Special issue on time-resolved vibrational spectroscopy

Time-resolved vibrational spectroscopy (TRVS), as a general approach to understanding molecular processes, has a long history dating back many decades. The International Conference on Time-Resolved Vibrational Spectroscopy (TRVS) has a more precise history that began in Lake Placid, NY, in 1982. This biennial meeting brings together researchers from all over the world who share a common interest in vibrational dynamics. From its origin focusing on vibrational spectroscopy per se, today the TRVS community is home to a wide variety of scientists who investigate phenomena where nuclear motions are at the center stage, such as in the ultrafast infrared or Raman response, or perhaps acting in a supporting role, when vibrations are coupled to electronic transitions. A quick perusal of the topics presented in the first edition of TRVS I in 1982 reveals an overwhelming majority of time-resolved resonance Raman to study such storied molecular systems as stilbene, bacteriorhodopsin, and chlorophyll. Whereas TRVS XX (2021) had less Raman, but a preponderance of multidimensional IR, visible, and combination spectroscopy, the latter provided new insight into the same systems as those discussed in 1982. There is no doubt that TRVS will continue to evolve as the scientific challenges change and new optical technology and theoretical tools advance. Nevertheless, the present Special Issue of the Journal of Chemical Physics provides a snapshot of work that was at the forefront of vibrational dynamics at one point in time.

The meeting, held in June from 13 to 19, 2021, occurred during the second year of the global pandemic due to COVID-19. Originally intended to take place in person on the tree-lined campus of the University of Michigan in Ann Arbor, we realized in late 2020 that such a dream would not come to pass. Thus began a process to plan a virtual conference—one of the first of its kind—that could attempt to mimic the sense of community and interaction that we expect of in-person gatherings. Through the dedicated work of graduate students and postdoctoral researchers, as well as faculty, we managed to carry out a version of TRVS that spanned time zones and continents, bringing together nearly 500 attendees at no financial cost and a minimal carbon footprint. None of this would have been possible without enthusiastic participants, especially the speakers and poster presenters (and sponsors), who shared their wonderful science during this troubling time. This special issue of J. Chem. Phys. commemorates the perseverance of scientific inquiry and global community in the face of seemingly insurmountable obstacles.

Below we summarize the main findings of the articles in this collection of 29 manuscripts spanning theory, experiment, and condensed and gas phases.

Besides the tried-and-true titanium–sapphire based regenerative amplifiers that have driven ultrafast science for over 20 years, there are new optical sources that enable a wide range of new experiments. New high-repetition rate sources, including frequency combs, are changing the way we think about optical spectroscopy implementations. In the infrared regime, dual-comb quantum cascade lasers provide access to single-shot protein conformational dynamics, as reported by Schubert et al.¹ Using two different mode locked lasers running at slightly different repetition frequencies, Han et al. used asynchronous optical sampling (ASOPS) to measure the transient absorption of thioflavin T.² A particularly noteworthy application of high repetition rate lasers is to measurements that can otherwise have prohibitively long acquisition times, such as 2D-Raman-THz spectroscopy, as reported by Duchi et al.³ in their article on liquid bromoform.

The ability to study the same molecular system in both the gas and solution phases provides a unique window into the role of solvation in mediating chemical dynamics. Solowan et al.⁴ study azulene using action-based 2D spectroscopy in both the liquid and gas phases, finding differences in the energetics and the rates of conical intersection (CI) and internal conversion in the two conditions.

A common theme in recent editions of TRVS has been the central role of water, both as a liquid and a solvent. This collection is no exception, featuring several different approaches to understanding water and hydration dynamics. Ahmed et al.⁵ reveal a strong isotope effect (OD vs OH) on the vibrational relaxation dynamics of free OD/OH at the air/water interface using heterodyne-detected vibrational sum frequency generation (VSFG) spectroscopy, attributing this effect to isotope-dependent reorientation. Highlighting the major advances in atomistic simulations of liquid water, Ishiyama⁶ reports on the non-equilibrium ab initio molecular dynamics simulations of the air/water interface, finding that the major relaxation pathway for pure water is intermolecular vibrational coupling.

Aqueous solutes also provide atomic-level probes of solvation dynamics, and this collection contains several articles that detail various aspects of small-molecule hydration. The increased attention to carbon dioxide is exemplified by the work of Gleim et al.⁷ that uses ultrafast IR spectroscopy to monitor the vibrational relaxation of CO₂ in water under various thermodynamic conditions. A promising vibrational probe, alkyl thiocyanate, has been thoroughly characterized by Zhao et al.⁸ in a broad range of solvents, including water, both experimentally and using sophisticated computational methods. In a twist on previous studies of denaturants in water, Marekha and Hunger⁹ use steady-state and ultrafast IR spectroscopy to monitor the dynamics of 1-methylurea, finding significant differences with non-methylated urea, highlighting the influence of a single methyl group on the local hydration environment. In this theme, Korotkevich and Bakker¹⁰ use 2D-IR spectroscopy to monitor the vibrational dynamics of carboxylate moieties in acetate and terephthalate in aqueous solution, revealing details of the anharmonic coupling between the symmetric and antisymmetric stretching modes.

Another way to access interfacial dynamics is to study vibrational transitions localized at an interface as reported by Lee et al.,¹¹ who use the ester carbonyl of model lipids in bilayers in conjunction with MD simulations to characterize hydration structural dynamics. The amide I mode of peptides and proteins offers a powerful means of accessing protein structural dynamics, and the contribution from Tan et al.¹² shows how the vibrational anharmonicity is a sensitive probe of protein–water coupling.

For chemists, the Born–Oppenheimer approximation provides the starting point for understanding reactivity and spectroscopy, even though many key chemical processes break the approximation at some point. Conical intersections (CIs) are essential in photochemistry and photophysics, and Hou et al.¹³ study a system, tetraphenylethylene, with at least two CIs, finding that the twisting CI’s barrier is greater than the cyclization CI.

Vibronic coherence has now been observed in nearly all photosynthetic proteins and complexes, although the significance remains somewhat unclear. Zhu et al.¹⁴ use high signal-to-noise ratio transient grating and 2D-ES to study the vibronic dynamics of light-harvesting complex II, finding evidence for vibronic mixing. Looking for specific vibrations that promote energy transfer, Patra and Tiwari¹⁵ use an effective mode Hamiltonian to identify a design principle that highlights vibronic resonance. In another theoretical contribution, Polley and Loring¹⁶ make use of a mapping Hamiltonian to develop a thermofield optimized mean trajectory approximation to compute 2D-ES as well as 2D vibrational/electronic (2D-VE) spectra.

Macromolecules exhibit structural dynamics on a large dynamic range of length and time scales. Since its inception, TRVS has been a venue to report advances in biomolecular dynamics and that tradition is carried on in this collection. A striking example is the study by van Wilderen et al.¹⁷ reporting femto-to-millisecond time-resolved IR spectroscopy of photoactive yellow protein, offering a residue-level view of protonation and deprotonation, events that are central to the photocycle. Site-specific information can be challenging in large biomolecules, but Yamashita et al.¹⁸ study model tryptophan molecules using time-resolved ultraviolet resonance Raman spectroscopy and computation to assess the nature of the thermal probe. Powerful simulation techniques provide a much-needed link between structure and spectrum, as exemplified by an article by Saxena et al.¹⁹ where cryo-electron microscopy structures of amyloid fibrils are used as input for spectral simulations.

Protein–DNA interactions underlie a vast array of biological processes, including replication, recognition, and regulation. Heussman et al.²⁰ employ rigidly inserted cyanine labels to reveal conformational distributions in single-strand/double-strand DNA fork constructs that are suspected to be important in protein–DNA complexation. Following in the theme of novel spectroscopic probes, Salehi and Meuwly²¹ use a simulation approach to characterize the environmental sensitivity of an azido-modified alanine residue, finding that the –N₃ label is a site-specific and sensitive dynamics probe.

There are solvents other than water, and they often confer interesting vibrational relaxation dynamics. Yang et al.²² have investigated an energetic material, cyclotetramethylene tetranitramine (HMX), using ultrafast transient absorption and 2D-IR spectroscopy of the nitro stretch as a probe, revealing structural dynamics as well as vibrational energy transfer. Spectral diffusion generally reflects solvation dynamics, whereas intramolecular vibrational energy redistribution (IVR) typically relates to intramolecular coupling, but 2D-IR spectroscopy of a tripod transition metal carbonyl complex reported by Crum et al.²³ shows that both correlate with solvent dynamics as captured by optical Kerr effect spectroscopy.

Besides liquids, solids exhibit complex and rich structural dynamics. Semiconductors in high magnetic fields produce Zeeman splittings of excitons, which Mapara et al.²⁴ resolve spectrally and temporally using 2D electronic spectroscopy. Highly confined molecules can exhibit altered reactivity, but represent a less widely explored aspect in ultrafast molecular spectroscopy, but as Pyles et al.²⁵ show, encapsulation may not always yield dynamical consequences that outweigh small-scale structural changes.

Plasmonic structures and semiconductor nanoparticles display rich quantum and classical dynamics and photonic phenomena. In the theme of plasmonic antennas, Cohn et al.²⁶ have used 2D-IR to study the ultrafast dynamics of a polymer coated resonant antenna array that couples strongly to the antennas producing delocalized polaritons. The interest in CdSe quantum dots is typically due to the effects of quantum confinement, but Wang et al.²⁷ observe coherent oscillations of 2D electronic spectroscopy that are size-independent, concluding that the origin of the beats is the longitudinal-optical phonon of the quantum dots.

New experimental methods are often susceptible to experimental complexities, and new methods do not always have clear predictions for signal magnitudes. Whaley-Mayda et al.²⁸ present useful guidelines for designing experiments that leverage their recently introduced fluorescence encoded IR spectroscopy, hoping to push the technique to the single-molecule sensitivity regime. Although the gold standard for VSFG spectroscopy is to use optical heterodyne detection, many researchers rely on homodyne detected vibrational SFG; Wang et al.²⁹ caution that the chirp of the broadband IR pulses can distort the VSFG spectra.