High resolution coherent multidimensional spectroscopy is a powerful tool that can be used to overcome difficulties encountered when using 1D spectroscopy. The 2D spectra have reduced congestion and show easily recognizable patterns, even for molecules that yield patternless 1D spectra. Furthermore, the peaks are automatically sorted by quantum number and species. A new infrared version of this technique has been developed. This talk provides background behind how the technique works and how to interpret the results.

In this presentation, we discuss a femtosecond tabletop extreme ultraviolet (XUV) source using high-harmonic generation in a gas target and its application towards studying the ultrafast dynamics of condensed matter systems. We will overview the building and design of the instrument, characterization of the source, preliminary data of systems containing first-row transition metals, and plans for future experiments.

Development of cryogenic ion traps have greatly enhanced the ability to control ion-neutral chemistry and spectroscopically probe the resulting reaction products. Currently, our lab uses a dual cryogenic ion trap instrument for controlled ion manipulation and subsequent tagging in preparation for infrared action spectroscopy studies. This allows, for example, the study of microsolvated ionic species and catalytic reaction intermediates. The first ion trap is a liquid nitrogen cooled “reaction trap”. It is the sole location for ion manipulation and thus limits us to a single chemical reaction or the addition of a single type of solvent molecule. To overcome this limitation, we have developed a cryogenic, mass selective, sequential multi-reaction trap setup with a modular housing design. Mass selectivity is achieved via frequency and duty cycle manipulations of the RF square wave trapping potential. In addition, the modular design reduces the cost and allows for easier adaptability and expansion. We show that such digital linear quadrupole can efficiently form clusters at low temperature and subsequently mass-select a single species within a series of different cluster sizes before ion transfer. Additionally, we show that manipulation of the square wave duty cycle during the clustering process can be used to enhance the formation of a specific cluster size. This single species enhancement and selection is expected to decrease the amount of time required for spectroscopic characterization. Careful ion transfer into a second reaction trap can be done to cluster a different species and simulate more complex ion environments. Future plans for the multi-reaction trap instrument include characterizing water networks around small peptides by inserting a D₂O as a position sensitive spectroscopic molecular probe.

The CRESU technique (French acronym for “reaction kinetics in uniform supersonic flows”) provides a wall-less gas-phase reactor to measure low-temperature reaction kinetics. In the past, probing methods such as laser-induced fluorescence (LIF), mass spectrometry, and chirped-pulse uniform flow (CPUF) microwave spectroscopy have been successfully used to measures reaction kinetics in CRESU flows, but the latter two call for sampling of the flow prior to detection. Here we show a selective, low-cost, and highly sensitive probing tool to measure the kinetics of reactions that involve both molecular and atomic species. This new detection method uses resonance-enhance multi-photon ionization (REMPI) and an electron capture probe, adapted from approaches successfully used in flames, to selectively identify atomic and molecular species. A negative-biased high voltage applied to two electrodes, that are placed on either side of a grounded probe, enables rapid capture of electrons produced by the selective ionization from the REMPI spectroscopy. The performance of this setup was verified by recording the (1+1) REMPI spectra of nitric oxide in 20 K and 50 K uniform supersonic flows. The REMPI probe response is proportional to the number of electrons produced by ionization, and therefore to the concentration of ionized species. Thus, the time-dependent REMPI signal can be used to measure the rate of decay or growth of a reactant or product in fast chemical reactions in CRESU flows.

The spectral resolution in Pulsed-Field Ionization Zero-Kinetic Energy (PFI-ZEKE) photoelectron spectroscopy is related to the state selectivity in the ionization process of the Rydberg states. The selectivity is determined by the applied electric field pulse sequence. Hollenstein et al.U. Hollenstein, R. Seiler, H. Schmutz, M. Andrist, and F. Merkt, J. Chem. Phys. 115, 5461-5469 (2001).sed discrete electric field pulses with increasing field strength in combination with a preceding field pulse of opposite polarity. By using such field pulse sequences with the smallest possible field step size (i.e., approximately 9 mV/cm), a spectral resolution of 0.06 cm⁻¹ could be achieved. To improve the resolution further Harper et al.Oliver J. Harper, Ning L. Chen, Séverine Boyé-Péronne, and Bérenger Gans, Phys. Chem. Chem. Phys. 24, 2777-2784 (2022).ecently suggested replacing the sequence of field steps by a linearly increasing field, as used earlier by Reiser et al. G. Reiser, W. Habenicht, K. Müller-Dethlefs, and E. W. Schlag, Chem. Phys. Lett. 152, 119–123 (1988). in combination with a prepulse of opposite polarity and obtained promising results on the PFI-ZEKE photoelectron spectrum of NO and CO₂. Using a home-built narrow-bandwidth long-pulse laser system (pulse lengths up to 50 ns) in combination with a field pulse ramp^{b, c}, we explore the resolution limit of this approach. To avoid overlap of spectral lines, we chose an atomic system, Ar, as test system and recorded PFI-ZEKE photoelectron spectra of transitions from the metastable states (3p)⁵(4s)[3/2]₂ (³\textP₂) and (3p)⁵(4s)′[1/2]₀ (³\textP₀) to the (3p)^{5 2}P_{3/2, 1/2} states of Ar⁺. This system also offers the advantage of a precisely known ionization energyV. L. Sukhorukov, I. D. Petrov, M. Schäfer, F. Merkt, M.-W. Ruf, and H. Hotop, J. Phys. B: At. Mol. Opt. Phys. 45, 092001 (2012) and references therein.ith which the ionization energy determined with the new method can be compared.

---

Footnotes:

U. Hollenstein, R. Seiler, H. Schmutz, M. Andrist, and F. Merkt, J. Chem. Phys. 115, 5461-5469 (2001).u Oliver J. Harper, Ning L. Chen, Séverine Boyé-Péronne, and Bérenger Gans, Phys. Chem. Chem. Phys. 24, 2777-2784 (2022).r G. Reiser, W. Habenicht, K. Müller-Dethlefs, and E. W. Schlag, Chem. Phys. Lett. 152, 119–123 (1988)., V. L. Sukhorukov, I. D. Petrov, M. Schäfer, F. Merkt, M.-W. Ruf, and H. Hotop, J. Phys. B: At. Mol. Opt. Phys. 45, 092001 (2012) and references therein.w

l0pt Figure Optical frequency comb (OFC) spectroscopy in the mid-infrared (MIR) promises faster, more precise, and more sensitive molecular spectroscopy. To date, demonstrations of MIR OFCs have suffered from low power, poor wavelength coverage, or low sensitivity. Systems that do excel in these areas have high cost and complexity. The technique and measurements reported here demonstrate that singly resonant, single frequency optical parametric oscillators (OPO’s) are a powerful platform for generating MIR OFC’s with properties not shown by other MIR light sources. An EOM frequency comb is first generated via phase modulation of CW light near 1064 nm. An inexpensive direct digital synthesizer (DDS)-based scheme is used to generate chirped modulation resulting in a  2 GHz-wide frequency comb with ultraflat comb teeth and frequency agile repetition rates between 1 MHz and 10 MHz. This comb pumps the OPO, resulting in an idler output that is a MIR OFC tunable between 2200 - 4000 nm with > 1 W output power (figure). This technique is utilized to perform frequency comb spectroscopy on select rovibrational features near 3 μm in methane and acetylene.

The spectrum of an optical frequency comb is composed of many equidistant lines, which is a natural match for enhancement cavities. Cavity ring-down spectroscopy is known to be a robust and highly sensitive technique, although it is challenging to implement with optical frequency combs. Here we demonstrate a new approach to performing direct frequency comb cavity ring-down spectroscopy in the CH stretching region using an interband cascade optical frequency comb. These chip-scale devices generate combs with large repetition rates (10 GHz), which enables mode-resolved detection using Vernier spectroscopy. The decay of each comb mode can be obtained as the comb is being scanned, providing sensitive and broadband detection. Here we demonstrate the effectiveness of this technique for trace gas detection and discuss the overall performance.

Optical frequency combs have become a promising tool for sensitive and broadband spectroscopy. They are especially attractive for investigations of the structure and reactivity of transient species, where multiplexed detection provides information regarding reactive intermediates and product branching ratios. However, the cost and complexity of conventional frequency combs has inhibited their widespread use in chemistry laboratories. Frequency combs generated using semiconductor lasers, such as quantum or interband cascade lasers, offer an alternative that is compact and less technically demanding. We have employed interband cascade lasers, which provide coverage in the CH stretching region, to monitor reactions initiated by pulsed-laser photolysis using Vernier spectroscopy. The reaction between 1-hydroxyethyl radical and oxygen forms acetaldehyde, and is an ideal test-case for the newly constructed instrument. We will discuss the performance characteristics and applications of this new and promising technique.

Dual Comb Spectroscopy (DCS) is an emerging technique for measuring infrared molecular absorption at higher speeds and spectral resolution than historically possible using Michelson interferometer-based Fourier Transform Spectroscopy. While Quantum Cascade Laser (QCL) DCS is capable of fast acquisition speeds and the ability to probe between 4-10 μm, these benefits also come at the cost of a low instantaneous bandwidth, spectral resolution, and coherence time over which measurements can be averaged. Here we present the development of a GHz repetition rate intra-pulse DFG mid-IR DCS system. This system is based on mode-locked lasers simultaneously spanning 3-5 μm with 0.03 cm⁻¹comb tooth spacing, μs acquisition speeds, and single-cycle residual noise below 10⁻¹. Spectra and noise characteristics from a static spectroscopy cell containing various hydrocarbons are reported and discussed. The system shows promise for sensing in rapid transient systems given the high single shot signal to noise ratio and rapid interferogram acquisition time afforded by the GHz pulse repetition rates.

r0pt Figure Delay-spectral focusing dual-comb coherent anti-Stokes Raman spectroscopy is proposed and performed, realizing a 40000 spectra/s acquisition rate, which is the fastest Raman spectral detection in the high-wavenumber region up to the present. The spectral resolution (~10 cm⁻¹) and the signal-to-noise ratio (~260) keep stable along the detection process. This novel spectroscopic technique avoids the invalid scanning time wasted in waiting for the superposition in time of the dual-comb pulses by actively modulating the repetition frequency difference and thus the relative delay. An intracavity electro-optic modulator (EOM), with high modulation amplitude and response frequency, is applied for fast repetition frequency modulation. Delay-spectral focusing method also helps to obtain the high-wavenumber region spectrum, which is difficult to realize for previously-used Fourier transform CARS (FT-CARS) due to the intrinsic coherence of ultrabroadband pulses. This technique shows huge potentials in which both high speed and high-wavenumber region detection are required, such as fast microspectroscopic imaging and flow cytometry.

Dual-comb spectroscopy with quantum cascade lasers is an inherently high-speed spectroscopic technique for the mid-infrared spectral region, as the sample is probed simultaneously at all the frequencies of the comb teeth. With typical beat note spacing of few MHz, the temporal resolution is of the order of microseconds and the spectral point spacing is typically 10 GHz. By interleaving several thousand spectra, the spectral point spacing is reduced to less than 10 MHz, suitable for spectroscopy of Doppler-broadened gases, within a measurement time of a few milliseconds. The above-mentioned interleaving is easily achieved by ramping the current of the lasers. We seek the frequencies of every comb tooth at every instant of the ramp in order to produce a frequency axis for the interleaved spectra. We continuously measure the spectral point spacing of the interrogating comb (i.e., its repetition frequency) by pointing a directional microwave antenna at the laser and picking up the intermode beat, which oscillates at the repetition frequency. We further measure the optical frequency of one comb tooth by beating the comb with a frequency-locked distributed feedback laser, acting as the optical reference frequency. We test the accuracy of the computed frequency axis by measuring the well-known positions of the P and R lines of the ν₁ fundamental band of N₂O near 1300 cm⁻¹. The spectral coverage is 1265-1305 cm⁻¹, and the measurement (scan) duration is varied from 27 to 215 ms. We find that the frequency scale is accurate within 1 MHz.