Coupling of Carbon Dioxide Stretch and Bend Vibrations Reveals Thermal Population Dynamics in an Ionic Liquid

The population relaxation of carbon dioxide dissolved in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf(2)) Was investigated using polarization-selective ultrafast infrared pump-probe spectroscopy and two-dimensional infrared (2D IR) spectroscopy. Due to the coupling of the bend with the asymmetric stretch, excitation of the asymmetric stretch of a molecule with a thermally populated bend leads to an additional peak, a hot band, which is red-shifted from: the main asymmetric absorption band by,the combination band shift. This hot band peak exchanges population with-the main peak-through the gain and loss of bend excitation quanta. The isotropic-pump probe signal originating from the unexcited bend state displays a fast, relatively small amplitude, initial growth followed by a longer time scale exponential decay. The signal is analyzed: over its full time range using a kinetic model to determine both the-vibrational lifetime (the-final decay) and rate constant for the loss of:the bend-energy. This bend relaxation manifests as the fast initial growth,of the stretch/no, bend signal because the hot-band (stretch with bend) is over pumped relative to the ground state equilibrium. The nonequilibrium pumping, occurs because the hot band has a larger transition dipole moment than the stretch/no bend peak. The system is then prepared, utilizing an acousto-optic mid-infrared pulse shaper to: cut a hole in the excitation pulse spectrum, such that the hot band is not pumped. The isotropic pump-probe signal from the stretch/no bend state is altered because the initial excited state population ratio has changed. Instead of-a growth due to relaxation of bend quanta, a fast initial decay is observed because of thermal excitation of the bend. Fitting thiS: curve gives the-rate constant for thermal excitation of the bend And the lifetime, which agree with those determined in the pump-probe experiments without frequency-selective pumping.