Frequency Modulation Absorption Spectroscopy Research Articles

Rotational Spectrum and First Interstellar Detection of 2-methoxyethanol Using ALMA Observations of NGC 6334I

We use both chirped-pulse Fourier transform and frequency-modulated absorption spectroscopy to study the rotational spectrum of 2-methoxyethanol (CH3OCH2CH2OH) in several frequency regions ranging from 8.7 to 500 GHz. The resulting rotational parameters permitted a search for this molecule in Atacama Large Millimeter/submillimeter Array (ALMA) observations toward the massive protocluster NGC 6334I, as well as source B of the low-mass protostellar system IRAS 16293−2422. A total of 25 rotational transitions are observed in the ALMA Band 4 data toward NGC 6334I, resulting in the first interstellar detection of 2-methoxyethanol. A column density of 1.3−0.9+1.4×1017 cm−2 is derived at an excitation temperature of 143−39+31 K. However, molecular signal is not observed in the Band 7 data toward IRAS 16293−2422B, and an upper-limit column density of 2.5 × 1015 cm−2 is determined. Various possible formation pathways—including radical recombination and insertion reactions—are discussed. We also investigate physical differences between the two interstellar sources that could result in the observed abundance variations.

Inelastic Scattering of CN Radicals at the Gas–Liquid Interface Probed by Frequency-Modulated Absorption Spectroscopy

We describe the results of the first application of frequency-modulated (FM) absorption spectroscopy to molecular scattering dynamics at a gas–liquid interface. A pulsed direct-current (dc) electri...

Frequency modulated circular dichroism spectroscopy: application to ICN photolysis

CN photofragments produced in the 248 nm photolysis of ICN are characterised by frequency-modulated absorption spectroscopy in the red A2Π ← X2Σ+ system. A range of high rotational states in both v = 0 and v = 1 were selected to isolate only those dissociation channels including coincident ground state I(2P3/2). Velocity-resolved alignment and orientation parameters are determined from the observed Doppler profiles. The alignment measurements for both v = 0 and v = 1 levels are very similar, and consistent with dissociation occurring via three distinct pathways, beginning with simultaneous excitation of the 4A′(3Π0+), 5A′(1Π1) and 5A″(1Π1) surfaces. While the incoherent alignment parameters for both v = 0 and v = 1 products are similar, the corresponding coherent orientation parameters are markedly different. CN (v = 0, N) exhibits a large positive orientation, whereas all rotational states probed in v = 1 show a much smaller orientation. This is consistent with the previously observed change in the sign of the coherent orientation parameter between v = 0 and v = 2.

Kinetics and Rate Constants of the Reaction CH2OH + O2→ CH2O + HO2in the Temperature Range of 236−600 K

The kinetics and absolute rate constants of the gas-phase reaction of the hydroxymethyl radical (CH2OH) with molecular oxygen have been studied using laser photolysis/near-IR absorption spectroscopy. The reaction was tracked by monitoring the time-dependent changes in the production of the hydroperoxy radical (HO2) concentration. For sensitive detection of HO2, two-tone frequency modulation absorption spectroscopy was used in combination with a Herriott-type optical multipass absorption cell. Rate constants were determined as a function of temperature (236 K<T<600 K) at 50 Torr of N2. The experimental results exhibit a slight negative temperature dependence in the measured temperature region. Microcanonical variational transition state theory was used to estimate the rate constants of the investigated reaction. The results of the theoretical calculations also suggest a negative temperature dependence of the reaction rate constant in the measured temperature region.

Wide temperature range (T = 295 K and 770–1305 K) study of the kinetics of the reactions HCO + NO and HCO + NO2 using frequency modulation spectroscopy

The rate constants for , HCO + NO --> HNO + CO, and , HCO + NO(2)--> products, have been measured at temperatures between 770 K < T < 1305 K behind reflected shock waves and, for the purpose of a consistency check, in a slow flow reactor at room temperature. HCO radicals were generated by 193 nm excimer laser photolysis of diluted gas mixtures containing glyoxal, (CHO)(2), and NO or NO(2) in argon and were monitored using frequency modulation (FM) absorption spectroscopy. Kinetic simulations based on a comprehensive reaction mechanism showed that the rate constants for the title reactions could be sensitively extracted from the measured HCO profiles. The determined high temperature rate constants are k(1)(769-1307 K) = (7.1 +/- 2.7) x 10(12) cm(3) mol(-1) s(-1) and k(2)(804-1186 K) = (3.3 +/- 1.8) x 10(13) cm(3) mol(-1) s(-1). The room temperature values were found to be in very good agreement with existing literature data and show that both reactions are essentially temperature independent. The weak temperature dependence of can be explained by the interplay of a dominating direct abstraction pathway and a complex-forming mechanism. Both pathways yield the products HNO + CO. In contrast to , no evidence for a significant contribution of a direct high temperature abstraction channel was found for . Here, the observed temperature independent overall rate constant can be described by a complex-forming mechanism with several product channels. Detailed information on the strongly temperature dependent channel branching ratios is provided. Moreover, the high temperature rate constant of , OH + (CHO)(2), has been determined to be k(7) approximately 1.1 x 10(13) cm(3) mol(-1) s(-1).

State-resolved thermalization of singlet and mixed singlet-triplet states of CH2

The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH(2) (~a (1)A1) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH(2) is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH(2) population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times.

Measurement of the sixth overtone band of nitric oxide, and its dipole moment function, using cavity-enhanced frequency modulation spectroscopy

We applied cavity-enhanced frequency modulation absorption spectroscopy (also known as noise-immune cavity-enhanced optical heterodyne molecular spectroscopy) to perform high-resolution spectroscopy of the sixth overtone band of nitric oxide near 797 nm. By using novel high-bandwidth balanced detectors, baseline variations caused by residual amplitude modulation were significantly reduced. The ultrahigh sensitivity (2 x 10(-10) cm(-1) minimum detectable absorption at 1 Hz detection bandwidth) of our spectrometer allowed accurate measurements of 15 individual line intensities of P- and R-branch transitions in the 2Pi(1/2)-2Pi(1/2) subband. A vibrational transition moment of 3.09(6) muD+/-13% and Herman-Wallis coefficients of a = -0.0078(26) and b = 0.001 25(45) were found by fitting the line intensities. Based on our measured transition moment, and those of other transitions from the literature, a new parametrization for the electric dipole moment function (EDMF) of nitric oxide, valid for 0.91 < or = r < or = 1.74 A, has been extracted. The residuals of this fit demonstrate that the derived EDMF is the most accurate representation to date of the dipole moment function. The new EDMF will be valuable for accurate intensity prediction of transitions that cannot be easily measured experimentally.

Water Dependence of the HO2 Self Reaction: Kinetics of the HO2−H2O Complex

Transient absorption spectra and decay profiles of HO2 have been measured using cw near-IR two-tone frequency modulation absorption spectroscopy at 297 K and 50 Torr in diluent of N2 in the presence of water. From the depletion of the HO2 absorption peak area following the addition of water, the equilibrium constant of the reaction HO2 + H2O <--> HO2-H2O was determined to be K2 = (5.2 +/- 3.2) x 10(-19) cm3 molecule(-1) at 297 K. Substituting K2 into the water dependence of the HO2 decay rate, the rate coefficient of the reaction HO2 + HO2-H2O was estimated to be (1.5 +/- 0.1) x 10(-11) cm3 molecule(-1) s(-1) at 297 K and 50 Torr with N2 as the diluent. This reaction is much faster than the HO2 self-reaction without water. It is suggested that the apparent rate of the HO2 self-reaction is enhanced by the formation of the HO2-H2O complex and its subsequent reaction. Results are discussed with respect to the kinetics and atmospheric chemistry of the HO2-H2O complex. At 297 K and 50% humidity, the concentration ratio of [HO2-H2O]/[HO2] was estimated from the value of K2 to be 0.19 +/- 0.11.

Hyperfine quantum beats from photolytic orientation and alignment

Using Doppler-resolved transient frequency modulation absorption spectroscopy, we have measured both alignment and orientation quantum beats arising from nuclear hyperfine depolarization of low rotational levels of CN X 2sigma+ photofragments produced in the ultraviolet photodissociation of ICN. Under our conditions of fully resolved fine-structure and completely unresolved hyperfine structure, the observed time evolution is insensitive to the rapid depolarization of angular momentum N that would be observed if the fine-structure components were not resolved. Non-statistical spin-rotation populations of CN photofragments, previously observed only for high rotational states, are now observed even for the lowest rotational states, indicating that the electron can not be considered a spectator spin in the depolarization.

A Shock Tube Study of Benzylamine Decomposition: Overall Rate Coefficient and Heat of Formation of the Benzyl Radical

The decomposition rate of benzylamine (C6H5CH2NH2) and the heat of formation of the benzyl radical (C6H5CH2) were determined in shock tube experiments combined with RRKM calculations. To obtain the decomposition rate of benzylamine, the NH2 mole fraction was measured using frequency-modulation absorption spectroscopy behind reflected shock waves. The initial slope of the NH2 concentration is directly proportional to the decomposition rate and the initial concentration of benzylamine. The rate expression for the decomposition reaction for the temperature range 1225−1599 K and the pressure range 1.19−1.47 bar is k1 = (5.49 × 1014)e-33110/[T(K)] s-1 with an uncertainty of ±15%. To obtain the high-pressure-limit rate expression for benzylamine decomposition, we performed RRKM calculations using the parameters obtained from the experimental data of this study and those of the VLPP study of Golden et al.4 The resulting high-pressure-limit rate for the temperature range 1000−1600 K is k∞ = (1.07 × 1016.0)e-36470...

A shock tube study of the NH 2+NO 2 reaction

The overall rate coefficient and the branching ratio of the NH 2 +NO 2 reaction were determined inshock tube experiments. Benzylamine (C 6 H 5 CH 2 NH 2 ) was used as a thermal source of the NH 2 radical. To obtain the overall rate coefficient, the NH 2 mole fraction was measured using frequency-modulation absorption spectroscopy. For the branching ratio deterrnination, the IR emission from N 2 O was mornitored. During the N 2 O emission experiments, the NO 2 mole fraction was measured simultaneously using a tunable diode laser. The mesured species profiles were interpreted using a detailed 118-reaction mechanism, which was based on the reaction mechanism from our previous study of the NH 2 +NO reaction with added reactions involving N 2 O and NO 2 . The overall rate coefficient is (5.5±0.8)×10 12 cm 3 mol −1 s −1 in the temperature range, 1330–1527 K, and pressure range 1.31–1.49 bar. This value is 1.6 times the result of Glarborg et al. and 2.5 times that extrapolated from the expression reported by Park and Lin. The measured branching ratio for the N 2 O+H 2 O product channel of the NH 2 +NO 2 reaction was 0.17±0.04 for the temperature range 1319–1493 K, which is consistent with the results of Park and Lin and Quandt and Hershberger at lower temperatures.

Rotationally resolved A 2Π←X 2Π electronic absorption spectrum of cyanotriacetylene cation in the gas phase

The origin band of the A 2Π←X 2Π electronic transition of cyanotriacetylene cation has been detected in the gas phase. The ions were generated in a liquid-nitrogen cooled hollow cathode discharge incorporated in a multiple reflection cell. The technique of frequency modulation absorption spectroscopy is used to observe the band at Doppler-limited resolution. Analysis of the rotational structure yields accurate values of the rotational constants in the two electronic states, confirming a linear geometry. Comparison with astronomical measurements of diffuse interstellar bands is made.

The unimolecular dissociation of 2-butenenitrile: measurements of the CN elimination channel using FM Doppler spectroscopy

The Doppler profiles of the nascent CN (X 2Σ +) fragments from the photodissociation of 2-butenenitrile, CH 3CHCHCN, at 193 nm have been measured using transient frequency-modulation absorption spectroscopy. State-dependent translational energy distributions were determined for selected CN states and can be modeled assuming a statistical partitioning of the available energy. The translational energy distributions suggest that the allyl radical rather than the 1-propenyl isomer is formed. No significant fragment v · j correlation was observed for any of the CN states measured. The results are consistent with dissociation on the ground electronic state without an exit barrier. Ab initio calculations employing MP2 gradient geometry optimizations and QCISD(T) single point energy calculations were performed for the relevant product species and transition states associated with isomerization and CN elimination. The calculations indicate that H-atom migration occurs prior to dissociation. The elimination of CN is a minor channel with a quantum yield of 0.01±0.01 at 193 nm.

Transient frequency modulation absorption spectroscopy of molecules produced in a laser ablation supersonic expansion source

Transient frequency modulation absorption spectroscopy has been applied to transition metal containing molecules generated in a laser ablation/chemical reaction supersonic expansion. The (0, 0) A″ 1 Σ + – X 1 Σ + band system of PtC ( ν=12 643 cm −1) and the (1,0) C 3 Δ 1 – X 3 Δ sub-band system of TiS ( ν=12 065 cm −1) were recorded in single pass through the expanding gas 4 cm from the source.

Shock tube determination of the overall rate of NH 2+NO→products at high temperatures

The rate coefficient of the reaction NH 2 +NO→products (R1) was determined in shock tube experiments using frequency modulation absorption spectroscopy for detection of NH 2 . Due to the sensitivity of the diagnostic system, very low reactant concentrations could be employed to reduce the influence of secondary reactions on the NH 2 profiles. The source of the NH 2 radicals in the experiments was the thermal decomposition of CH 3 NH 2 . To determine k 1 , a perturbation strategy was employed that is based on changes in the NH 2 profiles when NO is added to the CH 3 NH 2 /Ar mixtures. Analysis showed that NH 2 profiles in the CH 3 NH 2 /NO/Ar mixtures were sensitive primarily to reaction R1, with significantly lower sensitivity to the branching ratio and other NH 2 reactions. The measured NH 2 profiles were interpieted by detailed kinetic modeling to obtain k 1 values in the temperature range 1716–2507 K. The present, k 1 values are consistent with recent theoretical results of Miller and coworkers. There is no evidence for a positive activation energy for this reaction at elevated temperatures as report in several other high-temperature experimental studies. Combining the present high-temperature data with lower temperature determinations yields the following simple expression for the overall reaction rate. k 1 =2.08×10 16 T −1.34 cm 3 /mol s for the temperature range 300–2500 K.

Rotationally resolved A 2Π←X 2Π electronic spectra of cyanodiacetylene and dicyanoacetylene cations

The 000 bands of the A 2Π←X 2Π and A 2Πg←X 2Πu transitions of cyanodiacetylene and dicyanoacetylene cations have been rotationally analyzed at Doppler-limited resolution using frequency modulation absorption spectroscopy. The ions were generated in a liquid-nitrogen cooled hollow cathode modulated discharge incorporated in a White cell. Analysis of the rotational structure yields molecular parameters of the cations in the two electronic states, confirming linear geometries. The inferred spin–orbit and rotational constants are discussed in terms of the electronic structure of the isoelectronic cations. The spectrum of dicyanoacetylene cation reveals Λ-type doubling consistent with the presence of a nearby Σg+2 state. The second order spin-orbit splitting of the A 2Πg state is analyzed to predict the existence of the Σg+2 state below the A 2Πg state.

Vector signatures of adiabatic and diabatic dynamics in the photodissociation of ICN

Nascent Doppler profiles of CN (X 2∑+) fragments from the à band photodissociation of room temperature ICN have been measured using high-resolution transient frequency modulated absorption spectroscopy. Results for dissociation at 222 nm, 248 nm, 266 nm, and 308 nm are presented. From the Doppler profiles of multiple CN states, we determine branching ratios of the coincident atomic iodine states, and bipolar moments characterizing the CN velocity and angular momentum anisotropy. The measurements provide sensitive tests of the strengths of optical coupling to the excited states contributing to the à band continuum, and the adiabatic and diabatic dynamics leading to the observed product states. Precise velocity measurements resolve differences in the average energy of the ICN molecules leading to selected fragment channels. We find a bond energy for ICN of 26 980±100 cm−1, somewhat higher than previous literature values.

Rotationally resolved à 2Πg←X̃ 2Πu electronic spectrum of triacetylene cation by frequency modulation absorption spectroscopy

The spectrum of the à 2Πg←X̃ 2Πu 000 band system of the triacetylene cation and isotopic derivatives DC6H+ and C6D2+ have been studied at Doppler-limited resolution using frequency modulation absorption spectroscopy. The ions were generated in a liquid-nitrogen-cooled hollow cathode discharge incorporated in a White cell. A discharge modulation in combination with the frequency modulation technique was used to enhance the detection sensitivity. Analyses of the rotational structure yield accurate rotational and spin-orbit interaction constants of triacetylene cation in the two electronic states and information on its geometry.

Transient frequency-modulation absorption spectroscopy of free radicals in supersonic free jet expansions

A new technique combining supersonic jet expansion and transient frequency-modulation absorption experiments has been successfully developed for spectroscopic investigations of free radicals. The A ̃ ← X ̃ transition spectra of NH 2 and HCCl at near-infrared wavelengths have been obtained using this technique for demonstration. In addition, a combination of temporal waveform acquisition and frequency scan using this new technique has produced interesting information on the velocity and density distributions of supersonic jet expansion systems.

Diode laser spectroscopy of methane overtone transitions

With the aid of commercial room-temperature AlGaAs diode lasers, frequency modulation absorption spectroscopy was performed on the 7900 A and 8600 A rovibrational combination overtone bands of methane. Three weak transitions are reported in the range around 8610 A that, to our knowledge, have not yet been observed and measured. Self-broadening and pressure-broadening coefficients of one ofthese new absorption features (at 8608.93 A) were derived from CH(4) and for CH(4) immersed in N(2) and He buffer gases. An evaluation of the methane detection sensitivity is given for favorable laboratory conditions as well as for an open-path situation.