We present a compact and economical cw-OPO that covers 3.1 to 3.5 um (2857-3225 cm-1) with more than 30 mW of power. The linewidth of the laser was measured to be smaller than 1 MHz, enabling high resolution spectroscopy. Control of the laser is hands-free. Software has also been developed to allow a user to reach arbitrary wavelengths and scan the desired wavelength range easily. Wavelength monitoring can be achieved by using an inexpensive silicon-based wavemeter due to the design of the laser system. To demonstrate the ability of the laser system, we performed spectroscopy in a gas cell filled with CH₄ and C₂H₂. The result is in good agreement with HITRAN database.

In the search for extraterrestrial life in our solar system, new approaches for in situ sensing instrumentation must be developed to unambiguously detect prebiotic molecules, such as amino acids and their isomers, at low size, weight, and power cost. Ongoing work at Caltech and the Jet Propulsion Laboratory aims to explore how moderate power, continuous wave (CW) laser sources can be leveraged for messenger-assisted infrared photodissociation (IRPD) action spectroscopy adaptable to mass spectrometer (MS)-based in situ sensing platforms. Action spectroscopy could greatly enhance molecular identification and isomer differentiation by providing a mass-to-charge ratio, IR spectrum, and photofragment mass spectrum. This talk will present a series of IRPD spectra of water-tagged, protonated amino acids acquired from a Caltech IRPD instrument. Ions were generated from an electrospray source with both complexation and analysis performed in a single stage linear ion trap MS. The room temperature IRPD spectra were 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 (950-1600 cm⁻¹). Spectra are validated against quantum chemical calculations (DLPNO-CCSD(T)-F12/cc-pVTZ-F12//ωB97X-V/def2-QZVPP) and literature analog spectra collected using free electron laser radiation sources with presented results demonstrating a significant reduction in experimental complexity. Additional results to be presented include multiplexed IRPD spectra and the quantification of amino acid isomers in isomeric mixtures by least squares spectrum fitting and machine learning techniques. Overall, the current study demonstrates how low-power, CW QCL-based IRPD spectroscopy could be used to identify and quantify amino acids and their isomers for future search for life missions.

Probing the spectral evolution of molecules with both ultrafast time and high frequency resolution is essential for deciphering important dynamical processes in biological and catalytic systems. Two dimensional infrared spectroscopy (2D IR) provides rich structural and dynamical information, however, many challenges are faced in condensed-phase experiments due to spectral congestion, low sensitivity, and solvent background. The acquisition of ultrafast and multidimensional spectra of molecular systems isolated in the gas phase could provide an exciting new avenue for the study of molecular interactions and dynamics. Here, we present the development and application of two-dimensional cryogenic ion vibrational spectroscopy (2D-CIVS). First, demonstration spectra of Re(CO)₃(CH₃CN)₃⁺ reveal strong off diagonal cross-peak features between the coupled symmetric and asymmetric carbonyl stretch features. Next, the general applicability of 2D-CIVS is demonstrated by measuring the 2D IR spectrum of the tripeptide glutathioneH⁺ in the amide I-II region. Despite the congested spectrum, 2D-CIVS is able to recover ground-state bleach transitions with ample frequency resolution. Importantly, time evolution of the cross-peak features report on the coupling and intramolecular relaxation dynamics of the system.

Vibrational combination modes are a useful tool for understanding potential energy surfaces of simple and complex molecules. However, since combination modes are disallowed in the harmonic oscillator approximation, their amplitudes are commonly small. As such, it is difficult to perform linear and nonlinear spectroscopy on these moieties due to their relatively small transition dipole moments. Here, we use doubly vibrationally enhanced (DOVE) coherent multidimensional spectroscopy (CMDS) to probe coupling between strong and weak oscillators in CH₃CN. Ultrafast DOVE-CMDS spectra shows the ν(CC) mode at 918 cm⁻¹ couples to numerous modes within the 2750 – 6200 cm⁻¹ region, some of which are roughly two to three orders weaker in transition dipole strength than the ν(CC) mode. We identify coupling between ν(CC) and modes deep in the near infrared. The results suggest DOVE could be used to discern the presence of high wavenumber, small intensity vibrational transitions, and extend the use of nonlinear spectroscopy to the edges of the infrared region of the electromagnetic spectrum.

Applications from biological imaging to nuclear forensics require spectroscopic tools for nondestructive detection of chemical structure and isotope. Vibrational spectroscopies are an ideal method for detecting and distinguishing compounds, since they provide a molecular fingerprint that is unique and different for even similar compounds. However it can be challenging to observe isotope shifts in vibrational spectra, which depend on the reduced mass and are sometimes small. Measuring the shifts due to isotopes is especially challenging in a crowded spectrum of a mixture of chemicals and isotopes. For selective detection of molecules of specific isotopes, we use Coherent Anti-stokes Raman Spectoscopy (CARS) with quantum control by shaping the Stokes excitation pulse to excite and detect vibrational modes of one isotope and molecule at a time. We will show results applying this to mixtures of organic compounds for selective detection of carbon labeled compounds and for enhancing the signature of naturally occurring chlorine isotopes. This work could later be implemented in a CARS microscope for isotope and molecule specific imaging which has applications in biological imaging or nuclear forensics.

Investigating vibrionic wave packets in molecular systems helps understand the coupling between electronic and nuclear degrees of freedom, an important topic in physics and chemistry. Pump-probe techniques are powerful tools for studying various time-resolved photodissociation and photoionization processes. However, observing the electronic and fast vibrational motion using pump-probe strong field ionization of polyatomic molecules remains challenging in ultrafast spectroscopy. This is because ionic fragments are dominated by products arising from single ionization, thus making identifying coherence from pump-probe processes difficult. Coincidence technique can help resolve this issue. However, it is very time-consuming. In this work, we examine the vibrational wave packet dynamics on CH₃I cations by combining pump-probe and covariance mapping techniques. This approach correlates the time-of-flight spectrum of reaction product pairs using statistical methods. Experimentally, an IR pump pulse was used to ionize the methyl iodide molecules to produce vibrational wave packets; a second IR beam was then used to probe the dynamics. We observed various fragments arising from dissociative single-, double-, triple ionization, and non-dissociative double ionization. While the photoion-photoion coincidence (PIPICO) spectrum can identify the correlations between the dissociation double ionization channels of CH₃ and I, pump-probe with covariance mapping allows time-resolved measurements to detect vibrational coherence. Fast Fourier transform (FFT) analysis on individual fragments of CH₃⁺ or I⁺ failed to detect any vibrational wave packets. However, with time-resolved covariance mapping of the dissociative channels (CH₃⁺ and I⁺), a C-H stretching vibrational wave packet on the ground state of CH₃I⁺ was observed for the first time.

Angular streaking is a widely applied technique that was used to probe attosecond electronic dynamics such as tunneling delay (Attoclock). It was initially proposed to use circularly polarized light. Experimentally, elliptically polarized light is used because it is difficult to obtain a few-cycle circularly polarized light. However, elliptically polarized light has a profound effect on the measured attoclock angle shift and the results require sophisticated theoretical modeling to decipher. In this study, we developed a phase-resolved attoclock technique that enables the extraction of the attoclock angular shift for a circular polarized light for the first time. This was achieved by normalizing the phase-resolved electron angular distribution by the phase averaged angular distribution. The technique has been validated by numerically solving time-dependent Schrodinger’s equation in atomic systems. Furthermore, this technique has enabled us to investigate the effect of the coulomb potential on the attoclock angular shift. Two pairs of gases with similar (Kr/CO₂) and different (Ar/CO₂) ionization potential (IP) were used to study the phase-resolved attoclock. The result obtained shows a less than 2^° angular difference between the angle shifts for similar IP indicating the presence of an equalizing coulombic focusing with a vanishing tunneling delay. The electron momentum distributions of the pair of gas tagged from their cations differ indicating the contribution of their electronic structures to the nonadiabatic tunneling ionization of electrons under strong field. For the pair of gases with different IP, an angular shift difference of about 10^° was obtained, indicating that coulomb potential plays a crucial role in determining the angular shift. This technique will be used in future work to study the nonadiabaticity of tunneling ionization of other polyatomic systems to reveal the effect of electronic structures on the tunneling behavior of electrons under potential barrier.

Transient absorption spectroscopy, coupled with extreme ultraviolet (XUV) probe and visible/infrared pump, offers new avenues for understanding material-specific core-level electronic dynamics. Core-level spectroscopies provide detailed insights into electronic and vibrational dynamics, offering precise measurements on the femtosecond timescale. With the generation of XUV light through the high harmonic generation (HHG) process, such measurements are now achievable at a tabletop scale. At the University of Wisconsin - Madison, we commissioned X-Fast (XUV Femtosecond Absorption Spectroscopy Tabletop) to explore these capabilities further. Initial experiments conducted with this setup include studying the kinetics and dynamics of transition-metal perovskite oxides. These preliminary studies highlight the potential applications of the X-FAST instrument in probing ultrafast phenomena with high precision and material specificity.

Electro-Optic Frequency combs have many uses in telecom and spectroscopic applications. These combs are generated by modulating non-linear Electro-Optic crystals with RF frequencies. This modulation creates equally spaced sidebands centered around a CW seed laser. Many Electro-Optic frequency comb architectures either choose between optical bandwidth (cascaded GHz combs) or lower repetition rates (chirped RF generation). The former requiring multiple gigahertz Electro-optics, the latter is limited by available function generators. This work demonstrates an Electro-Optic frequency comb with >100 GHz of bandwidth and a repetition rate in the megahertz. Our comb has three cascaded Electro-optics driven at rigorous harmonics between 11.5-12.5 GHz, 1-2 GHz, and 75-150 MHz. The last modulation dictates the final repetition rate. These RF Modulations are generated by Phase-Locked Loops, which are phase and frequency locked to a 10 MHz Rubidium clock. This architecture is adaptable to the chosen harmonic modulation and repetition rate that is desired without changes to the components. This Comb can be used in dual comb spectroscopy where a highly stable repetition rate is needed along with a larger optical bandwidth. This architecture can be applied to any wavelength regime with a suitable seed laser using “off the shelf” components.