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Description
Cryogenic Infrared action spectroscopy has been proven to be effective for the experimental characterization of the vibrational modes of a large variety of molecular ions and ionic clusters in the gas phase[i]. Here, the ion of interest is isolated by a quadrupole mass filter and accumulated in a hexapole ion trap, where is traversed by a beam of helium nanodroplets that pick up the selected ions and when left the ion trap are probed by infrared light of a free electron laser (FHI-FEL). When the frequency of the laser is resonant with a vibrational transition of the ion the absorption of a photon occurs, with a subsequent intramolecular vibrational redistribution (IVR) among all vibrational modes in the picosecond (ps) timescale. This energy is quickly transfer to the helium droplet, which by evaporation of helium atoms (evaporative cooling) returns to its equilibrium temperature.. This has an outstanding consequence; as the absorption-dissipation process occurs at the ps timescale, every micro-pulse of the FEL (separated by 1 ns) encounters the ion at the equilibrium temperature of the helium droplet (0.37 K), always probing the ion at its vibrational ground state. The spectrum, then, is obtained by the detection of the bare ions in a time-of-flight analyzer as a function of the FEL wavenumber
Using two FEL pulse trains with a controlled pulse delay, we can perform one- and two-color IR-IR pump-probe spectroscopy on molecular ions in helium droplets. With the one-color pump-probe methodology, vibrational lifetimes can be measured by integrating the signal intensity as a function of the delay time between the two pulse trains. By employing a probe of a different wavelength (two-color), the flow of the energy from one mode to another can be followed, and the vibrational dynamics can be described.
