Bimolecular rate coefficients were determined for the reaction CN(v = 1) + NO and O-2 using continuous wave cavity ringdown spectroscopy in a uniform supersonic flow (UF-CRDS). The well-matched time scales for ringdown and reaction under pseudo-first-order conditions allow for the use of the SKaR method (simultaneous kinetics and ringdown) in which the full kinetic trace is obtained on each ringdown. The reactions offer an interesting contrast in that the CN(v = 1) + NO system is nonreactive and proceeds by complex-mediated vibrational relaxation, while the CN(v = 1) + O-2 reaction is primarily reactive. The measured rate coefficients at 70 K are (2.49 +/- 0.08) x 10(-11) and (10.49 +/- 0.22) x 10(-11) cm(3) molecule(-1) s(-1) for the reaction with O-2 and NO, respectively. The rate for reaction with O-2 is a factor 2 lower than previously reported for v = 0 in the same temperature range, a surprising result, while that for NO is consistent with extrapolation of previous high-temperature measurements to 70 K. The latter is also discussed in light of theoretical calculations and measurements of the rate constants for the association reaction in the high-pressure limit. The measurements are complicated by the presence of a metastable population of high-J CN formed by photolysis of the precursor BrCN, and a kinetic model is developed to treat the competing relaxation and reaction. It is particularly problematic for reactions at low temperatures where the rotational relaxation and reaction have similar rates, precluding a reliable determination of the rate coefficients at 30 K. Also presented are important modifications to the data acquisition and control for the instrument that have yielded considerably enhanced stability and throughput.

This second paper in a series of two describes the chirped-pulse ice apparatus that permits the detection of buffer gas cooled molecules desorbed from an energetically processed ice using broadband mm-wave rotational spectroscopy. Here, we detail the lower ice stage developed to generate ices at 4 K, which can then undergo energetic processing via UV/VUV photons or high-energy electrons and which ultimately enter the gas phase via temperature-programmed desorption (TPD). Over the course of TPD, the lower ice stage is interfaced with a buffer gas cooling cell that allows for sensitive detection via chirped-pulse rotational spectroscopy in the 60–90 GHz regime. In addition to a detailed description of the ice component of this apparatus, we show proof-of-principle experiments demonstrating the detection of H2CO products formed through irradiation of neat methanol ices or 1:1 CO + CH4 mixed ices.

We present an experimental and theoretical investigation of the reaction of vibrationally excited CN (v = 1) with isomers of butadiene at low temperature. The experiments were conducted using the newly built apparatus, UF-CRDS, which couples near-infrared cw-cavity ring-down spectroscopy with a pulsed Laval flow. The well-matched hydrodynamic time and long ring-down time decays allow measurement of the kinetics of the reactions within a single trace of a ring-down decay, termed Simultaneous Kinetics and Ring-down (SKaR). The pulsed experiments were carried out using a Laval nozzle designed for the 70 K uniform flow with nitrogen as the carrier gas. The measured bimolecular rates for the reactions of CN (v = 1) with 1,3-butadiene and 1,2-butadiene are (3.96 ± 0.28) × 10-10 and (3.06 ± 0.35) × 10-10 cm3 per molecule per s, respectively. The reaction rate measured for CN (v = 1) with the 1,3-butadiene isomer is in good agreement with the rate previously reported for the reaction with ground state CN (v = 0) under similar conditions. We report the rate of the reaction of CN (v = 1) with the 1,2-butadiene isomer here for the first time. The experimental results were interpreted with the aid of variable reaction-coordinate transition-state theory calculations to determine rates and branching of the addition channels based on a high-level multireference treatment of the potential energy surface. H-abstraction reaction rates were also theoretically determined. For the 1,2-butadiene system, theoretical estimates are then combined with literature values for the energy-dependent product yields from the initial adducts to predict overall temperature-dependent product branching. H loss giving 2-cyano-1,3-butadiene + H is the main product channel, exclusive of abstraction, at all energies, but methyl loss forming 1-cyano-prop-3-yne is 15% at low temperature growing to 35% at 500 K. Abstraction forming HCN and various radicals is important at 500 K and above. The astrochemical implications of these results are discussed.

A new instrument is described that will employ buffer gas cooling with mm-wave rotational spectroscopy (60-90 GHz) to probe molecules desorbed from astrochemical ices prepared in an ultrahigh vacuum environment. Here the design and performance of the buffer gas cell, mm-wave spectrometer and data acquisition system are reported, while application to molecules desorbed from ice surfaces will be described in a future publication. The effective temperature of the neon-cooled buffer gas cell is determined by monitoring a range of rotational lines of propyl cyanide introduced into the cell. Its number density is estimated from comparison to room temperature measurements and the effective collision cross section with neon is estimated by monitoring the free induction decay (FID) lifetimes. The spectrometer and data acquisition system described are capable of acquiring and time-domain averaging the FIDs at 10 Gs/s, 10 bit vertical resolution and 98% duty cycle.

Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 lm. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground (X) over bar R-2(+) state and its first excited bending vibrational level of 2P symmetry. The upper states, of 2R(+) symmetry at 6055.6 cm-1 and 2P 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 2P level and J = 29/2 for the 2R one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (0 2 0), level of 2R symmetry, to a previously observed state of 2P 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. (C) 2020 Elsevier Inc. All rights reserved.

The pressure dependence of the OH + C2H4 addition reaction has been investigated using frequency-modulated laser absorption spectroscopy to monitor OH kinetics. Bimolecular rate coefficients for the title reaction are reported in argon bath gas at room temperature and total pressures ranging from 2 to 361 Torr. The pressure-dependent rate coefficients measured here agree well with the majority of published kinetic studies under similar conditions. Previous high-level ab initio calculations have identified a prereaction complex on the OH + C2H4 potential energy surface. The influence of this complex on the OH + C2H4 kinetics has been investigated using one-dimensional master equation analyses of the current and previous experimental measurements.

The photodissociation of O3 at 266 nm has been studied using velocity mapped ion imaging. We report temperature-dependent vector correlations for the O2(a1Δg, v = 0, j = 18-20) fragments at molecular beam temperatures of 70 K, 115 K, and 170 K. Both the fragment spatial anisotropy and the v-j correlations are found to be increasingly depolarized with increasing beam temperature. At all temperatures, the v-j correlations for the j = 19 state were shown to be reduced compared to those of j = 18 and 20, while no such odd/even rotational state difference was observed for the spatial anisotropy, consistent with previous measurements. We find that temperature-dependent differences in the populations and v-j correlations between the odd and even rotational states can be explained by a Λ-doublet propensity model. Although symmetry conservation should lead to formation of only the A' Λ-doublet component, and only even rotational states, out-of-plane rotation of the parent molecule breaks the planar symmetry and permits the formation of the A″ Λ-doublet component and odd rotational states. A simple classical model to treat the effect of parent rotation on the v-j correlation and the odd/even rotational population alternation reproduces both the current measurements and previously reported rotational distributions, suggesting that the "odd" behavior originates from a Λ-doublet propensity, and not from a mass independent curve crossing effect, as previously proposed.

We report on one-color experiments near 214 nm involving the photodissociation of jet-cooled OCS to produce high rotational states (40 < J < 80) of CO (X 1Σ+, v = 0, 1) which were then ionized by 2+1 resonance-enhanced multiphoton ionization via the E 1Π state. The nominally forbidden Q-branch of the two-photon E 1Π-X 1Σ+ transition is observed with intensity comparable to the allowed R-branch. The bright character of the high- J Q-branch lines can be described quantitatively as intensity borrowing due to mixing of the E 1Π and C 1Σ+ states, using J-dependent mixing coefficients extrapolated from the observed Λ-doubling in the lower rotational levels of the E state. In addition to the significant enhancement of Q-branch intensities above the values predicted by conventional two-photon line strengths for a 1Π-1Σ+ transition, the high- J lines of the R- and P-branches appear to be suppressed in intensity by approximately a factor of 3 compared to the unperturbed low- J line strengths, most likely due to perturbations associated with a 1Σ- state. The E-state rotational term values for J < 80, v = 0 derived from the present spectra agree within our measurement and calibration uncertainties with the extrapolations based on the molecular constants previously derived from rotational levels with J < 50. The E-X transition is attractive for future application to photodissociation dynamics and rotational polarization measurements of CO photofragments, with convenient access to state-selective probing on multiple rotational branches, which exhibit different sensitivity to fragment alignment.

We report the observation and analysis of spectra in part of the near-infrared spectrum of C2H, originating in rotational levels in the ground and lowest two excited bending vibrational levels of the ground X̃2Σ+ 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, v2=1 and the 2Σ+ component of v2=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.

The photodissociation of H2O2 dilute in argon at 193 nm has been investigated using near infrared frequency modulation laser absorption spectroscopy on selected rotational lines of the OH vibrational overtone bands between 6500 and 7100 cm(-1) to detect the OH and HO2 radical products with the same laser. Sequential OH and HO2 time-dependent profiles have been measured at room temperature and total pressures ranging from 3 to 56 Torr. Kinetic analysis gives the product distribution following H2O2 photolysis of (82 +/- 1) % OH and (18 +/- 1) % HO2 under the present conditions, in excellent agreement with previously reported measurements.

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.

Frequency comb-referenced measurements of sub-Doppler laser saturation dip absorption lines in the v 1 + v 3 band of acetylene near 1.5 μm are reported. These measurements include transitions involving higher rotational levels than previously frequency measured in this band. The accuracy of the measured frequencies is typically better than 10 kHz. Measurements of the observed sub-Doppler line widths as a function of pressure showed that the self-pressure-broadening coefficients are about 3.5 times larger than those derived from conventional pressure broadening of unsaturated Doppler-limited spectra. This is attributed to the contribution of velocity-changing collisions to the total dephasing rate in the low pressure sub-Doppler measurements. At higher pressures, when the homogeneous broadening becomes comparable to the typical Doppler shift per elastic collision, the velocity changing collisions cease to contribute significantly to the incremental pressure broadening. A time-dependent soft collision model is developed to illustrate the transition between low and high pressure regimes of sub-Doppler pressure-broadening.

Ethynyl, C 2 H, is an important intermediate in combustion processes and has been widely observed in interstellar space.Spectroscopically, it is of particular interest because it possesses three low-lying electronic surfaces: a ground 2 Σ + state, and a low-lying 2 Π excited electronic state, which splits due to the Renner-Teller effect.Vibronic coupling among these states leads to a complicated, mixed-character, energy level structure.We have previously reported work b on three bands originating from the X(0, 0, 0) 2 Σ ground state to excited vibronic states: two 2 Σ -2 Σ transitions at 6696 and 7088 cm -1 and a 2 Π -2 Σ transition at 7108 cm -1 .In this work, the radicals were formed in a hot, non-thermal, population distribution by u.v.pulsed laser photolysis of a precursor.Kinetic measurements of the time-evolution of the ground state populations following collisional relaxation and reactive loss were also made, using some of the stronger rotational lines observed.Time-dependent signals in mixtures containing a variable concentration of precursor in argon suggested that vibronically hot C 2 H radicals were less reactive than the relaxed, thermalized, radical.Two additional hot bands originating in states X(0, 1 1 , 0) 2 Π and X(0, 2 0 , 0) 2 Σ, have now been identified in the same spectral region.In a new series of experiments, we have measured the kinetics of formation and decay of representative levels involving all the assigned transitions, i.e. originating in X(0, v 2 , 0), with v 2 = 0, 1, and 2, in various concentrations of mixtures of precursor, inert gas and hydrogen.The new spectra also show greatly improved signal-to-noise ratio in comparison to our previous work, due to the use of a transient FM detection scheme, and additional spectral assignments seem likely.Both kinetics and spectroscopic results will be described in the talk.

! ( . * / (* . Saturation dip spectra of acetylene in the v1 + v3 band have been obtained for rotational lines with J = 31 − 37 inclusive, using a diode laser referenced to a frequency comb. The estimated accuracy and precision of the measurements is better than 10 kHz in 194 THz. Data were obtained as a function of sample pressure to investigate the broadening of the saturation features. The observed line shapes are well modeled by convolution of a fixed Gaussian transit-time and varying Lorentzian lifetime broadening, i.e. a Voigt-type profile. The lines exhibit a significantly larger collisional (lifetime) broadening than has been measured in conventional Doppler and pressure-broadened samples at ambient temperatures. The figure shows the fitted Lorentzian width versus sample pressure for P(31). The slope of this plot gives the pressure broadening coefficient, γself = 9.35(13) MHz/mbar. For comparison, the coefficient derived from conventional Doppler and pressure broadened spectra for this transition is 2.7 MHz/mbarc. The sub-Doppler broadening coefficients are all significantly larger than the conventionally measured ones, due to the increased importance of velocity-changing collisions. The measurements therefore give information on the balance between hard phaseor state-changing and large cross-section velocity-changing collisions. Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-SC0012704 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences and Biosciences within the Office of Basic Energy Sciences.

We demonstrate a method of combining a supercontinuum light source with a commercial Fourier transform spectrometer, using a novel approach to dual-beam balanced detection, implemented with phase-sensitive detection on a single light detector. A 40 dB reduction in the relative intensity noise is achieved for broadband light, analogous to conventional balanced detection methods using two matched photodetectors. Unlike conventional balanced detection, however, this method exploits the time structure of the broadband source to interleave signal and reference pulse trains in the time domain, recording the broadband differential signal at the fundamental pulse repetition frequency of the supercontinuum. The method is capable of real-time correction for instability in the supercontinuum spectral structure over a broad range of wavelengths and is compatible with commercially designed spectrometers. A proof-of-principle experimental setup is demonstrated for weak absorption in the 1500-1600 nm region.

Work at Brookhaven National Laboratory was carried out under Contract No. DE-SC0012704 with the U.S. Department of Energy, Office of Science, and supported by its Division of Chemical Sciences, Geosciences, and Biosciences.