Time-resolved Infrared Emission Spectroscopy Research Articles

Aromatic and Acetylenic C-H or C-D Stretching Bands Anharmonicity Detection of Phenylacetylene by UV Laser-Induced Vibrational Emission.

The anharmonic infrared (IR) emission spectra of phenylacetylene C6H5CCH and an isotopologue C6H5CCD induced by 193 nm UV excitation have been investigated in the gas phase. The study has been operated with a homemade IR spectrometer enabling to record time- and wavelength-resolved spectra between 2.5 and 4.5 μm, emitted all along the collisional cooling. The analysis is supported by a kinetic Monte Carlo simulation in the vibrational harmonic approximation. For both species, the anharmonic shifts of the acetylenic C-H or C-D stretching modes and the aromatic C-H stretching modes are studied for band positions and bandwidths in terms of the internal energy. For C6H5CCD, the internal energy dependence of the emission intensity band ratio is investigated and rationalized. This work demonstrates the potential of time-resolved IR emission spectroscopy to explore anharmonicity of astrophysically relevant molecules.

Radiative Relaxation of Gold Nanorods Coated with Mesoporous Silica with Different Porosities upon Nanosecond Photoexcitation Monitored by Time-Resolved Infrared Emission Spectroscopy.

Gold nanorods (AuNRs) have been widely used in photothermal conversion, and a coating of silica (SiO2) provides higher thermal stability, better biocompatibility, and versatile chemical functionalization. In this work, two gold nanorods coated with surfactant-templated mesoporous silica layers of the same thickness but different porosities, and thus different specific surface areas, were prepared. Upon irradiation with 1064 nm nanosecond pulsed laser, the transient infrared emissions of AuNR@SiO2 enveloped the stretching mode of the Si-O-Si bridge (1000-1250 cm-1), the bending mode of adsorbed H2O (1600-1650 cm-1) within the mesoporous silica layer, and blackbody radiation, in terms of an underlying broad band (1000-2000 cm-1) probed with a step-scan Fourier transform spectrometer. The mesoporous silica shell and the adsorbed H2O gained populations of their vibrationally excited states, and the whole AuNR@SiO2 was heated up via the photothermal energy of the core AuNRs. An average temperature after 5-10 μs within 80% of the emission intensity was ca. 200 °C. The decay of the emission at 1000-1250 and 1500-1750 cm-1 was both accelerated, and the blackbody radiation components were negatively correlated with the porosity of the mesoporous silica layer. Higher porosity of the mesoporous silica layer was associated with more effective depopulation of the vibrationally excited states of the silica layers on the AuNRs via the nonradiative thermal conduction of the adsorbed H2O, since H2O has a larger thermal conduction coefficient than that of silica, in concomitance with the accelerated emission kinetics. This work unveils the roles of the porosity, capping materials, and entrapping molecules of a core-shell nanostructure during the thermalization after photoexcitation.

Is Photolytic Production a Viable Source of HCN and HNC in Astrophysical Environments? A Laboratory-based Feasibility Study of Methyl Cyanoformate

Motivated by the possibility that cyano-containing hydrocarbons may act as photolytic sources for HCN and HNC in astrophysical environments, we conducted a combined experimental and theoretical investigation of the 193 nm photolysis of the cyano-ester, methyl cyanoformate (MCF). Experimentally, nanosecond time-resolved infrared emission spectroscopy was used to detect the emission from nascent products generated in the photolysis reaction. The time-resolved spectra were analyzed using a recently developed spectral reconstruction analysis, which revealed spectral bands assignable to HCN and HNC. Fitting of the emission band shape and intensity allowed determination of the photolysis quantum yields of HCN, HNC, and CN (A22 Π1)and an HNC/HCN ratio of ∼0.076 ± 0.059. Additionally, multiconfiguration self-consistent field calculations were used to characterize photoexcitation-induced reactions in the ground and four lowest singlet excited states of MCF. At 193 nm excitation, dissociation is predicted to occur predominantly on the repulsive S 2 state, with minor pathways via internal conversion from S 2 to highly excited ground state. An automated transition-state search algorithm was employed to identify the corresponding ground-state dissociation channels, and Rice-Ramsperger-Kassel-Marcus and Kinetic Monte Carlo simulations were used to calculate the associated branching ratios. The proposed mechanisms were validated using the experimentally measured and quasi-classical trajectory-deduced nascent internal energy distributions of HCN and HNC. This work, along with previous studies, illustrates the propensity for cyano-containing hydrocarbons to act as photolytic sources for astrophysical HCN and HNC and may help explain the observed overabundance of HNC in astrophysical environments.

Monitoring the Transient Thermal Infrared Emission of Gold Nanoparticles upon Photoexcitation with a Step-Scan Fourier-Transform Spectrometer

The transient photothermal process of gold nanoparticles (AuNP) capped with different molecules, namely, citrate, cetyltrimethylammonium bromide (CTAB), and methoxyl-polyethylene glycol thiol (mPEG), has been investigated by time-resolved infrared emission spectroscopy monitored with a step-scan Fourier-transform spectrometer. Upon photoexcitation of the surface plasmonic resonance band of AuNPs with a 532 nm nanosecond pulsed laser, the transient infrared emission was observed within about 1 μs, referring to the duration of the laser heating and thermalization of AuNPs. Comparing the infrared emission contours with the blackbody radiation spectra at different temperatures revealed that the temperature reached 400 ± 100 °C in 90–120 ns as the 24 nm mPEG-capped AuNPs were excited by a peak power of 5 × 1010 W m–2 (25 mJ cm–2 from a 5 ns pulsed laser) at 532 nm. The insignificant changes in the morphology and size distribution of mPEG-AuNP suggested that the surface modification via covalent bonding helped ...

Large cross section for super energy transfer from hyperthermal atoms to ambient molecules

The experimentally measured cross section for super energy transfer collisions between a hyperthermal H atom and an ambient molecule is presented here. This measurement substantiates an emerging energy transfer mechanism with significant cross section, whereby a major fraction of atomic translational energy is converted into molecular vibrational energy through a transient collision-induced reactive complex. Specifically, using nanosecond time-resolved infrared emission spectroscopy, it is revealed that collisions between hyperthermal hydrogen atoms (with 59 kcal/mol of kinetic energy) and ambient ${\mathrm{SO}}_{2}$ result in the production of vibrationally highly excited ${\mathrm{SO}}_{2}$ with $g14\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ of internal energy. The lower limit of the cross section for this super energy transfer process is determined to be $0.53\ifmmode\pm\else\textpm\fi{}0.05$ \AA{}${}^{2}$, i.e., 2% of all hard-sphere collisions. This cross section is orders of magnitude greater than that predicted by the exponential energy gap law, which is commonly used for describing collisional energy transfer through repulsive interactions.

The reaction of formyl radical with chlorine atom

The radical–radical reaction of formyl radical with chlorine atom has been investigated by the time-resolved infrared emission spectroscopy and by the theoretical calculations at the UB3LYP/6-31++G(d,p) and single-point UCCSD(T)/6-311++G(d,p) levels. The products have been verified as the vibrationally excited CO ( v⩽4) and HCl. The reaction is initiated by radical–radical recombination forming an intermediate of formaldehyde chloride, which further dissociates into the products of HCl and CO.

The reaction of formaldehyde with chlorine atom

The elementary reaction of formaldehyde (H 2CO) with chlorine atom has been experimentally investigated by the time-resolved infrared emission spectroscopy. The nascent products HCO and HCl ( v⩽3) are observed. The major product channel has been identified as Cl+H 2CO→HCl+HCO. Theoretical calculations have revealed that the mechanism is a nearly barrierless peripheral hydrogen abstraction reaction.

Reaction of vinyl radical with O2 studied by time-resolved infrared emission spectroscopy

Nascent products of C2H3+O2 reaction have been studied by time-resolved FTIR emission spectroscopy. Vibrational excitation of reaction products H2CO, HCO, CO, and CO2 was observed, providing the evidence of existing two exothermic reaction channels: HCO+H2CO and CH3+CO2 for the reaction. The latter channel was firstly observed in the experiment. Simulation of the spectrum of the products reveals that the highest excitation of CO and CO2(ν3) at 5 μs is v=4 and 6, respectively; the branching ratio of the channel HCO+H2CO to the channel CH3+CO2 is estimated to be 17.

Vibrational band strengths and temperatures of nitric oxide by time-resolved infrared emission spectroscopy in a shock tube

Infrared emission spectra from the fundamental vibration—rotation band of NO are observed as functions of time in a shock tube using a specially constructed infrared array spectrometer. Emission is observed from shock-heated, dilute NO/Ar mixtures behind reflected shocks at temperatures between 800 and 2500 K and pressures of ≈1 atm. The spectrally resolved measurements cover the range 4.5–6.6 μm at a spectral resolution of 0.2 μm/pixel. The spectral intensity distributions on the long wavelength side of the band sample higher vibrational levels and thus provide a sensitive measure of vibrational temperature. Absolute spectral intensities in optically thin portions of the band determine the NO number density once the temperature is known. The integrated band intensities determine the temperature-dependent photon yields for vibrationally excited NO( v), which are in turn reduced by a rigorous anharmonic oscillator analysis to determine the thermally averaged Einstein coefficient for the optical transition from v = 1 to v = 0. The resulting value is A( v = 1) = 13.2(± 10%) sec -1, which is in excellent agreement with several determinations at temperatures near and below 300 K. This result establishes temperature independence for the NO Einstein coefficients up to 2500 K.

Time-resolved infrared emission spectroscopy in high-enthalpy supersonic air flows

We describe a spectrally resolved infrared emission method for observing path-integrated number densities and temperatures in high-temperature supersonic flow on a time-continuous basis. We have conducted infrared emission measurements of the fundamental vibrational band of nitric oxide (NO) to determine NO number densities and vibrational temperatures in a high-temperature, Mach 3 air flow in the Physical Sciences, Inc. (PSI) shock-tunnel facility. A specially constructed infrared array spectrometer was used to observe spectral distributions of NO vibrational radiation between 5 and 7 μm. The instrument was calibrated on a conventional shock tube and was implemented on the PSI shock tunnel to diagnose the flow at the exit of a Mach 3 nozzle, as driven by a plenum of shock-heated air at 3000-4000 K and 10-40 atm