Solvent response to fluorine-atom reaction dynamics in liquid acetonitrile.

Time-resolved infra-red (IR) absorption spectroscopy is used to follow the production of HF from the reaction of fluorine atoms in liquid acetonitrile (CH3CN). Photolysis of dissolved XeF2 using ∼50 fs duration, 267 nm laser pulses generates F atoms and XeF on prompt (sub-ps) timescales, as verified by broadband transient electronic absorption spectroscopy. The fundamental vibrational band of HF in solution spans more than 400 cm(-1) around the band centre at 3300 cm(-1), and analysis of portions of the time-resolved spectra reveals time constants for the rise in HF absorption that become longer to lower wavenumber. The time constants for growth of 40 cm(-1) wide portions of the IR spectra centred at 3420, 3320 and 3240 cm(-1) are, respectively, 3.04 ± 0.26, 5.48 ± 0.24 and 7.47 ± 0.74 ps (1 SD uncertainties). The shift to lower wavenumber with time that causes these changes to the time constants is attributed to evolution of the micro-solvation environment of HF following the chemical reaction. The initial growth of the high-wavenumber portion of the band may contain a contribution from relaxation of initially vibrationally excited HF, for which a time constant of 2.4 ± 0.2 ps is deduced from IR pump and probe spectroscopy of a dilute HF solution in acetonitrile.