Continuous probing of cold complex molecules with infrared frequency comb spectroscopy

Abstract

Abstract-Cavity-enhanced frequency comb spectroscopy for molecule detection in the mid-infrared powerfully combines high resolution, high sensitivity, and broad spectral coverage [1]- [3]. However, this technique, and essentially all spectroscopic methods, is limited in application to relatively small, simple molecules. At room temperature, even molecules of modest size can occupy many millions of rovibrational states, resulting in highly congested spectra. Here we integrate comb spectroscopy with continuous, cold samples of molecules produced via buffer gas cooling [4], thus enabling the study of significantly more complex molecules. Until now, high resolution mid-IR spectroscopy of such molecules has been primarily performed with supersonic jets using single frequency lasers. In both sensitivity and spectral resolution, buffer gas sources are advantageous: they can be operated continuously, in contrast to most jets that are typically pulsed with a low duty cycle [5], [6]; and they provide samples moving slowly in the lab frame, with no observation of buffer gas-molecule clustering. Both of these features help to improve spectral resolution and detection sensitivity, as well as facilitate tracking of molecular interaction kinetics. Here 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 (CH3NO2), a model system that presents challenging questions to the understanding of large amplitude vibrational motion [7]-[21], as well as several large organic molecules with fundamental spectroscopic and astrochemical relevance, including naphthalene (C10H8) [22], adamantane (C10H16) [23], and hexamethylenetetramine (C6N4H12) [24]-[27]. 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 [28], weakly bound clusters [29], and cold chemistry [30].For over half a century, high-resolution infrared spectroscopy has provided a broad foundation for the fundamental understanding of molecular structure and dynamics. Spectroscopic analysis of vibrational, rotational, fine, and hyperfine structure yields extraordinarily detailed information about the molecular Hamiltonian. The success of such studies, however, requires well resolved and assignable spectra, which have been largely limited to relatively small, simple molecular systems. While the study of large and more complex molecules is an emerging front that provides fundamental insights into the nature of energy flow in a strongly correlated system, acquiring useful and tractable spectra for complicated molecular species poses many experimental challenges. Large molecules with many internal...

Perspective: Ultracold molecules and the dawn of cold controlled chemistry

“…The method has been demonstrated for cold beams of O 2 , 36 CaH, 37 SrF, 38 and benzonitrile. 39 Buffer gas cooled molecules have also been used as a continuous source of cold molecules for cavity-enhanced direct frequency comb spectroscopy 40 and enantiomer-specific detection of chiral molecules via microwave spectroscopy. 41,42 The direct observation of conformational relaxation by collisions with He atoms in a cryogenic environment (∼6 K) has also been reported for the 1,2-propanediol molecule.…”

“…The reactivity of the cis-conformer was found to be a factor of 2 larger than the trans-conformer (differentiated by the O-H bond orientation). Rellergert et al 61 reported a chemical reaction rate coefficient of about 2.0 × 10 −10 cm 3 s −1 in collisions of 40 Ca atoms with 174 Yb + ions confined in a hybrid trap. While initial calculations by the authors suggested a radiative charge transfer mechanism for the large rate coefficient, subsequent calculations by Zygelman et al 120 indicated an upper bound of 1.5 × 10 −15 cm 3 s −1 for radiative transfer in the 1 mK-1 K range.…”

Mid-infrared continuous-filtering Vernier spectroscopy using a doubly resonant optical parametric oscillator

“…Both approaches enable measurements with frequency resolution and precision provided by the comb with no influence of the instrumental line shape and are suitable for broadband precision spectroscopy [4,14]. Mechanical MIR FTS has been successfully combined with enhancement cavities [6,15] and enabled, e.g., acquisition of rotationally resolved spectra of cold complex molecules [16]. The use of the cavities with mechanical FTS requires a high-bandwidth lock and the transmitted spectral range is limited by the cavity dispersion [6].…”

Sensitive and broadband measurement of dispersion in a cavity using a Fourier transform spectrometer with kHz resolution

“…Optical frequency combs, whose spectra consist of thousands of equidistant narrow lines, can probe cavity modes over a much broader bandwidth. In cavity-enhanced optical frequency comb absorption spectroscopy, spectra of entire molecular bands can be acquired with high resolution in short acquisition times [7][8][9][10]. Combs are also an ideal tool for measurements of broadband cavity dispersion induced either by the cavity mirror coatings or intracavity samples.…”