Infrared frequency comb spectroscopy of CH₂I₂: Influence of hot bands and pressure broadening on the ν₁ and ν₆ fundamental transitions

Direct frequency comb spectroscopy was utilized to measure the vibrational absorption spectrum of diiodomethane, CH₂I₂, from 2960 to 3125 cm⁻¹. The data were obtained using a CH2I2 concentration of (6.8 ± 1.3) × 10¹⁵ molecule cm⁻³ and a total pressure of 10–300 mbar with either nitrogen or argon as the bath gas. The rovibrational spectra of two fundamental transitions, ν₆ and ν₁, were recorded and analyzed. We suggest that a significant contribution to the observed congested spectra is due to the population in excited vibrational states of the low energy ν₄ I–C–I bend, resulting in transitions 6¹₀4ⁿ_n and 1104nn, where the integer n is the initial vibrational level v = 1–5. PGOPHER was used to fit the experimental spectrum, allowing for rotational constants and other spectral information to be reported. In addition, it was found that the peak widths for the observed transitions were limited by pressure broadening, resulting in a pressure broadening parameter of (0.143 ± 0.006) cm⁻¹ atm⁻¹ by N₂ and (0.116 ± 0.006) cm⁻¹ atm⁻¹ by Ar. Further implications for other dihaloalkane infrared spectra are discussed.