L. W. Farrar

Sensitive Detection and Identification of Isovanillin Aerosol Particles at the pg/cm3 Mass Concentration Level Using Raman Spectroscopy

A compact Raman spectroscopy system with high sensitivity to chemical aerosols has been developed. This system has been used to detect isovanillin aerosols with mass concentration of 12 pg/cm3 in a 15 s signal integration period with a signal-to-noise ratio of 32. We believe this represents the lowest chemical aerosol concentration and signal integration period product ever reported for a Raman spectroscopy system. The Raman system includes (i) a 10 W, 532-nm cw laser, (ii) an aerosol flow cell, (iii) a 60× aerosol concentrator, (iv) an f/1.8 Raman spectrometer with a spectral range of 400–1400 cm−1 and a resolution of 4 cm−1, and (v) a low-noise CCD camera (1340 × 400 pixels). The collection efficiency of the Raman system has been determined to be 2.8%. Except for the laser cooling subsystem, the Raman system fits in a 0.61 m × 0.61 m × 0.61 m box. Copyright 2015 American Association for Aerosol Research

Measurement of the surface-enhanced coherent anti-Stokes Raman scattering (SECARS) due to the 1574 cm-1 surface-enhanced Raman scattering (SERS) mode of benzenethiol using low-power (<20 mW) CW diode lasers

The surface-enhanced coherent anti-Stokes Raman scattering (SECARS) from a self-assembled monolayer (SAM) of benzenethiol on a silver-coated surface-enhanced Raman scattering (SERS) substrate has been measured for the 1574 cm−1 SERS mode. A value of 9.6 ± 1.7 × 10−14 W was determined for the resonant component of the SECARS signal using 17.8 mW of 784.9 nm pump laser power and 7.1 mW of 895.5 nm Stokes laser power; the pump and Stokes lasers were polarized parallel to each other but perpendicular to the grooves of the diffraction grating in the spectrometer. The measured value of resonant component of the SECARS signal is in agreement with the calculated value of 9.3 × 10−14 W using the measured value of 8.7 ± 0.5 cm−1 for the SERS linewidth Γ (full width at half-maximum) and the value of 5.7 ± 1.4 × 10−7 for the product of the Raman cross section σSERS and the surface concentration Ns of the benzenethiol SAM. The xxxx component of the resonant part of the third-order nonlinear optical susceptibility |3χ(3)R xxxx | for the 1574 cm−1 SERS mode has been determined to be 4.3 ± 1.1 × 10−5 cmg−1·s2. The SERS enhancement factor for the 1574 cm−1 mode was determined to be 3.6 ± 0. 9 × 107 using the value of 1.8 × 1015 molecules/cm2 for Ns.

Measurement of the Raman Line Widths of Neat Benzenethiol and a Self-Assembled Monolayer (SAM) of Benzenethiol on a Silver-Coated Surface-Enhanced Raman Scattering (SERS) Substrate

Raman line widths of neat benzenethiol and a self-assembled monolayer (SAM) of benzenethiol on a surface-enhanced Raman scattering (SERS) substrate have been measured using a mini spectrometer with a resolution (full width at half-maximum) of 3.3 ± 0.2 cm−1. Values of 7.3 ± 0.7, 4.6 ± 0.6, 2.4 ± 0.6, 3.2 ± 0.5, 8.8 ± 0.9, and 11.0 ± 1.1 cm−1 have been determined for the Raman line widths of the 414, 700, 1001, 1026, 1093, and 1584 cm−1 modes of neat benzenethiol. Values of 13.3 ± 0.7, 9.1 ± 0.7, 5.1 ± 0.6, 5.9 ± 0.6, 13.3 ± 0.5, and 8.7 ± 0.5 cm−1 have been determined for the SERS line widths of a benzenethiol SAM on a silver-coated SERS substrate for the corresponding frequency-shifted modes at 420, 691, 1000, 1023, 1072, and 1574 cm−1. The line widths for the SERS modes at 420, 691, 1000, 1023, and 1072 cm−1 are about a factor of two larger than those of the corresponding Raman modes. However, the line width of the SERS mode at 1574 cm−1 is slightly smaller than the corresponding Raman mode at 1584 cm−1.

Raman spectra and cross sections of ammonia, chlorine, hydrogen sulfide, phosgene, and sulfur dioxide toxic gases in the fingerprint region 400-1400 cm−1

Raman spectra of ammonia (NH3), chlorine (Cl2), hydrogen sulfide (H2S), phosgene (COCl2), and sulfur dioxide (SO2) toxic gases have been measured in the fingerprint region 400-1400 cm−1. A relatively compact (<2′x2′x2′), sensitive, 532 nm 10 W CW Raman system with double-pass laser and double-sided collection was used for these measurements. Two Raman modes are observed at 934 and 967 cm−1 in NH3. Three Raman modes are observed in Cl2 at 554, 547, and 539 cm−1, which are due to the 35/35 35/37, and 37/37 Cl isotopes, respectively. Raman modes are observed at 870, 570, and 1151 cm−1 in H2S, COCl2, and SO2, respectively. Values of 3.68 ± 0.26x10−32 cm2/sr (3.68 ± 0.26x10−36 m2/sr), 1.37 ± 0.10x10−30 cm2/sr (1.37 ± 0.10x10−34 m2/sr), 3.25 ± 0.23x10−31 cm2/sr (3.25 ± 0.23x10−35 m2/sr), 1.63 ± 0.14x10−30 cm2/sr (1.63 ± 0.14x10−34 m2/sr), and 3.08 ± 0.22x10−30 cm2/sr (and 3.08 ± 0.22x10−34 m2/sr) were determined for the differential Raman cross section of the 967 cm−1 mode of NH3, sum of the 554, 547, and 539 cm−1 modes of Cl2, 870 cm−1 mode of H2S, 570 cm−1 mode of COCl2, and 1151 cm-1 mode of SO2, respectively, using the differential Raman cross section of 3.56 ± 0.14x10−31 cm2/sr (3.56 ± 0.14x10−35 m2/sr) for the 1285 cm−1 mode of CO2 as the reference.

Chemical aerosol Raman detector

A sensitive chemical aerosol Raman detector (CARD) has been developed for the trace detection and identification of chemical particles in the ambient atmosphere. CARD includes an improved aerosol concentrator with a concentration factor of about 40 and a CCD camera for improved detection sensitivity. Aerosolized isovanillin, which is relatively safe, has been used to characterize the performance of the CARD. The limit of detection (SNR = 10) for isovanillin in 15 s has been determined to be 1.6 pg/cm3, which corresponds to 6.3 × 109 molecules/cm3 or 0.26 ppb. While less sensitive, CARD can also detect gases. This paper provides a more detailed description of the CARD hardware and detection algorithm than has previously been published.

Raman detection of single airborne aerosol particles of isovanillin

Single airborne aerosol particle of isovanillin were detected using a compact Raman spectroscopy system. The Raman system consisted of a 10 W, 532-nm cw laser, a 50x aerosol concentrator, an aerosol flow cell, an f/1.0 single-sided collection optics, an f/1.8 Raman spectrometer with a spectral range of 400-1400 cm-1, and a low-noise CCD camera (1340 x 400 pixels; 20 x 20 μm/pixel). The combined collection and detection efficiency of the Raman system was 1.0%. The diameters of eleven particles were determined to be 3.4, 3.1, 3.5, 3.4, 2.3, 3.1, 2.5, 2.6, 2.5, 3.0, and 3.1μm based on the fundamental Raman equation. The accuracy of the particle diameter is estimated to be ±0.1 μm using measured concentration of the atmospheric CO2.