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Register a new userThe near-infrared spectrum of ethynyl radical
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Transient diode laser absorption spectroscopy has been used to measure three strong vibronic bands in the near infrared spectrum of the C$_2$H, ethynyl, radical not previously observed in the gas phase. The radical was produced by ultraviolet excimer laser photolysis of either acetylene or (1,1,1)-trifluoropropyne in a slowly flowing sample of the precursor diluted in inert gas, and the spectral resolution was Doppler-limited. The character of the upper states was determined from the rotational and fine structure in the observed spectra and assigned by measurement of ground state rotational combination differences. The upper states include a $^2Sigma ^+$ state at 6696 cm$^{-1}$, a second $^2Sigma ^+$ state at 7088 cm$^{-1}$, and a $^2Pi$ state at 7110 cm$^{-1}$. By comparison with published calculations (R. Tarroni and S. Carter, textit{J. Chem. Phys} textbf{119}, 12878 (2003) and textit{Mol. Phys}. textbf{102}, 2167 (2004)), the vibronic character of these levels was also assigned. The observed states contain both $X^2Sigma^+$ and $A^2Pi$ electronic character. Several local rotational level perturbations were observed in the excited states. Kinetic measurements of the time-evolution of the ground state populations following collisional relaxation and reactive loss of the radicals formed in a hot, non-thermal, population distribution were made using some of the strong rotational lines observed. The case of C$ _{2} $H may be a good place to investigate the behavior at intermediate pressures of inert colliders, where the competition between relaxation and reaction can be tuned and observed to compare with master equation models, rather than deliberately suppressed to measure thermal rate constants.
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Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 $mu$m. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground $tilde{X},^2Sigma^+$ state and its first excited bending vibrational level of $^2Pi$ symmetry. The upper states, of $^2Sigma^+$ symmetry at 6055.6 cm$^{-1}$ and $^2Pi$ symmetry at 6413.5 cm$^{-1}$, respectively, had not previously been observed and the data were analyzed in terms of an effective Hamiltonian representing their rotational and fine structure levels to derive parameters which can be used to calculate rotational levels up to J = 37/2 for the $^2Pi$ level and J = 29/2 for the $^2Sigma$ one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (020), level of $^2Sigma$ symmetry, to a previously observed state of $^2Pi$ symmetry near 6819 cm$^{-1}$. These strong absorption bands at convenient near-IR laser wavelengths will be useful for monitoring CCH radicals in chemical systems.
We report the observation and analysis of spectra in part of the near-infrared spectrum of C$_2$H, originating in rotational levels in the ground and lowest two excited bending vibrational levels of the ground $tilde{X},^2Sigma^+$ state. In the analysis, we have combined present and previously reported high resolution spectroscopic data for the lower levels involved in the transitions to determine significantly improved molecular constants to describe the fine and hyperfine split rotational levels of the radical in the zero point, $v_2=1$ and the $^2Sigma^+$ component of $v_2=2$. Two of the upper state vibronic levels involved had not been observed previously. The data and analysis indicate the electronic wavefunction character changes with bending vibrational excitation in the ground state and provide avenues for future measurements of reactivity of the radical as a function of vibrational excitation.
Sub-Doppler, saturation dip, spectra of lines in the $v_1 + v_3$, $v_1 + 2v_4$ and $v_3 + 2v_4$ bands of $^{14}$NH$_3$ have been measured by frequency comb-referenced diode laser absorption spectroscopy. The observed spectral line widths are dominated by transit time broadening, but show resolved or partially-resolved hyperfine splittings that are primarily determined by the $^{14}$N quadrupole coupling. Modeling of the observed line shapes based on the known hyperfine level structure of the ground state of the molecule shows that, in nearly all cases, the excited state level has hyperfine splittings similar to the same rotational level in the ground state. The data provide accurate frequencies for the line positions and easily separate lines overlapped in Doppler-limited spectra. The observed hyperfine splittings can be used to make and confirm rotational assignments and ground state combination differences obtained from the measured frequencies are comparable in accuracy to those obtained from conventional microwave spectroscopy. One upper state level shows very clear differences from the expected splittings. Examination of the known vibration-rotation level structure shows there is a near degeneracy between this level in $v_1+v_3$ and a rotational level in the $v_1 + 2v_4$ manifold which is of the appropriate symmetry to be mixed by magnetic hyperfine terms that couple ortho- and para- forms of the molecule.
We analyse a sample of 85 luminous (log(nuLnu(3um)/erg s-1)>45.5) quasars with restframe ~2-11 um spectroscopy from AKARI and Spitzer. Their high luminosity allows a direct determination of the near-infrared quasar spectrum free from host galaxy emission. A semi-empirical model consisting of a single template for the accretion disk and two blackbodies for the dust emission successfully reproduces the 0.1-10 um spectral energy distributions (SEDs). Excess emission at 1-2 um over the best-fitting model suggests that hotter dust is necessary in addition to the ~1200 K blackbody and the disk to reproduce the entire near-infrared spectrum. Variation in the extinction affecting the disk and in the relative strength of the disk and dust components accounts for the diversity of individual SEDs. Quasars with higher dust-to-disk luminosity ratios show slightly redder infrared continua and less prominent silicate emission. We find no luminosity dependence in the shape of the average infrared quasar spectrum. We generate a new quasar template that covers the restframe range 0.1-11 um, and separate templates for the disk and dust components. Comparison with other infrared quasar composites suggests that previous ones are less reliable in the 2-4 um range. Our template is the first one to provide a detailed view of the infrared emission on both sides of the 4 um bump.
Resonant inelastic x-ray scattering (RIXS) provides remarkable opportunities to interrogate ultrafast dynamics in liquids. Here we use RIXS to study the fundamentally and practically important hydroxyl radical in liquid water, OH(aq). Impulsive ionization of pure liquid water produced a short-lived population of OH(aq), which was probed using femtosecond x-rays from an x-ray free-electron laser. We find that RIXS reveals localized electronic transitions that are masked in the ultraviolet absorption spectrum by strong charge-transfer transitions -- thus providing a means to investigate the evolving electronic structure and reactivity of the hydroxyl radical in aqueous and heterogeneous environments. First-principles calculations provide interpretation of the main spectral features.
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