Continuous probe of cold complex molecules with infrared frequency comb spectroscopy

TL;DR

This paper introduces a novel method combining cavity-enhanced frequency comb spectroscopy with buffer gas cooling to enable high-resolution, sensitive, and broad-spectrum analysis of complex, cold molecules, including those relevant to astrochemistry.

Contribution

It demonstrates the first rotationally resolved absorption spectra of complex molecules in the mid-infrared, expanding spectroscopic capabilities to larger, more complex species.

Findings

Achieved simultaneous improvements in resolution, sensitivity, and bandwidth.

First rotationally resolved spectra of complex molecules like nitromethane, naphthalene, and adamantane.

Potential to study complex molecules, clusters, and cold chemistry with enhanced precision.

Abstract

Cavity-enhanced frequency comb spectroscopy for molecule detection in the mid-infrared powerfully combines high resolution, high sensitivity, and broad spectral coverage. However, this technique, and essentially all spectroscopic methods, is limited in application to relatively small, simple molecules. Here we integrate comb spectroscopy with continuous, cold samples of molecules produced via buffer gas cooling, thus enabling the study of significantly more complex molecules. We report simultaneous gains in resolution, sensitivity, and bandwidth and demonstrate this combined capability with the first rotationally resolved direct absorption spectra in the CH stretch region of several complex molecules. These include nitromethane (CH$_3$NO$_2$), a model system that presents challenging questions to the understanding of large amplitude vibrational motion, as well as several large organic molecules with fundamental spectroscopic and astrochemical relevance, including naphthalene (C$_{10}$H$_8$), adamantane (C$_{10}$H$_{16}$), and hexamethylenetetramine (C$_{6}$N$_4$H$_{12}$). This general spectroscopic tool has the potential to significantly impact the field of molecular spectroscopy, simultaneously improving efficiency, spectral resolution, and specificity by orders of magnitude. This realization could open up new molecular species and new kinetics for precise investigations, including the study of complex molecules, weakly bound clusters, and cold chemistry.