Vibrational Relaxation Dynamics Research Articles

Unraveling the excited-state vibrational cooling dynamics of chlorophyll-a using femtosecond broadband fluorescence spectroscopy.

Understanding the dynamics of excited-state vibrational energy relaxation in photosynthetic pigments is crucial for elucidating the mechanisms underlying energy transfer processes in light-harvesting complexes. Utilizing advanced femtosecond broadband transient fluorescence (TF) spectroscopy, we explored the excited-state vibrational dynamics of Chlorophyll-a (Chl-a) both in solution and within the light-harvesting complex II (LHCII). We discovered a vibrational cooling (VC) process occurring over ∼6ps in Chl-a in ethanol solution following Soret band excitation, marked by a notable ultrafast TF blueshift and spectral narrowing. This VC process, crucial for regulating the vibronic lifetimes, was further elucidated through the direct observation of the population dynamics of higher vibrational states within the Qy electronic state. Notably, Chl-a within LHCII demonstrated significantly faster VC dynamics, unfolding within a few hundred femtoseconds and aligning with the ultrafast energy transfer processes observed within the complex. Our findings shed light on the complex interaction between electronic and vibrational states in photosynthetic pigments, underscoring the pivotal role of vibrational dynamics in enabling efficient energy transfer within light-harvesting complexes.

Dynamics of Formamide-Water Mixtures Investigated by Linear and Nonlinear Infrared Spectroscopy.

Formamide (FA) exhibits complete miscibility with water, offering a simplified model for exploring the solvation dynamics of peptide linkages in biophysical processes. Its liquid state demonstrates a three-dimensional hydrogen bonding network akin to water, reflecting solvent-like behavior. Analyzing the microscopic structure and dynamics of FA-water mixtures is expected to provide crucial insights into hydrogen bonding dynamics─a key aspect of various biophysical phenomena. This study is focused on the dynamics of FA-water mixtures using linear and femtosecond infrared spectroscopies. By using the intrinsic OD stretch and extrinsic probe SCN-, the local vibrational behaviors across various FA-water compositions were systematically investigated. The vibrational relaxation of OD stretch revealed a negligible impact of FA addition on the vibrational lifetime of water molecules, underscoring the mixture's water-like behavior. However, the reorientational dynamics of OD stretch slowed with increasing FA mole fraction (XFA), plateauing beyond XFA > 0.5. This suggests a correlation between OD's reorientational time and the strength of the hydrogen bond network, likely tied to the solution's changing dielectric constant. Conversely, the vibrational relaxation dynamics of SCN- was strongly correlated with XFA, highlighting a competition between water and FA molecules in solvating SCN-. Moreover, a linear relationship between rising viscosity and the prolonged correlation time of SCN-'s slow dynamics indicates that the solution's macroscopic viscosity is dictated by the extended structures formed between FA and water molecules. The relation between the reorientation dynamics of the SCN- and the macroscopic viscosity in aqueous FA-water mixture solutions was analyzed by using the Stokes-Einstein-Debye equations. The direct viscosity-diffusion coupling is observed, which can be attributed to the homogeneous dynamics feature in FA-water mixture solutions. The inclusion of these intrinsic and extrinsic probes not only enhances the comprehensiveness of our analysis but also provides valuable insights into various aspects of the dynamics within the FA-water system. This investigation sheds light on the fundamental dynamics of FA-water mixtures, emphasizing their molecular-level homogeneity in this binary mixture solution.

Full-Dimensional Quantum Dynamics Studies of Ro-vibrationally Inelastic Scattering of H2O with Ar: A Benchmark Test of the Rigid-Rotor Approximation

While the rigid-rotor (RR) approximation is usually considered to be accurate for describing pure rotationally inelastic scattering involving diatoms in their ground or low-lying vibrational states, its validity in scattering involving polyatomic molecules has not been fully examined. The existence of soft/anharmonic vibrational modes in polyatomic molecules could make rotational-vibrational energy transfer rather efficient, thus undermining the premise of the RR approximation. In this work, we conduct a benchmark test of the RR approximation in the rotationally inelastic scattering of the H2O(v2 = 0, 1) + Ar system by comparing with full-dimensional quantum scattering calculations. We demonstrate that the error in the RR rate coefficient for v2 = 0 is less than 5%, while it can reach up to 20% for some initial states within the v2 = 1 manifold. These results indicate that the RR approximation gradually deteriorates with increasing quantum number v2. Vibrational relaxation dynamics of this system was also studied, and it is found that transitions from initial states with a large rotational quantum number of projection on the a principal axis are more efficient. These results shed valuable light on ro-vibrationally inelastic scattering involving polyatomic molecules.

Rapid vibrational relaxation dynamics and strong Solute-Solvent interactions of the nitrite anion

In this work, we have used steady-state and ultrafast vibrational spectroscopy to measure the absorption band centers and widths, as well as vibrational energy relaxation (VER) lifetimes for the antisymmetric ν3 stretching band of the nitrite (NO2–) anion in H2O, D2O, and DMSO. The nitrite IR absorption bands are very broad and the excited state has an extremely short VER lifetime in all three solvents, suggesting that NO2– interacts quite strongly with all of them. We also measured and analyzed reflection spectra for NaNO2 pressed pellets and determined that the damping is fast in the reststrahlen band.

Distinct Hydrogen Bonding Dynamics Underlies the Microheterogeneity in DMF-Water Mixtures.

The hydrogen bonding interaction between the amide functional group and water is fundamental to understanding the liquid-liquid heterogeneity in biological systems. Herein, the structure and dynamics of the N,N-dimethylformamide (DMF)-water mixtures have been investigated by linear and nonlinear IR spectroscopies, using the hydroxyl stretch and extrinsic probe of thiocyanate as local vibrational reporters. According to vibrational relaxation dynamics measurements, the orientational dynamics of water is not directly tied to those of DMF molecules. Wobbling-in-a-cone analysis demonstrates that the water molecules have varying degrees of angular restriction depending on their composition due to the formation of specific water-DMF networks. Because of the preferential solvation by DMF molecules, the rotational dynamics of the extrinsic probe is slowed significantly, and its rotational time constants are correlated to the change of solution viscosity. The unique structural dynamics observed in the DMF-water mixtures is expected to provide important insights into the underlying mechanism of microscopic heterogeneity in binary mixtures.

High-resolution CH stretch spectroscopy of jet-cooled cyclopentyl radical: First insights into equilibrium structure, out-of-plane puckering, and IVR dynamics.

First, high-resolution sub-Doppler infrared spectroscopic results for cyclopentyl radical (C5H9) are reported on the α-CH stretch fundamental with suppression of spectral congestion achieved by adiabatic cooling to Trot ≈ 19(4) K in a slit jet expansion. Surprisingly, cyclopentyl radical exhibits a rotationally assignable infrared spectrum, despite 3N - 6 = 36 vibrational modes and an upper vibrational state density (ρ ≈ 40-90 #/cm-1) in the critical regime (ρ ≈ 100 #/cm-1) necessary for onset of intramolecular vibrational relaxation (IVR) dynamics. Such high-resolution data for cyclopentyl radical permit detailed fits to a rigid-rotor asymmetric top Hamiltonian, initial structural information for ground and vibrationally excited states, and opportunities for detailed comparison with theoretical predictions. Specifically, high level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples (CCSD(T))/ANO0, 1 level are used to calculate an out-of-plane bending potential, which reveals a C2 symmetry double minimum 1D energy surface over a C2v transition state. The inversion barrier [Vbarrier ≈ 3.7(1) kcal/mol] is much larger than the effective moment of inertia for out-of-plane bending, resulting in localization of the cyclopentyl wavefunction near its C2 symmetry equilibrium geometry and tunneling splittings for the ground state too small (<1MHz) to be resolved under sub-Doppler slit jet conditions. The persistence of fully resolved high-resolution infrared spectroscopy for such large cyclic polyatomic radicals at high vibrational state densities suggests a "deceleration" of IVR for a cycloalkane ring topology, much as low frequency torsion/methyl rotation degrees of freedom have demonstrated a corresponding "acceleration" of IVR processes in linear hydrocarbons.

Intramolecular vibrational redistribution in formic acid and its deuterated forms.

The intramolecular vibrational relaxation dynamics of formic acid and its deuterated isotopologues is simulated on the full-dimensional potential energy surface of Richter and Carbonnière [J. Chem. Phys. 148, 064303 (2018)] using the Heidelberg MCTDH package. We focus on couplings with the torsion vibrational modes close to the trans-cis isomerization coordinate from the dynamics of artificially excited vibrational mode overtones. The bright C-O stretch vibrational mode is coupled to the out-of-the plane torsion mode in HCOOH, where this coupling could be exploited for laser-induced trans-to-cis isomerization. Strong isotopic effects are observed: deuteration of the hydroxyl group, i.e., in HCOOD and DCOOD, destroys the C-O stretch to torsion mode coupling whereas in DCOOH, little to no effect is observed.

Ultrafast vibrational dynamics of the free OD at the air/water interface: Negligible isotopic dilution effect but large isotope substitution effect.

Vibrational relaxation dynamics of the OH stretch of water at the air/water interface has been a subject of intensive research, facilitated by recent developments in ultrafast interface-selective nonlinear spectroscopy. However, a reliable determination of the vibrational relaxation dynamics in the OD stretch region at the air/D2O interface has not been yet achieved. Here, we report a study of the vibrational relaxation of the free OD carried out by time-resolved heterodyne-detected vibrational sum frequency generation spectroscopy. The results obtained with the aid of singular value decomposition analysis indicate that the vibrational relaxation (T1) time of the free OD at the air/D2O interface and air/isotopically diluted water (HOD-H2O) interfaces show no detectable isotopic dilution effect within the experimental error, as in the case of the free OH in the OH stretch region. Thus, it is concluded that the relaxation of the excited free OH/OD predominantly proceeds with their reorientation, negating a major contribution of the intramolecular energy transfer. It is also shown that the T1 time of the free OD is substantially longer than that of the free OH, further supporting the reorientation relaxation mechanism. The large difference in the T1 time between the free OD and the free OH (factor of ∼2) may indicate the nuclear quantum effect on the diffusive reorientation of the free OD/OH because this difference is significantly larger than the value expected for a classical rotational motion.

Non-Markovian vibrational relaxation dynamics at surfaces.

Vibrational dynamics of adsorbates near surfaces plays both an important role for applied surface science and as a model lab for studying fundamental problems of open quantum systems. We employ a previously developed model for the relaxation of a D-Si-Si bending mode at a D:Si(100)-(2 × 1) surface, induced by a "bath" of more than 2000 phonon modes [Lorenz and P. Saalfrank, Chem. Phys. 482, 69 (2017)], to extend previous work along various directions. First, we use a Hierarchical Effective Mode (HEM) model [Fischer et al., J. Chem. Phys. 153, 064704 (2020)] to study relaxation of higher excited vibrational states than hitherto done by solving a high-dimensional system-bath time-dependent Schrödinger equation (TDSE). In the HEM approach, (many) real bath modes are replaced by (much less) effective bath modes. Accordingly, we are able to examine scaling laws for vibrational relaxation lifetimes for a realistic surface science problem. Second, we compare the performance of the multilayer multiconfigurational time-dependent Hartree (ML-MCTDH) approach with that of the recently developed coherent-state-based multi-Davydov-D2 Ansatz [Zhou et al., J. Chem. Phys. 143, 014113 (2015)]. Both approaches work well, with some computational advantages for the latter in the presented context. Third, we apply open-system density matrix theory in comparison with basically "exact" solutions of the multi-mode TDSEs. Specifically, we use an open-system Liouville-von Neumann (LvN) equation treating vibration-phonon coupling as Markovian dissipation in Lindblad form to quantify effects beyond the Born-Markov approximation.

Solution structures and ultrafast vibrational energy dissipation dynamics in cyclotetramethylene tetranitramine.

Steady-state and time-resolved infrared (IR) studies of cyclotetramethylene tetranitramine (HMX) were carried out, using the asymmetric nitro-stretch as probe, to investigate its solution structures and vibrational energy transfer processes in pure dimethyl sulfoxide (DMSO) and in a DMSO/water mixture. A linear IR spectrum in the nitro-stretching mode region shows two major bands and one minor band in DMSO but changes to the two major bands mainly picture when adding water as an antisolvent of HMX, suggesting a transition from well-solvated and less perfect β-conformation to a less-solvated and close-to-perfect β-conformation. The latter bears a similar asymmetric nitro-stretch vibration profile to the β-polymorph in the crystal form. Density functional theory computations of the nitro-stretching vibrations suggest that HMX in DMSO may be in a NO2 group rotated β-conformation. Two-dimensional IR cross-peak intensity reveals intramolecular energy transfer between the axial and equatorial nitro-groups in the β-HMX on the ps time scale, which is slightly faster in the mixed solvent case. The importance of water as an antisolvent in influencing the equilibrium solvation structure, as well as the vibrational and orientational relaxation dynamics of HMX, is discussed.

Dynamics in tris(pentafluoroethyl)trifluorophosphate (FAP) anion based ionic liquids: A 2D-IR study with tungsten hexacarbonyl

Ultrafast two-dimensional infrared spectroscopy (2D-IR) measurements for tungsten hexacarbonyl, W(CO)6, has been performed in two fluoroalkylphosphate ([FAP])-based ultrahigh hydrophobic, ionic liquids, namely, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate ([EMIM][FAP]) and 1-(hexyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM][FAP]) to probe their local structure and dynamics. To understand the effect of FAP anion on the dynamics, a relatively less hydrophobic [PF6-] anion has been introduced in the form of [HMIM][PF6]. While the linear IR spectra records some influence of anion in terms of spectral shifts, the alkyl chain length of the cation seems to have no significant influence on the spectral peak position; and even the spectral broadening is found to be in the similar range. Further, vibrational population relaxation dynamics and vibrational anisotropy decay also does not show any significant variation due to difference in constituent of these ionic liquids as well as its solvent properties, such as, viscosity. Finally, 2D-IR spectra show large variation in the spectral shapes with waiting times, which qualitatively remains similar in all the tested ionic liquids. The frequency-fluctuation correlation function measured from 2D-IR spectra displays a bi-exponential decay with the shorter time-constant dependent on the identity of the anion, and has been attributed to the ion-rattling motion, whereas, the longer component has been found to be invariant with the identity of ionic liquid, and has been attributed to ion-cage motions which is in line with previous MD simulations as well as previous experimental measurements, using ionic vibrational probe. Overall, 2D-IR spectroscopy, in combination with this neutral and hydrophobic probe, W(CO)6 provides a unique opportunity to understand the local structural dynamics of this ultrahigh hydrophobic ionic liquid which are otherwise not accessible by hitherto explored ionic vibrational probes.

Vibrational relaxation of carbon dioxide in water.

The dynamics of vibrational relaxation of carbon dioxide in water has been studied using femtosecond mid-infrared pump-probe spectroscopy with excitation of the anti-symmetric stretching (ν3) fundamental state of the solute. The relaxation dynamics were recorded at a constant pressure of 500 bars and in the temperature range between 300 and 600K, thereby covering the liquid-to-near-critical region of the solvent. The excited state of the ν3-mode is deactivated in two competing pathways: (i) direct relaxation to the ground state with resonant transfer of the excess vibrational energy into the bending-librational continuum of the water solvent and (ii) relaxation to the bending fundamental state with transfer into the intramolecular bending mode of H2O. The rate of pathway (i) decreases with increasing temperature, from ∼1/(9ps) at 300K to ∼(1/16ps) at 600K and obeys Fermi's golden rule strictly, provided that the spectral density of energy-accepting solvent states is derived from the stationary infrared absorption profile of H2O. The rate of pathway (ii) is 1/(23ps) and assumed to be temperature-independent within our data analysis. Finally, the bending fundamental of CO2 can also relax to the ground state by resonantly transferring the remaining excess energy to the librational fundamentals of the solvent.

Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface.

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T1 = 0.13 ps, which is slightly larger than that in the bulk (T1 = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T1 = 0.74 ps in the bulk decreases to T1 = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OH⋯OH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Vibrational relaxation dynamics in layered perovskite quantum wells

Organic-inorganic layered perovskites, or Ruddlesden-Popper perovskites, are two-dimensional quantum wells with layers of lead-halide octahedra stacked between organic ligand barriers. The combination of their dielectric confinement and ionic sublattice results in excitonic excitations with substantial binding energies that are strongly coupled to the surrounding soft, polar lattice. However, the ligand environment in layered perovskites can significantly alter their optical properties due to the complex dynamic disorder of the soft perovskite lattice. Here, we infer dynamic disorder through phonon dephasing lifetimes initiated by resonant impulsive stimulated Raman photoexcitation followed by transient absorption probing for a variety of ligand substitutions. We demonstrate that vibrational relaxation in layered perovskite formed from flexible alkyl-amines as organic barriers is fast and relatively independent of the lattice temperature. Relaxation in layered perovskites spaced by aromatic amines is slower, although still fast relative to bulk inorganic lead bromide lattices, with a rate that is temperature dependent. Using molecular dynamics simulations, we explain the fast rates of relaxation by quantifying the large anharmonic coupling of the optical modes with the ligand layers and rationalize the temperature independence due to their amorphous packing. This work provides a molecular and time-domain depiction of the relaxation of nascent optical excitations and opens opportunities to understand how they couple to the complex layered perovskite lattice, elucidating design principles for optoelectronic devices.

Vibrational couplings and energy transfer pathways of water\u2019s bending mode

Coupling between vibrational modes is essential for energy transfer and dissipation in condensed matter. For water, different O-H stretch modes are known to be very strongly coupled both within and between water molecules, leading to ultrafast dissipation and delocalization of vibrational energy. In contrast, the information on the vibrational coupling of the H-O-H bending mode of water is lacking, even though the bending mode is an essential intermediate for the energy relaxation pathway from the stretch mode to the heat bath. By combining static and femtosecond infrared, Raman, and hyper-Raman spectroscopies for isotopically diluted water with ab initio molecular dynamics simulations, we find the vibrational coupling of the bending mode differs significantly from the stretch mode: the intramode intermolecular coupling of the bending mode is very weak, in stark contrast to the stretch mode. Our results elucidate the vibrational energy transfer pathways of water. Specifically, the librational motion is essential for the vibrational energy relaxation and orientational dynamics of H-O-H bending mode.

Effect of isotope substitution on the Fermi resonance and vibrational lifetime of unnatural amino acids modified with IR probe: A 2D-IR and pump-probe study of 4-azido-L-phenyl alanine.

The infrared (IR) probe often suffers from an unexpected complex absorption profile due to the Fermi resonance and short vibrational lifetime, which restricts the application of time-resolved IR spectroscopy to investigate the site-specific structural dynamics of the protein. Researchers have found that isotope substitution to the IR probe not only removes the Fermi resonance but also extends the dynamic observation window with a prolonged vibrational lifetime. This method has been successfully applied to modify the vibrational properties of many IR probes for time-resolved spectroscopy and imaging. In this study, the effect of isotope substitution (15N) on the vibrational properties of the azide stretching band in 4-azido-L-phenylalanine has been investigated using ultrafast pump-probe and 2D-IR spectroscopy. In contrast to the earlier reports, it has been observed that the Fermi resonance remains unchanged even after isotope substitution, and there is very little change in the vibrational relaxation dynamics as well. Anharmonic frequency analysis reveals that the α-N atom of N3 is being shared between the two transitions participating in the Fermi resonance and gets affected similarly due to isotope labeling. Hence, this study unveils the specific circumstance at which the isotope labeling strategy may not be successful in eliminating the Fermi resonance band and explains the molecular origin behind it. This study also suggests definitive approaches on how to overcome the limitations related to the Fermi resonance to extend the development and application of this IR probe for biological research.

Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy

The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears. Although this free OH is the most characteristic feature of interfacial water, the molecular-level understanding of its dynamic property is still limited due to the technical difficulty. We study ultrafast vibrational relaxation dynamics of the free OH at the air/water interface using time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. With the use of singular value decomposition (SVD) analysis, the vibrational relaxation (T1) times of the free OH at the neat H2O and isotopically-diluted water interfaces are determined to be 0.87 ± 0.06 ps (neat H2O), 0.84 ± 0.09 ps (H2O/HOD/D2O = 1/2/1), and 0.88 ± 0.16 ps (H2O/HOD/D2O = 1/8/16). The absence of the isotope effect on the T1 time indicates that the main mechanism of the vibrational relaxation of the free OH is reorientation of the topmost water molecules. The determined sub-picosecond T1 time also suggests that the free OH reorients diffusively without the switching of the hydrogen-bond partner by the topmost water molecule.

Specific Host-Guest Interactions in the Crown Ether Complexes with K+ and NH4+ Revealed from the Vibrational Relaxation Dynamics of the Counteranion.

The specific host-guest interactions in the corresponding complexes of K+ and NH4+ with typical crown ethers were investigated by using FTIR and ultrafast IR spectroscopies. The counteranions, i.e., SCN-, were employed as a local vibrational probe to report the structural dynamics of the complexation. It was found that the vibrational relaxation dynamics of the SCN- was strongly affected by the cations confined in the cavities of the crown ethers. The time constant of the vibrational population decay of SCN- in the complex of NH4+ with the 18-crown-6 was determined to be 6 ± 2 ps, which is ∼30 times faster than that in the complex of K+ with the crown ethers. Control experiments showed that the vibrational population decay of SCN- depended on the size of the cavities of the crown ethers. A theoretical calculation further indicated that the nitrogen atom of SCN- showed preferential coordination to the K+ ions hosted by the crown ethers, while the NH4+ can form hydrogen bonds with the oxygen atoms in the studied crown ethers. The geometric constraints formed in the complex of crown ethers can cause a specific interaction between the NH4+ and SCN-, which can facilitate the intermolecular vibrational energy redistribution of the SCN-.

A hierarchical effective mode approach to phonon-driven multilevel vibrational relaxation dynamics at surfaces

We discuss an efficient Hierarchical Effective Mode (HEM) representation of a high-dimensional harmonic oscillator bath, which describes phonon-driven vibrational relaxation of an adsorbate-surface system, namely, deuterium adsorbed on Si(100). Starting from the original Hamiltonian of the adsorbate-surface system, the HEM representation is constructed via iterative orthogonal transformations, which are efficiently implemented with Householder matrices. The detailed description of the HEM representation and its construction are given in the second quantization representation. The hierarchical nature of this representation allows access to the exact quantum dynamics of the adsorbate-surface system over finite time intervals, controllable via the truncation order of the hierarchy. To study the convergence properties of the effective mode representation, we solve the time-dependent Schrödinger equation of the truncated system-bath HEM Hamiltonian, with the help of the multilayer extension of the Multiconfigurational Time-Dependent Hartree (ML-MCTDH) method. The results of the HEM representation are compared with those obtained with a quantum-mechanical tier-model. The convergence of the HEM representation with respect to the truncation order of the hierarchy is discussed for different initial conditions of the adsorbate-surface system. The combination of the HEM representation with the ML-MCTDH method provides information on the time evolution of the system (adsorbate) and multiple effective modes of the bath (surface). This permits insight into mechanisms of vibration-phonon coupling of the adsorbate-surface system, as well as inter-mode couplings of the effective bath.

Vibrational Relaxation Dynamics of an Azido-Cobalt(II) Complex from Femtosecond UV-Pump/MIR-Probe Spectroscopy and Model Simulations with Ab Initio Anharmonic Couplings.

Vibrational energy relaxation is of critical importance for the light-controlled reactivity of transition-metal complexes. In time-resolved optical spectroscopies, it gives rise to pronounced spectral redistributions with complex band shifts and thus to nonexponential kinetics, all of which are very difficult to quantify. Here we study the vibrational relaxation dynamics of a pentacoordinated azido-cobalt(II) complex in liquid solution following its ultrafast charge-transfer excitation in the near-ultraviolet (UV). The complex is photochemically remarkably stable and returns within the experimental time resolution back to its quartet electronic ground state via internal conversion. The nonadiabatic transition effectively instantaneously converts the entire photon energy into kinetic energy of the vibrational degrees of freedom. The ensuing relaxation dynamics of the vibrationally highly excited complex are monitored as a function of time using femtosecond mid-infrared (MIR) spectroscopy in the antisymmetric stretching region of the azido ligand and occur on a time scale of a few tens of picoseconds. The dynamic evolution of the MIR spectrum due to vibrational cooling of the complex can be understood quantitatively within the framework of an anharmonic coupling model, which relies on an ab initio intramolecular cubic/quartic force field from density functional theory combined with second-order vibrational perturbation theory. The simulations suggest that the primary internal conversion preferentially dumps the excess energy into the low-frequency bending modes of the azido ligand, whereas its high-frequency stretching modes are barely affected by the initial nonadiabatic transition. Surprisingly, the two bending vibrations appear to relax independently of one another, each with its own characteristic cooling time.