A cylindrical quadrupole ion trap in combination with an electrospray ion source for gas-phase luminescence and absorption spectroscopy

Abstract

A relatively simple setup for collection and detection of light emitted from isolated photo-excited molecular ions has been constructed. It benefits from a high collection efficiency of photons, which is accomplished by using a cylindrical ion trap where one end-cap electrode is a mesh grid combined with an aspheric condenser lens. The geometry permits nearly 10% of the emitted light to be collected and, after transmission losses, approximately 5% to be delivered to the entrance of a grating spectrometer equipped with a detector array. The high collection efficiency enables the use of pulsed tunable lasers with low repetition rates (e.g., 20 Hz) instead of continuous wave (cw) lasers or very high repetition rate (e.g., MHz) lasers that are typically used as light sources for gas-phase fluorescence experiments on molecular ions. A hole has been drilled in the cylinder electrode so that a light pulse can interact with the ion cloud in the center of the trap. Simulations indicate that these modifications to the trap do not significantly affect the storage capability and the overall shape of the ion cloud. The overlap between the ion cloud and the laser light is basically 100%, and experimentally >50% of negatively charged chromophore ions are routinely photodepleted. The performance of the setup is illustrated based on fluorescence spectra of several laser dyes, and the quality of these spectra is comparable to those reported by other groups. Finally, by replacing the optical system with a channeltron detector, we demonstrate that the setup can also be used for gas-phase action spectroscopy where either depletion or fragmentation is monitored to provide an indirect measurement on the absorption spectrum of the ion.

Storage time dependent photodissociation action spectroscopy of polycyclic aromatic hydrocarbon cations in the cryogenic electrostatic storage ring DESIREE

“…For example, a species whose He-tagged action spectrum shows a transition coincident with a diffuse interstellar band, but with an incommensurate band width at the temperature of interstellar clouds, is unlikely to be the carrier of that DIB. Further development of single-photon action spectroscopies of bare ions such as laser-induced uorescence 29,35,36 or laser-induced vibrational emission 37 is needed to reduce any ambiguity associated with multi-photon processes like photodissociation.…”

The Effect of an Electric Field on the Spectroscopic Properties of the Isolated Green Fluorescent Protein Chromophore Anion

“…We subjected the betaine complexes to dispersed fluorescence experiments using the newly built LUNA setup . Experiments on mass‐selected ions were done at two different excitation wavelengths within the absorption band (445 nm and 487 nm), but no emitted photons were detected.…”

Roadmap on dynamics of molecules and clusters in the gas phase

“…Still, it is non-trivial to extract the emitted photons after photo-excitation, and collection efficiencies are often less than half a percent. In our luminescence setup (denoted LUNA [231]) we use a cylindrical Paul trap where the exit electrode is a mesh grid to allow as many photons as possible to exit the "fluorescence cell." This approach increases the collection efficiency to about 5%.…”

“…Future measurements of emission spectra in the visible and near-infrared sectors will provide much more detailed information about the emitting molecules, as will time resolved measurements. This development will take place at traps and storage rings [185,226,231]. Although photon detection efficiencies will remain below those of massive particles for some time, the technique promises ultimately nondestructive information on a par with or surpassing the one obtained by measurements of unimolecular fragmentation.…”