Figure 7.
Comparison of types of IR action spectroscopy. (A) Instrumentation and scheme of action for infrared multiple photon dissociation (IRMPD) spectroscopy. With resonant irradiation, multiple IR photons excite the intact parent ion until the fragmentation threshold is reached. Fragment and parent ions are detected. (B) Instrumentation and scheme of action for messenger (tagging) spectroscopy. Ions are tagged with buffer gas atoms or molecules, e.g., N2, He, or Ar, in a cryogenic ion trap. With resonant irradiation typically with a single photon, the tag is detached, and the bare ion is detected. (C) Instrumentation and scheme of action for cryogenic spectroscopy in helium nanodroplets. The ions are picked up by helium nanodroplets in an ion trap and cooled to 0.4 K. With resonant irradiation, the ion is excited and immediately cooled again by evaporative cooling. After several iterations, the ion is released from the nanodroplet and detected.