Frequency Modulation Absorption 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.

Direct measurement of high-lying vibrational repumping transitions for molecular laser cooling

Molecular laser cooling and trapping requires addressing all spontaneous decays to excited vibrational states that occur at the $\gtrsim 10^{-4} - 10^{-5}$ level, which is accomplished by driving repumping transitions out of these states. However, the transitions must first be identified spectroscopically at high-resolution. A typical approach is to prepare molecules in excited vibrational states via optical cycling and pumping, which requires multiple high-power lasers. Here, we demonstrate a general method to perform this spectroscopy without the need for optical cycling. We produce molecules in excited vibrational states by using optically-driven chemical reactions in a cryogenic buffer gas cell, and implement frequency-modulated absorption to perform direct, sensitive, high-resolution spectroscopy. We demonstrate this technique by measuring the spectrum of the $\tilde{A}^2\Pi_{1/2}(1,0,0)-\tilde{X}^2\Sigma^+(3,0,0)$ band in $^{174}$YbOH. We identify the specific vibrational repump transitions needed for photon cycling, and combine our data with previous measurements of the $\tilde{A}^2\Pi_{1/2}(1,0,0)-\tilde{X}^2\Sigma^+(0,0,0)$ band to determine all of the relevant spectral constants of the $\tilde{X}^2\Sigma^+(3,0,0)$ state. This technique achieves high signal-to-noise, can be further improved to measure increasingly high-lying vibrational states, and is applicable to other molecular species favorable for laser cooling.

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...

Rotational spectroscopy of the HCCO and DCCO radicals in the millimeter and submillimeter range

Context. The ketenyl radical HCCO has recently been detected in the interstellar medium (ISM) for the first time. Further astronomical detections of HCCO will help us understand its gas-grain chemistry, and subsequently revise the oxygen-bearing chemistry towards dark clouds. Moreover, its deuterated counterpart DCCO has never been observed in the ISM. A broad spectroscopic investigation is still lacking for both HCCO and DCCO, although they exhibit a significant astrophysical relevance.Aims. In this work we aim to measure the pure rotational spectra of the ground state of HCCO and DCCO in the millimeter and submillimeter region, considerably extending the frequency range covered by previous studies.Methods. The spectral acquisition was performed using a frequency-modulation absorption spectrometer between 170 and 650 GHz. The radicals were produced in a low-density plasma generated from a select mixture of gaseous precursors. We were able to detect and assign more than 100 rotational lines for each isotopolog.Results. The new lines have significantly enhanced the previous data set allowing the determination of highly precise rotational and centrifugal distortion parameters. In our analysis we took into account the interaction between the ground electronic state and a low-lying excited state (Renner–Teller pair) which enables the prediction and assignment of rotational transitions withKaup to 4.Conclusions. The present set of spectroscopic parameters provides highly accurate, millimeter, and submillimeter rest-frequencies of HCCO and DCCO for future astronomical observations. We also show that towards the pre-stellar core L1544, ketenyl peaks in the region wherec-C3H2peaks, suggesting that HCCO follows a predominant hydrocarbon chemistry, as already proposed by recent gas-grain chemical models.

Laboratory measurements and astronomical search for cyanomethanimine.

C-cyanomethanimine (HNCHCN), existing in the two Z and E isomeric forms, is a key prebiotic molecule, but, so far, only the E isomer has been detected toward the massive star-forming region. Sagittarius B2(N) using transitions in the radio wavelength domain. With the aim of detecting HNCHCN in Sun-like-star forming regions, the laboratory investigation of its rotational spectrum has been extended to the millimeter-/submillimeter-wave (mm-/submm-) spectral window in which several unbiased spectral surveys have been already carried out. High-resolution laboratory measurements of the rotational spectrum of C-cyanomethanimine were carried out in the 100-420 GHz range using a frequency-modulation absorption spectrometer. We then searched for the C-cyanomethanimine spectral features in the mm-wave range using the high-sensitivity and unbiased spectral surveys obtained with the IRAM 30-m antenna in the ASAI context, the earliest stages of star formation from starless to evolved Class I objects being sampled. For both the Z and E isomers, the spectroscopic work has led to an improved and extended knowledge of the spectroscopic parameters, thus providing accurate predictions of the rotational signatures up to ~700 GHz. So far, no C-cyanomethanimine emission has been detected toward the ASAI targets, and upper limits of the column density of ~ 1011-1012 cm-2 could only be derived. Consequently, the C-cyanomethanimine abundances have to be less than a few 10-10 for starless and hot-corinos. A less stringent constraint, ≤ 10-9, is obtained for shocks sites. The combination of the upper limits of the abundances of C-cyanomethanimine together with accurate laboratory frequencies up to ~ 700 GHz poses the basis for future higher sensitivity searches around Sun-like-star forming regions. For compact (typically less than 1″) and chemically enriched sources such as hot-corinos, the use of interferometers as NOEMA and ALMA in their extended configurations are clearly needed.

Accurate sub-millimetre rest frequencies for HOCO+ and DOCO+ ions

HOCO$^+$ is a polar molecule that represents a useful proxy for its parent molecule CO$_2$, which is not directly observable in the cold interstellar medium. This cation has been detected towards several lines of sight, including massive star forming regions, protostars, and cold cores. Despite the obvious astrochemical relevance, protonated CO$_2$ and its deuterated variant, DOCO$^+$, still lack an accurate spectroscopic characterisation. The aim of this work is to extend the study of the ground-state pure rotational spectra of HOCO$^+$ and DOCO$^+$ well into the sub-millimetre region. Ground-state transitions have been recorded in the laboratory using a frequency-modulation absorption spectrometer equipped with a free-space glow-discharge cell. The ions were produced in a low-density, magnetically-confined plasma generated in a suitable gas mixture. The ground-state spectra of HOCO$^+$ and DOCO$^+$ have been investigated in the 213-967 GHz frequency range, with the detection of 94 new rotational transitions. Additionally, 46 line positions taken from the literature have been accurately remeasured. The newly-measured lines have significantly enlarged the available data sets for HOCO$^+$ and DOCO$^+$, thus enabling the determination of highly accurate rotational and centrifugal distortion parameters. Our analysis showed that all HOCO$^+$ lines with Ka $\geq$ 3 are perturbed by a ro-vibrational interaction that couples the ground state with the v$_ 5$ = 1 vibrationally-excited state. This resonance has been explicitly treated in the analysis in order to obtain molecular constants with clear physical meaning. The improved sets of spectroscopic parameters provide enhanced lists of very accurate, sub-millimetre rest-frequencies of HOCO$^+$ and DOCO$^+$ for astrophysical applications. These new data challenges a recent tentative identification of DOCO$^+$ toward a pre-stellar core.

The Application of a High Sensitive Laser Frequency Modulation Absorption Specroscopy

Due to the weak absorption of H2O in the near infra red region, the frequency modulation (FM) is one of good technologies to increase the sensitivity of detection. This method is used to study the absorption spectra of water-vapor in the region around 814.65 nm. Compared to direct absorption method, the signal-to-noise of spectral signal is enhanced. Therefore, the laser FM is a good potential spectroscopic technology in the application of laser sensor for trace gas.

Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for possible application in cavity-enhanced absorption spectroscopy.In order to estimate an absorbance baseline noise of laser intensity and frequency modulated light triplet passed through the V-cavity in open air, a 1.5-cm long optical cell filled by C2H2 at low pressure was placed behind the cavity output mirror. The performance of the setup was evaluated from the experimental bandwidth normalised relative intensity noise on the cavity output and the frequency modulation absorption signals induced by C2H2 absorption in the 1.5-cm cell. From these data we estimate that the noise-equivalent absorption sensitivity of 2.1*10^-11 cm^-1 Hz^-1/2 by a factor of 11.7 above a shot-noise limit can be achieved for C2H2 absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM00 resonances.

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 the Product Branching Ratio of the NH2 + NO Reaction at High Temperatures

The product branching ratio of NH2 + NO → NNH + OH and NH2 + NO → N2 + H2O has been determined in the temperature range 1826−2159 K in a shock tube study. The pressure range behind reflected shock waves was 1.10 to 1.21 bar. The time history of the NH2 radical was measured using a frequency modulation absorption technique. The initial gas mixture compositions were 8−28 ppm monomethylamine (CH3NH2), 0.4−1.2% ammonia (NH3), and 0.4% NO, and the balance Ar. According to sensitivity analysis using a detailed 125-reaction mechanism, the NH2 profiles are mainly sensitive to the branching ratio α = k1a/(k1a + k1b) under the experimental conditions of this study, and they exhibit only a small sensitivity to secondary reactions and the overall rate coefficient, k1 = k1a + k1b. The branching ratio increases from 0.59 ± 0.02 at 1826 K to 0.66 ± 0.04 at 2159 K. These results are consistent with our earlier measurements of the branching ratio at lower temperature and agree with the theoretical result of Miller and Klippenstein.

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.