An ion mobility mass spectrometer coupled with a cryogenic ion trap for recording electronic spectra of charged, isomer-selected clusters

Abstract

Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio. Cluster ions generated by laser ablation of a solid sample are selected according to their collision cross sections with helium buffer gas using a drift tube ion mobility spectrometer and their mass-to-charge ratio using a quadrupole mass filter. The mobility- and mass-selected target ions are introduced into a cryogenically cooled, three-dimensional quadrupole ion trap where they are thermalized through inelastic collisions with an inert buffer gas (He or He/N2 mixture). Spectra of the molecular ions are obtained by tagging them with inert atoms or molecules (Ne and N2), which are dislodged following resonant excitation of an electronic transition, or by photodissociating the cluster itself following absorption of one or more photons. An electronic spectrum is generated by monitoring the charged photofragment yield as a function of wavelength. The capacity of the instrument is illustrated with the resonance-enhanced photodissociation action spectra of carbon clusters ([Formula: see text]) and polyacetylene cations (HC2 nH+) that have been selected according to the mass-to-charge ratio and collision cross section with He buffer gas and of mass-selected [Formula: see text] and Au2Ag+ clusters.

Disentangling Electronic Spectra of Linear and Cyclic Hydrogenated Carbon Cluster Cations, C2n+1H+ (n = 3–10)

“…In each trace, there is a fast peak and a slow peak that can be assigned to cyclic and linear C n H + isomers, respectively. The cyclic clusters are more compact and have smaller CCSs than the linear clusters and pass more quickly through the He buffer gas in the drift region (see ref ). The peak assignments are supported by CCS calculations, which predict that CCSs for linear clusters should be 15–20% larger than CCSs for cyclic clusters (see Table S1 in the Supporting Information), consistent with the ATD data.…”

“…The experimental apparatus has been described previously, with a comprehensive description available in ref . Briefly, protonated carbon clusters (C n H + ) were generated by ablating the surface of a rotating graphite disk using the output of a pulsed Nd:YAG laser (λ = 532 nm, 14 mJ/pulse, 100 Hz).…”

“…We have recently used this isomer-selective scheme to obtain electronic spectra of monocyclic C 2n + clusters over the n = 6−14 range. 8,9 Small linear C n H + clusters are known to exist in extraterrestrial environments, with C 3 H + and C 5 H + recently discovered in the interstellar medium (ISM) through their microwave transitions. 10−12 The C 7 H + cluster is predicted to exist in an even greater abundance than C 3 H + .…”

“…To deal with this potential ambiguity, we have adopted an experimental approach whereby the target isomer population is selected according to the ions’ collision cross-section (CCS) with He buffer gas prior to mass selection and spectroscopic interrogation in a cryogenically cooled quadrupole ion trap (QIT). We have recently used this isomer-selective scheme to obtain electronic spectra of monocyclic C 2 n + clusters over the n = 6–14 range. , …”

The combination of laser photodissociation, action spectroscopy, and mass spectrometry to identify and separate isomers

“…A subsequent experimental arrangement that combines IMS and cryogenic PD action spectroscopy has presented the electronic spectra of several cyclic carbon cluster ions. 158,159…”