Resonant multi-photon IR dissociation spectroscopy of a trapped and sympathetically cooled biomolecular ion species

TL;DR

This study demonstrates vibrational spectroscopy of trapped, sympathetically cooled biomolecular ions using resonant multi-photon IR dissociation, revealing insights into their vibrational and rotational dynamics at low temperatures.

Contribution

It introduces a method for performing IR spectroscopy on biomolecular ions in a trap without UV lasers, and measures their dissociation cross section and transition broadening effects.

Findings

Measured R-IRMPD cross section is about two orders smaller than vibrational excitation cross section.

Observed rotational bandwidth exceeds expectations from known broadening factors.

Indicates increased internal and rotational temperature of ions during spectroscopy.

Abstract

In this work we demonstrate vibrational spectroscopy of polyatomic ions that are trapped and sympathetically cooled by laser-cooled atomic ions. We use the protonated dipeptide tryptophane-alanine (HTyrAla+) as a model system, cooled by Barium ions to less than 800mK secular temperature. The spectroscopy is performed on the fundamental vibrational transition of a local vibrational mode at 2.74 {\mu}m using a continuous-wave optical parametric oscillator (OPO). Resonant multi-photon IR dissociation spectroscopy (without the use of a UV laser) generates charged molecular fragments, which are sympathetically cooled and trapped, and subsequently released from the trap and counted. We measured the cross section for R-IRMPD under conditions of low intensity, and found it to be approximately two orders smaller than the vibrational excitation cross section. The observed rotational bandwidth of the vibrational transition is larger than the one expected from the combined effects of 300 K black-body temperature, conformer-dependent line shifts, and intermolecular vibrational relaxation broadening (J. Stearns et al., J. Chem. Phys., 127, 154322-7 (2007)). This indicates that as the internal energy of the molecule grows, an increase of the rotational temperature of the molecular ions well above room temperature (up to on the order of 1000K), and/or an appreciable shift of the vibrational transition frequency (approx. 6-8 cm$^{-1}$) occurs.