Initially developed as a tool for metrology, frequency combs are most often used for precision, frequency-resolved spectroscopy. The utility of frequency combs in ultrafast spectroscopy is just beginning to be explored. By exploiting the properties of frequency combs, we are improving the sensitivity, spectral resolution, and detection of ultrafast spectroscopies. The first technique discussed will be cavity-enhanced transient absorption spectroscopy, which uses fiber-laser frequency combs coupled to external enhancement cavities to increase the sensitivity of ultrafast transient absorption spectroscopy. A home-built Ytterbium fiber-laser frequency comb and amplifier system provide a stable source of ultrafast pulses. External enhancement cavities increase both the laser power and effective absorption path length, thus improving the signal by several orders of magnitude over traditional transient absorption spectroscopy. Altogether the sensitivity is more than four orders-of-magnitude better than the previous best transient absorption techniques, which allows for the study dilute samples in molecular beams on the femtosecond timescale with transient absorption spectroscopy. The second technique discussed will be the application of cavity-enhancement and frequency-comb techniques, including dual-comb spectroscopy, to two-dimensional spectroscopy. Initial results and current progress towards ultrafast two-dimensional spectroscopy of dilute species in molecular beams will be presented.

Molecular vibrations, involving both ground and excited electronic states, are at the heart of chemical transformations which necessitates understanding the origin of these vibrations. Femtosecond infrared spectroscopy and frequency- domain coherent Raman spectroscopy successfully captured full vibrational spectra but bear a few limitations like interfering lineshapes, background signals, etc. Time-domain measurement, i.e. impulsive stimulated Raman scattering employs a short Raman pump pulse, creating a nuclear wavepacket which evolves as a function of time and is interrogated using a probe pulse and Fourier transform of the temporal interferogram yields the Raman spectrum whereas the scattering background can be easily removed as a zero frequency component. However, separation of vibrational coherences in ground and excited electronic states even for small chromophores in condensed phase still remains challenging. Recently, we showed how ‘spectrally dispersed’ impulsive stimulated Raman spectroscopy can be employed to track time evolution of vibrational coherences in ground as well as excited states, distinctly, under non-resonant/resonant impulsive excitation [1-4]. More specifically, separation of excited-state, ground-state and solvent coherences for diatomic as well as polyatomic molecules in solution is demonstrated, which I will discuss in this presentation. The origin of spectral patterns corresponding to certain vibrational modes of the solute as well as the solvent will be presented. In addition to this, density functional theory is employed to identify the Raman active modes, which nicely correlates with the experimental observations. Details of this method as an emerging technique will be discussed. References: [1] S. Dhamija, G. Bhutani, and A. K. De, Asian Journal of Physics, 255-260, 29 (3 and 4), 2020. [2] S. Dhamija, G. Bhutani, and A. K. De, Frontiers in Optics + Laser Science 2021, paper JW7A.70, 2021. [3] S. Dhamija, G. Bhutani, A. Jayachandran, and A. K. De, The Journal of Physical Chemistry A, 1019-1032, 126(7), 2022. [4] S. Dhamija, G. Bhutani, and A. K. De, Chemical Physics Letters, Under review (Manuscript ID: CPLETT-22-308).

Ni catalysis has garnered much attention over the past decades as a low-cost, abundant alternative to Pd catalysis. Ni cycles are thought to be critically important to the cross-coupling step in the catalytic cycle, but typical Ni catalysts contain bipyridine ligands which are generally unable to stabilize high-valent Ni. Tridentate pyridinophane ligands, on the other hand, are able to stabilize both high- and low-valent Ni, making them optimal ligands to use in Ni catalysis. Tridentate pyridinophane Ni dichloride complexes form a highly reactive Ni catalyst upon photoexcitation of the metal-to-ligand charge transfer (MLCT) band. This process is thought to occur from the d-d charge transfer state, though there is no experimental evidence for this and little is known about the photophysics following photoexcitation of the MLCT transition. Optical transient absorption gives insight into the photophysics of the catalyst formation. We report highly-efficient back electron transfer (BET) following MLCT excitation. We will vary the excitation energy to investigate if the BET is suppressed or enhanced. We will perform transient XUV spectroscopy to probe the dynamics at the Ni M-edge and determine the states that are involved in the Ni catalyst formation.

We report ultrafast transient absorption and 2D IR spectra of the light and deuterated isotopologues of acetylacetone to study the vibrational coupling and dynamics of the strong intramolecular hydrogen bond. Strong 2D IR cross-peaks in the fingerprint region reveal a high level of OH bend character throughout this region. This mode mixing gives rise to a large manifold of OH bend overtone and combination bands in the OH stretch region as evidenced by a highly elongated OH bend excited state absorption transition. As a consequence, strong OH stretch/bend Fermi resonance interactions contribute to a broad OH stretch absorption band that exhibits ultrafast population dynamics on a time scale less than 100 fs. The deuterated species displays similarly strong anharmonic coupling and relaxation dynamics, in addition to coherent oscillations corresponding to the O-O hydrogen bond stretch motion which are absent in the light isotopologue. Polarization anisotropy measurements shows a fast 200 fs reorientation relaxation of the OH stretch while the OD stretch displays a slow 1 ps component. The large isotopic dependence of the anisotropy dynamics is attributed to a combination of differences in anharmonic couplings and proton/deuteron transfer dynamics in the vibrationally hot molecules.

Time-resolved spectroscopy with optical frequency combs combines rapid acquisition with high sensitivity, broad bandwidth, and high resolution.A. J. Fleisher, B. J. Bjork, T. Q. Bui, K. C. Cossel, M. Okumura, J. Ye, J. Phys. Chem. Lett. 5, 2241-2246 (2014)his presents an opportunity to study chemistry on a microsecond timescale with molecular specificity and multiplexing. Here we introduce two cross-dispersed frequency comb spectrometers operating in two wavelength regions of the infrared: one from 1.5 μm to 1.7 μm and another from 4.4 μm to 4.7 μm. In the latter mid-infrared region, we resolve the ro-vibrational lines of several isotopocules of nitrous oxide (N₂O), demonstrating a spectrometer-limited resolution of 725 MHz.D. M. Bailey, G. Zhao, A. J. Fleisher, Anal. Chem. 92, 13759-13766 (2020)mprovements in spectrometer design, beginning in the former near-infrared region, allow for individual frequency-comb teeth to be resolved. Applied in combination with fast-frame-rate camera technology and emerging solid-state or frequency-agile comb sources, the result is a high-throughput spectroscopic technique that is well-suited for investigating the dynamic chemistry of individual events.

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Footnotes:

A. J. Fleisher, B. J. Bjork, T. Q. Bui, K. C. Cossel, M. Okumura, J. Ye, J. Phys. Chem. Lett. 5, 2241-2246 (2014)T D. M. Bailey, G. Zhao, A. J. Fleisher, Anal. Chem. 92, 13759-13766 (2020)I

Vibronic spectra simulated under the harmonic Franck-Condon approximation are an ubiquitous tool for interpreting the structure of optical, photoelectron, and photoionization spectra. For transitions to bound final states located in a well on their potential energy surface, it is straightforward to calculate overlap integrals between discrete vibrational eigenstates, but this approach breaks down when the surface has locally negative curvature, such as near transition states. Time-dependent methods based on the vibrational autocorrelation function alleviate many of these difficulties, but suffer from certain analytical and technical deficiencies, including branch-cut discontinuities (associated with bound, periodic vibrations) and unstable finite-precision arithmetic (associated with unbound, imaginary-frequency modes). In this talk, we present a new derivation of the multidimensional, harmonic autocorrelation function that resolves these issues. An application is illustrated with the cyclopropyl radical, c-C₃H₅, which undergoes prompt ring-opening to allyl upon photoionization. We will also discuss progress towards perturbative anharmonic corrections within the time-dependent approach.

Recent work in both experiment and ab-initio theory indicates that intersystem crossing (ISC) can occur on ultrafast timescales in certain organic compounds, offering a relaxation channel competing with internal conversion. In particular, many nitroaromatic compounds are being investigated for this behavior. 2-nitrophenol (2NP) is one such system; after UV excitation the S1 state has both strong spin-orbit coupling to neighboring triplet states allowing for fast ISC and a low barrier to excited-state intramolecular proton transfer. Recent trajectory surface hopping calculations indicate that both of these relaxation channels occur on similar sub-picosecond timescalesC. Xu et al., Sci Rep 6, 26768 (2016). Both transient absorption spectroscopy (TAS) in solution and time-resolved photoelectron spectroscopy (TRPES) were used to probe the dynamics but the measured time constants were not consistent between methods which makes interpretation more difficultH. A. Ernst et al., J. Phys. Chem. A 119, 9225 (2015) To further elucidate the dynamics in 2NP, we perform gas-phase TAS measurements using a newly-developed broadband cavity-enhanced ultrafast transient absorption spectrometerM. C. Silfies et al., PCCP 23, 9743 (2021) The spectrometer has a pump wavelength of 350 nm and a tunable probe from 450 to 700 nm with a demonstrated detection limit of ∆OD < 1 ×10⁻⁹/√{Hz}. This technique serves as a complement to both solution-phase TAS and TRPES and provides additional information for comparison with theory. Using molecular beam techniques we are able to vary the sample vibrational/rotational temperature or change the solvent environment with clustering to observe the effects on 2NP relaxation dynamics. In this talk, we will discuss results from 2NP under various conditions and compare to previous experiments and theory.

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Footnotes:

C. Xu et al., Sci Rep 6, 26768 (2016).. H. A. Ernst et al., J. Phys. Chem. A 119, 9225 (2015). M. C. Silfies et al., PCCP 23, 9743 (2021).

r0pt Figure The environment has a profound effect on the ultrafast photophysics of tryptophan due to radically different electronic nature of the lowest two singlets(La/Lb) which make up the first absorption band. In aqueous environment the polar La state becomes fluorescent. Therefore previous works have attributed the ultrafast dynamics to a sub-50fs Lb→La internal conversion followed by picosecond relaxation of solvent around the La-state. We have investigated the primary photoinduced processes in solvated tryptophan by combining UV transient absorption spectroscopy with sub-30 fs temporal resolution and CASPT2/MM calculations and unveil a richer mechanism comprising of two population transfer events involving the La and Lb electronic state. Our results reveal two consecutive coherent population transfer events involving the lowest two singlet states: a sub-50-fs nonadiabatic La →Lb through a conical intersection and a subsequent 220 fs reverse Lb→La due to solvent assisted adiabatic stabilization of La state. Vibrational fingerprints present in the transient spectra show compelling evidence of the vibronic coherence established between the two states from the earliest times after excitation and lasting till the back-transfer to La is complete. I will present how the delayed response of solvent causes a dynamic inversion of the energetic order of the vibronically coupled states, which determines the direction of the population transfer.