A sub-4 Kelvin radio frequency linear multipole wire trap.

A linear cryogenic 16-pole wire ion trap has been developed and constructed for cryogenic ion spectroscopy at temperatures below 4 K. The trap is temperature-variable, can be operated with different buffer gases, and offers large optical access perpendicular to the ion beam direction. The housing geometry enables temperature measurement during radio frequency operation. The effective trapping potential of the wire-based radio frequency trap is described and compared to conventional multipole ion trap designs. Furthermore, time-of-flight mass spectra of multiple helium tagged protonated glycine ions that are extracted from the trap are presented, which prove very low ion temperatures and suitable conditions for sensitive spectroscopy.

Low frequency vibrational anharmonicity and nuclear spin effects of Cl-(H2) and Cl-(D2).

Low frequency combination bands of 35Cl-(H2) and 35Cl-(D2) have been measured in the region between 600 and 1100 cm-1 by infrared predissociation spectroscopy in a cryogenic 22-pole ion trap using a free electron laser at the FELIX Laboratory as a tunable light source. The 35Cl-(H2) (35Cl-(D2)) spectrum contains three bands at 773 cm-1 (620 cm-1), 889 cm-1 (692 cm-1), and 978 cm-1 (750 cm-1) with decreasing intensity toward higher photon energies. Comparison of the experimentally determined transition frequencies with anharmonic vibrational self-consistent field and vibrational configuration interaction calculations suggests the assignment of the combination bands v1 + v2, 2v1 + v2, and 3v1 + v2 for 35Cl-(H2) and 2v1 + v2, 3v1 + v2, and 4v1 + v2 for 35Cl-(D2), where v1 is the 35Cl-⋯H2 stretching fundamental and v2 is the Cl-(H2) bend. The observed asymmetric temperature dependent line shape of the v1 + v2 transition can be modeled by a series of ∑+-∏ ro-vibrational transitions, when substantially decreasing the rotational constant in the vibrationally excited state by 35%. The spectrum of 35Cl-(D2) shows a splitting of 7 cm-1 for the strongest band which can be attributed to the tunneling of the ortho/para states of D2.

Vibrational Predissociation Spectroscopy of Cold Protonated Tryptophan with Different Messenger Tags.

Vibrational spectra of protonated tryptophan were recorded by predissociation of H2 messenger tags using cryogenic ion traps. We explore the issue of messenger induced spectral changes by solvating TrpH+(H2) n with n = 1-5 to obtain single photon vibrational spectra of TrpH+ and of its partly deuterated isotopomer in the spectral region of 800-4400 cm-1. Depending on the number of messenger molecules, the spectra of several conformational isomers associated with multiple H2 binding locations along with two natural conformations of TrpH+ were found using the two photon MS3IR2 conformational hole burning method. Most probable messenger positions were established by comparison with predictions from DFT calculations on various candidate structures. Mechanical anharmonicity effects associated with the charged amino group were modeled by Born-Oppenheimer ab initio molecular dynamics. The spectra of TrpH+(H2O) m=1,2, recorded by infrared multiphoton dissociation (IRMPD), reveal broad features in the NH stretching region of the NH3+ group, indicating strong hydrogen bonding in acceptor-donor configuration with the benzene ring for the first water molecule, while the second water appears to attach to a less strongly perturbing site, yielding unique transitions associated with the free OH stretching fundamentals. We discuss the structural deformations induced by the water molecules and compare our results to recent experiments on similar hydrated cationic systems.

Isomeric Broadening of C60+ Electronic Excitation in Helium Droplets: Experiments Meet Theory.

Helium is considered an almost ideal tagging atom for cold messenger spectroscopy experiments. Although helium is bound very weakly to the ionic molecule of interest, helium tags can lead to shifts and broadenings that we recorded near 963.5 nm in the electronic excitation spectrum of C60+ solvated with up to 100 helium atoms. Dedicated quantum calculations indicate that the inhomogeneous broadening is due to different binding energies of helium to the pentagonal and hexagonal faces of C60+, their dependence on the electronic state, and the numerous isomeric structures that become available for intermediate coverage. Similar isomeric effects can be expected for optical spectra of most larger molecules surrounded by nonabsorbing weakly bound solvent molecules, a situation encountered in many messenger-tagging spectroscopy experiments.