3.508μ-HeXe-laser line absorption by formaldehyde monomer and dimer and by nitrogen dioxide monomer and dimer without and in the presence of argon

Abstract

Transmission of a single-mode HeXe-laser line at ω L = 2850.633 cm −1 through formaldehyde at room-temperature reveals off-center single-line absorption in the 5 1,5( ν 5 = 0) → 6 0,6( ν 5 = 1) transition by formaldehyde monomer (Fm) and line-center absorption by a formaldehyde dimer (Fd) which is Lorentzian for formaldehyde partial pressures of p ≳ 0.1 atm. Coefficients for self- and foreign-broadening of the monomer by dimer, argon, nitrogen dioxide monomer, and nitrogen dioxide dimer were determined by taking into account the laser-absorption by formaldehyde dimer and, where applicable, by nitrogen dioxide monomer (1) and dimer (2). The results at T 0 = 300°K are b Fm 0 ≡ b Fm Fm 0 (⋍b Fm Fd 0) = 6.13 × 10 −1 cm −1- atm −1 (±3.0%) , b Fm Ar 0 = 3.92 × 10 −2 cm −1-atm −1 (±5.5%), b Fm 2 0 ⪡ b Fm 1 0 = 3.22 × 10 −1 cm −1-atm −1 (±8.8%) based upon a dimer absorption governed by SK/ b Ed 0 = 36.7 cm −1-atm −2 (±0.7%) with the equilibrium constant K Ed 0 = ( p Fd/ p Fm 2) eq at 300°K and a previously established 9 high-precision value for the monomer line-strength of S Fm 0 = 6.81 × 10 −1 cm −2-atm −1 (±0.5%). Best data correlations were obtained with b Fd 0 ≡ b Fd Fm 0 ⋍ b Fd Fd 0 ⋍ b Fd Ar 0 ⋍ b Fd 1 0 ⋍ b Fd 2 0 for (H 2CO) x Ar and (H 2CO) x Ar mixtures. A high-precision value of ω Fm − ω L = 5.80 × 10 −3 cm −1 (±0.55%) was obtained by low-pressure ( p ≲ 1.6 × 10 −3 atm) pure formaldehyde laser-absorption with the above values for b Fm 0 and S Fm 0. Room-temperature results of HeXe-laser absorption at 3.508μ by NO 2 monomer (1) and dimer (2) at total pressures p ≳ 0.13 atm could best be interpreted by far-wing line-absorption for NO 2 and Lorentzian line-center absorption for N 2O 4. The corresponding absorption coefficients at T 0 = 300°K are Sb/ Δω 2 π) 1 0 p = 4.90 × 10 −2/( p/atm) cm −1-atm −1 (±5.2%) and S/ bπ) 2 0/ p = 8.44 × 10 −3/( p/atm) cm −1-atm −1 (±4.6%) with b 1 0 ≡ b 11 0 ⋍ b 12 0 and b 12 0 ≡ b 22 0 ⋍ b 21 0 . The present low-temperature results and previous high-temperature shock-tube laser-absorption data for formaldehyde-argon mixtures were combined and interpreted in terms of formaldehyde monomer and dimer absorption. The resulting temperature dependences for the line-strengths are S Fm( T) = S Fm 0( T 0/ T) m with m = 1.79 (±7.3%) for the monomer and (SK/b) Fd (T) = (SK/b) Fd 0(T/T 0) n + 1 2 exp[+( ΔH 0/ RT 0) × ( T 0/ T − 1)] with n = 1.36 (±20%) and ΔH 0 = 940 cal/mole 12 for the dimer for T ⪅ 1600°K.