Time-resolved IR Absorption Spectroscopy Research Articles

(Invited) Charge Separation Processes in Non-Fullerene Acceptors Studied by Time-Resolved IR

The mechanism of charge transfer and charge dissociation processes are investigated by femtosecond time-resolved IR absorption spectroscopy.

Waveguide-enhanced 2D-IR spectroscopy in the gas phase

A method for obtaining high-quality 2D-IR spectra of gas-phase samples is presented. Time-resolved IR absorption spectroscopy techniques, such as 2D-IR spectroscopy, often require that beams are focused into the sample. This limits the exploitable overlapped path length through samples to a few millimeters. To circumvent this limitation, 2D-IR experiments have been performed within a hollow waveguide. This has enabled acquisition of 2D-IR spectra of low-concentration gas-phase samples, with more than an order of magnitude signal enhancement compared with the equivalent experiment in free space. The technique is demonstrated by application to the 2D-IR spectroscopy of iron pentacarbonyl.

Potential-Dependent Recombination Kinetics of Photogenerated Electrons in n- and p-Type GaN Photoelectrodes Studied by Time-Resolved IR Absorption Spectroscopy

Recombination kinetics of photogenerated electrons in n-type and p-type GaN photoelectrodes active for H(2) and O(2) evolution, respectively, from water was examined by time-resolved IR absorption (TR-IR) spectroscopy. Illumination of a GaN film with UV pulse (355 nm and 6 ns in duration) gives transient interference spectra in both transmittance and reflection modes. Simulation shows that the interference spectra are caused by photogenerated electrons. We observed that recombination in the microsecond region is greatly affected by the applied potentials, the lifetime becoming longer at negative and positive potentials for n- and p-type GaN electrodes, respectively. There is a good correlation between potential dependence of the steady-state reaction efficiency and that of the number of surviving electrons in the millisecond region. We also performed potential jump measurement to examine the shift in Fermi level by photogenerated charge carriers. In the case of n-type GaN, the electrode potential jumps to the negative side by accumulation of electrons in the bulk. However, in the case of p-type GaN, the electrode potential first jumps to the negative side within 20 μs and gradually shifts to the positive side in a few milliseconds, while the number of charge carriers is constant at >0.2 ms. This two-step process is ascribed to electron transport from the bulk to the surface of GaN, because the electrode potential is sensitive to the number of electrons in the bulk. The results confirm that TR-IR combined with potential jump measurement provides useful information for understanding the behavior of charge carriers in photoelectrochemical systems.

Reactions of the transient species Cr(CO)5(cyclohexane) with C4HnE (n = 4, 8; E = O, NH, S) studied by time-resolved IR absorption spectroscopy

Time-resolved IR absorption spectroscopy is used to investigate substitution of the cyclohexane (CyH) molecule of the photolytically generated alkane-solvated transient intermediate Cr(CO)5(CyH) by heterocyclic ligands C4HnE (n=4, 8; E=O, NH, S). From the concentration and temperature dependences of the pseudo-first order rate constants, we obtain activation parameters for the reactions, and find that they are consistent with an associative (A) or interchange (I) mechanism. As was the case with ligand substitution reactions at W(CO)5(CyH), a ligand's reactivity depends both on its electron-donating ability and on its polarizability. We also find that for a reaction with a given DeltaH, the activation entropy is higher for reaction of Cr(CO)5(CyH) than it is for reaction of W(CO)5(CyH). Comparison of the present results with ligand substitution reactions of W(CO)5(CyH), CpMn(CO)2(CyH), and Cr(CO)5(n-heptane) indicates that for ligand substitution reactions at alkane-solvated transition-metal intermediates, the solvent's effect upon the reaction rate is primarily entropic.

Time‐Resolved Infrared Spectroscopy of K3Ta3B2O12 Photocatalysts for Water Splitting.

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Time-Resolved Infrared Spectroscopy of K3Ta3B2O12 Photocatalysts for Water Splitting

Electrons photoexcited in K(3)Ta(3)B(2)O(12), an efficient photocatalyst for the water-splitting reaction driven by ultraviolet light, were observed using time-resolved IR absorption spectroscopy with microsecond resolution. When the catalyst was irradiated with 266 nm light pulses, a structureless absorption appeared at 3000-1500 cm(-1). The absorption was assigned to the optical transition of electrons that were band gap-excited and then trapped in mid-gap states. The absorbance decayed with a time delay because of the electron-hole recombination. The rate of recombination in an argon atmosphere was sensitive to the composition of the starting material used in the catalyst preparation. The electron decay was accelerated by exposing the catalyst to water vapor. The degree of acceleration was qualitatively correlated with the H(2) production rate observed during steady-state light irradiation.

Phototoxic Phytoalexins. Processes that Compete with the Photosensitized Production of Singlet Oxygen by 9-Phenylphenalenones†

Experiments were performed to elucidate the excited-state behavior of 9-phenylphenalenones, which are phototoxic plant secondary metabolites involved in mechanisms of light-mediated plant defense. Using a combination of time-resolved and steady-state UV/visible spectroscopies, time-resolved IR absorption spectroscopy, time-resolved singlet oxygen phosphorescence measurements and cyclic voltammetry, we provide evidence of an intramolecular charge-transfer process in the excited singlet and the triplet states of 9-phenylphenalenones that modulates the photosensitized production of singlet oxygen.

Unusual behavior in the 308 nm flash photolysis of Vaska's complex

Time-resolved IR absorption spectroscopy was used to investigate the photolysis of Vaska's Complex (VC), trans-(PPh 3) 2Ir(CO)(Cl). Upon 308 nm photolysis, an intermediate was formed that regenerated VC on a millisecond timescale. This intermediate was not formed at the laser flash, but was generated over the course of ∼10 μs. Most unusually, there was no evidence for prompt bleach of the C–O stretch of VC upon photolysis. Evidence was presented that the intermediate from which regeneration of VC occurred was a dimeric species. Possible pathways for the generation of the dimeric intermediate are discussed.

Time-Resolved Infrared Absorption Studies of Surface OH Groups on TiO2 Particles Irradiated by UV Pulses

Abstract The shift of vibrational frequency of surface OH groups on TiO2 particles induced by UV pulse irradiation has been studied by time-resolved IR absorption spectroscopy. By the UV irradiation of the TiO2 particles, the absorption intensity at 3696 cm-1 was decreased and that at 3654 cm-1 was increased. These changes were gradually recovered on a millisecond time scale. The transient changes were interpreted by a small (< 0.1 cm-1) shift of a ν(O–H) band at 3677 cm-1. The temperature jump triggered by the UV pulse caused the shift.

Diagnostics and modeling of glow discharges by time-resolved IR absorption spectroscopy

In this work, several applications of time resolved absorption spectroscopy, with different time scales and spectral resolutions, are described for the diagnostic and modeling of cold plasmas produced in square wave modulated hollow cathode discharges. These methods have revealed a very sensitive and efficient way to test the relevance of the different individual mechanisms, in comparison with the usual plasma diagnostic methods of the stationary state.

Water- and Oxygen-Induced Decay Kinetics of Photogenerated Electrons in TiO2 and Pt/TiO2: A Time-Resolved Infrared Absorption Study

Electrons photogenerated in TiO2 and Pt/TiO2 catalysts were observed by time-resolved IR absorption spectroscopy. Transient IR absorption of an identical spectrum appeared on TiO2 and Pt/TiO2 irradiated by a 355 nm pump pulse. The absorption was attributed to optical transition of photogenerated electrons trapped in shallow midgap states. Electron- and hole-consuming reactions of adsorbates controlled the decay kinetics of the electrons competing with the electron−hole recombination. On TiO2, O2 from the gas-phase captured the electrons and accelerated the decay rate at a delay time of 10−100 μs. In water vapor, holes reacted with surface hydroxyls within 2 μs, and the recombination decay of the electrons was obstructed. When Pt/TiO2 was exposed to water, the oxidation and reduction steps of the water splitting reaction affected the decay kinetics in different time domains. Photogenerated holes oxidized water within 2 μs, whereas electrons reduced water at 10−900 μs.

Time-resolved IR-absorption spectroscopy of hot-electron dynamics in satellite and upper conduction bands in GaP.

The relaxation dynamics of hot electrons in the ${\mathit{X}}_{6}$ and ${\mathit{X}}_{7}$ satellite and upper conduction bands in GaP was directly measured by femtosecond UV-pump--IR-probe absorption spectroscopy. From a fit to the induced IR-absorption spectra the dominant scattering mechanism giving rise to the absorption at early delay times was determined to be intervalley scattering of electrons out of the ${\mathit{X}}_{7}$ upper conduction-band valley. For long delay times the dominant scattering mechanism is electron-hole scattering. Electron transport dynamics of the upper conduction band of GaP has been time resolved.

Time-resolved diode laser infrared absorption spectroscopy of the nascent HCl in the infrared laser chemistry of 1,2-dichloro-1,1-difluoroethane

The IR multiphoton excitation and the frequency, fluence and intensity dependence of the IR-laser chemical yields of CF 2ClCH 2Cl have been studied in the fluence range of 1 to 10 J cm −2 yielding a steady-state constant k(st)/ I=0.74×10 6 s −1 MW −1 cm 2 which is approximately independent of intensity. Time-resolved IR absorption spectroscopy with diode laser sources has been used to observe the nascent HCl during the first few 100 ns indicating a population inversion between the levels ν=1, J=4 and ν=2, J=5. At low reactant pressures ( p⩽10 Pa) the time-resolved measurement gives a steady-state rate constant consistent with the theoretical result adjusted to the static yield measurements. The capability of state-selective and time-resolved IR spectroscopy is thus demonstrated, giving real-time determinations of rate constants.

Time-resolved IR spectral photography

The non-linear optics basis and experimental realization of time-resolved IR spectral photography (TRISP), a new technique of nanosecond time-resolved IR absorption spectroscopy, are described. TRISP has been applied to the study of transients. It was used to study the time evolution of the rotational temperature of HCI during CO 2-laser-induced explosions of HN 3–HCI mixtures. In a recent experiment tert-butyl radicals were formed by ruby laser photolysis of (CH 3) 3CNO. TRISP was used to obtain the gas phase IR spectrum of the radicals in the 3 μm region. By following the decay of this transient IR absorption, the rate coefficients for the combination and disproportionation reactions between tert-butyl radicals were deduced.