Perspective: Electrospray photoelectron spectroscopy: From multiply-charged anions to ultracold anions

Abstract

Electrospray ionization (ESI) has become an essential tool in chemical physics and physical chemistry for the production of novel molecular ions from solution samples for a variety of spectroscopic experiments. ESI was used to produce free multiply-charged anions (MCAs) for photoelectron spectroscopy (PES) in the late 1990 s, allowing many interesting properties of this class of exotic species to be investigated. Free MCAs are characterized by strong intramolecular Coulomb repulsions, which create a repulsive Coulomb barrier (RCB) for electron emission. The RCB endows many fascinating properties to MCAs, giving rise to meta-stable anions with negative electron binding energies. Recent development in the PES of MCAs includes photoelectron imaging to examine the influence of the RCB on the electron emission dynamics, pump-probe experiments to examine electron tunneling through the RCB, and isomer-specific experiments by coupling PES with ion mobility for biological MCAs. The development of a cryogenically cooled Paul trap has led to much better resolved PE spectra for MCAs by creating vibrationally cold anions from the room temperature ESI source. Recent advances in coupling the cryogenic Paul trap with PE imaging have allowed high-resolution PE spectra to be obtained for singly charged anions produced by ESI. In particular, the observation of dipole-bound excited states has made it possible to conduct vibrational autodetachment spectroscopy and resonant PES, which yield much richer vibrational spectroscopic information for dipolar free radicals than traditional PES.

Electron diffraction of CBr4 in superfluid helium droplets: A step towards single molecule diffraction

“…12,13 Superfluid helium droplets with a terminal temperature of 0.38 K and a frictionless environment thus emerge as an ideal coolant.…”

Sensitivity of Photoelectron Angular Distributions to Molecular Conformations of Anions

“…Nevertheless, as most of the pEP -molecules are at energies well below the barrier, the out-of-plane conformation will have the highest probability and therefore, qualitatively, we expect the overall PAD to predominantly represent that of the lowest-energy conformation in agreement with our observations. The use of temperature controlled cryogenic ion traps coupled with PE imaging 37,38 would allow the molecules to be cooled so that only the lowest modes are populated which would greatly simplify a quantitative analysis.…”

Communication: Vibrationally resolved photoelectron spectroscopy of the tetracyanoquinodimethane (TCNQ) anion and accurate determination of the electron affinity of TCNQ

“…A photoelectron spectrum was also obtained at 3.100 eV, in resonance with the second excited state of TCNQ − , as a result of two-photon detachment, following fast internal conversion. The experiment was done using our recently developed high-resolution photoelectron imaging apparatus, 20 which consists of an electrospray ionization (ESI) source, 21 a cryogenic ion trap, 22 a time-of-flight (TOF) mass spectrometer, and a high-resolution velocity-map imaging (VMI) analyzer. 23 Briefly, the TCNQ − anions were produced by electrospray of a 1 mM solution of TCNQ (Sigma-Aldrich) dissolved in acetonitrile and spiked with a small amount of NaI.…”