Consecutive photodissociation of a single complex molecular ion

Photodissociation is a general tool for gaining information about the structure and dynamics of molecules 1, and it is known to play an important role for the physics and chemistry of, e.g., planetary atmospheres 2 and in interstellar clouds 3. Photodissociation can also be a pathway to create otherwise not easily accessible neutral molecules 4,5 in the gas phase. Furthermore, some specific molecular anions 6 and various positively charged radicals 7 have been produced very efficiently through photodissociation. In general, for charged photofragments, both the absolute number of ions produced as well as the chemical composition of the charged fragments can efficiently be determined by the use of time-of-flight TOF8,9, quadrupole 8,10, or cyclotron 11,12 mass spectrometers. However, for complex molecular ions which often disintegrate into smaller fragments through a series of consecutive dissociation steps when exposed to light, the use of the above mass spectrometers makes it difficult to identify the individual steps in the photodissociation chains since experiments typically involve larger ensembles of ions 13,14. If only a single ion is present in the experiment at the time, and its composition can be determined nondestructively at various times, one will avoid this problem. Recently, we demonstrated a technique, based on rf excitation of center-of-mass resonant modes, that allows nondestructive identification of a single molecular ion trapped and sympathetically cooled by a laser-cooled atomic ion 15. Besides allowing for consecutive measurements of the molecular ion mass with a relative mass resolution of better than 10 �3 during an experiment, the sympathetic cooling leaves the molecular ion translationally very cold 10 mK and spatially localized to better than a few microns 15. In this Rapid Communication, we describe how our method can be used to study the consecutive photodissociation of a single complex molecular ion. Using the aniline ion irradiated by the combination of cw light at 397 nm and nanosecond pulsed light at 294 nm as an example, we show how the sequential breakage into lighter fragments can be monitored nondestructively on time scales up to several minutes, and potentially up to several hours. Among the variety of ions produced and identified, the lightest species, robust against further photodissociation, is found to be c-C3H3 + cyclopropenylium. Our motivation for the present study is manifold. First, we want to prove that our nondestructive detection method of a single trapped molecular ion allows detailed studies of the time evolution of light-induced consecutive fragmentation at time scales from milliseconds up to several hours, a time span unmatched by other techniques. Second, we want to prove that the non-destructive detection technique of consecutive fragmentation is a viable way for probabilistic preparation of a wealth of single molecular ions which could be used as targets for, e.g., astrophysical studies. Third, more generally we want to prove that studies of spatially localized and very cold single ions can be extended from atomic and diatomic systems 15 to complex molecular systems. This opens new opportunities in molecular science, including molecular rotational dynamics and chemical reaction dynamics on ultralong time scales. Our experimental setup is sketched in Fig. 1. The central part of the experiments is a linear Paul trap consisting of four rods, each divided into three sections. By keeping the central section of each rod at a lower voltage than the end sections, confinement of positively charged ions along the trap axis