High-Resolution Absorption Studies of the ùA₂−X̃¹A₁2₀²4₀¹Band of Formaldehyde

Abstract

Absolute peak absorption cross sections and pressure broadening coefficients have been recorded with sub-Doppler limited instrumental resolution for selected rotational lines in the 2(0)(2)4(0)(1) vibronic band of the formaldehyde A(1)A2-X(1)A1 electronic transition. The measured absorption cross sections range between (0.18 +/- 0.01) and (10.1 +/- 0.08) x 10(-19) cm2 molecule(-1) and are considerably larger than values from the literature recorded using apparatus where instrumental broadening was significant. However, comparisons with spectral simulations with equivalent resolution from Smith et al. (J. Phys. Chem. A 2006, 110, 11645-11653) are in excellent agreement. Pressure broadening was studied for the collision partners CH2O, CO2, N2, O2, Ne, Kr, Ar, and He, and the resulting broadening coefficients were found to be reduced in comparison to equivalent values measured in infrared regions, consistent with the reduced dipole moment of the upper state probed in this work. Cavity-enhanced absorption spectroscopy (CEAS) measurements were undertaken using calibrated low concentration (2.9-4.6 ppmv) samples from a permeation source and demonstrate a noise equivalent absorption of 1.2 x 10(-6) cm(-1) Hz(-1/2). This implies a minimum detectable formaldehyde concentration with the current system in atmospheric air of 172 ppbv Hz(-1/2).

High-Resolution Absorption Cross Sections of Formaldehyde in the 30285–32890 cm–1 (304–330 nm) Spectral Region

“…A comparison of the rotational resolution of this study with recent work is shown in Figure 4, which shows a 9 cm −1 section of the 2 0 2 4 0 1 band. The figure shows the current work and, for comparison, recent studies by Smith et al, 35 Motsch et al, 37 and Crow et al 38 where the four spectra overlap. The reported resolutions of the three preceding 35 measured at a resolution of 0.35 cm −1 , the current study measured at a resolution of 0.09 cm −1 , and Crow et al 38 with a reported resolution of 3.3 × 10 −4 cm −1 and the relative absorption of Motsch et al 37 with a reported (rebinned) resolution of 0.003 cm −1 , which is plotted on an arbitrary scale.…”

“…The figure shows the current work and, for comparison, recent studies by Smith et al, 35 Motsch et al, 37 and Crow et al 38 where the four spectra overlap. The reported resolutions of the three preceding 35 measured at a resolution of 0.35 cm −1 , the current study measured at a resolution of 0.09 cm −1 , and Crow et al 38 with a reported resolution of 3.3 × 10 −4 cm −1 and the relative absorption of Motsch et al 37 with a reported (rebinned) resolution of 0.003 cm −1 , which is plotted on an arbitrary scale. studies were 0.35, 0.003 (rebinned from an initial resolution of 1.67 × 10 −5 cm −1 ), and 3.3 × 10 −4 cm −1 , respectively.…”

“…The resolution of the Motsch et al 37 spectrum is sufficient to fully resolve the Doppler broadened lines but focused on line positions and did not determine absolute cross sections. The Crow et al 38 spectrum fully resolves the absorption features but was limited to two 10 cm −1 sections of the 2 0 2 4 0 1 band and only obtained absolute cross section values for eight individual HCHO lines. The line width of the laser used in the current study is slightly broader than the Doppler width of a rotational line of HCHO at 300 K and so we were able to resolve all but the most closely spaced lines.…”

“…Motsch et al 37 investigated the absorption features of HCHO at a resolution of 1.67 × 10 −5 cm −1 , which enabled precise determinations of line positions and line strengths of the 2 0 1 4 0 3 and 2 0 2 4 0 1 rovibrational bands between 30140 and 30800 cm −1 (325−328 nm). Crow et al 38 studied two approximately 10 cm −1 sections of the 2 0 2 4 0 1 vibronic band of HCHO at high-resolution (3.3 × 10 −4 cm −1 ) and measured absolute peak absorption cross sections at select rotational lines across the band. That study also examined the pressure broadening for several collisional partners and determined broadening coefficients.…”

Laser Spectroscopy for Atmospheric and Environmental Sensing

“…The new instrument was also fully automated, including the sample pre-concentration. A CEAS has been used to study the absorption spectrum of formaldehyde in high resolution [204]. The spectrometer uses two laser sources: a diode pumped Nd:YVO 4 laser at 532 nm and an external cavity diode laser at 841 nm.…”