 ± 6 (Δz= 63 ± 3). We estimate upper limits on the H i column density frequency distribution function that are consistent with measurements from prior surveys for redshifted optical DLAs, and nearby 21-cm emission and absorption. By cross-matching our sample of radio sources with optical spectroscopic identifications of galaxies in the GAMA 23 field, we were able to detect 21-cm absorption at z = 0.3562 towards NVSS J224500−343030, with a column density of $N_{\rm H\,\small{I}} = (1.2 \pm 0.1) \times 10^{20}\, (T_{\rm spin}/100\, \mathrm{K})$ cm−2. The absorber is associated with GAMA J22450.05−343031.7, a massive early-type galaxy at an impact parameter of 17 kpc with respect to the radio source and which may contain a massive (MH i ≳ 3 × 109 M⊙) gas disc. Such gas-rich early types are rare, but have been detected in the nearby Universe.

On-chip cavity-enhanced absorption spectroscopy using a white light-emitting diode and polymer mirrors

We have integrated disposable polymer mirrors within a microfluidic chip to form a multi-pass cell, which increases the absorption path length by a maximum of 28 times, providing micromolar detection limits in a probed volume of 10 nL.

Measurement of rovibrational ΔJ = 0, 4, and 6 transitions in solid H2 together with their associated pair spectra

Weak features in the infrared spectum of solid para-hydrogen containing a small amount of orth-hydrogen impurity were measured with high resolution (0.01–0.036 cm−1) and high sensitivity using a Fourier transform spectrometer. Effective absorption path lengths in the solid of 3.75 cm (for the range 4 000–10 000 cm−1) and 15.0 cm (for the range 5000–8000 cm−1) were used to observe the discrete zero-phonon rovibrational transitions U1←0(0), U1←0(1), and W1←0(0), for which accurate line positions and intensity parameters are given. Line positions for the associated satellite spectra for each of those transitions and for the Q1←0(0) and Q1←0(1) transitions were also determined. New features in the satellite spectra of the Q1←0(0), Q1←0(1), U1←0(0), and U1←0(1) transitions are reported. A novel ortho-hydrogen to para-hydrogen converter allowed convenient variation of the temperature for conversion. A multipass optical system external to the head of the cryostat was designed to extend the absorption path length through the hydrogen crystal.

An Infrared Spectrum of CO2 Dimers in the "Locked" Configuration

The two components of the (ν1, 2ν2) Fermi doublet of gaseous CO2 in an absorption path length of 56 m at 192 °K show a complex structure. When the allowed C16O18O absorption and the pressure-induced CO2 absorption are removed by computational procedures, the residual spectrum consists of two similar symmetric patterns of five maxima. These are interpreted in terms of the rotation and vibration of (CO2)2 dimers held by quadrupole–quadrupole interaction in the locked T position at an intermolecular distance of 4.1 Å.

Collision-induced spectra of hydrogen in the first and second overtone regions with applications to planetary atmospheres

The pressure-induced spectra of hydrogen in the regions of the first and second overtones have been recorded with an absorption path length of 137 m in the temperature range 85 to 116 K. For pure hydrogen the structure and integrated intensities of the bands are in good agreement with calculations based on the theory of quadrupole induction, except that the second overtone shows evidence of a small amount of overlap induction. The enhancements of the first overtone in hydrogen + argon and hydrogen + nitrogen mixtures have structures in accordance with quadrupole induction, but the calculated intensities are somewhat too high. The enhancement of the first overtone in the hydrogen + helium mixture was too weak to be recorded. Calculated profiles for the third overtone in pure hydrogen are given. The implication of these measurements for studies of the hydrogen and helium content of planetary atmospheres is discussed.

The induced infrared fundamental band of hydrogen dissolved in solid argon

The induced infrared fundamental band of hydrogen dissolved (~1:100) in solid argon was studied with a 20-cm absorption path length at −191 °C. Transparent crystals were prepared by slow cooling of the liquid solution saturated with hydrogen at ~25 atm pressure. The H2 transitions, Q, S(0), and S(1), show similar patterns of five maxima, each of which can be analyzed as a zero-phonon line at the H2 frequency and summation and difference tones with lattice transition frequencies, 112 and 22 cm−1. The 112-cm−1 frequency is interpreted as arising from a localized lattice vibration involving an H2 molecule on a substitutional lattice site. Calculation from a model of an H2 molecule moving in the field of its argon neighbors, considered stationary, gave 109 cm−1 for this frequency. The origins of the zero-phonon lines and the 22-cm−1 lattice transition frequency are not so clear, and several possibilities are discussed. The H2 frequencies are shifted from their free-molecule values by the sum of a vibrational shift, Δνvlb = −17 cm−1, and rotational shifts corresponding to ΔB = −0.52 cm−1.