New Method for Double-Resonance Spectroscopy in a Cold Quadrupole Ion Trap and Its Application to UV–UV Hole-Burning Spectroscopy of Protonated Adenine Dimer

A novel method for double-resonance spectroscopy in a cold quadrupole ion trap is presented, which utilizes dipolar resonant excitation of fragment ions in the quadrupole ion trap. Photofragments by a burn laser are removed by applying an auxiliary RF to the trap, and a probe laser detects the depletion of photofragments by the burn laser. By scanning the wavelength of the burn laser, conformation-specific UV spectrum of a cold ion is obtained. This simple and powerful method is applicable to any type of double-resonance spectroscopy in a cold quadrupole ion trap and was applied to UV–UV hole-burning spectroscopy of protonated adenine dimer. It was found that protonated adenine dimer has multiple conformers/tautomers, each with multiple excited states with drastically different excited state dynamics.