Optical frequency combs pose a promising approach to molecular spectroscopy and trace gas detection, offering the benefits of both broadband and high-resolution sources. When combined with enhancement cavities, extremely long effective pathlengths can be achieved in a benchtop system. Combining the benefits of enhancement cavities and combs can be technically challenging, and often requires multiple feed-back loops which preclude measurements in the field. There is also desire to push direct frequency comb spectroscopy into the mid-infrared where strong fundamental vibrational bands exist, which adds additional difficulty. Here, we demonstrate cavity-enhanced dual-comb spectroscopy in the mid-infrared for the first time, covering 60 cm⁻¹ centered at 9.4 μm using quantum cascade lasers (QCLs). The cavity length was set such that the repetition rate of the probing comb matched an integer multiple of the cavity's free spectral range. In order to avoid feedback from reflections off the cavity, a bow-tie geometry was used. The frequency comb that was transmitted from the cavity was heterodyned with a second QCL frequency comb and the signals were processed using a commercial dual-comb spectrometer. The high power-per-mode and heterodyne detection provided high signal-to-noise on the transmitted light, while the large difference in repetition rate between the combs allowed for high temporal resolution. The system was completely free running, allowing for robust and sensitivity measurements. A demonstration using trace methanol was performed to characterize the sensitivity of the system. The effective pathlength was increased to 285 m (cavity Finesse of 800), and the system reached a sensitivity of 2.7×10⁻⁸ cm⁻¹ per spectral element.

The lack of powerful and tunable sources in the terahertz frequency (100 GHz to 10 THz) can limit the accuracy and resolution of rotational molecular spectroscopy. While the ground state rotational spectrum of molecules is easily measured, thanks to the large thermal population of lower rotational levels at room temperature, measuring the rotational spectra in sparsely populated excited vibrational states can be much harder. Unlike molecular infrared lasers, quantum cascade lasers offer continuous tunabilty across the ro-vibrational transitions of most molecules. Here, we exploit this tunability to demonstrate a new type of laser and a new type of spectroscopy. Pumping molecules using a quantum cascade laser dramatically enhances the population of sparsely populated vibrational states, and the resulting enhancement of their rotational absorption or emission lines enables the direct measurement of their otherwise weak spectra. In some molecules, this pumping even exceeds the lasing threshold, and every line in the rotational spectrum may be made to lase.

In the search for life on other worlds in our solar system, overlooked approaches to in situ sensing instrumentation can increase science return relating to the detection of prebiotic molecules such as amino acids, lipids, and nucleic acids at low size, weight, and power cost. Ongoing work at Caltech and the Jet Propulsion Laboratory (JPL) aims to explore how moderate power, continuous wave (CW) laser sources can be leveraged to execute single- and multi-photon photodissociation schemes adaptable to mass spectrometer (MS)-based in situ sensing platforms. These techniques could serve as potent analysis tools that provide unambiguous molecular identification through combined initial m/z, the IR or UV/Vis spectrum, as well as the photofragment mass spectrum. This talk will present an initial study of a low-power CW variant of messenger-assisted infrared photodissociation (IRPD) spectroscopy of singly hydrated, protonated phospho-tyrosine (pTyrH⁺(H₂O)) generated from an electrospray source with both complexation and analysis performed in a single stage linear ion trap MS. The room temperature IRPD spectrum of pTyrH⁺(H₂O) was recorded by monitoring the loss of water tag when irradiated with a moderate power ( < 100 mW) quantum cascade laser (QCL) source in the fingerprint region (865-950 cm⁻¹ and 1000-1670 cm⁻¹). Findings are compared against literature analog spectra collected using a free electron laser radiation source with presented results constituting a dramatic reduction in experimental complexity. Discussions will be aided with the support of quantum chemical calculations (DLPNO-CCSD(T)-F12/cc-pVTZ-F12//ωB97X-V/def2-QZVPP). Additionally, this presentation will briefly describe efforts to adapt flight heritage MS hardware at JPL to execute similar analysis schemes. Overall, the current study is the first demonstration of using low-power, CW QCL-based action spectroscopy for the goal of analyzing a broad range of prebiotic molecules on future search for life missions.

Optical frequency combs have proven to be a powerful tool in precision molecular spectroscopy due to their unique blend of a coherent, low-noise spectral source and the broad bandwidth they offer. One particularly useful technique to take full advantage of the high-resolution potential of these devices is dual-comb spectroscopy, in which two frequency combs with slightly different repetition rates are heterodyned together to generate an interferogram, akin to the signal an FTIR produces, through which the optical spectrum can be retrieved. We have constructed a pair of fiber-based optical frequency combs in-house at Connecticut College, one of which has an adjustable repetition rate, for use in a dual-comb spectrometer. We will present details on the two frequency combs, report our progress toward this dual-comb spectrometer, and discuss its future implementation in conducting rotationally-resolved measurements of the 1.65 μm vibrational overtone of benzene in a supersonic beam apparatus.

l0pt Figure Two-photon absorption spectroscopy is an ideal method for precision measurements of molecular energy levels and trace gas concentration. We report a two-photon absorption of the Q(36) rovibrational transition of ¹³CO₂, measured by cavity-enhanced absorption spectroscopy (CEAS) at λ = 2.8 μm. The high signal-to-noise ratio of two-photon absorption spectra was obtained by optical locking of a narrow-linewidth mid-infrared laser to a high finesse cavity. One longitudinal mode of the cavity was shifted to scan the frequency, calibrated by an optical frequency comb. We studied the properties of the two-photon absorption spectrum. It is expected that this method can be applied in the field of molecular frequency reference and high-sensitivity detection.

In the troposphere, alkene ozonolysis produces carbonyl oxide intermediates, known as Criegee intermediates, which can undergo unimolecular decay to generate hydroxyl (OH) radicals, an important atmospheric oxidant. This study focuses on the syn conformer of the methyl-substituted Criegee intermediate (syn-CH₃CHOO) that decays via 1,4 hydrogen transfer from the methyl group to the terminal oxygen, followed by O-O bond fission to yield OH radical products. IR excitation of jet-cooled syn-CH₃CHOO in the overtone CH stretch (2ν_CH) region facilitates this process, leading to rapid dissociation to OH + vinoxy radicals within 10 ns. Y. Fang, F. Liu, V. P. Barber, S. J. Klippenstein, A. B. McCoy and M. I. Lester, J. Chem. Phys. 145, 234308 (2016).ere, we demonstrate ion-dip infrared spectroscopy of syn-CH₃CHOO by combining IR activation in the 2ν_CH region with VUV photoionization (10.5 eV, preferentially probing the syn conformer) C. A. Taatjes, O. Welz, A. J. Eskola, J. D. Savee, A. M. Scheer, D. E. Shallcross, B. Rotavera, E. P. F. Lee, J. M. Dyke, D. K. W. Mok, D. L. Osborn and C. J. Percival, Science 340, 177-180 (2013).nd time-of-flight mass spectrometry detection (m/z 60). The resultant ground state depletion spectrum is recorded and compared with that previously obtained by IR action spectroscopy with detection of OH products using laser-induced fluorescence. F. Liu, J. M. Beames, A. S. Petit, A. B. McCoy and M. I. Lester, Science 345, 1596-1598 (2014).he ion-dip infrared spectrum reproduces the two main features at 5987 and 6081 cm⁻¹, but with notable changes that are being explored. In addition, the experimental results are compared with anharmonic frequency calculations at various levels of theory, which predict a single strong absorption in the 2ν_CH region. Further calculations of vibrational couplings are needed to interpret the experimental observations.

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Footnotes:

Y. Fang, F. Liu, V. P. Barber, S. J. Klippenstein, A. B. McCoy and M. I. Lester, J. Chem. Phys. 145, 234308 (2016).H C. A. Taatjes, O. Welz, A. J. Eskola, J. D. Savee, A. M. Scheer, D. E. Shallcross, B. Rotavera, E. P. F. Lee, J. M. Dyke, D. K. W. Mok, D. L. Osborn and C. J. Percival, Science 340, 177-180 (2013).a F. Liu, J. M. Beames, A. S. Petit, A. B. McCoy and M. I. Lester, Science 345, 1596-1598 (2014).T

HR-2DIR spectroscopy is a new technique that can be used to resolve congested spectra, create 2D patterns, and assign rovibrational peaks in the near infrared region of the spectrum. This talk describes the use HR-2DIR to assign peaks in the CH overtone region of chloromethane.

Electro-optic frequency combs are attractive light sources for dual comb spectroscopy because they are turn-key, robust lasers. The EOM comb presented in this work has a novel architecture, utilizing multiple modulators, to optimize spectral bandwidth with MHz comb tooth spacing. The repetition rate is dictated by the final EOM operating at 70-150 MHz, which matches many commercial mode-locked ultrafast laser systems. Each frequency source is phase and frequency locked using a phase locked loop, which offers long term stability without optics. The frequencies can be tuned without any change to the optics or electrical hardware. The comb tooth linewidth is controlled by the seed laser, a commercial Nd:YAG with a kHz linewidth. Integration of this laser into a dual comb spectroscopy experiment will also be discussed.

Ultrafast Two-Dimensional (2D) Spectroscopy is a powerful technique that has provided valuable insight into diverse systems from protein folding to isomerization of metal complexes. We aim to bring this technique to the realm of small molecules in molecular beams. 2D spectroscopy is unique because it allows for the direct measurement of coupled excitations, represented by off diagonal peaks within the spectra. However, because 2D spectroscopy is a third-order process, the signals are necessarily weak, requiring a combination of higher laser power and concentrated sample to obtain detectable signals. To increase the sensitivity of this technique, our lab built a frequency comb laser and coupled it to an external enhancement cavity. This combination of a frequency comb and optical cavity allows for Cavity Enhanced 2D Spectroscopy to be performed with broadband ultrafast pulses. Even with a modest cavity finesse, the resonant cavity pulses see a 2-3 order increase in their electric fields, greatly increasing the sensitivity of the 2D experiment. We explore the use of enhancement cavities on 2D spectroscopy signal levels, as well as trade-offs inherent to the technique.

When paired with cavity enhancement, interband cascade optical frequency combs offer a highly sensitive method for broadband spectroscopy. We present a new method for the detection of UV photolysis-generated transient species using cavity enhanced Vernier spectroscopy with a mid-IR interband cascade frequency comb. The cavity is used to generate an enhanced spectrum by extending the pathlength. This technique allows for the real-time high-resolution detection of short-lived species. We will discuss the benchmark reaction between hydroxymethyl radicals and oxygen to form formaldehyde, as well as the application of this new technique for the detection of resonance-stabilized radicals.

The extension of nonlinear coherent spectroscopies to the hard X-ray regime could be achieved through the creation of intense, phase stable, femtosecond X-ray pulse pairs, thus potentially further revolutionizing studies of chemistry. XFEL pulses are inherently stochastic, structurally and temporally, which is a limiting factor in advancing nonlinear spectroscopy, but stimulated emission generated by XFEL pulses can result in the pulse pairs necessary for this progress. One potential approach to creating these pairs is highlighted in previous work describing the generation of intense, coherent, X-ray pulse pairs on the femtosecond timescale. These pulse pairs were realized based on the observation of spectral fringes that appeared in the X-ray superfluorescence and seed stimulated emission. The aim of this work is to establish the optimal pump and seed pulse conditions that lend to repeatable generation of femtosecond X-ray pulse pairs. The first step in doing so is to determine the correlation between the parameters of the incoming SASE pulses and the generated pulse pairs. Here we present an overview of the correlation of measured pump pulses and stimulated emission generated from Cu foils, and MnCl₂, and NaMnO₄ jets.