Intramolecular vibrational energy redistribution and intermolecular energy transfer of benzene in supercritical CO2: measurements from the gas phase up to liquid densities

Light-induced relaxation dynamics of the ferricyanide ion revisited by ultrafast XUV photoelectron spectroscopy

The photoexcited ferricyanide undergoes an ultrafast spin crossover followed by Jahn–Teller distortion.

Hydrogen bond dynamics of superheated water and methanol by ultrafast IR-pump and EUV-photoelectron probe spectroscopy

Snapshot of superheated water 40 ps after fs-IR laser excitation; representative aggregates formed during the simulation (close-up) compared to one obtained from superheated methanol phase (inset).

Vibrational energy relaxation of large-amplitude vibrations in liquids

Given the limited intermolecular spaces available in dense liquids, the large amplitudes of highly excited, low frequency vibrational modes pose an interesting dilemma for large molecules in solution. We carry out molecular dynamics calculations of the lowest frequency ("warping") mode of perylene dissolved in liquid argon, and demonstrate that vibrational excitation of this mode should cause identifiable changes in local solvation shell structure. But while the same kinds of solvent structural rearrangements can cause the non-equilibrium relaxation dynamics of highly excited diatomic rotors in liquids to differ substantially from equilibrium dynamics, our simulations also indicate that the non-equilibrium vibrational energy relaxation of large-amplitude vibrational overtones in liquids should show no such deviations from linear response. This observation seems to be a generic feature of large-moment-arm vibrational degrees of freedom and is therefore probably not specific to our choice of model system: The lowest frequency (largest amplitude) cases probably dissipate energy too quickly and the higher frequency (more slowly relaxing) cases most likely have solvent displacements too small to generate significant nonlinearities in simple nonpolar solvents. Vibrational kinetic energy relaxation, in particular, seems to be especially and surprisingly linear.

Ultrafast excited state dynamics and spectroscopy of 13,13'-diphenyl-β-carotene

Ultrafast transient broadband absorption spectroscopy based on the Pump-Supercontinuum Probe (PSCP) technique has been applied to characterize the excited state dynamics of the newly-synthesized artificial β-carotene derivative 13,13'-diphenyl-β-carotene in the wavelength range 340-770 nm with ca. 60 fs cross-correlation time after excitation to the S(2) state. The influence of phenyl substitution at the polyene backbone has been investigated in different solvents by comparing the dynamics of the internal conversion (IC) processes S(2)→ S(1) and S(1)→ S(0)* with results for β-carotene. Global analysis provides IC time constants and also time-dependent S(1) spectra demonstrating vibrational relaxation processes. Intramolecular vibrational redistribution processes are accelerated by phenyl substitution and are also solvent-dependent. DFT and TDDFT-TDA calculations suggest that both phenyl rings prefer an orientation where their ring planes are almost perpendicular to the plane of the carotene backbone, largely decoupling them electronically from the polyene system. This is consistent with several experimental observations: the up-field chemical shift of adjacent hydrogen atoms by a ring-current effect of the phenyl groups in the (1)H NMR spectrum, a small red-shift of the S(0)→ S(2)(0-0) transition energy in the steady-state absorption spectrum relative to β-carotene, and almost the same S(1)→ S(0)* IC time constant as in β-carotene, suggesting a similar S(1)-S(0) energy gap. The oscillator strength of the S(0)→ S(2) transition of the diphenyl derivative is reduced by ca. 20%. In addition, we observe a highly structured ground state bleach combined with excited state absorption at longer wavelengths, which is typical for an "S* state". Both features can be clearly assigned to absorption of vibrationally hot molecules in the ground electronic state S(0)* superimposed on the bleach of room temperature molecules S(0). The S(0)* population is formed by IC from S(1). These findings are discussed in detail with respect to alternative interpretations previously reported in the literature. Understanding the dynamics of this type of artificial phenyl-substituted carotene systems appears useful regarding their future structural optimization with respect to enhanced thermal stability while keeping the desired photophysical properties.

Chemical dynamics of vibrationally excited molecules: Controlling reactions in gases and on surfaces

Experimental studies of the chemical reaction dynamics of vibrationally excited molecules reveal the ability of different vibrations to control the course of a reaction. This Perspective describes those studies for the prototypical reaction of vibrationally excited methane and its isotopologues in gases and on surfaces and looks to the prospects of similar studies in liquids. The influences of vibrational excitation on the C-H bond cleavage in a single collision reaction with Cl and in dissociative adsorption on a Ni surface bear some striking similarities. Both reactions are bond-selective processes in which the initial preparation of a molecular eigenstate containing a large component of C-H stretching results in preferential cleavage of that bond. It is possible to cleave either the C-H bond or C-D bond in the reaction of Cl with CH3D, CH2D2, or CHD3 and, similarly, to use initial excitation of the C-H stretch to promote dissociation of CHD3 to CD3 and H on a Ni surface. Different vibrational modes, such as the symmetric and antisymmetric stretches in CH3D or CH4, lead to very different reactivities, and molecules with the symmetric stretching vibration excited can be as much as 10 times more reactive than ones with the antisymmetric stretch excited. The origin of this behavior lies in the change in the vibrational motion induced by the interaction with the atomic reaction partner or the surface.

CONNECTING CHEMICAL DYNAMICS IN GASES AND LIQUIDS

▪ Abstract  Modern ultrafast spectroscopic techniques provide new opportunities to study chemical reaction dynamics in liquids and hold the possibility of obtaining much of the same detailed information available in gases. Vibrational energy transfer studies are the most advanced of the investigations and demonstrate that it is possible to observe state-specific pathways of energy flow within a vibrationally excited molecule (intramolecular vibrational relaxation) and into the surrounding solvent molecules (intermolecular energy transfer). Energy transfer in liquids and gases share many common aspects, but the presence of the solvent also alters the relaxation in both obvious and subtle ways. Photodissociation is amenable to similarly detailed study in liquids, and there are informative new measurements. Bimolecular reactions have received the least attention in state-resolved measurements in liquids, but the means to carry them much further now exist. Studying photodissociation and bimolecular reaction of molecules prepared with initial vibrational excitation in liquids is a realistic, but challenging, goal.

Product Screening of Fast Reactions in IR-Laser-Heated Liquid Water Filaments in a Vacuum by Mass Spectrometry

In the present article a novel approach for rapid product screening of fast reactions in IR-laser-heated liquid microbeams in a vacuum is highlighted. From absorbed energies, a shock wave analysis, high-speed laser stroboscopy, and thermodynamic data of high-temperature water the enthalpy, temperature, density, pressure, and the reaction time window for the hot water filament could be characterized. The experimental conditions (30 kbar, 1750 K, density approximately 1 g/cm3) present during the lifetime of the filament (20-30 ns) were extreme and provided a unique environment for high-temperature water chemistry. For the probe of the reaction products liquid beam desorption mass spectrometry was employed. A decisive feature of the technique is that ionic species, as well as neutral products and intermediates may be detected (neutrals as protonated aggregates) via time-of-flight mass spectrometry without any additional ionization laser. After the explosive disintegration of the superheated beam, high-temperature water reactions are efficiently quenched via expansion and evaporative cooling. For first exploratory experiments for chemistry in ultrahigh-temperature, -pressure and -density water, we have chosen resorcinol as a benchmark system, simple enough and well studied in high-temperature water environments much below 1000 K. Contrary to oxidation reactions usually present under less extreme and dense supercritical conditions, we have observed hydration and little H-atom abstraction during the narrow time window of the experiment. Small amounts of radicals but no ionic intermediates other than simple proton adducts were detected. The experimental findings are discussed in terms of the energetic and dense environment and the small time window for reaction, and they provide firm evidence for additional thermal reaction channels in extreme molecular environments.

Intramolecular vibrational energy redistribution in bridged azulene-anthracene compounds: Ballistic energy transport through molecular chains

Intramolecular vibrational energy flow in excited bridged azulene-anthracene compounds is investigated by time-resolved pump-probe laser spectroscopy. The bridges consist of molecular chains and are of the type (CH(2))(m) with m up to 6 as well as (CH(2)OCH(2))(n) (n=1,2) and CH(2)SCH(2). After light absorption into the azulene S(1) band and subsequent fast internal conversion, excited molecules are formed where the vibrational energy is localized at the azulene side. The vibrational energy transfer through the molecular bridge to the anthracene side and, finally, to the surrounding medium is followed by probing the red edge of the azulene S(3) absorption band at 300 nm and/or the anthracene S(1) absorption band at 400 nm. In order to separate the time scales for intramolecular and intermolecular energy transfer, most of the experiments were performed in supercritical xenon where vibrational energy transfer to the bath is comparably slow. The intramolecular equilibration proceeds in two steps. About 15%-20% of the excitation energy leaves the azulene side within a short period of 300 fs. This component accompanies the intramolecular vibrational energy redistribution (IVR) within the azulene chromophore and it is caused by dephasing of normal modes contributing to the initial local excitation of the azulene side and extending over large parts of the molecule. Later, IVR in the whole molecule takes place transferring vibrational energy from the azulene through the bridge to the anthracene side and thereby leading to microcanonical equilibrium. The corresponding time constants tau(IVR) for short bridges increase with the chain length. For longer bridges consisting of more than three elements, however, tau(IVR) is constant at around 4-5 ps. Comparison with molecular dynamics simulations suggests that the coupling of these chains to the two chromophores limits the rate of intramolecular vibrational energy transfer. Inside the bridges the energy transport is essentially ballistic and, therefore, tau(IVR) is independent on the length.