Proton transfer at subkelvin temperatures

TL;DR

This paper introduces a new method for ionizing molecules at subkelvin temperatures using proton transfer inside helium nanodroplets, enabling studies of delicate molecules and astrochemical reactions with minimal secondary reactions.

Contribution

The authors develop a novel proton transfer technique inside helium nanodroplets for ionizing molecules at temperatures below 1 K, reducing secondary reactions and enabling new spectroscopic and astrochemical investigations.

Findings

Demonstrated proton transfer ionization inside helium nanodroplets at <1 K.

Showed the method's ability to determine proton affinities of molecules.

Produced XH$^{+}$ ions suitable for action spectroscopy.

Abstract

We demonstrate a novel method to ionize molecules or molecular clusters by proton transfer at temperatures below 1 K. The method yields nascent ions and largely eliminates secondary reactions, even for notoriously "delicate" molecules. Protonation is achieved inside liquid helium nanodroplets (HNDs) and begins with the formation of (H$_{2}$)$_{m}$H$^{+}$ ions as the proton donors. In a separate and subsequent step the HNDs are doped with a proton acceptor molecule, X. Proton transfer occurs between X and the cold proton donor ions inside a helium droplet, an approach that avoids the large excess energy that is released if HNDs are first doped and then ionized. Mass spectra, recorded after stripping excess helium and hydrogen in a collision cell, show that this method offers a new way to determine proton affinities of molecules and clusters by proton-transfer bracketing, to investigate astrochemically relevant ion-molecule reactions at sub-kelvin temperatures, and to prepare XH$^{+}$ ions that are suitable for messenger-tagging action spectroscopy.