Time-resolved Infrared Absorption Research Articles

Electron- and Hole-transfer from TiO2 Particles to Adsorbates Studied by Time-Resolved Infrared Absorption Spectroscopy.

Behavior of the electrons and holes photogenerated in TiO2 particles was observed by time-resolved infrared absorption spectroscopy in the presence of oxygen, water, and methanol vapor. The electrons photogenerated by a band-gap excitation displayed a structureless, broad absorption of IR light from 3000 to 900 cm−1, which was assigned to the intra-conduction-band transition and/or excitation from mid-gap traps to the conduction band. This electron-induced absorption was probed as a function of time delay after the photoexcitation. Electron decay caused by the recombination with holes and by the charge-transfer reactions with adsorbates were kinetically analyzed. The electron decay was accelerated in the presence of oxygen gas due to an electron-capture reaction at the interface, whereas was decelerated in methanol vapor due to an effective hole-capture by methoxy groups. On platinized TiO2 particles exposed to water vapor, a hole-capture reaction completed within 2 μs after band-gap excitation, whereas the electron-capture reaction occurred in 10 μs or later. These results demonstrate the effectiveness of this method to identify individual steps of photo-induced reactions at interfaces.

Electron- and Hole-Capture Reactions on Pt/TiO2 Photocatalyst Exposed to Methanol Vapor Studied with Time-Resolved Infrared Absorption Spectroscopy

The decay kinetics of photogenerated electrons in a Pt/TiO2 catalyst has been studied by time-resolved infrared absorption (TR-IR) spectroscopy. The electron decay was drastically affected by exposure to methanol vapor, and the electron−hole pair recombination was prevented. A certain fraction of the electrons remained beyond 1 s and slowly decayed from ∼1−9 s. The prevented recombination was interpreted with the effective capture of the holes. The extent of the hole capture was insensitive to the pressure of methanol vapor. An irreversibly adsorbed methoxy groups was proposed to capture the holes on the basis of the steady-state FT-IR spectra of the catalyst exposed to methanol vapor of different pressures. However, the slow decay of the electrons exhibited a positive-order response to the methanol pressure. Undissociated methanol physisorbed and equilibrated with the gas phase, which was identified in the steady-state spectra, should play a key role in the electron capture process. Possible mechanisms o...

Excited State Structure of N-(4-cyanophenyl)carbazole by Time-Resolved Infrared Absorption Spectroscopy

Abstract Nanosecond time-resolved infrared absorption spectroscopy has been used to study the charge transfer characteristics of the title compound in electronically excited states. The transient infrared bands observed at 2090 cm-1 and 2060 cm-1 have been attributed to the CN stretch modes in the first excited singlet and triplet states of the molecule. These frequencies are in conformity with enhanced charge separation in the excited states. Comparison of the present result with the existing data for 4-(N,N-dimethylamino) benzonitrile has indicated that the CN stretch frequency is not a direct measure of the polarity of the excited states of these molecules.

Electronic and Steric Effects in Ligand Substitution at a Transient Organometallic Species: The Reaction of W(CO)5(Cyclohexane) with (CH3)nTHF and (CH3)nFuran (n = 1, 2)

Time-resolved infrared absorption spectroscopy is used to study the reactions of the solvated transient intermediate W(CO)5(cyclohexane) with the molecules (L) 2-methyltetrahydrofuran (MeTHF), 2,5-dimethyltetrahydrofuran (Me2THF), 2-methylfuran (MeFur), and 2,5-dimethylfuran (Me2Fur). In all four cases, the only reaction observed on the microsecond to millisecond time scale is substitution of the cyclohexane molecule to form W(CO)5(L). From the temperature dependence of the reaction rate constant, measured over the range 15−60 °C, we determine the following activation parameters: for L = MeTHF, ΔH⧧ = 4.0 ± 0.3 kcal mol-1, ΔS⧧ = −14.1 ± 1.0 eu; for L = Me2THF, ΔH⧧ = 4.3 ± 0.3 kcal mol-1, ΔS⧧ = −14.8 ± 1.2 eu; for L = MeFur, ΔH⧧ = 6.0 ± 0.2 kcal mol-1, ΔS⧧ = −10.2 ± 1.2 eu; and for L = Me2Fur, ΔH⧧ = 6.3 ± 0.4 kcal mol-1, ΔS⧧ = −9.8 ± 1.0 eu. We find that the relative rates of reaction of MeTHF and Me2THF can be explained solely in terms of a steric effect, while for MeFur and Me2Fur, the inductive effect of the electron-donating substituents leads to an electronic effect that competes with the steric effect.

Real Time Infrared Spectroscopic Probe of the Reactions of Fe(CO)3 and Fe(CO)4 with N2 in the Gas Phase

Time-resolved infrared absorption spectroscopy has been used to study the gas-phase reactions of Fe(CO)3, Fe(CO)3N2, and Fe(CO)4 with N2, where Fe(CO)3 is generated by 308 nm laser photolysis of Fe...

Time-resolved infrared absorption spectroscopy of photogenerated electrons in platinized TiO 2 particles

Kinetics of photogenerated electrons in platinized TiO 2 (P-25) particles were observed by time-resolved infrared (IR) absorption spectroscopy with AC-coupled detection of cw probe light. The sample irradiated by UV pulse (355 nm and 10 ns) gave transient absorption which monotonously increased in intensity with decreasing wavenumber from 3000 to 900 cm −1 . Photogenerated electrons trapped in shallow mid-gap states were proposed to originate the transient absorption. The decay kinetics of the trapped electrons were fitted to a multi-exponential formula with six lifetimes ranging from 1×10 −7 to 3×10 −1 s.

Fast Processes in a N2O-Modulated Hollow Cathode Discharge: Excitation and Diffusion

In this work the changes in the infrared absorption of N2O observed in a hollow cathode discharge modulated by a 45 Hz square wave are studied by time-resolved infrared absorption spectroscopy, with a Fourier transform infrared (FTIR) spectrometer working in the step-scan mode. These variations are attributed to alternative population and depopulation of the ground state of N2O; on the other hand, no changes are observed in the total concentration of N2O and the major products of the discharge. The experimental results are explained by means of a simple kinetic model: vibrational excitation processes are assumed to be the main cause of the observed effects, but an inhomogeneous distribution of the stable species and the incorporation of diffusion terms between the plasma volume and the rest of the discharge cell are shown to be crucial in order to justify the quick and sharp variations in the N2O ground-state population seen along the path of the IR beam. The influence of these effects has been verified ...

Dynamics of the dissolution of an underpotentially deposited Cu layer on Au(111): a combined time-resolved surface-enhanced infrared and chronoamperometric study

It is well known that an ordered phase consisting of 2/3 monolayer (ML) of Cu and 1/3 ML of sulfate is formed by the underpotential deposition of Cu on Au(111) in sulfuric acid. Dynamic processes involved in the dissolution of the ordered phase for a potential step have been investigated by time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS) and chronoamperometry. The results demonstrate that the dissolution occurs through two successive steps. The initial step involves the desorption of 1/3 ML of Cu and 1/3 ML of sulfate via a Langmuir-type kinetics and the remaining 1/3 ML of Cu is desorbed in the subsequent step via a nucleation-and-growth kinetics. The observation is partly consistent with dynamic Monte Carlo simulations reported in the literature.

Picosecond Time-Resolved Infrared Absorption Studies on the Photoexcited States of Poly(p-phenylenevinylene)

Picosecond time-resolved infrared absorption spectra of the photoexcited states of poly(p-phenylenevinylene) [(C6H4CHCH)n, PPV] have been observed with tunable picosecond light pulses obtained by difference-frequency generation between signal and idler waves from an optical parametric generator and amplifier. The time-resolved photoinduced infrared absorption bands due to a vibrational transition observed at 1550 cm-1 and an electronic transition at 3000 cm-1 of the photoexcited PPV have fast (≈1.7 ps) and slow (≈50 ps) decay components. By comparison of the picosecond time-resolved infrared absorption spectra of short-lived (fast decay component) and long-lived (slow decay component) transient species with the photoinduced and doping-induced infrared difference spectra of PPV, the short-lived transient species is assigned to bound polaron pairs, and the long-lived transient species is assigned to positive and negative polarons.

Spatially and time-resolved infrared absorption for optical and electrical characterization of indirect band gap semiconductors

The current status of the spatially and time-resolved free-carrier absorption (FCA) method is provided. The FCA technique allows monitoring carrier dynamics in a time scale from nanoseconds to miliseconds by employing either collinear or orthogonal geometry between pump and probe beams. A high spatial resolution is achieved allowing in-depth carrier profiles to be extracted. The method is particularly suited for investigation of injection-dependent optical and recombination phenomena: band gap optical absorption, Shockley–Read–Hall (SRH) lifetime, Auger recombination coefficient, and the injection-dependent surface (interface) recombination velocity. We summarize important aspects of the technique demonstrating numerous measurements that have been implemented in studies of bulk Si, epilaxial 4H-SiC and porous silicon.

Experimental and theoretical determination of the magnetic dipole transition moment for the Br(4p5)(2P1/2←2P3/2) fine-structure transition and the quantum yield of Br(2P1/2) from the 193 nm photolysis of BrCN

The integrated-absorption coefficients of several hyperfine lines of the magnetic dipole allowed transition of the bromine atom, Br, center at 3685.2 cm−1 were measured, and a value for the square of the magnetic dipole transition moment of the Br atom was determined. A theoretical calculation for the magnetic dipole transition moment was also carried out using a relativistic ab initio atomic structure formulation. The theoretical value was in excellent agreement with the value predicted assuming pure LS coupling, and in reasonable agreement with experiment. The Br atom was generated in equal concentration with the cyano radical (CN) by the 193 nm photolysis of cyanogen bromine, BrCN. The CN radicals were titrated by the rapid reaction with C3H8 to generate HCN and a small amount of HNC. Both time-resolved and frequency-scanned infrared absorption spectroscopy were used to monitor the Br, HCN, and HNC species. The photolysis of BrCN at 193 nm produced both the ground state Br(2P3/2) and the spin-orbit excited Br(2P1/2) atoms, and the yield for the production of Br(2P1/2) atoms was measured to be 0.31±0.01. The rate constants for the quenching of Br(2P1/2) by BrCN and C3H8 at 293 K were also determined.

Experimental determination of the υ 1 fundamental vibrational transition moment for HNC

Hydrogen isocyanide, HNC, was generated by the reaction of translationally energetic H atoms with ClCN, and the resulting products, HCl, HCN and HNC, followed by time-resolved infrared absorption spectroscopy. The subsequent reaction of the Cl and CN species with the H atom precursor, CH 3SH, resulted in a definite stoichiometric relationship between the stable products and HNC. This stoichiometric relationship was used to relate the HNC absorption to those for HCN and HCl. The υ 1 fundamental vibrational transition moment was determined to be 0.141±0.007 Debye.

Products of the ultraviolet photodissociation of trifluoroacetic acid and acrylic acid

The photodissociation of trifluoroacetic and acrylic acids by the ultraviolet light from a flashlamp has been investigated by measuring the relative yields of some of the major products by time-resolved infrared absorption using tunable, narrow band diode lasers. Yields of CO2 were measured both in the absence and presence of added O2. The former experiments measure the CO2 produced directly by decarboxylation of the acid, channel (2) below, the latter the sum of the yields from channels (1) and (2) since HOCO is rapidly converted to CO2. The yields of CO from the decarbonylation channel (3) have also been measured. For trifluoroacetic acid, the relative yields are found to be [HOCO]:[CO2]:[CO]=(0.28±0.07):(0.61±0.09):(0.11±0.06) and, for acrylic acid, [HOCO]:[CO2]:[CO]=(0.32±0.08):(0.37±0.08):(0.31±0.09). The results are discussed in relation to the other, limited, measurements on the photodissociation of these acids and whether these three processes are likely to occur independently of one another.

Initial vibrational level distribution of HCN[X̃ 1Σ+(v10v3)] from the CN(X 2Σ+)+H2→HCN+H reaction

The reaction of the cyano radical (CN) with hydrogen was studied by time-resolved infrared absorption spectroscopy of individual rovibrational states of HCN. The initial vibrational level distribution of HCN(v10v3) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. The experiments were carried out at two temperatures, 293 and 324 K, with similar results. It was estimated that about 50% of the available reaction exothermicity was deposited as vibrational excitation of the HCN product. Surprisingly, the HCN(101) vibrational level received a significant fraction of the observed vibrational population, implying that the CN vibration was not really a spectator bond in the reaction dynamics. Furthermore, the observed HCN(v10v3) vibrations only account for about 27% of the initial HCN population produced in the title reaction. A significant fraction of the product HCN molecules must have been produced with the bending mode excited, likely in combination with the H–C stretch vibrations.

Thermal Rate Constant and Branching Ratio for CN + HD → HCN/DCN + D/H from T = 293 to 375 K

The reaction rate constant for the cyano (CN) radical with deuterium hydride (HD) has been determined over the temperature range from 293 to 375 K. As well, the branching ratio for the partitioning into HCN + D or DCN + H has also been measured. The CN radical was detected by time-resolved near-infrared absorption spectroscopy using the CN (A{sup 2}{Pi} {l_arrow} X{sup 2}{Sigma}) (2,0) band near 789.5 nm while the HCN molecule was monitored by time-resolved infrared absorption spectroscopy using the HCN(001) {l_arrow} (000) {upsilon}{sub 3} fundamental band near 3.0 {micro}m. Both species were monitored following the same photolysis laser pulse used to generate the CN radical. The measurements were carried out in low pressures of Ar or He as carrier gas. The frequencies and structure of the transition states for all four isotopomers for CN + H{sub 2} were calculated using ab initio quantum chemistry methods and a normal-mode analysis. Reasonable agreement was found between the experimental results and the rate constants predicted by conventional transition state theory using the theoretical transition state properties. These measurements should provide for an interesting test for theoretical predictions of thermal rate constants for this prototypical four-atom reaction system.

Time-resolved infrared study of ground-state phototautomer formed in the excited-state proton transfer of 7-hydroxyquinoline in methanol

Time-resolved infrared absorption spectra of 7-hydroxyquinoline (7-HQ) in methanol were measured to investigate the relaxation processes following S 1 → S′ 1 proton transfer tautomerization. Deuterium and nitrogen isotope effects were observed for the transient infrared spectra of 7-HQ- 14N and - 15N in MeOD and MeOH. The 1644 (1628) cm −1 band in MeOH (MeOD) is ascribed to the H(D)-bonded C = O stretching of the phototautomer in the ground state (St́ 0). Transient absorption decay exhibits a remarkable deuterium isotope effect. It is thus demonstrated that the ground-state reverse proton transfer of S′ 0 is responsible for the observed transient decay.

Coordination reaction of NO onto W(CO) 5 in the gas phase

The reaction of the coordinatively unsaturated mono-tungsten center W(CO) 5 with NO was investigated by time-resolved infrared absorption spectroscopy in the wavenumber region 1650–2035 cm −1 in the gas phase at room temperature. Initially, W(CO) 5 reacted with NO to make a nitrosyl coordination compound W(CO) x (NO) where x is probably 4 or 5. The coordinated nitrosyl and carbonyl stretching frequencies were found to be around 1726 and 1999 cm −1, respectively. W(CO) x (NO) reacted successively with NO to form a relatively stable complex W(CO) 3(NO) 2 with one CO stretching band at 2015 cm −1 and two NO stretching bands at 1676 and 1712 cm −1.

The CH3C(O)O2Radical. Its UV Spectrum, Self-Reaction Kinetics, and Reaction with CH3O2

The reactions of CH3C(O)O2 with itself and with CH3O2 are investigated using flash photolysis combined with time-resolved UV spectroscopy and transient infrared absorption. The UV spectrum of CH3C(O)O2 exhibits two bands; the stronger short wavelength component has a maximum cross section of 6.5 × 10-18 cm2 at 206 nm and the weaker one a cross section of 2.9 × 10-18 cm2 at 250 nm. These bands are used to monitor the disappearance of CH3C(O)O2 and the secondary formation of CH3O2, yielding a self-reaction rate constant of × 10-12 e(504±114)/T cm3 s-1 and a cross reaction rate constant of × 10-13 e(726±75)/T cm3 s-1. Transient IR monitoring of the formation of CH2O reveals CH3C(O)O2 + CH3O2 → CH2O + CH3COOH + O2 to be the dominant channel for the cross reaction over the entire 209−358 K temperature range. This contrasts with previous studies that indicate a steep temperature dependence for the branching ratio, with a channel leading to CH3C(O)O + CH3O + O2 dominant at high temperatures.

The CF3C(O)O2Radical. Its UV Spectrum, Self-Reaction Kinetics, and Reaction with NO

Flash photolysis combined with time-resolved UV spectroscopy and transient infrared absorption is used to investigate the reactions of CF3C(O)O2 with itself, CF3O2, and nitric oxide. The UV spectrum of CF3C(O)O2 exhibits two bands, the stronger short wavelength component of which has a maximum cross section of 7.1 × 10-18 cm2 at 207 nm. These bands are used to monitor the disappearance of CF3C(O)O2 and the secondary formation of CF3O2, yielding a self-reaction rate constant of × 10-12 e(270±200)/T cm3 s-1. The cross reaction between CF3C(O)O2 and CF3O2 is found to be slow, having a rate constant of ≤2 × 10-12 cm3 s-1. Transient IR monitoring of the loss of NO and concomitant formation of NO2 leads to a rate constant of × 10-12 e(563±115)/T cm3 s-1 for the reaction between CF3C(O)O2 and NO. This result implies that CF3C(O)O2 radicals formed as intermediates in the atmospheric degradation of hydrofluorocarbons (HFCs) are rapidly converted into CF3O2 radicals, which are in turn converted into carbonyl fluoride and FNO.

Formate Ester Norrish Type II Elimination: Diode Laser Probing of Gas-Phase Yields

Time-resolved infrared absorption spectroscopy was utilized to monitor the production of HCOOH, CO2, and CO following ultraviolet laser excitation of gas-phase formate esters. Excitation of ethyl formate at 227.5 nm resulted in formation of HCOOH and CO2. The CO2 quantum yield was estimated to be 0.5 ± 0.1. No evidence for CO formation was obtained at this wavelength. Relative quantum yields for the Norrish Type II elimination of HCOOH from ethyl, n-propyl, n-butyl, isopropyl, isobutyl, and tert-butyl formate were obtained at 227.5 and 222 nm. Normalization of the observed HCOOH yields with respect to the number of γ-hydrogen atoms resulted in reactivity trends at 227.5 nm of 1:3:9 for the abstraction of primary, secondary, and tertiary hydrogen atoms, respectively. At 222 nm, a similar reactivity trend was observed with yields per available γ-hydrogen of 1:3:7 for abstraction of primary, secondary, and tertiary hydrogen atoms. Yields were found to be independent of ester pressure over the range 100−550 m...